英文原版教材选编geology1-n


ACKNOWLEDGMENTS
The author thanks the following organization for providing photographs for this book: the National Aeronautics and Space Administration(NASA), the Nation Oceanic and Atmospheric Administration (NOAA), the U.S. Army Corps of Engineers, the USDA Forest Service, the USDA Soil Conservation Service, the U.S. Defense Nuclear Agency, the U.S. Department of Energy, the U.S. Geological Survey (USGS), the U.S. Maritime Administration, the U.S. Navy, and the Woods Hole Oceanographic Institution (WHOI).
The author also thanks Sir Ernest Shackleton, Jon Erickson, S. Jeffress Williams, Peter Barnes, Ellen J. Prager, Pro. Zhinan Sun, and Mrs. Luqin Chen.

0 INTRODUCTION
Our planet contains so much water that perhaps it would have been better named Oceania. Ours is the only known body in the Solar System that is surrounded by water filled with unique geologic structures and teeming with a staggering assortment of marine life forms. Some of the strangest creatures on Earth, whose ancestors go back several hundred million years, live on the deep ocean floor. Much of the world’s untapped wealth lies under the waves. And the seabed is a new frontier for the discovery of mineral resources.
The floor of the ocean features a rugged landscape unmatched anywhere on the continents. Vast undersea mountain ranges much more extensive than those on land crisscross the seabed. Although deeply submerged, the midocean ridges are easily the most pronounced features on the planet, extending over an area larger than that covered by all the major terrestrial mountain ranges combined. The ocean floor is continuously being created at spreading ridges, where molten rock oozes out of the mantle, and being destroyed in deep-sea trenches off the continents and island arcs in the open ocean. The subduction of the ocean crust in deep-sea trenches plays a fundamental role in global tectonics and accounts for powerful geologic forces that continuously shape the planet.
An extraordinary number of volcanoes are hidden under the waves, and most of the volcanic activity that continually remarks the surface of the Earth occurs on the ocean floor. Active volcanoes rising up from the bottom of the ocean create the tallest mountains, dwarfing even those on the continents. Most of the world’s islands in fact began as undersea volcanoes that broke the surface of the sea. However, the preponderance of marine volcanoes are not exposed at the surface and remain as isolated seamounts. Many ridges host an eerie world that time forgot, a cold, dark abyss comprised of tall chimneys spewing hot, mineral-rich water that support unusual animals previously unknown to science.
Rivers of strong, flowing ocean currents are the main transport system for distributing water and heat to all corners of the world. Chasms that challenge the largest terrestrial canyons plunge to great depths. Massive submarine sides gouge deep depressions into the seabed and deposit enormous heaps of sediment on the ocean floor. Undersea slides also occasionally generate tall waves that pound nearby shores, causing much destruction to seaside communities. Abyssal storms with strong currents sculpt the ocean bottom, churning up huge clods of sediment, dramatically modifying the seafloor. The scouring of the seabed and the deposition of large amounts of sediment results in a highly complex marine geology.
The most intriguing terrain features exist on the bottom of the ocean. The ocean floor hosts a myriad of unique geologic formations. Unusual seamounts erupt mud instead of lava like most volcanoes. Scattered along the seafloor are remarkable volcanic deposits, including piles of pillow lavas, forests of black and white smokers, and undersea geysers, whose warm, mineral-laden water rises to the surface in massive plumes. The active undersea world sports a variety of sea caves, blue holes, calderas, and craters formed by undersea explosions and meteorite impacts. These are just a few examples among the many wonders of the seabed.
0.1 DIFFICULTIES ARE JUST THINGS TO OVERCOME
Marine geology is concerned with the character and history of that part of the earth covered by seawater. The importance of marine geology is evident when we consider that three-fourths of the earth’s surface is covered by water. Areas of concern range from the beach to marine marshes and lagoons, across the continental shelf, and down to the deepest parts of the ocean. Marine geologists and geophysicists rarely restrict their research to areas below sea level because a considerable body of important information about the history of the earth and the oceans is gained from rocks exposed above sea level. Marine stratigraphers and paleontologists often examine uplifted marine sediments. Likewise, those interested in the evolution of the oceanic crust often visit oceanic crust. The little of one of the papers by the marine geologist Philip Kuenen [1958] was “ No Geology without Marine Geology.”
The primary aim of marine geology and geophysics is to develop an understanding of the structure of the earth beneath the oceans, the history and character of processes that have shaped the earth beneath the sea, and oceanic and global history itself. The roles that geophysicists and geologists play in developing this knowledge overlap considerably, although geophysicists traditionally have been more concerned with the earth’s structure, while geophysicists traditionally have been concerned mainly with the earth’s history. Nevertheless, the discoveries by geophysicists about oceanic structure have directly led to some of our more fundamental advances in knowledge of the history of the earth. Marine geologists differ from land-based investigators mainly because they use different tools. Because marine geologists are unable to walk over the outcrops directly and sample the ocean floor (except from submersibles), special methods have been developed for submarine sampling. Virtually all marine research requires a vessel of some sort. Because the methods employed by marine geologists are so different from those of land geologists and because the ocean tends to act as one major geochemical system, the lines along which marine geologists reason tend to be different from those of land geologists. Marine geologists also differ from marine geophysicists in that they are concerned primarily with the study of rocks and sediments. Geophysicists, on the other hand, work mainly with data related to the earth’s gravity, heat flow, magnetism, earthquakes, and with artificially generated sound waves transmitted through sediment and rock sequences.
Almost everything we know about the geology of the oceans has been discovered during the last 40 years. During the first half of the twentieth century, considerable intuition existed concerning the potential of the oceans toward providing critical information about the character and history of the earth. The dominant feature of the surface of the earth is its oceans, covering about 72 percent of the surface. The distribution of this water and land is not even, since about 81 percent of the Southern Hemisphere is covered by ocean, compared with 61 percent of the Northern Hemisphere. A hemisphere with a pole in New Zealand contains 89 percent sea and only 11 percent land surface. Continental areas of the earth are almost always (95 percent) antipodal to oceanic areas. The total volume of water in the oceans is 1350 million cubic kilometer (km3) [Menard and Smith,1966], while the average depth of the ocean is about 3700 meters(m). This is in contrast with the average elevation of the land surface, which is only 850 m. However, both continents and ocean form only a veneer upon the vast globe. An examination of a globe of the earth also shows that the ocean is broken up by large continental areas into a few major oceans and numerous smaller oceans or seas. The longest uninterrupted oceanic stretch lies south of about 50°S----Eurasia-Africa, North and South America, and Australia. This configuration of the earth’s surface is only the most recent, since the relative positions of continents and land have been in a state of dynamic change through all of geologic time. To a large extent this book is the story of this dynamic oceanic evolution: the story of tectonic and of geographic change and of the interactions of the global environmental system, especially marine sediments and life.
0.2 HISTORY OF MARINE GEOLOGY
Most of the earth’s surface has been explored only during the last few decades as marine geology developed in conjunction with oceanography. This period represents one of the greatest ages of geographic and scientific exploration. The foundations for this era of oceanic exploration were begun about 150 years ago by a few investigators involved in some of the earliest scientific expeditions. During the voyage of the H.M.S beagle from 1831 to 1836, Charles Darwin’s observations about the origin of species by evolution and its implications concerning antiquity of life established a scientific base for the further exploration of the earth’s history. Darwin also provided some of the earliest speculations concerning movement of the ocean floor during his attempts to explain the origin of coral atolls.
Development of knowledge about the bathymetry of the oceans began in the middle 1800s. Before that time there was little concept of oceanic depths because of the short lengths of rope used in most soundings. The first accurate deep-sea soundings seem to have been made by Sir James Ross in 1840 during a voyage to the Antarctic with H.M.S Erebus and Terror. During this expedition, Ross discovered bottom at 14,550 ft at one location, demonstrating the considerable depths of the ocean basins. In response to the needs of shipping, extensive bathymetric surveys began in the nearshore areas of the eastern United States. By 1843 about 30,000 km2 of the inner part of the continental shelf had been sounded by the “Survey of the Coast,” which was later renamed the U.S. Coast and Geodetic Survey . In 1830 a parallel bureau was organized within the U.S. Navy Department to compile charts of deeper waters, under the direction of Lt. Charles Wilkes. In 1842 Lt. Matthew Fontain Maury succeeded Lt. Wilkes and expanded operations. This depot became the U.S. Navy Hydrographic Office in 1866. The first deep-sea bathymetric chart was published by Maury. Because of his influence within the Navy, Maury had the naval ships equipped with 10,000-fathom reels of baling twine and 64-lb cannonballs for use as sinkers on the sounding lines. He used the data generated from these surveys (180 deep soundings) to construct the first deep-sea bathymetric chart of the oceans in the Atlantic from 52°N to 10°S. This map served as a basis for laying the first trans-Atlantic telegraph cables. Because of these efforts, Maury can be regarded as the first marine geologist.
As the need increased to lay trans-Atlantic and other submarine telegraph cables, there was interest in the character of the deep-sea floor and whether life could exist at such depths. An influential British biologist of the time, Edward Forbes, claimed that no life existed in the oceans below about 600 m. he had developed these ideas during an expedition on H.M.S. Beacon in the Mediterranean in 1841 even though bottom-living organisms had been dredged to depths as great as 1380 ft. he gave the name azoic zone to the deep parts of the ocean inferred to contain no life. The azoic zone was thought to have been formed by the depletion of oxygen because of reduced circulation and stagnation. The azoic theory was not refuted until the 1860s, when ocean cables, on being raised to the surface for repairs, were found to have living organisms attached to them. This discovery of life in the deep sea in turn stimulated interest in the search for primitive life (living fossils) that might have survived in what was then considered to be the relatively unchanging deep-sea environment. Another major concept related to the deep sea resulted from the study of deep-sea calcareous oozes by Thomas Huxley, a close friend of Charles Darwin. Huxley had noticed that the chalk cliffs on land were composed of tiny calcareous shells of planktonic organisms like those preserved in deep-sea sediments and suggested that the chalks could be uplifted deep-sea sediments.
By the 1860s, a number of questions had been formulated about the deep ocean and a climate had developed in which an extensive expedition could be mounted to answer these questions. One of the most influential advocates of such an expedition was Charles Wyville Thomson, the successor of the Edward Forbes’s chair in natural history at Edinburgh University. Thomson had not accepted the theory of evolution and believed that the vertical range of various groups of organisms in the ocean corresponded closely with their vertical range in strata. In other words, he believed more primitive creatures become dominant with increase in oceanic depths. The Royal Society of London was persuaded to sponsor the most ambitious and innovative scientific project ever attempted—a global survey of the deep ocean. This was the Challenger expedition (1872-1876), under the direction of Thomson. The Challenger was a corvette of 2300 tons with auxiliary steam power, and her assignment was to determine “the conditions of the deep-sea throughout all the great oceanic basins.” The expedition, still the longest, covered nearly 70,000 nautical miles in 4 years, carried out almost 500 deep soundings and 133 dredgings and obtained various data from 362 stations (one every 200 miles). As a result of the expedition, 715 new genera and about 4500 new species were described. The introduction of steel cable in 1870 greatly facilitated deep-sea sampling operations. A deep-sea sounding of 1800m in the Mariana Trench was the deepest up to that time.
The Challenger expedition provided a solid foundation for marine geology. Samples of bottom sediment provided the basis for the recognition of the major types of marine sediments, their classifications, and their general distribution patterns throughout the oceans. The formation of deep-sea oozes was extensively studied by Sir John Murray. He developed accurate ideas about the role of planktonic organisms in determining the character of the deep-sea sediments at different depths and latitudes throughout the oceans. He was also able to differentiate various planktonic microfossil assemblages from different latitudes, thus providing a foundation for the biogeography of oceanic plankton. Murray, because of the breadth and brilliance of his geological work, is generally considered the father of modern submarine geology. He tool major responsibility for the publication of the challenger reports, volumes of results of the expedition, which remained the main source of knowledge of the ocean floor until the 1930s. Murray capped his career by jointly publishing The Depth of the Ocean with J.Hjort in 1812. For many years, it was one of the most widely read books on oceanography. Although the challenger expedition added much knowledge about the deep ocean, it did little to alter the false concept that had reigned for years before the expedition—that the deep sea is a tranquil environment with uninterrupted sediment deposition. This myth was not destroyed for almost another 100 years.
For the 70 years following the challenger expedition, few new data were gathered about the geology and geophysics of the ocean floor. The oceans were largely left to the biologists. In America, charts of sediment patterns had been drawn by Delesse in the 1860s and Pourtales in the 1870s from samples collected along the continental margin from New England to Florida, but little else was done for many years. In 1912 the theory of continental drift was promoted by the German meteorologist, Alfred Wegener. But, after early heated debate, the theory fell into disrepute due to a lack of essential information about the geology and geophysics of the oceans. One of the major advances during the time period before World War II was the development of the electronic echo sounder to measure ocean depths, replacing the time-consuming and often incorrect wire soundings. The development of the echo sounder resulted from efforts at submarine detection and was first employed during the German Meteor expedition in the South Atlantic in the 1930s. Thus topographic mapping of the oceans made a rapid leap forward. The Meteor expedition demonstrated, through a number of survey lines across the South Atlantic, that a ridge occurred throughout the length of the mid-Atlantic region. The ridge had been discovered in the North Atlantic during cable-laying operations, then it was called the Telegraph Plateau, but its length was unknown. It has been renamed the mid-Atlantic ridge and has been found to be one part of an ocean-wide ridge system.
A second major development between the world wars was the development of techniques to measure gravity at sea. This was pioneered by the Dutch physicist F.A.Vening Meinesz and employed from submarines, representing relatively stable platforms. Vening Meinesz’s most important discovery was large negative gravity anomalies associated with the deep oceanic trenches, such as the one off Indonesia. This suggested that tectonic activity prevents the earth’s crust from reaching isostatic equilibrium in these regions. His observations played an important role in the development of theories of global tectonism. In 1932 Veninng Meinesz carried out investigations using a U.S. submarine in the trench off Puerto Rico in association with the U.S. Navy-Princeton Gravity expedition. One of the members of the scientific party was a graduate student, Harry H. Hess, who later played a principal role in the development of the theory of sea-floor spreading.
Surprisingly little additional work was conducted on deep-sea sediments during this time. The German South Polar expedition of 1901-1903 had collected short sediment cores, which were studies by Pjilippi in 1912. In 1929-1930 during the Snellius expedition, Dutch investigators employed an explosive coring device called a Piggot gun, which obtained sediment cores up to 2m long. Cores such as these allowed geological history to be determined from changes in the sediments and fossils. Similar sores taken from the North Atlantic in the 1930s showed that glaciations can be distinguished from interglacial episodes based on changes in planktonic microfossil assemblages and sediment type [Schott, 1935; Stetson, 1939; Phleger, 1939; Bramlette and Bradley, 1940]. Unfortunately, too few sediment studies had been carried out to disclose the environment of the deep oceans. Until World War II it was still believed that the deep-sea floor was monotonous, uninteresting, and a place of virtually no water movement. In 1942 it was stated in the volume The oceans, by H.Sverdrup,N.Johnson, and R.Fleming [1942], that, “From the oceanographic point of view the chief interest in the topography of the sea floor is that it forms the lower and lateral boundaries of the water.”
World War II had a profound effect on the development of oceanography and its branch, marine geology. Oceanographic research is expensive and the major advances must be sponsored by governments or wealthy individuals. Before the war, funds from governments were meager. Some solid financial support was provided by non profit institutions established by wealthy individuals. These set the stage for later developments that accelerated because of the war effort. World War II radically reordered the priorities of the United States and other nations; the importance of science grew and research appropriations soared. In relation to antisubmarine warfare, research was expanded on sound transmission through water, helping to lay a foundation for seismic studies of marine sediments. Numerous studies of the sea floor were initiated because they influenced the behavior of sound, and work on sediment charts began in early 1943. But for scientists, the most significant out come of the war was an increased confidence in their potential for contributing to the nation’s welfare [Schlee, 1973]. After the war, oceanography emerged as a modern science with a well established base for growth in all disciplines. The concentrated exploration of the oceans began. This also coincided with the completion of geographic exploration of the landmass other than Antarctica. A redistribution of interests occurred within the field; the preeminent position of marine biology was replaced by a more balanced approach emphasizing the physical sciences. Oceanography became tied to government support and government policy. In 1946, the Office of Naval Research (ONR) was established to sponsor long-term research including oceanography. The ONR played the leading role in the rapid development of the oceanographic institutions from the middle 1940s through the late 1960s. In 1950 the U.S. National Science Foundation was established and has taken on increasing responsibility for the support of marine geological research in the United States. Other governments, including the USSR, Britain, France, Canada, New Zealand and, slightly later, West Germany and Japan, also began to support marine geological research on a larger scale. The Swedish deep-sea expedition (1947-48) provided valuable additions to marine geology.
In the postwar environment marine geology developed rapidly. Marine geology and geophysics of the oceans could be studied systematically because of greatly increased funding levels. Most of the world was being explored for the first time. Expansion occurred in many of the large U.S. marine laboratories. Scripps Institution of Oceanography in La Jolla, California, expanded to become the largest oceanographic institution in the United States. It was established for marine research at the turn of the century and incorporated in the University of California in 1912. Roger Revelle, its director from 1948 to 1964, enhanced its emphasis on ocean-floor research. Woods Hole Oceanographic Institution in Woods Hole, Massachusetts, another large institution to grow rapidly during World War II, was chartered in 1930 as a private, nonprofit research institution. Ocean mapping was developed on a large scale by Soviet scientists, who have produced detailed atlases of ocean-floor features and sediments.
By the 1950s the echo-sounding technique had been highly refined, and it was possible to measure ocean depths throughout the world accurately and cheaply to a depth of 10,000m. The resolving power of the modern precision depth recorder is better than one part in 5000, so that a change as small as 1 m can be detected at depths of 5000 m. The oceanographic fleet increased at this time, making the expansion of bathymetric mapping of the oceans possible. After initial work by Maurice Ewing (Fig. 1-1) and Bruce Heezen at the Lamont Geological Observatory of Columbia University (now the Lamont-Doherty Geological Observatory) the mapping project was led by Bruce Heezen and Marine Tharp and led to the publication of the well-known maps of the Pacific, Atlantic, Indian, and Arctic oceans in National geographic of the National Geographic Society, Washington D.C. These maps are so detailed that the have provided many scientists with stimulation in maps are so detailed that they have provided many scientists with stimulation in their individual areas of interest. Heezen concentrated his efforts on exploring the oceanic-ridge systems. The mid-ocean ridge system, more than 3000 m in height, practically encircles the earth. It surpasses the Alps and the Himalayas in scale, yet was discovered in its entirety only 25 years ago. It was Ewing and Heezen who first realized that the earth is encircled by this ridge system [Ewing and Heezen, 1956]. They also discovered that at the crest of the ridges is a narrow trough or rift valley [Ewing and Heezen, 1956]. This central rift valley was interpreted by Carey [1958] to be a narrow block, sinking under tension as the sea floor on either side of the valley moves apart. H.W.Menard of Scripps Institution of Oceanography was concurrently exploring the crests of the East Pacific rise but found no rift valleys.
The Lamont-Doherty Geological Observatory, which was built by Maurice Ewing, played an important postwar role in the growth of knowledge of the geology and geophysics of the oceans. These activities “changed the subject from a polite today; that the floors of the oceans are no longer terra incognita is largely because of Maurice Ewing’s curiosity, which was relentless” [Wertenbaker, 1974]. Very early in his career, Ewing recognized the fundamental differences between oceanic and continental crust, a difference which Ewing called a brutal fact. To develop an understanding about the character of the oceanic lithosphere, including its sediments, Ewing and his colleagues developed techniques using seismic waves artificially created by underwater explosions near the surface of the oceans. This area of investigation is called marine-explosion seismology and provided data on sediment thickness in the ocean and on the suboceanic crustal structure. Later, the air-gun method was developed; it has become the most widely used method for studying sediment character. Sound waves are generated by shooting an air gun into surface water. Continuous reflections of sound waves off buried layers of sediment show the detailed configurations of oceanic sediment. After the war, large quantities of explosives were available for such studies at sea. Ewing began his seismic investigations at Woods Hole in the years immediately preceding World War II and continued them at Lamont after the war. In 1953 he purchased the Vema, the first research vessel for the observatory, which enabled the scientists at Lamont to develop seismic techniques more rapidly. Seismic-reflection techniques eventually understanding of sediment distributions in the ocean basins and deep oceanic processes. Ewing made sure that the Lamont ships were always engaged in data collection of a wide variety including magnetic and gravity data, bottom photographs, and piston cores. Huge libraries of data and sediments accumulated for current and future investigations. David Ericson and Goesta Wollin carried out crucial pioneering micropaleontological studies of the ocean basins. At the same time, Emiliani sediments. Other major structures of the ocean floor were discovered and mapped. Among the most important were the extensive, linear fracture analysis of deep-sea sediments. Other major structures of the ocean basins. At the same time, Emiliani pioneered the use of ocean isotopes for paleotemperature analysis of deep-sea sediments. Other major structures of the ocean floor were discovered and mapped. Among the most important were the extensive, linear fracture zones discovered by Dietz [1952] and Menard [1953] in the Pacific. These have been found to be widespread in all of the ocean basins and have played a critical role in tectonic theory. The mapping of the ocean basins during the 1950s produced rather simple of sea-floor topography, providing a basis for the development of theories phase of the exploration of the oceans is described in the following.
Many recent developments have occurred, including the development of methods for navigation using fixes from satellites, which provide accuracy for an oceanographic station to within 100 m. Deep-sea drilling has been continuing since 1968 and is discussed in Chapter 3. More recently, deep-diving submersibles have been used to observe the features of the ocean floor, especially on the oceanic ridges. Pioneering work on deep-diving developments has been carried out by Jacques-Yves Cousteau, who also initiated the use of scuba diving. Important centers of marine geological research outside the United States include the National Oceanographic Institute, Wormley, United Kingdom; Keil University,West Germany; Centre National pour l’ Exploration des oceans (CNEXO), Brest France; Institute of Oceanology in Moscow, USSR; Bedford Institute and Dalhousie lington, New Zealand; University of Cape Town, South Africa; and the University of Tokyo, Japan. A large number of institutions within the United States carry out marine geological and geophysical research.










1 THE BLUE PLANET
Throughout our planet’s long history, as many as 20 oceans have come and gone, as continents drifted apart and reconverged into supercontinents. The present ocean basins formed after a supercontinent named Pangaea, Greek meaning “all lands,” broke apart into today’s continents about 170 million years ago. Before the breakup, a single large ocean called Panthalassa, Greek meaning “universal sea,” surrounded the supper continent. Prior to the assembly of Pangaea, all continents surrounded an ancient Atlantic Ocean called the Iapetus Sea. Deeper into the past, the continents again formed a supercontinent, and its breakup created entirely new seas, which participated in a great exploration of life. Life itself possibly evolved at the bottom of a global ocean not long after the Earth was created.
1.1 ORIGIN OF SEA AND SKY
During the Earth’s formative years, a barrage of asteroids and comets pounded the infant planet and its moon (Fig.1-1). Some meteorites were stony, with rock and metal; others were icy, with frozen gases and water ice; many contained carbon, as though coal rained down from the heavens. It is possible that these carbon-rich meteorites bore the organic molecules from which life sprang forth. Comets comprising rock debris and ice also plunged into the Earth, releasing tremendous quantities of water vapor and gases. The degassing of these cosmic invaders produced mostly carbon dioxide, ammonia, and methane, major constituents of the early atmosphere, which began to form about 4.4 billion years ago.
Most of the water vapor and gases originated within the Earth itself by volcanic outgassing. Magma contains heavy amounts of volatiles, mostly water and carbon dioxide. Tremendous pressures deep inside the Earth held the volatiles within the magma until it rose to the surface, where the trapped water and gases escaped violently as the magma depressurized. The early volcanoes were extremely explosive because the Earth’s interior was hotter and the magma contained more abundant volatiles than is now the case.
The Earth soon acquired a primordial atmosphere composed of carbon dioxide, nitrogen, water vapor, and other gases belched from a profusion of volcanoes. Water vapor so saturated the air that atmospheric pressure was many times greater than it is today. The early atmosphere contained up to 1,000 times the current level of carbon dioxide, which was fortunate because the sun’s output was only about 75 percent of its present value and a strong greenhouse effect kept the Earth from freezing. The planet also retained its warmth by a high rotation rate, with days only 14 hours long.













Figure 1-1 The surface of the moon viewed from the Apollo 8 spacecraft, with the Earth rising above the lunar horizon. Courtesy of NASA and USGS
The surface of the earth was scorching hot and in a constant rage. Winds blew with tornadic force, and fierce dust storm raging across the dry surface blanketed the planet with suspended sediment, much as do Martian dust storms today (Fig.1-2). Huge lightning bolts darted back and forth, and earth-shaking thunder sent gigantic shock waves reverberating through the air. Volcanoes erupted in one gigantic outburst after another, lighting the sky with white-hot sparks of ash and sending red-hot lava flowing across the land.
The restless Earth rent apart as massive quakes cracked open the thin crust, and huge batches of magma bled through the fissures. Voluminous lava flows flooded the surface, forming flat, featureless plains dotted with towering volcanoes. The intense volcanism also lofted massive quantities of volcanic debris into the atmosphere, giving the sky an eerie red glow. The dust cooled the planet and provided particles around which water vapor coalesced.
With a further drop in atmospheric temperatures, water vapor condensed into heavy clouds that shrouded the planet, completely blocking out the sun and plunging the surface into darkness. As the atmosphere continued to cool, sheets of rain fell from the sky, and deluge upon deluge overflowed the landscape. Raging floods cascaded down steep volcanic slopes and the sides of large meteorite craters, gouging out deep ravines in the rocky plain. Around 4 billion years ago, when the rains ceased and the skies finally cleared, the Earth emerged as a giant blue orb covered by a global ocean nearly 2 miles deep, with scattered chains of volcanic islands.

TABLE 1-1 THE GEOLOGIC TIME SCALE

Era

Period

Epoch

Age (millions of years)

First Life Forms

 

 

 

Cenozoic

 

Quaternary

Holocene

0.01

 

 

 

 

Pleistocene

2

Man

Pliocene

7

Mastodons

Miocene

26

Saber-tooth

 

 

Tigers

Tertiary

Oligocene

37

 

Eocene

54

Whales

Paleocene

65

Horses

 

 

Alligators

 

 

Mesozoic

Cretaceous

 

135

 

 

 

 

Birds

Jurassic

 

190

Mammals

 

 

 

Dinosaurs

Triassic

 

250

 

 

 

 

 

Paleozoic

 

Permian

280

Reptiles

Pennsylvanian

310

 

 

trees

Caboniferous

 

 

Mississippian

345

Amphibians

Devonian

400

Insects

Silurian

435

Sharks

Ordovician

500

Land plants

Cambrian

570

Sea Plants

Shelled animals

 

Proterozoic (Eon)

700

Invertebrates

2500

Metazoans

 

Archean(Eon)

3500

Earliest life

4000

Oldest rocks

4600

Meteorites

Figure 1-2 A boulder-strewn field showing rocks embedded in fine sediment from Martian dust storms. Courtesy of NASA
1.2 THE UNIVERSAL SEA
In a remote mountainous area in southwest Greenland (Fig.1-3), metamorphosed marine sediments of the Isua Formation furnish strong evidence for an early ocean. The continental crust was perhaps only one-tenth of its current size and contained slivers of granite that drifted freely over the Earth’s water face. The Isua rocks originated in volcanic island arcs and therefore lend credence to the idea that plate tectonics operated early in the history of the Earth. The rocks are among the most ancient, dating to about 3.8 billion years, and indicate that the planet had abundant surface water by this time.
Between the end of the great meteorite bombardment and the formation of the first sedimentary rocks about 3.8 billion years ago, large volumes of water flooded the Earth’s surface. Seawater probably began salty due to the volcanic outgassing of chlorine and sodium, but the ocean did not reach its present concentration of salts until about 500 million years ago. The salt level has remained generally constant ever since. However, major changes in seawater chemistry often correlated with biological radiation and extinction.
Most of the crust was deeply submerged during the early history of the Earth, as evidenced by an abundance of chert, which is among the hardest minerals and appears to have precipitated from silica-rich water in deep oceans. Modern seawater is deficient in silica because such organisms as sponges and diatoms extract it to build their skeletons (Fig.1-4). Massive deposits of diatomaceous earth, or diatomite, are a tribute to the great success of these organisms during the last 600 million years. Between 10 and 4 million years ago, mats of diatoms spread across vast areas of the eastern tropical Pacific. When the mats sank, they were preserved in the bottom ooze that slowly accumulated during millions of years.


Figure 1-3 Location of the Isua Formation in southwestern Greenland, which contains some of the oldest rocks on the Earth.

TABLE 1-2 RADIATION AND EXTINCTION OF SPECIES

Organism

Radiation

Extinction

Mammals

Paleocene

Pleistocene

Reptiles

Permian

Upper Cretaceous

Amphibians

Pennsylvanian

Permian-Triassic

Insects

Upper Paleozoic

 

Land plants

Devonian

Permian

Fishes

Devonian

Pennsylvanian

Crinoids

Ordovician

Upper Permian

Trilobites

Cambrian

Carboniferous and Permian

Ammonoids

Devonian

Upper Cretaceous

Nautiloids

Ordovician

Mississippian

Brachiopods

Ordovician

Devonian and Carboniferous

Graptolites

Ordovician

Silurian and Devonian

Foraminiferans

Silurian

Permian and Triassic

Marine invertebrates

Lower Paleozoic

Permian

Figure 1-4 Diatoms from the Choptank Formation, Calvert County, Maryland. Photo by G.W.Andrews, courtesy of USGS
Sulfur was particularly abundant in the early ocean and combined easily with metals like iron to form sulfates. The earliest organisms were sulfur-metabolizing bacteria similar to those living symbiotically in the tissues of tubeworms existing near sulfurous hydrothermal vents on the East Pacific Rise and on a dozen other midocean ridges scattered around the world. Because the atmosphere and ocean lacked significant amounts of oxygen, the bacteria obtained energy by the reduction of sulfate ions. The growth of primitive bacteria was also limited by the amount of organic molecules produced in the ocean.
Iron that was leached from the continents and dissolved in seawater consequently reacted with oxygen in the ocean and precipitated in massive ore deposits on shallow continental margins. Alternating layers of iron-rich and iron-poor sediments gave the ore a banded appearance, called a banded iron formation (BIF). The average ocean temperature was probably warmer than it is today. When warm ocean currents rich in iron and silica flowed toward the polar regions, the suddenly cooled waters could no longer hold minerals in solution; and the minerals precipitated, forming alternating layers because of the difference in settling rates between silica and iron, the heavier of the two minerals. BIF deposits mined extensively throughout the world provide more than 90 percent of the minable iron reserves. In effect, primitive plant life, which generated oxygen by photosynthesis, indirectly created the Earth’s iron deposits.
Oxygen, which currently comprises 21 percent of the atmosphere, was practically nonexistent when life first appeared. Oxygen levels remained low because of the oxidation of dissolved metals in seawater and reduced gases emitted from submarine hydrothermal vents. The seas contained much iron, which reacted with oxygen generated by photosynthesis- a fortunate circumstance, since oxygen was also poisonous to primitive life forms.
About 2 billion years ago, after most of the dissolved iron had been locked up in the sediments, the level of oxygen began to rise and replace carbon dioxide in the ocean and atmosphere. Major plate tectonic cataclysms caused oxygen levels to surge between 2.1 and 1.7 billion years ago, and again between 1.1 and 0.7 billion years ago, during the breakup of supercontinents and the formation of new ocean basins.
Greenstone belts comprising ancient metamorphic rocks in the interiors of continents (Fig. 1-5) are among the best evidence for plate tectonics operating early in the Earth’s history. Ophiolite complexes in greenstones are slices of ocean floor shoved up on the continents by drifting plates and date as old as 3.6 billion years. Blueschists are metamorphosed rocks of subducted ocean crust thrust onto the continents by plate motions. Pillow lavas, which are tubular bodies of basalt extruded undersea, also appear in the greenstone belts, signifying that the volcanic eruptions took place on the ocean floor.


Fig.1-5 Archean greenstone belts comprise the ancient cores of the continents.
Plate tectonics have played a prominent role in shaping the Earth virtually from the very beginning of the planet. Continents were adrift from the time they originated, within a few hundred million years after the formation of the Earth. This tectonic activity is manifested by 4-billion-year-old Acasta gneiss, a metamorphosed granite, in Canada’s Northwest Territories, suggesting that the formation of the crust was well underway by this time. The discovery of this gneiss leaves little doubt that at least small patches of continental crust existed on the Earth’s surface during the first half billion years of its existence.

TABLE 1-3 EVOLUTION OF THE BIOSPHERE

Event

Billions 

of Years 

Ago

Biological Consequence

Percent 

Oxygen

Results

Full oxygen 

conditions

0.4

Fishes, land plants, 

and animals

100

Approach present 

biological conditions

Appearance of 

shelly animals

0.6

Cambrian fauna

10

Burrowing habitat

Metazoans appear

0.7

Ediacarian fauna

7

First metazoan fossils 

& tracks

Eukaryotic cells 

appear

1.4

Larger cells

>1

Red beds, multicellular 

organisms

Blue-green algae

2.0

Algal filaments

1

Oxygen metabolism

Algal precursors

2.8

Stromatolites

<1

Beginning photosynthesis

Origin of life

4.0

Light carbon

0

Evolution of biosphere

The proto-North American continent called Laurentia assembled from a half dozen major crustal fragments called cratons (Fig. 1-6) some 1.8 billion years ago, making it one of the oldest continents. At Cape Smith on Hudson Bay lies a piece of oceanic crust that was squeezed onto the land during this time, a telltale sign that continents collided and enclosed an ancient sea. Arcs of volcanic rock weave through central and eastern Canada down into the Dakotas. In a region between Canada’s Great Bear Lake and the Beaufort Sea lie the roots of an ancient mountain range running through the basement rock, formed by the collision of Laurentia with an unknown landmass more than a billion years ago.
Toward the end of the Precambrian era, some 700 million years ago, all landmasses assembled into a supercontinent centered over the equator. A superocean located approximately in the region of the present Pacific Ocean surrounded the supercontinent. Between 630 and 560 million years ago, the supercontinent rifted apart and 4 or 5 continents rapidly separated. As the continents dispersed and subsided, seas flooded the interiors, creating large continental shelves. Most of the continents huddled around the tropics. This setting heralded an explosion of new life forms in the warm Cambrian seas.






















Figure 1-6 The cratons that comprise the North American continent.
1.3 THE IAPETUS SEA
Approximately 500 million years ago, the continents surrounded a large body of water called the Iapetus Sea (Fig.1-7), which opened as a result of the breakup of the late Precambrian supercontinent. In the Southern Hemisphere, continental motions assembled the present continents of South America, Africa, Australia, Antarctica, and India into the supercontinent Gondwana, named for a geologic province in east-central India.
Present-day Australia sat at the northern edge of Gondwana on the Antarctic Circle. Fossils of the tropical fern glossopteris, whose leaf impressions look like features, appeared in coal beds on the southern continents and India. The plant is absent on the northern continents, which suggests the previous existence of two large continents, one in the Southern Hemisphere and the other in the Northern Hemisphere, separated by a large open sea.
The formation of the Iapetus created extensive inland seas that inundated most of the ancestral North American continent (Laurentia) and the ancient European continent, called Baltica. The Iapetus Sea was similar in size to the present North Atlantic and occupied the same general location. A continuous coastline running from Georgia to Newfoundland between about 570 and 480 million years ago suggests that this ancient east coast faced a wide, deep sea, which stretched at least 1,000 miles across from east to west and bordered a much larger body of water to the south.
















Figure 1-7 About 500 million years ago, the continents surrounded an ancient sea called the Iapetus.
Volcanic island dotted the Iapetus Sea, which resembled the present Pacific Ocean between Southeast Asia and Australia. About 460 million years ago, the shallow waters of the near-shore environment of this ancient sea contained abundant invertebrates, including trilobites-small, oval arthropods that accounted for about 70 percent of all species and a favorite among fossil collectors. Eventually, the trilobites faded, while mollusks and other invertebrates expanded throughout the seas. The vase-shaped archaeocyathans (Fig. 1-8) resembled both sponges and corals and built the earliest limestone reefs, eventually becoming extinct in the Cambrian period.
Between about 420 million and 380 million years ago, Laurentia collided with Baltica, closing off the Iapetus. The collision fused the two continents into the megacontinent Laurasia, named after the Laurentian province of Canada and the Eurasian continent. The Eurasian continent, the largest landmass in the world, formed when some dozen individual continental blocks welded together approximately a half billion years ago (Fig. 1-9).


















Figure 1-8 Archaeocyathans built the earliest limestone reefs.











Figure 1-9 The cratons that comprise Eurasia.
During the formation of Laurasia, island arcs between the two landmasses were scooped up and plastered against continental edges as the oceanic crustal plate carrying the islands subducted under Baltica. This subduction rafted the islands into collision with the continent and deposited the formerly submerged rocks on the present west coast of Norway. Slices of land called terranes, situated in western Europe, drifted into the Iapetus from ancient Africa. Likewise, slivers of crust from Asia traveled across the ancestral Pacific Ocean to form much of western North America.
1.4 THE PANTHALASSA SEA
Throughout geologic history, smaller continental blocks collided and merged into larger continents. Millions of years after assembly, the continents rifted apart, and the chasms filled with seawater to form new oceans. However, the regions presently bordering the Pacific Basin apparently have not collided with each other. Rather, the Pacific Ocean is a remnant of an ancient sea called the Panthalassa, which narrowed and widened in response to continental breakup, dispersal, and reconvergence in the area occupied by today’s Atlantic Ocean.
So, while oceans have repeatedly opened and closed in the vicinity of the Atlantic Basin, a single ocean has existed continuously at the site of the Pacific Basin. Following the breakup of Pangaea in the early Jurassic period about 170 million years ago, the Pacific plate was hardly larger than the present-day United States. The rest of the ocean floor was comprised of other unknown plates that disappeared as the pacific plate grew; consequently, no existing oceanic crust is older than Jurassic in age.
Laurentia, comprising the interior of North America, Greenland, and northern Europe, assembled during the collision of several microcontinents beginning about 1.8 billion years ago and evolved during a relatively brief period of only150 million years. Laurentia continued to grow by garnering bits and pieces of continents and chains of young volcanic islands. About 700 million years ago, Laurentia collided with another large continent on its southern and eastern edges, creating a new supercontinent centered over the equator. A superocean positioned in the approximate location of today’s Pacific Ocean surrounded this supercontinent.
When Laurentia fused with Baltica to form the megacontinent Laurasia, island arcs in the Panthalassa Sea began colliding with the western margin of the present North America. Erosion leveled the continents, and shallow seas flowed inland, flooding more than half the land surface. The inland seas and wide continental margins, along with a stable environment, enabled marine life to flourish and spread throughout the world.
From 360 million to 270 million years ago, Gondwana and Laurasia converged into Pangaea, which straddled the equator and extended almost from pole to pole (Fig.1-10). This massive continent reached its peak size about 210 million years ago, with an area of about 80million square miles or 40 percent of the Earth’s total surface area. More than a third of the landmass was covered with water. An almost equal amount of land existed in both hemispheres, whereas today two thirds of the continental landmass is located north of the equator; below the equator the breakdown is 10 percent landmass and 90 percent ocean. A single great ocean stretched uninterrupted across the planet, with the continents huddled to one side of the globe.















Figure 1-10 The supercontinent Pangaea extended almost from pole to pole.
The sea level fell substantially after the formation of Pangaea, draining the interiors of the continents and causing the inland seas to retreat. A continuous shallow-water margin ran around the entire perimeter of Pangaea, and no major physical barriers hampered the dispersal of marine life. Moreover, the seas were largely restricted to the ocean basins, leaving the continental shelves mostly exposed.
The continental margins were less extensive and narrower than they are now, due to a drop in sea level as much as 500 feet, which confined marine habitats to the near-shore regions. Consequently, habitat area for shallow-water marine organisms was limited, causing low species diversity. Permian ocean life was sparse, with many immobile animals and few active predators. Ocean temperatures remained cool following a late Permian ice age. Marine invertebrates that managed to escape extinction lived in a narrow margin near the equator. Corals, which require warm, shallow water for survival, were particularly hard hit, as evidenced by the lack of coral reefs at the beginning of the Mesozoic era.
1.5 THE TETHYS SEA
With Laurasia occupying the Northern hemisphere and its counterpart Gondwana located in the Southern hemisphere, the two landmasses were separated by a large shallow equatorial body of water called the Tethys Sea (Fig.1-11), named for the mother of the seas in Greek mythology. After the assembly of Pangaea, the Tethys became a huge embayment separating the northern and southern arms of Pangaea, which resembled a gigantic letter C straddling the Equator.












Figure 1-11 About 400 million years ago, the continents surrounded an ancient sea called the Tethys.
The Tethys was a broad tropical seaway that extended from western Europe to southeast Asia and harbored diverse and abundant shallow-water marine life. Reef-building was intense in the Tethys Sea, forming thick deposits of limestone and dolomite laid down by prolific lime-secreting organisms. The tropics served as an evolutionary cradle because they had a greater area of shallow seas than other regions, providing an exceptional environment for new organisms to evolve.
During the Mesozoic era, an interior sea flowed into the west-central portions of North America and inundated the area that now comprises eastern Mexico, southern Texas, and Louisiana. Seas also invaded South America, Africa, Asia, and Australia. The continents were flatter, mountain ranges were lower, and sea levels were higher than at present. Thick beds of limestone and dolomite were deposited in interior seas of Europe and Asia; these rock beds later uplifted to form the Alps and Himalayas.
At the beginning of the Cenozoic era, high sea levels continued to flood continental margins and formed great inland seas, some of which split continents in two. Seas divided North America in the Rocky Mountain and high plains regions, South America was cut in two in the region that later became Amazon basin, and Eurasia was split by the joining of the Tethys Sea and the newly-formed Arctic Ocean.
The oceans were interconnected in the equatorial regions by the Tethys Sea, providing a unique circumglobal oceanic current system that distributed heat to all parts of the world and maintained an unusually warm climate. The higher sea levels reduced the total land surface to perhaps half its present size. About 80 million years ago, the Western Interior Cretaceous Seaway (Fig.1-12) was a shallow body of water that divided the north American continent into the western highlands, comprising the newly forming Rocky Mountains and isolated volcanoes, and the eastern uplands, consisting of the Appalachian Mountains.
During the final stages of the Cretaceous period, when waters receded from the land as sea levels dropped, temperatures in the Tethys Sea began to fall. Most warmth-loving species, especially those living in the tropical Tethys Sea, disappeared when the Cretaceous ended. The most temperature sensitive Tethyan fauna suffered the heaviest extinction rates. Species that were amazingly successful in the warm waters of the tethys dramatically declined when ocean temperatures dropped.














Figure 1-12 The paleogeography of the Cretaceous period, showing the interior sea. Dashed lines indicated ancient landmasses.
Major marine groups that disappeared at the end of the Cretaceous included marine dinosaurs, the ammonoids (ancestors of the nautilus) the rudists (huge coral-shaped clams), and other types of calms and oysters. All the shelled cephalopods were absent in the Cenozoic seas, except the nautilus and shell-less species, including cuttlefish, octopus, and squid. The squid competed directly with fish, which were little affected by the extinction.
Marine species that survived the great extinction were much the same as those of the Mesozoic era. The ocean has a moderating effect on evolutionary processes because it has a longer “memory” of environmental conditions than does land, taking much longer to heat up or cool down. Species that inhabited unstable environments, such as those regions in the higher latitudes, were specially successful. Offshore species fared much better than those in the turbulent inshore waters.
About 50 million years ago, the Tethys Sea narrowed as the African and Eurasian continents collided, closing off the sea entirely beginning about 20 million years ago. Thick sediments in the Tethys Sea separating Gondwana and Laurasia buckled and uplifted into mountain belts on the northern and southern flanks (Fig.1-13). The contact between the continents spurred a major mountain building episode that rised the Alps and other ranges in Europe and squeezed out the tethys Sea.










Figure 1-13 Active fold belts result from crustal compression where continental tectonic plates collide, such as the collision of Africa with Eurasia.
When the Tethys linking the Indian and Atlantic oceans closed as Africa rammed into Eurasia, the collision resulted in along chain of mountains and two major inland seas, the ancestral Mediterranean and a composite of the Black, Caspian, and Aral seas, called the Paratethys, which covered much of what is now Eastern Europe. About 15 million years ago, the Mediterranean separated from the Paratethys, which became a brackish sea. Then, about 6 million years ago, the Mediterranean evaporated, leaving a huge gapping pit whose floor baked in the desert sun. The adjacent Black Sea, a remnant of the ancient Tethys, might have experienced a similar fate.
1.6 THE ATLANTIC
About 170 million years ago, a great rift that developed in the present Caribbean Sea began to separate Pangaea into today’s continents (Fig.1-14). The rift sliced northward through the continental crust that connected North America, Northwest Africa, and Eurasia, separating the continents. In the process, this area breached and flooded with seawater, forming the infant North Atlantic. The rifting occurred over a period of several million years along a zone hundreds of miles wide. At about the same time, India nested to Australia, Antarctica swung away from Africa toward the southeast, forming the proto-Indian Ocean.













Figure 1-14 The breakup of Pangaea: (1) 225, (2) 180, (3) 135, and (4) 65 million years ago.
About 50 million years after rifting began, the infant North Atlantic had achieved a depth of 2 miles or greater. It was bisected by an active midocean ridge system that produced new oceanic crust as the plates carrying the surrounding continents separated. Meanwhile, the South Atlantic began to form, opening up like a zipper from south to north. The rift propagated northward at a rate of several inches per year, similar to the separation rate of the two plates carrying South America and Africa. The entire process of opening the South Atlantic took place in a span of just 5 million years.
The breakup of Pangaea compressed the ocean basins, causing a rise in sea levels and a transgression of the seas onto the land. After this breakup, the continents drifted apart in spurts instead of a constant speed. The rate of seafloor spreading in the Atlantic matches the rate of plate subduction in the Pacific, where one plate dives under another, forming a deep trench. The subduction of old oceanic crust explains why the ocean floor is no older than 170 million years.
By 80 million years ago, the North Atlantic was fully developed ocean. Some 20 million years later, the Mid-Atlantic rift progressed into the Arctic basin, detaching Greenland from Europe. North America was no longer connected to Europe except for a land bridge across Greenland that continued to allow the migration of species between the two continents. The strait between Alaska and Asia narrowed, creating the nearly landlocked Arctic Ocean.
The South Atlantic continued to widen, with more than 1,500 miles of ocean separating South America and Africa. Africa moved northward, leaving Antarctica (still joined to Australia), behind, and began to close the Tethys Sea. In the early Tertiary, Antarctica and Australia broke away from South America and moved eastward. When the two continents rifted apart, Antarctica moved toward the South Pole, while Australia continued moving northeastward.
When Antarctica separated from South America and Australia and drifted over the South Pole some 40 million years ago, the polar vortex formed a circumpolar Antarctic ocean current (Fig. 1-15). This current isolated the frozen continent, preventing it from receiving warm poleward flowing waters from the tropics. Deprived of warmth, Antarctica became a frozen wasteland. During this time, warm salt water filled the ocean depths, while cooler water covered the upper layers.












Figure 1-15 The circum-Antarctic current isolates the waters off Antarctica.
Because of high evaporation rates and low rainfall, warm water in the Tethys Sea became top-heavy with salt and sank to the sea’s bottom. Meanwhile, ancient Antarctica, whose climate was warmer than it is today, generated cool water that filled the sea’s upper layers, causing the deep ocean to circulate from the tropics to the poles, just the opposite of today’s patterns. About 28 million years ago, Africa collided with Eurasia and blocked warm water from flowing to the poles, thereby allowing a major ice sheet to form on Antarctica. Ice flowing into the surrounding sea cooled the surface waters, which sank to the ocean depths and flowed toward the equator, generating the present ocean circulation system.
The early Tertiary coincided with changes in the deep-ocean circulation. Land gathering in one area affects the shapes of ocean basins. The ocean bottom influences how much heat ocean currents carry from the tropics to the poles. The change in ocean circulation eliminated many species of marine life on the European continent, which flooded with shallow seas.
When Greenland separated from North America and Eurasia, beginning about 57 million years ago, it opened the North Atlantic. The separation of Greenland from Europe might have drained frigid arctic waters into the North Atlantic, significantly lowering its temperature. The climate grew much colder and the seas withdrew from the land as the ocean fell by about 1,000 feet to perhaps its lowest level in the last several hundred million years; it remained depressed for the next 5 million years. The drop in sea level also coincided with the accumulation of massive ice sheets atop Antarctia.
Greenland was largely ice-free until about 4 million years ago, when a sheet of ice up to 2 miles thick buried this large island. Alaska connected with Siberia and closed off the Arctic Basin from warm water currents originating in the tropics, resulting in the formation of pack ice in the Arctic Ocean.
About 3 million years ago, the Isthmus of Panama separating North and South America uplifted as oceanic plates collided. The barrier created by the land bridge isolated Atlantic and pacific species, and Extinction impoverished the once rich fauna of the western Atlantic. The new landform halted the flow of cold water currents from the Atlantic into the Pacific, that, along with the closing of the Arctic Ocean from warm Pacific currents, might have initiated the Pleistocene ice ages, when massive glaciers swept out of the polar regions and buried the northern lands.




2 MARINE EXPLORATION
Early geologists thought the ocean floor was a barren desert, covered by thick, muddy sediments washed off the land and by debris of dead marine organisms raining down from above. After billions of years, it was assumed that the sediments had accumulated to a depth of several miles, making the deep waters of the ocean a vast, featureless plain, unbroken by ridges or valleys and interspersed with a few scattered volcanic islands.
As technology improved, the view of the seabed grew much more accurate and complex, revealing midocean ridges grander than terrestrial mountain ranges and chasms deeper than any canyon on the continents. The midocean ridges, with their vigorous volcanic activity, appeared to generate new oceanic crust. The deep-sea trenches, with their extensive earthquakes were found on the deep seafloor, where previously no life was thought to exist. Indeed, the ocean floor was much more complicated than ever imagined.
2.1 EXPLORING THE OCEAN FLOOR
In the mid-1800s, depth soundings of the ocean floor were taken in preparation for laying the first transcontinental telegraph cable linking the United States with Europe. The depth recordings indicated hills, valleys, and a middle Atlantic rise (named Telegraph Plateau) where the ocean was supposed to be the deepest. Sometimes, section of the telegraph cable became buried under submarine slides and had to be brought to the surface for repair.
In 1874, the British cable-laying ship HMS Faraday was attempting to mend a broken telegraph cable in the North Atlantic. The cable rested on the ocean floor at a depth of 2.5 miles, where it passed over a large rise. While grappling for the cable, the claws of the grapnel snagged on a rock. When the grapnel was finally freed and brought to the surface, clutched in one of its claws was a large chunk of black basalt, a common volcanic rock. What made this discovery astonishing was that volcanoes were not supposed to be on the Atlantic Ocean floor.
The British corvette HMS challenger, the first fully equipped oceanographic research vessel, was commissioned in 1872 to explore the world’s oceans. The crew took depth soundings, water samples, and temperature readings, and dredged bottom sediments for evidence of animal life living on the deep seafloor. The challenger ‘s nets hauled up large numbers of deep-sea and bottom-dwelling animals, many from the deepest trenches. The catch included some of the strangest creatures scientists had yet found. Many species were unknown to science, and some were thought to have long gone extinct.
During nearly 4 years of exploration, the challenger charted 140 square miles of ocean bottom and sounded every ocean except the Arctic. The deepest sounding was taken off the Mariana Islands in the western Pacific. While recovering samples in the deep water off the Marianas Trench,the research vessel encountered a deep trough known as the Mariana Trench. This trench forms a long line northward from the Island of Guam and is the lowest place on earth, reaching a depth of nearly 7 miles below sea level.
While dredging the deep ocean floor in the Pacific, the Challenger recovered rocks resembling dense lumps of coal. Mistaken for fossils or meteorites, the rocks were put on display in the British Museum as geological oddities. Almost a century later, future analysis showed the true value of the dark, potato-size clumps. The nodules contained large quantities of valuable metals, including manganese, copper, nickel, cobalt, and zinc. Scientists realized that the world’s largest reserve of manganese nodules lay on the bottom of the North Pacific, about 16,000 feet below the surface. Fields thousands of miles long contained nodules estimated at 10 billion tons.
Other valuable minerals were found on the deep-sea floor. In 1978, the French research submersible Cyana discovered unusual lava formations and mineral deposits on the seabed in the eastern Pacific, more than 1.5 miles deep. These deposits were sulfide ores in 30-foot-high mounds of porous gray and brown material. The massive sulfide deposits contained abundant iron copper and zinc. The French research vessel Sonne found another sulfide ore field nearly 2,000 miles long on the floor of the East Pacific. The sediments contained as much as 40 percent zinc along with deposits of other metals, some in greater concentrations than their landbased counterparts.
Research vessels discovered valuable sediments more than 7,000 feet deep on the bed of the Red Sea between Sudan and Saudi Arabia. The largest deposit was in an area 3.5 miles wide known as the Atlantis II Deep, named for the research vessel that discovered it. The rich bottom ooze was estimated contain about 2 million tons of zinc, 400,000 tons of copper, 9,000 tons of silver, and 80 tons of gold. The sea undoubtedly provided unheard of mineral riches.
In the early 1970s,knowledge of the seafloor and the capacity to explore it were still rudimentary. Shipboard sonar was inadequate for mapping the rugged topography of the midocean ridges. The imagery improved substantially when sonar devices were mounted on a vehicle and towed at a considerable depth behind a ship. A system called SeaBeam made highresolution sonar maps of the midocean ridge crests. Its sonar covered a broad swath of seafloor, allowing a ship to map an entire area by tracking back and forth in well-spaced lines.
Cameras were also mounted on sleds (Fig.2-1) and pulled through elaborate obstacle courses in the dark abyss, but the instruments were damaged or lost at an alarming rate. A massive camera vehicle called Angus weighed 1.5 tons, allowing it to be towed almost directly beneath the ship for better navigational control. The most sophisticated device, called Deep Tow, carried sonar, television cameras and sensors for measuring temperature, pressure, and electricity. During operation over the East Pacific Rise off the coast of Ecuador, the camera sled “flew” into a hot plume of water. Upon further exploration, photographs taken by Angus revealed a lava field scattered with large white clams.












Figure 2-1 A deep-sea camera and color video system used to photograph sulfide ore deposits on the seafloor. Photo by Hank Chezar, courtesy of USGS
The submersible Alvin was sent down to investigate this phenomenon. It discovered an oasis of hydrothermal vents (Figs. 2-2a&2-2b) and exotic deep-sea creatures 1.5 miles below sea level. At the base of jagged basalt cliffs was evidence of active lava flows, including fields strewn with pillow lavas. Unusual chimneys called black smokers spewed out hot water blackened with sulfide minerals. Others called white smokers, ejected hot water that was milky white. Species previously unknown to science lived in total darkness among the hydrothermal vents. Tubeworms up to 10 feet tall swayed in the volcanic terrain. Huge clams up to a foot long and clusters of mussels formed large communities around the vents.
















Figure 2-2a The deep submersible Alvin at the port of Wood’s Hole, Massachusetts. Photo by R. Wahl, courtesy of U.S. Navy
2.2 SURVEYING THE SEABED
The more scientists probed the ocean floor, the more complex they learned it to be. The ocean covers about 70 percent of the Earth’s surface to an average depth of over 2 miles. It is shallowest in the Atlantic Basin and deepest in the Pacific Basin. If Mount Everest, the world’s tallest mountain, were placed in the deepest part of the Pacific Basin, the water would still rise over a mile above its peak. Yet relation to the overall size of the Earth, the ocean is merely a thin layer of water like the outer skin of an onion.
Early methods of sampling the seabed included dragging a dredge behind a ship to scoop up the bottom sediments, or using a snapper (Fig.2-3) whose jaws closed when the instrument struck the bottom. But these techniques only sampled the topmost layers, which were not recovered in the order of their original deposition. In the early 1940’s, a piston cover was developed, which when dropped to the seabed retrieved a vertical section of the ocean floor intact. The corer consisted of a long barrow that plunged into the bottom mud under its own weight. A piston firing upward from the samples were then brought to the surface (Fig.2-4).












Figure 2-2b A hydrothermal vent with sulfide-laden hot water pouring out into cold seawater on the ocean floor. The photograph is taken from Alvin, whose claw holds a temperature probe. Photo by N.P. Edgar, courtesy of USGS
The bottom of the ocean was at first thought to contain sediments washed off the continents, forming deposits several miles thick after billions of years of accumulation. However, core drilling at several sites revealed that the oldest sediments were less than 200 million years old. The sediments were measured with an undersea that used seismic waves similar to sound waves to locate sedimentary structures.
An ocean-bottom seismograph dropped to the seafloor (Fig.2-5) recorded microearthquakes in the Earth’s submarine crust and rose automatically to the surface for recovery. Seismic instruments towed behind ships also detected geologic structures deep within the sub-oceanic crust. These surveys provided important information about the ocean floor that could not be obtained by direct means, and revealed that instead of miles of silt and mud, the oceanic crust contained only a few thousand feet of sediments.



















Figure 2-3 A snapper sampling instrument, whose jaws close when striking the ocean bottom. Photo by K. O. Emery, courtesy of USGS
During the height of the cold war in the late 1950’s, American and Russian oceanographic vessels mapped the ocean floor so that ballistic missile submarines would be able to navigate in deep water without grounding on uncharted seamounts. When Russian aircraft shot down a Korean airliner over Sakhalin Island on August 30,1983,killing all 269 passengers and crew, a search for the downed aircraft was conducted using the unmanned submersible Deep Drone (Fig.2-6) operated by the U.S. Navy.
Sonar depth-ranging was another important tool for mapping undersea terrain. SeaMarc, a side-looking sonar system towed in a “fish” about 1,000 feet above the ocean floor, provided a sonar image of the ocean bottom by bouncing sound waves off the seabed (Fig.2-7). As ships traversed the Atlantic Ocean, onboard sonographs painted a remarkable picture of the ocean floor. Lying 2.5 miles deep in the middle of the Atlantic Ocean was a huge submarine mountain range , surpassing in scale the Alps and the Himalayas. The range ran down the middle of the ocean, weaving halfway between the continents that surrounded the Atlantic Basin. This massive ridge was the side of volcanic activity so intense that it seemed as though the Earth’s insides were coming out.












Figure 2-4 Piston coring in the Gulf of Alaska. Photo by P.R. Carlson, courtesy of USGS
The midocean ridges were found to be a string of seamounts in a region where it was assumed that the deep seafloor should have been flat and barren. With more detailed mapping of the ocean floor, scientists found that the Mid-Atlantic Ridge was the most peculiar mountain range yet discovered. The ridge was 10,000 feet above the ocean floor, with a deep trough running through the middle like a giant crack in the Earth’s crust. It was 4 miles deep in places, or four times deeper than the Grand Canyon, and up to 15 miles wide, making it the grandest canyon Earth.
Undersea survey has shown that the submerged mountains and undersea ridges form a continuous chain 45,000miles long, several hundred miles wide, and up to 10,000 feet high that winds around the globe like the stitching on a baseball. Although the midocean ridge system lies deep beneath the sea, it is easily the most dominant feature on the face of the planet, extending over an area greater than that covered by all major terrestrial mountain ranges combined.















Figure 2-5 An ocean bottom seismograph provides direct observations of earthquakes on midocean ridges. Courtesy of USGS
When advanced instrumentation was developed, the view of seafloor came into better focus. The ocean floor proved to be far more active and younger than previously imagined. Additional surveys conducted across the extensive undersea mountain ranges included rock sampling, sonar depth-finding, thermal measurements, magnetic readings, and seismic surveys. The resulting data suggested that the oceanic crust was spreading outward at the midocean ridge. Magma rising the mantle erupted onto the ocean floor, adding new oceanic crust to the ridge crest as both sides pulled apart.
Temperature surveys showed anomalous amounts of heat seeping out of the Earth in the mountainous regions of the middle Atlantic. It was as though magma were bleeding from the mantle through cracks in the oceanic crust. Volcanic activity in the ridges suggested that new material was being added to the seafloor. This activity appeared more intense in the Atlantic Ocean, where the midocean ridge is steeper and jagged, than in the Pacific or the Indian oceans, where branches of ridges were overridden by continents.



















Figure 2-6 The unmanned submersible Deep Drone being launched to search for the wreckage of Korean Air Lines Flight 007, shot down near Sakhalin Island on August 30, 1983, by Russian aircraft. Photo by F.Barbante, courtesy of U.S. Navy
Deep-sea trenches off continental margins and volcanic inland arcs were initially thought to have been created by the tremendous weight of sediments washed off the continents and pulled down into the mantle by a dense underlying material. The downward pull on the sediments formed vast bulges in the ocean floor called geosynclines. However gravity surveys conducted over the trenches indicated that the pull of gravity was much too weak to account for the sagging of the seafloor.
The trenches were also found to be sites of almost continuous earthquake activity deep in the bowels of the earth. The deep-seated earthquake acted like beacons marking the boundaries of a large slab of crust descending into the mantle. The unusual activity of the trenches suggested that they were sites where old oceanic crust subducted into the Earth’s interior. Perhaps here at last was the engine that drove the continents around the surface of the Earth.












Figure 2-7 Sonograph of the lower continental slope off the Atlantic coast from SeaMarc. Photo by N. P. Edgar, courtesy of USGS
2.3 GEOLOGIC OBSERVATIONS
Observations of these and other fascinating geologic features on the ocean floor led to the seafloor spreading theory. The hypothesis described the creation and destruction of the ocean floor at specific regions around the world. The seafloor spreading theory resolved many problems connected with the mysterious characteristics on the seafloor, including the midocean ridges, the relatively young ages of rocks in the oceanic crust, and the formation of island arcs. But more importantly, here at last was the long-sought mechanism for continental drift. The continents do not plow through the ocean crust like ice breakers slicing through frozen seas, as previously thought, but instead tide above mantle like ships caught in mobile icefloes.
Exploration of the ocean floor bought a new understanding of the forces that shaped the planet. After overwhelming geological and geophysical evidence was collected from the floor of the ocean in support of the theory of continental drift, geologists finally abandoned the archaic thinking of the past century. By the late 1960s, most geologists in the Northern Hemisphere, who had long rejected the theory, finally joined their southern colleagues, who had been for some time convinced of the reality of continental drift because of overwhelming evidence in South America and the opposing African continent.
The discovery of many mysteries on the seabed, including spreading ridges and deep-sea trenches, led geologists to develop an entirely new way of looking at the Earth: the theory of plate tectonics (Fig.2-8).Tectonics (from the Greek word tekton, meaning “to built”) is the geologic process responsible for all features on the Earth’s surface. The theory incorporated the process of seafloor spreading and continental drift into a comprehensive model. Therefore, all aspects of the Earth’s history and structure could be unified by the revolutionary concept of movable plates. Well-defined earthquakes zones marked the boundaries of the plates, and analysis of earthquakes around the Pacific Ocean revealed a consistent direction of crustal movement.
The Atlantic Ocean is bisected by the Mid-Atlantic Ridge, which manufactures new oceanic crust as the continents surrounding the Atlantic Basin spread apart. The Mid-Atlantic Ridge is the center of intense seismic and volcanic activity and the focus of high heat flow from the lithosphere and erupts on the ocean floor, adding new oceanic crust to both sides of the ridge crest.
As the Atlantic Basin widens, the surrounding continents separate at a rate of about 1 inch per year. In response to the widening seafloor in the Atlantic and the separation of continents around the Atlantic Basin, the Pacific Basin shrinks at a corresponding rate. The Pacific is ringed by subduction zones that destroy old oceanic crust in deep-sea trenches (Fig.2-9). Spreading ridges in the Pacific are also much more active than those in the Atlantic. These features on the ocean floor are responsible for most of the geologic activity that surrounds the Pacific Ocean.
2.4 OCEAN DRILLING
The oceans have an average depth of over 2 miles and are blanketed by layers of thick sediments. To correctly date these sediments, they had to be recovered in the order they were laid down; thus, dredging technique were of little use. Fortunately, a technique known as seafloor coring was developed, enabling scientists to take accurate sediment samples. A hollow pipe is drilled into the sediments and a long cylindrical sample is brought to the surface. Early attempts at coring in deep water, however, only penetrated a few feet into the upper sediments of the ocean floor.

TABLE 2-1 CONTINENTAL DRIFT

Geologic division

(millions of years)

Gondwana

Laurasia

Quaternary       3 Opening of Gulf of California

 

Pliocene         11  Spreading begins near Galapagos Islands  

Spreading changes directions in Eastern Pacific

                    Opening of the Gulf of Aden

                                                        

Birth of Iceland

Miocene         26

                    Opening of Red Sea

Oligocene        37  

                     Collision of India with Eurasia       

Spreading begins in Arctic Basin

Eocene          54                                   

Separation of Greenland 

Paleocene        65  Separation of Australia from Antarctica   

from Norway

                                                       

Opening of the Labrador Sea

                     Separation of New Zealand from Antarctica

                                                       

Opening of the Bay of Biscay

                     Separation of Africa from Madagascar   

Major rifting of North

and South America                  America from Africa begins

Cretaceous     135   

Separation of Africa from India,        Separation of North America

Australia, New Zealand, and Antarctica   from Africa begins

Jurassic        180   

Triassic        250   Assembly of all continents into the 

                      Supercontinent Pangaea

Figure 2-8 The plate tectonics model. New oceanic crust is generated at spreading ridges and old oceanic crust is destroyed in subduction zones, which moves the continents around the face of the Earth.










Figure 2-9 The subduction of the ocean floor provides new molten magma for volcanoes that fringe the deep-sea trenches.
In the mid-1960s the National science Foundation sponsored a deep-sea drilling program called project Mohole. The moho, named after the Yugoslav seismologist Andrija Mohorovicic, is the point of contact between the Earth’s crust and mantle. The crust is the thinnest in the ocean, measuring only about 3 to 5 miles thick. Scientists hoped that the moho would provide new clues about the origin, age, and composition of the Earth’s interior, which land-based drilling could not obtain. Unfortunately, the task of drilling through miles of oceanic crust in waters as much as 2 or more miles deep become expensive and time-consuming.
In 1968, the British research vessel Glomar Challenger was commissioned for the Deep Sea Drilling Project, a consortium of American oceanographic institution. The project’s objective was to drill a large number of shallow hole in widely scattered parts of the ocean floor in an attempt to prove the theory of seafloor spreading. A similar deep-sea drillship called the Glomar Pacific (Figs.2-10a&b) was the first to begin drilling on the Atlantic outer continental shelf and slope of the United States. Both ships were designed with a 140-foot drilling derrick amidships; computerized thrusters located fore and aft maintained station over the drill hole even in rough seas.











Figure 2-10a The Glomar Pacific drilling on the Atlantic outer continental shelf and slope of the United States. Courtesy of USGS
A string of drill pipe dangled as 4 miles beneath the ship, with the drill bit cutting through the sediment by the force of its own weight. The core, which is a cylindrical vertical section of rock, was retrieved through the drill stem by a removable inner barrel, allowing the drill bit to remain in the hole. When the drill bit dulled, it and the drill pipe had to be brought back up to the surface for replacement. The drill string was then lowered back over the drill hole and a special funnel-like apparatus guided the drill bit into the hole.
The primary purpose of the international Ocean Drilling Program (ODP) and the Joint Oceanographic Investigation for Deep Earth Sampling (JOIDES) was to take rotary core samples of the ocean floor at hundreds of sites around the world. However, special precaution were taken not to drill in potentially productive oil fields, where drilling might cause blowouts that would result in hazardous oil spills. Just the opposite occurred on the south flank of the Gosta Rica Rift east of the Galapagos Islands in1979 when the Challenger drilled a hole into the crust. Instead of blowing out hot water, which is often the case, the well sucked in a powerful, steady stream of seawater. The suction resulted from the downward convection of circulating water within the oceanic crust as it descended toward a magma chamber, acquiring heat during hydrothermal activity.














Figure 2-10b Deep-sea drilling. Drill pipe dangles beneath the drillship and cuts through the bottom sediments by its own weight.
The deepest hole was bored into the ocean floor in the eastern Pacific near the Galapagos Islands by the drillship JOIDES Resolution. The purpose was to sample a section of the entire oceanic crust from top to bottom in an area where the crust was thought to be thinnest. During a 14-year period beginning in 1979, the ship made 7 trips to the drill site to deepen the hole, with each session lasting up to 2 months. On the sixth trip, the ship first had to recover drill pipe lost in the hole during the previous effort. When this task was accomplished, the hole was extended further to a depth of more than 6,500 feet beneath the seabed. In January 1993, the Resolution returned again to deepen the hole another 370 feet only to lose the drill bit. This mishap forced the crew to abandon the drill hole perhaps only a few hundred feet short of their goal.
Taking a shortcut to the bottom of the ocean crust, ODP scientists found a site where the lower crust is uncovered along the Atlantis II Fracture Zone in the Indian Ocean, which is part of the midocean ridge that forms the boundary between the African and Antarctic tectonic plates. Running down the middle of the ridge is a feature called a spreading center, which periodically breaks apart, leaving a gap that fills with molten magma. As the magma cools and hardens, the rock forms new oceanic crust that joins to the ends of the plates.
The structure of a spreading center resembles steps in a staircase, with short, straight segments roughly parallel to each other (see fig.3-2). The fracture zones are valleys that connect adjacent segments like the vertical jumps between the steps. When the scientists drilled through the valley floor of the fracture zone, they recovered coarsely crystalline rocks called gabbros, which are known to make up the lower segment of the oceanic crust.
After recovered and dating cores from several midocean ridges, the Challenger made a truly remarkable discovery. The farther away from the deep-sea ridges the ship drilled, the thicker and older the sediments became. But even more surprising was that the thickest and oldest sediments were not billions of years old as expected, but were in fact younger than 200 millions years. Near the continental shelves, where thick layers of sediment form flat abyssal plains, the drill cores revealed thin beds of calcium carbonate just above hard volcanic rock that was buried under thousands of feet of red clay and other sediments. The discovery of abyssal red clay, whose color signifies a terrestrial origin, provided additional evidence for seafloor spreading.
The deepest abysses in the world are adjacent to continental margins, the actual boundaries of continents, where the oceanic lithosphere is the oldest. The calcium carbonate layer located by the Challenger was about 4 miles deep, far below the depth where the crush of cold water dissolves calcium carbonate. Well protected from the corrosive effect of seawater by the overlying sediments, the calcium carbonate originating in shallower water near midocean ridges was somehow transported to the edges of the continents.
The floor of the Atlantic conveys lithosphere, the rigid layer of the upper mantle, away from its point of origin at the Mid-Atlantic Ridge. The ocean floor at the crest of the midocean ridges consists mostly of basalt, a volcanic rock. Continuing away from the crest, the bare rock is blanketed by an increasing thickness of sediments, composed mostly of red clay from detritus material washed off the continents and from windblown desert sediment that has landed in the sea. Some large sandstorms over the Sahara Desert blow dust so high into the atmosphere that prevailing air currents carry the dust all the way across the Atlantic Ocean to South America (Fig.2-11).













Figure 2-11 During the summer of 1976, drought conditions in West Africa and a prevailing easterly wind resulted in a dust surge- an enormous cloud of dust blowing out over the Atlantic Ocean (shown south of the dashed line) from the Sahara Desert. Courtesy of NOAA
Near the ridge crest, the sediments are predominantly composed of calcareous ooze built up by a rain of decomposed shells and skeletons of microorganisms. Farther away from the ridge crest, the slope falls below the calcium carbonate compensation zone generally about 3 miles deep. Below this depth, calcium carbonate, whose solubility increase with pressure, dissolves in seawater. Therefore, only red clay should exist in the deep abyssal waters far from the crest of the midocean ridge. Yet drill cores taken from the abyssal near continental shelves, where oceanic crust is the oldest and deepest, clearly show thin layers of calcium carbonate below thick beds of red clay and above hard volcanic rock. Geologists concluded that the red clay protected the calcium carbonate from dissolving in the deep waters of the abyss. The discovery implies that the midocean ridge was the source of the calcium carbonate near continental margins and that the seafloor has moved across the Atlantic Basin.
2.5 MAGNETIC SURVEYS
Geologists looking for a decisive test for seafloor spreading stumbled upon magnetic reversals on the ocean floor. Experiments using sensitive magnetic recording instruments called magnetometers towed behind ships over the midocean ridges (Fig.2-12) revealed magnetic patterns locked in the volcanic rocks on the seafloor. These patterns alternated from north to south and were mirror images of each other on opposite sides of the ridge crest. The magnetic fields captured in the rocks also showed the past position of the magnetic poles as well as their polarities.











Figure 2-12 A crewmember lowers a magnetometer over the stern of the oceanographic research ship USNS Hayes. Courtesy of U. S. Navy
Two or three times every million years, the Earth’s geomagnetic field reverses polarity, with the north and south magnetic poles switching places. Over the last 4 million years, the field has reversed 11 times. As the iron-rich basalts of the midocean ridges cool, the magnetic fields of their iron molecules line up in the direction of the Earth’s magnetic field at the time of their deposition. As the ocean floor spreads out on both sides of the ridge, the basalts solidify, establishing a record of the geomagnetic field at each successive reversal. This process produced parallel bands of magnetic rocks of varying width and magnitude on both sides of the ridge crest that were mirror images of each other (Fig.2-13). Here at last was clinching proof for seafloor spreading: in order for the magnetic stripes to form in such a manner, the ocean floor had to be pulling apart.








Figure 2-13 As volcanic rock cools at midocean ridges, it is polarized in the direction of the Earth’s magnetic field, providing a series of magnetic stripes on the ocean floor.
The magnetic stripes also provided a means of dating virtually the entire ocean floor, because the magnetic reversal occurred randomly, and any set of patterns is unique in geologic history. The rate of seafloor spreading was calculated by determining the age of the magnetic stripes by dating drill cores taken from the midocean ridge and measuring the distance from their points of origin at the ridge crest. During the past 100 million years, the rate of seafloor spreading has changed little. Periods of increased acceleration have been accompanied by an increase in volcanic activity. In the past 10 to 20 years, there has been progressive acceleration, reaching a peak about 2 million years ago.
The spreading rates on the East Pacific Rise are upward of 6 inches per year, which results in less topographical relief on the ocean floor. The active tectonic zone of a fast-spreading ridge is usually quite narrow, generally less than 4 miles wide. In the Atlantic, the rates are much slower, only about 1 inch per year, which allows taller ridges to form. Using the rate of seafloor spreading, the Atlantic appears to have opened around 170 million years ago-a time span remarkably concurrent with the estimated date for the breakup of the continents.
2.6 SATELLITE MAPPING
In 1978, the radar satellite Seasat (Fig.2-14) precisely measured the distance to the ocean surface over most of the globe. Among the astonishing discoveries was the fact that ridges and trenches on the ocean bottom produce corresponding hills and valleys on the surface of the ocean because of variations in the pull of gravity. The topography of the ocean surface shows bulges and depressions, with several hundred feet of relief. But because these surface variations range over wide areas, they are unrecognized on the open sea.

TABLE 2-2 COMPARISON OF MAGNETIC REVERSAL WITH OTHER PHENOMENA (Dates in Millions of Years)

Magnetic Reversal

Unusual Cold

Meteorite Activity

Sea Level Drops

Mass Extinctions

0.7

0.7

0.7

 

 

1.9

1.9

1.9

 

 

2.0

2.0

 

 

 

10

 

 

 

11

40

 

 

37-20

37

70

 

 

70-60

65

130

 

 

132-125

137

160

 

 

165-140

173

The pull of gravity from undersea mountains, ridges, trenches, and other structures of varying mass distributed over the seafloor controls the shape of the surface water. Undersea mountain ranges produce large gravitational forces that cause seawater to pile up around them, resulting in gentle swells on the ocean surface. Conversely, submarine trenches with less mass to attract water form shallow troughs in the sea surface. For example, a trench 1 mile deep can cause the ocean to drop dozens of feet.
The satellite altimetry data produced a map of the entire ocean surface (Fig.2-15). Chains of midocean ridges and deep-sea trenches were delineated with a clarity greater than had been achieved by any other method of mapping the ocean floor. The seafloor maps also revealed many new features such as rifts, ridges, seamounts, and fracture zones and better defined several known features. These maps provided additional support for the theory of plate tectonics, which holds that the crust is broken in to several plates whose constant shifting is responsible for the geologic activity on the Earth’s surface, including the growth of mountain rangers and the widening of ocean basins.















Figure 2-14 The Seasat satellite radar mapped the ocean surface over most of the globe.








Figure 2-15 Radar altimeter data from the Geodynamic Experimental Ocean Satellite (GEOS-3) and Seasat was used to produce this map of the ocean floor. (1) Mid-Atlantic Ridge, (2) Mendocino Fracture Zone, (3) Hawaiian Island chains, (4) Tonga Trench, (5) Emperor Seamounts, (6) Aleutian Trench, (7) Mariana Trench, (8) Ninety East Ridge. Courtesy of NASA
The satellite imagery also revealed long-buried fracture zones undiscovered by conventional seafloor mapping techniques. The faint lines running like a comb through the central Pacific seafloor might be influenced by convection currents in the mantle 30 to 90 miles beneath the oceanic crust. Each circulating loop consists of hot material rising and cooler material sinking back into the depths, tugging on the ocean floor as it descends.
Even buried structures came into full view for the first time. One example is an ancient midocean ridge that formed when South America, Africa, and Antarctica began separating around 125 million years ago. The seafloor spreading center was buried deep under thick layers of sediment. The boundary between the plates moved westward, leaving behind the ancient ridge, which began to subside. The ridge’s discovery might help geologists trace the evolution of the oceans and continents over the last 200 million years. The satellite’s discoveries are further proof that the deep-sea floor remains, in large part, uncharted territory, and that the exploration of inner space is as important as the exploration of outer space.
















3. THE DYNAMIC SEAFLOOR
The ocean’s crust is constantly changing. It is comparatively young, less than 5 percent of Earth’s age. The age difference is due to the recycling of oceanic crust into the mantle; almost all the ocean floor has disappeared into the Earth’s interior over the last 170 million years. The oceanic crust is continuously being created at midocean ridges, where basalt oozes out of the mantle through rifts in the crust, and destroyed in deep-sea trenches, where the lithosphere plunges into the mantle and remelts in a continuous cycle.
The divergence of lithospheric plate creates new oceanic crust at spreading ridges, while convergence destroys old oceanic crust in subduction zones. When two plates collide, the less buoyant oceanic crust subducts under continental crust. The lithosphere and the overriding oceanic crust recycle through the mantle to make new crust. The lithospheric plates act like rafts riding on a sea of molten rock, slowly carrying the continents around the surface of the globe.
3.1 LITHOSPHERIC PLATES
The Earth’s outer shell is fractured like a broken egg into several large plates (Fig.3-1). The shifting lithospheric plates range in size from a few hundred to several million square miles. They comprise the crust and the upper brittle mantle called the lithosphere. The lithosphere consists of the rigid outer layer of the mantle and under the continental and underlies the continental and oceanic crust. The thickness of the lithosphere is about 60 miles under the continents and averages about 25 miles under the ocean.
The lithospheric plates ride on a hot pliable layer of the mantle called the asthenosphere, in a manner similar to hard wax riding on melted wax. They carry the crust like drifting slabs of rock. The plates diverge at midoceaan spreading ridges and converge at subduction zones, which lie at the edges of lithospheric plates. The lithospheric plates subduct into the mantle in a continuous cycle of crustal regeneration. Their constant interaction with each other shapes the surface of the planet. This structure of the upper mantle is important for the operation of plate tectonics, which is responsible for all geologic activity on the Earth.
The plate boundaries are zones of active deformation that absorb the force of impact between nearly rigid plates. These boundary zones vary from a few hundred feet where plates slide past each other at transform faults to several tens of miles at midocean ridges and subduction zones. The divergent plate margins are midocean spreading ridges, where basalt welling up from within the upper mantle new oceanic crust as part of the process of seafloor spreading (Fig.3-2).











Figure 3-1 The major lithospheric plates.








Figure 3-2 Creation of oceanic crust at a spreading ridge.
Oceanic crust does not form as a single homogenous mass, but rather, is made in long narrow ribbons laid side by side and interspersed with fracture zones. The midocean ridge system, which does not always lie in the middle of the ocean, snakes 45,000 miles around the globe, making it the longest structure on Earth. The lateral plate margins are transform faults, where plates slide past each other accompanied by little or no tectonic activity such as the upwelling of magma and the generation of earthquakes.
The convergent plate margins are the subduction zones represented by deep-sea trenches, where old oceanic crust sinks into the mantle to provide magma for volcanoes fringing the trenches. If tied end to end, the subduction zones would stretch around the world. The convergence rates between plates range from less than 1 inch to more than 5 inches per year, corresponding to the rates of plate divergence, However, subduction zones and associated spreading ridges on the margins of a plate do not operate at the same rates; this disparity causes the plates to travel across the surface of the Earth. If subduction overcomes seafloor spreading, the lithospheric plate shrinks and eventually disappears altogether.
3.2 OCEANIC CRUST
The crust of the ocean is remarkable for its consistent thickness and temperature, averaging about 4 miles thick and not varying more than 20 degrees Celsius over most of the globe. By comparison, the continental crust is on average 25 to 30 miles thick, and in the domain of mountain ranges, it reaches a thickness of 45 miles. The continents also have thick roots of relatively cold mantle material extending down to a depth of about 250 miles. The average density of continental crust is 2.7 times the density of water, compared with 3.0 for oceanic crust and 3.4 for the mantle. The difference in density buoys up the continental and oceanic crust.
The oceanic crust is like a layer cake with 3 distinct strata. It has an upper layer of pillow basalts, formed when lava extruded undersea at great depths; a middle layer of a sheeted-dike complex, consisting of a tangled mass of feeder that bring magma to the surface; and a lower layer of gabbros, coarse-grained rocks that crystallized slowly under high pressure in deep magma chambers. The same rock formation is found on the continents. This similarity has led geologists to speculate that these formations were pieces of ancient oceanic crust called ophiolites (Fig.3-3).
Most oceanic crust is less than 170 million years old with a mean age of 100 million years, compared with the continental crust, which is about 4 billion years old. The difference in ages is due to the recycling of oceanic crust into the mantle, as described previously. Almost all the ocean floor has since disappeared into the Earth’s interior to provide the raw materials for the continued growth of the continents.
As mentioned earlier, new oceanic crust forms at spreading ridges, where basalt oozes out of the mantle through rifts on the ocean floor. Some molten magma erupts as lava on the surface of the ridge through a system of vertical passages. Once at the surface, the liquid rock down the ridge and hardens into sheets or rounded forms of pillow lavas, depending on the rate of extrusion and the slope of the ridge. Periodically, lava overflows onto the ocean floor in gigantic eruptions, adding several square miles of new oceanic crust yearly. As the oceanic crust cools and hardens, it contracts, forming fractures through which water circulates.

Not all magma extrudes onto the ocean floor. Most of it cools and bonds to the edges of separating plates. Much of the magma solidifies within the conduits above the magma chambers, forming massive vertical sheets called dikes that resemble a deck of cards standing on end. Individual dikes measure about 10 feet thick, about 1 mile high, and approximately 3 miles long.










Figure 3-3 Worldwide distribution of ophiolites, which are slices of oceanic crust shoved up on land by the action of plate tectonics.
The oceanic plates thicken with age, from a few miles thick after formation at midocean spreading ridges to more than 50 miles thick in the oldest ocean basins next to the continents. The depth at which an oceanic plate sinks as it moves away from a midocean spreading ridge varies at a ratio of the square root of its age. For example, a plate that is 2 million years old lies about 2 miles deep; a plate that is 20 million years old lies about 2.5 miles deep; and a plate that is 50 million years old lies about 3 miles deep.
A typical oceanic plate starts out thin and gradually thickens by the underplating of new lithosphere from the upper mantle and the accumulation of overlying sediment layers. The ocean floor at the summit of a midocean ridge consists almost entirely of hard basalt and acquires a thickening layer of sediments farther outward from the ridge crest. By the time the oceanic plate extends as wide as the Atlantic Ocean, the portion near continental margins where the sea is the deepest is about 60 miles thick. Eventually, the oceanic plate becomes so thick and heavy that it can no longer remain on the surface. It then bends downward and subducts beneath continent into the Earth’s interior (Fig.3-4).
As the oceanic plate dives into a subduction zone, it remelts and acquires new minerals from the mantle, providing the raw material for new oceanic crust in the form of molten magma that reemerges at volcanic spreading centers along midocean ridges. Sediments deposited on the ocean floor and the water trapped between sediment grains are also caught in the subduction zones. But the lower melting points and lesser density of these molten sediments makes them rise toward the surface to supply nearby volcanoes with magma and recycled seawater.







Figure 3-4 The Earth’s crust is composed of continental granites and ocean basin basalts.
The fluid portion of the upper mantel is called the asthenosphere. Here rocks are semimolten or plastic, enabling them to slowly flow. After millions of years, the molten rocks reach the topmost layer of the mantle, or lithosphere. With a reduction of pressure within the Earth, the rocks melt and rise through fractures in the lithosphere. As the molten magma passes through the lithosphere, it reaches the bottom of the oceanic crust, where it forms magma chambers that further press against the crust, which continues to widen the rift. Molten lava pouring out of the rift forms ridge crest on both sides and adds new material to the spreading ridge system (Fig.3-5).








Figure 3-5 The structure of a spreading ridge, where material from the asthenosphere produces new lithosphere.
The mantle material below spreading ridges where new oceanic crust forms is mostly peridotite, a strong, dense rock composed of iron and magnesium silicates. As the peridotite, melts on its journey to the base of the oceanic crust, a portion becomes highly fluid basalt, which on the most common magma erupted on the surface of the Earth. About 5 cubic miles of basaltic magma is removed from the mantle and added to the crust every year. Most of this volcanism occurs on the ocean floor at spreading centers, where the oceanic crust pulls apart. Gabbro containing higher amounts of silica solidifies out of the basaltic melt and accumulates lower layer of the oceanic crust.
The oceanic crust, composed of basalts originating at spreading ridges and sediments washed off continents and islands, gradually increases density and finally subducts into the mantle. On its way deep into the Earth’s interior, the lithosphere and its overlying sediments melt. The molten magma rises toward the surface in huge bubble like structures called diapirs, from the Greek word meaning “to pierce.” When the magma reaches the base of the crust, it provides new molten rock for magma chambers beneath volcanoes and granitic bodies called plutons, which often form mountains. In this manner, plate tectonics is continually changing and rearranging the face of the Earth.
3.3 THE ROCK CYCLE
The entire volume of the world’s oceans circulates through the crust at spreading ridges every 10 million years, a volume approximately equivalent to the annual flow of the Amazon, the world’s largest river. This action accounts both for the unique chemistry of seawater and for the efficient thermal and chemical exchanges between the crust and the ocean. The magnitude of some of these chemical exchangers is comparable in volume to the input of elements into the oceans by all the world’s rivers, which carry materials weathered from the continents .The most important of these chemical elements is carbon, which controls many of the life processes on the planet.
When the seafloor subducts into the Earth’s interior, the intense heat of the mantle drives out carbon dioxide from carbonaceous sediments. The molten rock, with its contingent of carbon dioxide, works its way upward through the mantle and fills the magma chambers that underly its volcanoes and spreading ridges. The consequent eruption of volcanoes and the flow of molten rock midocean ridges resupplies the atmosphere and ocean with new carbon dioxide, making the Earth in effect one great carbon dioxide recycling plant (Fig.3-6).
The geochemical carbon cycle –the transfer of carbon within the Earth—involves the interactions between the crust, ocean, atmosphere, and life. The biological carbon cycle is only a small component of this cycle, and is the transfer of carbon from the atmosphere to vegetation by photosynthesis, returning carbon to the atmosphere when plants respire or decay. The vast majority of carbon is not stored in organic matter, however, but is locked up in sedimentary rocks on the ocean floor and on the continents.








Figure 3-6 The geochemical carbon cycle. Carbon dioxide in the form of bicarbonate is washed off the land and enters the ocean, where organisms convert it to carbonate sediments, which are thrust into the mantle, become part of magma, and escape into the atmosphere from volcanoes.

TABLE 3-2 RELATIVE AMOUNTS OF CARBON

Source

Relative Amount

Calcium carbonate in sedimentary rocks

Ca-Mg carbonate in sedimentary rocks

Sedimentary organic matter in the remains of animal tissues

Bicarbonate and carbonate dissolved in ocean

Coal and petroleum

Soil humus

Atmospheric carbon dioxide

All living plants and animals

60,000

45,000

25,000

75

7

5

1.5

1

The oceans play a critical role in the carbon cycle by regulating the level of carbon dioxide in the atmosphere. In the upper layers of the ocean, the concentration of gases is in equilibrium with the atmosphere: the mixed layer of the ocean within the upper 300 feet (Fig.3-7) contains as much carbon dioxide as the entire atmosphere. The gas dissolves into seawater mainly by the agitation of surface waves. Without marine photosynthetic organisms to absorb dissolved carbon dioxide, much of the reservoir of this gas would escape into the atmosphere, more than tripling the present carbon dioxide content and causing a runaway greenhouse effect.










Figure 3-7 Turbulence in the upper layers of the ocean induces the mixing of gases.
Atmospheric carbon dioxide combines with rain to form carbonic acid. The acid reacts with surface rocks, producing dissolved calcium and bicarbonate, which are carried by streams to the ocean. Marine organisms use these substances to build calcium carbonate skeletons and other supporting structures. When the organisms die, their skeletons sink to the bottom of its ocean, where they dissolve in the deep abyssal waters. Because of its large volume, the abyss holds the largest reservoir of carbon dioxide in the world.
In shallow water, the carbonate skeletons build deposits of limestone (Fig.3-8), which buries carbon dioxide in the geologic column comprising all sedimentary rocks. The burial of carbonate is responsible for about 80 percent of the carbon deposited on the ocean floor. The remainder of the carbonate originates from dead organic matter washed off the continents. Half the carbonate transforms back into carbon dioxide , which escapes into the cease and all life would end.
The deep water, which represents about 90 percent of the ocean’s volume, circulates very slowly, with a residence time (time in place) of about 1,000 years. It comes into direct contact with the atmosphere only in the polar regions. Thus, the deep water’s absorption of carbon dioxide is limited. The abyss receives most of its carbon in the form of the shells of dead organisms and fecal matter that sink to the ocean bottom. Carbon dioxide returns to the atmosphere by upwelling currents in the tropics, which is why the concentration of this gas is greater near the equator.











Figure 3-8 Formation of carbonate sediments on the ocean floor from the burial of shells and skeletons of marine organisms.
Volcanic activity on the ocean floor and on the continents plays a vital role in restoring the carbon dioxide content of the atmosphere. Carbon dioxide escapes from carbonaceous sediments that melt in the Earth’s interior to provide new magma. The molten magma, along with volatiles including water and carbon dioxide, rises to the surface to feed magma chambers beneath midocean ridges and volcanoes. When the volcanoes erupt, carbon dioxide is released from the magma and returns to the atmosphere, completing the cycle.
3.4 OCEAN BASINS
Ocean basins are the largest depressions on Earth. The ocean floor lies much deeper below sea level than the continents rise above it. If the oceans were completely drained of water, the planet would look much like the rugged surface of Venus, which lost its oceans eons ago. The deepest parts of the dry seabed would lie several miles below the surrounding continental margins. The floor of the desiccated ocean would be traversed by the longest mountain ranges and fringed in many places by the deepest trenches. Vast empty basins would divide the continents, which would stand out like thick slabs of rock.
Most of the seawater that surrounds the continents lies in a single great basin in the Southern Hemisphere, which is nine-tenths ocean. It branches northward into the Atlantic, Pacific, and Indian basins in the Northern Hemisphere, where most of the continental landmass exists. The Arctic Ocean is a nearly landlocked sea connected to the Atlantic and Pacific only by narrow straits. The Bering Sea (Fig.3-9) separates Alaska and Asia by only 56 miles at its narrowest point. About 20 million years ago, a ridge near Iceland subsided, allowing cold water from the recently formed Arctic Ocean to surge into the Atlantic, giving rise to the oceanic circulation system in existence today.











Figure 3-9 St. Lawrence Island in the Bering Sea, showing cinder cones at the northwest end of the Kookooligit Mountains. Photo by H. B. Allen, courtesy of USGS
The oceans expand across some 70 percent of the Earth’s surface, covering an area about 140 million square miles with more than 300 million cubic miles of water. About 60 percent of the planet is covered by water no less than 1 mile deep, with an average depth of about 2.3 miles. The midocean ridges lie at an average depth of 1.5 miles. And the ocean bottom slopes away on both sides to a depth of about 3.5 miles. In the Pacific Basin, the ocean is up to 7 miles deep, the lowest point on Earth.
With only marine-born sedimentation and no bottom currents to stir up the seabed, an even blanket of material would settle onto the original volcanic floor of the oceans. Instead, however, the rivers of the world contribute a substantial amount of the sediment deposited on the deep ocean floor. The largest rivers of North and South America empty into the Atlantic, which receives considerably more river-borne sediment than does the Pacific. The burial of organic material also greatly aids the formation of offshore petroleum reserves.

Because the Atlantic is smaller and shallower than the Pacific, its marine sediments are buried more rapidly and therefore are more likely to survive than those in the Pacific. The floor of Atlantic accumulates at a rate of about an inch every 2,500 years. The deep-ocean trenches around the Pacific trap much of the material reaching its western edge, where it subducts into the mantle.
Strong near-bottom currents redistribute sediments in the Atlantic on a greater scale than in the Pacific. Abyssal storms with powerful currents occasionally sweep patches of ocean floor clean of sediments and deposit the debris elsewhere. On the western side of the Pacific, Atlantic, and Indian ocean basins, periodic undersea storms skirt the foot of the continental rise and transport huge loads of sediment, dramatically modifying the seafloor. The scouring of the seabed and deposition of thick layers of the seafloor sediment results in a much more complex marine geology than would be developed simply by a constant rain of sediments from above.
3.5 SUBMARINE CANYONS
The ocean floor presents a rugged landscape unmatched anywhere else on Earth. Chasms dwarfing even the largest continental canyons plunge to great depths. Rivers emptying into the sea eroded the exposed seabed when the sea level lowered dramatically during the last ice age.
At the height of the last ice age, about 10 cubic miles of the Earth’s water were held in the continental ice sheets, which covered about a third of the land surface with an ice volume three times greater than it present size. The accumulated ice dropped the level of the ocean by about 400 feet, advancing the shoreline hundreds of miles seaward. The coastline of the eastern seaboard of the United States extended about halfway to the edge of the continental shelf, which runs eastward more than 600 miles. The drop in sea level exposed land bridges and linked continents.
Numerous canyons slice through the continental shelf beneath the Bering Sea between Alaska and Siberia. About 75 million years ago, continental movements created the broad Bering shelf rising 8,500 feet above the deep ocean floor. The shelf was exposed as dry land at several times during the ice ages when sea levels dropped hundreds of feet, and terrestrial canyons cut deep into the shelf. When the ocean refilled again at the end of the last ice age, massive landslides and mudflows swept down steep slopes on the shelf’s edge, gouging out 1,400 cubic miles sediment and rock.
A step resembling a sea cliff on the continental shelf off the eastern United States has been traced for nearly 200 miles. It appears to represent the former ice age coastline, now completely submerged under seawater. The massive continental glaciers that sprawled over much of the Northern Hemisphere held enough water to lower the sea by several hundred feet. When the glaciers melted, the sea returned to near its present level. Submarine canyons carved into bedrock 200 feet below sea level can be traced to rivers on land.
Several submarine canyons slice through the continental margin and ocean floor off eastern North America (Fig.3-10). Submarine canyons on continental shelves and slopes posses many features identical to those of river canyons, and some rival even the largest on the continents. These canyons are characterized by high, steep walls and an irregular floor that slopes continually outward. The canyons are upward of 30 miles or more in length, with an average wall height of about 3,000 feet. The Great Bahamas Canyon is one of the largest submarine canyons, with a wall height of 14,000 feet, making it more than twice deep as the Grand Canyon.














Figure 3-10 A seismogram of the Mid-ocean Canyon in the Newfoundland Basin. Photo by R.M. Pratt, courtesy of USGS
Rivers flowing across the exposed land gouged out several submarine canyons in the ocean floor when sea levels were much lower than they are today. Many submarine canyons have heads near the mouths of large rivers. Some submarine canyons extend to depths of over 2 miles, too deep for a terrestrial river origin. They formed instead by undersea slides, which carve out deep gashes in the ocean floor.
The Mediterranean Sea appears to have almost completely dried up 6 million years ago, making its seafloor a desert basin more than a mile below the surrounding continental plateaus. Rivers draining into the desiccated basin gouged out deep canyons. A deep sediment-filled gorge follows the course of the Rhone River in southern France for more than 100 miles and extends to a depth of 3,000 feet below the surface where the river drains into the Mediterranean Sea. Under the sediments of the Nile Delta is buried a mile-deep canyon that can be traced as far south as Aswan, 750 miles away, and is comparable in size to the Grand Canyon.
Submarine slides move rapidly down steep continental slopes and are responsible for excavating deep submarine canyons. The surface of the slopes is covered mainly with fine sediments swept off the continental shelves by submarine slides. The slides consist of sediment-laden water that is denser than the surrounding seawater. The turbid water moves swiftly along the ocean floor, eroding the soft bottom material. These muddy waters, called turbidity currents, move down steep slopes and play a major role in shifting the sands of the deep sea (Fig.3-11).











Figure 3-11 Sonar profile of the continental slope south of Nantucket, showing slumped debris from 5,000- to 7,500-foot depths. Photo by R.M. Pratt, courtesy of USGS
3.6 MICROPLATES AND TERRANES
During the breakup of Pangaea in the early Jurassic period, the Pacific plate-the largest in the world-was hardly bigger than the present-day continental United States. About 190 million years ago, the Pacific plate might have begun as a tiny microplate, a small block of oceanic crust that sometimes lies at the junction between two or three major plates. The rest of the ocean floor consisted of other unknown plates that have long since disappeared as the Pacific plate continued to grow. This is why no oceanic crust is older than Jurassic in age.
A microplate about the size of Ohio sits at the junction of the Pacific, Nazca, and Antarctic plates in the South Pacific about 2,000 miles west of South America. Seafloor spreading along the boundary zone between the plates adds new oceanic crust onto their edges, causing the plates to diverge. The different rates of seafloor spreading have caused the microplate at the hub of the spreading ridges, which fan out like the spokes of a bicycle wheel, to rotate one quarter-turn clockwise in the last 4 million years. A similar microplate near Easter Island to the north has spun nearly 90 degrees over the last 3 to 4 million years, suggesting that most microplates behave in this manner.
Three lithospheric plates bordering the Pacific Ocean- the Nazca, Antarctic, and South American plates-come together in an unusual triple junction. The first two plates spread apart along a boundary called the Chile Ridge, off the west coast of South America, similar to the way the Americas drift away from Eurasia and Africa along the mid-Atlantic Ridge. The Chile Ridge lies off the Chilean continental shelf at a depth of more than 10,000 feet. Along its axis, magma rises from deep within the Earth and piles up into mounds forming undersea volcanoes.
The Nazca plate moving northeast subducts beneath the westward- moving South American plate at the Peru-Chile Trench. The eastern edge of the Nazca plate is subducting at a rate of about 50 miles every million years, which is faster than its western edge is growing. In essence, the Chile Trench is consuming the Chile Ridge, which will eventually disappear altogether. Several times in the past 170 million years, other plates and their associated spreading centers have vanished beneath the continents that surround the Pacific Basin. This activity had a substantial impact on the coastal geology.
A high degree of geologic activity around the rim of the Pacific Basin formed virtually all the mountain ranges facing the Pacific, as well as the island arcs along its perimeter. Much of western North America assembled from island arcs and other crustal debris skimmed off the Pacific plate as the North American plate moved westward. Northern California is a jumble of crustal fragments assembled over 100 million years ago. Rock formations in San Francisco came from as far as 2,500 miles across the Pacific Ocean.
A nearly complete slice of ocean crust, the type that shoves up on the continents by drifting plates, sits in the middle of Wyoming. The entire state of Alaska is an assemblage of about 50 terranes set adrift over the past 160 million years by the wanderings and collisions of crustal plates (Fig.3-12). Similarly, the Andes might have thrust upward by the accretion of oceanic plates along the continental margin of South America.











Figure 3-12 Maclaren Glacier on the south side of the Alaskan Range. Photo by T. L. Pewe, courtesy of USGS
Terranes are patches of oceanic crust originating from faraway sources shoved up onto the continents and assembled into geologic collages. They are distinct from their geologic surroundings and are usually bounded by faults. The composition of terranes generally resembles that of an oceanic island or plateau, although some comprise a consolidated conglomerate of pebbles, sand, and silt that accumulated in an ocean basin between colliding crustal fragments.
Most terranes created on an oceanic plate are elongated bodies that deformed when colliding with a continent, which subjects them to crustal movements that modify their shape. They exist in a variety of shapes and sizes, from small slivers to subcontinents such as India. The assemblage of terranes in China is being stretched and displaced in an east-west direction because of the continuing pressure that India exerts on southern Asia as it raises the Himalayas (Fig.3-13). North of the Himalayas lies a belt of ophiolites, which appears to mark the boundary between the sutured continents. Ophiolites (from the Greek word ophis, meaning “serpent”) are slices of ocean floor shoved up on the continents by drifting plates and date as old as 3.6 billion years.














Figure 3-13 The frontier between India and China from the space shuttle, showing the Himalaya Mountains. Courtesy of NASA
Suspect terranes are fault-bounded blocks whose geologic histories are distinct from those of neighboring terranes and of adjoining continental masses. They range in age from less than 200 million years old to well over a billion years old. Different species of fossil radiolarians---marine protozoans with skeletons of silica and abundant from about 500 million to 160 million years ago-determine the age of the terranes and also defined specific regions of the ocean where the terranes originated.
Many terranes that comprise western North America have rotated clockwise as much as 70 degrees or more, with the oldest terranes having the greatest degree of rotation. Terrane boundaries, called suture zones, are commonly marked by ophiolite belts, consisting of marine sedimentary rocks, pillow basalts, sheeted dike complexes, gabbros, and peridotites. Suspect terranes were displaced over great distances before finally coming to rest at a continental margin. Some North American suspect terranes have a western Pacific origin and were displaced thousands of miles to the east. At their usual rate of travel, terranes could make a complete circuit of the globe in only half a billion years.




























4 RIDGES AND TRENCHES
Vast undersea mountain ranges, much more extensive than those on the continents, crisscross the ocean stretches. A continuous system of midocean ridges girdles the planet, and is by far the longest geologic structure. Although deeply submerged, the midocean ridge system is easily the Earth’s most dominant feature, extending over a large area than all major terrestrial mountain ranges combined.
The subduction of the lithosphere in deep-sea trenches plays a fundamental role in global tectonics and accounts for powerful geologic forces that continuously shape the surface of the Earth. Major mountain ranges and most volcanoes are associated with the subduction of lithospheric plates. The subduction of the oceanic crust into the mantle produces strain in the descending lithosphere, causing powerful earthquakes to rumble across the landscape.
4.1 THE MIDOCEAN RIDGES
The shifting lithospheric plates create new oceanic crust in a continuous cycle of crustal rejuvenation. The subducing lithosphere circulates through the mantle and reemerges as magma at a dozen or so midocean ridges around the world, generating more than half the Earth’s crust. The addition of new basalt to the ocean floor is responsible for the growth of the lithospheric plates upon which the continents ride.
A large part of this activity takes place in the middle of the Atlantic Ocean, where molten rock welling up from the upper mantle generates new sections of oceanic crust. The floor of the Atlantic acts like two conveyor belts, whose rollers are convection lips in the upper mantle, transporting oceanic crust in opposite directions outward from its point of origin at the Mid-Atlantic Ridge (Fig.4-1).












Figure 4-1 The Mid-Atlantic spreading ridge system separated the New World from the Old World.
The spreading ridge system runs from Iceland in the north to Bovet Island (about 1,000 miles off Antarctica) in the south. The midocean ridge is a string of volcanic seamounts, created by molten magma upwelling from within the mantle. Running down the middle of the ridge crest is a deep trough like a giant crack in the ocean’s crust. This trough reaches 4 miles deep and is up to 15 miles wide, making it the greatest chasm on Earth.
The submerged mountains and undersea ridges form a continuous chain 45,000 miles long (Fig.4-2). The mountainous belt is several hundred miles wide and rises upward of 10,000 feet above the ocean floor. Starting out from the Arctic Ocean, the ridge system spans southward across the Atlantic Basin, continues around Africa, Asia, and Australia, runs under the Pacific Ocean, and terminates at the west coast of North America.











Figure 4-2 Midocean ridges that wind around the world’s ocean basin are composed of individual volcanic spreading centers.
The ocean floor at the crest of the ridge consists mainly of basalt, the most common magma erupted on the surface of the Earth. About 5 cubic miles of new basalt is added to the crust annually, mostly on the ocean floor at spreading ridges. With increasing distance from the crest, a thickening layer of sediments shrouds the bare volcanic rock. As the two newly separated plates move away from the rift, material from the asthenosphere adheres to their edges to form new lithosphere. The lithosphere plate thickens as it propagates from a midocean rift system, causing the plate to sink deeper into the mantle; this is why the seafloor near the continental margins surrounding the Atlantic Basin is the deepest part of the ocean.
Intense seismic and volcanic activity along the midocean ridges manifests itself as a high heat flow from the Earth’s interior. Molten magma originating from the mantle rises through the lithosphere and adds new basalt to both sides of the ridge crest. The greater the flow of magma, the more rapid the seafloor spreading and the lower the relief. The spreading ridges in the Pacific Ocean are more active than those in the Atlantic and therefore are less elevated. Rapid spreading ridges do not achieve the heights of slower ones because the magma does not have the opportunity to pile up into tall heaps. The axis of a slow-spreading ridge is characterized by a rift valley several miles deep and about 10 to 20 miles wide.
In the Pacific Ocean, a rift system called the East Pacific Rise stretches 6,000 miles from the Antarctic Circle to the Gulf of California. It lies on the eastern edge of the Pacific plate marking the boundary between the Pacific and Cocos plates. It is the counterpart of the Mid-Atlantic Ridge and a member of the word’s largest undersea mountain chain. The East Pacific Rise is a network of midocean ridges, which lie mostly at a depth of about 1.5 miles. Each rift is a narrow fracture zone, where plates of the oceanic crust diverge at an average rate of about 5 inches a year.










Figure 4-3 The Romanche Fracture Zone, which offsets the Mid-Atlantic Ridge.
A set of closely spaced fracture zones dissects the Mid-Atlantic Ridges in the equatorial Atlantic. The larges of these structures is the Romanche Fracture Zone, which offsets the axis of the ridge in an east-west direction by nearly 600 miles (Fig.4-3). The floor of the Romanche trench is as much as 5 miles below sea level, and the highest parts of the ridges on either side of the trench are less than a mile below sea level, providing a vertical relief four times that of the Grand Canyon.
The shallowest portion of the ridge is capped with a fossil coral reef, suggesting it was above sea level some 5 million years ago. Many similar and equally impressive fracture zones span the area, culminating in a sequence of troughs and transverse ridges several hundred miles wide. The resulting terrain is unmatched in size and ruggedness anywhere else in the world.
4.2 THE HEAT ENGINE
All geologic activity that shapes the surface of the Earth is outward expression of the great heat engine in the interior of the planet. The motion of the mantle churning over ever so slowly below the crust brings heat from the core to the surface in convection loops (Fig.4-4), the main driving force behind plate tectonics. Convection is the motion within a fluid medium that results when temperatures differ at the bottom and the top. The core transfers heat to mantle rocks, whose increased buoyancy causes them to rise to the surface.









Figure 4-4 Convection currents in the mantle spread lithospheric plates apart.
Convection currents and mantle plumes of hot rock transport molten magma to the underside of the lithosphere, which is responsible for most of the volcanic activity on the ocean floor and on the continents. Most mantle plumes originate from within the mantle, and some arise from the very bottom of the mantle, making the Earth’s interior a huge bubbling pot stirred throughout its entire depth.
The formation of molten rock and the rise of magma to the surface results from an exchange of heat within the planet’s interior. Fluid rocks in the mantle acquire heat from the core, ascend, dissipate heat to the lithosphere, cool, and descend back to the core where they are heated once again. The mantle currents travel very slowly, completing a single convection loop in several hundred million years.
The Earth is steadily losing heat from its interior to the through the lithosphere. About 70 percent of this heat lose results seafloor spreading, while most of the rest is due to volcanism at subduction zones (Fig.4-5). Lithospheric plates created at spreading ridges and destroyed at subduction zones are the final products of convection currents in the mantle.












Figure 4-5 Volcanism at spreading ridges and subduction zones is responsible for most of the Earth’s heat loss.
Most of the mantle’s heat originates from internal radiogenic sources. The rest comes from the core, which has retained much of its original heat since the early accretion of the Earth some 4.6 billion years ago. The temperature difference between the mantle and the core is nearly 1,000 degrees. Material from the mantle might be mixing with the fluid outer core to form a distinct layer on the surface that could block heat flowing from the core to the mantle and interfere with mantle convection.
The asthenosphere is the semi-molten region of the upper mantle upon which the rigid lithospheric plates ride. The asthenosphere is constantly losing material, which escapes from midocean ridges and adheres to the undersides of lithospheric plates. If the asthenosphere were not continuously fed new material from mantle plumes, the plates would grind to a complete halt, and the Earth would become, in all respects, a dead planet because all geologic activity would cease.
4.3 SEAFLOOR SPREADING
Seafloor spreading, which creates new lithosphere at spreading ridges on the ocean floor, begins with hot rocks rising from deeper portions of the mantle by convection currents. After reaching the underside of the lithosphere, the mantle rock spreads out laterally, dissipates heat near the surface, cools, and descends back into the deep interior of the Earth, where it receives more heat in a repeated cycle.
The constant pressure again the bottom of the lithosphere fractures the plate and weakens it. Convection currents flowing outward either side of the fracture carry the separated parts of the lithosphere along with them, widening the gap in the plate. The rifting reduces the pressure in the underlying mantle, allowing mantle rocks to melt and rise the fracture zone.
The molten rock passes through the lithosphere and forms magma chambers that supply molten rock for the generation of new lithosphere. Crustal material is sometimes introduced into the deep magma sources by subduction or off-scraping of a continental margin. The magma reservoirs resemble a mushroom up to 6 miles wide and 4 miles thick. The greater the supply of magma to the chambers, the higher the chambers elevate the overlying spreading ridge.
As magma flows outward from the trough between crests, it adds new layers of basalt to both sides of the spreading ridge, creating new lithosphere. Some molten rock overflows onto the ocean floor in tremendous eruption that generate additional ocean crust. The continents ride passively on the lithospheric plates created at spreading ridges and destroyed at subduction zones. Therefore, the engine that drives the birth and evolution of rifts and, consequently, the breakup of continents and the formation of oceans, ultimately originates in the mantle.
The spreading ridges are the sites of frequent earthquakes and volcanic eruption (Fig4-6). Over much of its length, the ridge system is carved down the middle by a sharp break or rift that is the center of an intense heat flow. Magma oozing out at spreading ridges erupts basaltic lava through long fissures in the trough between ridge crests and along lateral faults. The faults usually occur at the boundary between lithospheric plates, where the oceanic crust pulls apart by the plate separation. Magma welling up along the entire length of the fissure forms large lava pools that harden to seal the fracture.












Figure 4-6 Birth of a new Icelandic island, Surtsey, in November 1963, 7 miles south of Iceland. Courtesy of U.S. Navy









Figure 4-7 Transform faults at spreading centers on the ocean floor.
The spreading ridge system is not a continuous mountain chain but broken into small, straight sections called spreading centers (Fig.4-7). The movement of new lithosphere generated at the spreading centers produces a series of fracture zones, long, narrow linear up to 40 miles wide that consist of irregular ridges and valleys aligned in a stairstep shape. When lithospheric plates slide past each other as the seafloor spreads apart, they create transform faults ranging from a few miles to several hundred miles long. The transform faults transform from active faults between spreading ridge axes to inactive fracture zones past the ridge axes. The transform faults partition the midocean ridge system into independent segments, each with its own volcanic sources.
The transform of the Mid-Atlantic Ridge are offset laterally in a roughly east-west direction. The faults occur every 20 to 60 miles along the midocean ridge, where the longer offsets each consist of a deep trough joining the tips of two segments of the ridge. Friction between segments produces strong shearing forces, wrenching the ocean floor into steep canyons. Other types of offsets up to 15 miles wide separate several spreading center, which are each 20 to 30 miles along. The end of one spreading center often runs past the end of another, and sometimes the tips of the segments bend toward each other.
Transform faults appear to result from lateral strain on the ocean floor, which is how rigid lithospheric plates are expected to react surface of a sphere. This activity is more intense in the Atlantic, where the spreading ridge system is steeper and more jagged than in the Pacific and Indian oceans. Transform faults dissecting the Mid-Atlantic Ridge generally are more rugged than those of the East Pacific Rise. Moreover, fewer widely spaced transform faults exist along the East Pacific Rise, where the rate of seafloor spreading is 5 to 10 times faster than at the Mid-Atlantic Ridge. Therefore, the crust affected by transform faults is younger, hotter, and less rigid in the Pacific than in the Atlantic, giving the Pacific undersea terrain much less relief.
4.4 BASALTIC MAGMA
The seafloor on the crest of the midocean ridge consists of hard volcanic rock. About 80 percent of all oceanic volcanism occurs along spreading ridges, where magma welling up from the mantle spews out onto the ocean floor. The spreading crustal plates grow by the steady accretion of solidifying magma. The molten magma beneath the spreading ridges consists mostly of peridotite, an iron-magnesium silicate. As the peridotite melts on its way through the lithosphere a portion becomes highly fluid basalt. More than one square mile of new ocean crust, comprising about 5 cubic miles of basalt, forms throughout the world annually in this manner.
Magma rising toward the surface fills shallow reservoirs or feeder pipes that are the immediate source of volcanic activity. The magma chambers closest to the surface exist under spreading ridges, where the oceanic crust is only 6 miles or less thick. Large magma chambers lie under fast-spreading ridges where the lithosphere forms at a high rate, as in the Pacific. Narrow magma chambers lie under slow-spreading ridges such as in the Atlantic.
As the magma chamber swells with molten rock and begins to expand, the crest of the spreading ridge bulges upward because of the buoyant forces generated by the magma. The greater the supply of molten magma, the higher it elevates the overlying ridge segment. The magma rises in narrow plumes that balloon out along the spreading ridge, upwelling as a passive response to the release of pressure from plate divergence, somewhat like what happens when the lid is taken off a pressure cooker. Only the center of the plume is hot enough to rise all the way to the surface, however. If the entire plume erupted, it would build a massive volcano several miles high that would rival the tallest volcanoes found on other planets in the Solar System.
The main types of lava formations associated with midocean ridges are sheet flows and tube flows which form pillow lavas (Fig. 4-8). Sheet flows are more prevalent in the active volcanic zone of fast-spreading ridge segments like those of the East Pacific Rise, where in some places the plates separate at a rate of 5 or more inches per year. These flows consist of flat slabs of basalt usually less than a foot thick. The basalt that forms sheet flows is more fluid than that responsible for pillow structures. Pillow lavas often occur at slow-spreading ridges, such as the Mid-Atlantic Ridge, where plates separate at a rate of only about an inch per year and the lava is much more viscous. The manufacture of oceanic crust in this manner explains why some of the most intriguing terrain features lie on the bottom of the ocean.









Figure 4-8 Pillow lava on Knight Island, Alaska. Photo by F.H. Moffit, courtesy of USGS
4.5 THE CIRCUM-PACIFIC BELT
Deep-sea trenches, where the ocean floor disappears into the Earth’s interior, ring the Pacific Basin. Lithospheric plates descent sheetlike into the mantle at subduction zones, lying off continental margins and adjacent to island arcs. Plate subduction is responsible for the intense seismic activity that fringes the Pacific Ocean in a region known as the circum-Pacific, a chain of subduction zones flanking the Pacific Basin.
Most earthquakes originate at plate boundaries (Fig.4-9). Wide bands of earthquakes mark continental plate margins, and narrow bands of earthquakes mark many major oceanic plate boundaries. The most powerful quakes are associated with plate subduction where one plate thrusts under another in deep subduction zones. The greatest amount of seismic energy occurs along the rim of the Pacific Ocean. In the western Pacific, the circum-Pacific belt encompasses volcanic island arcs that fringe the subduction zones, producing some of the largest earthquakes in the world.








Figure 4-9 Most earthquakes occur in broad zones associated with plate boundaries.
The circum-Pacific belt is also known for extensive volcanic activity. Subduction zone volcanoes form island arcs, mostly in the Pacific, and most volcanic mountain ranges on the continents. The circum-Pacific belt coincides with the “ring of fire”, which explains why the Pacific rim also contains the majority of the world’s active volcanoes: the same tectonic forces that produce earthquakes are responsible for volcanic activity. The area of greatest seismicity is on the plate boundaries associated with deep trenches and volcanic island arcs, where an ocean plate dives under a continental plate.
Starting from New Zealand, which is divided by faults (Fig.4-10), the circum-Pacific belt runs northward, encompassing the islands of Tonga, Samoa, Fiji, the Loyalty Islands, the New Hebrides, and the Solomons. The belt then runs westward to embrace New Britain, New Guinea, and the Moluccas Islands. One segment continues westward over Indonesia, while the principal arm travels northward to encompass the Philippines, where a large fault zone runs from one end of the islands to the other. The seismic belt continues on to Taiwan and the Japanese archipelago, which has been hard hit by major earthquakes.












Figure 4-10 Arrows indicate the Wellington Fault in New Zealand. Courtesy of USGS
An inner belt runs parallel to the main belt and takes in the Marianas, a string of volcanic islands characterized by a massive trench system in place over 30,000 feet deep. The belt continues northward and follows the seismic arc across the top of the Pacific, comprising the Kuril Islands (devastated by an 8.2 magnitude earthquake on October 4, 1994), the Kamchatka Peninsula, and the Aleutian Island, which constantly rock and roll. The Aleutian Trench, the largest on Earth, is responsible for the many great earthquakes that strike Alaska. A 200-mile-long stretch called the Shumagin gap, which is accumulating huge stresses in the descending Pacific plate, is poised for a massive earthquake.
Crossing over to the eastern side of Pacific Basin, the seismic belt continues along the Cascadia subduction zone, extending along the coast from southern British Columbia to northern California. This belt has severely shaken the Pacific Northwest in the geologic past is responsible for numerous powerful volcanoes. The tectonic activity is generated by the Juan de Fuca and Gorda plates slipping under the North American plate. The San Andreas Fault (Fig.4-11), which marks the boundary between the North American and Pacific plates, rattles much of California.













Figure 4-11 View south along the San Andreas Fault in the Carriao Plains, California. Photo by R.E. Wallace, courtesy of USGS
The Anders Mountain regions of Central and South America, especially in Chile and Peru, have been lashed by some of the world’s strongest and most destructive earthquakes. The 1960 Chilian earthquake of 9.5 magnitude, the largest in modern history, elevated a California-size chunk of crust some 30 feet. In this century alone, some two dozen earthquakes of 7.5 magnitude or greater have devastated the region.
An immense subduction zone lying just off the coast influences the while western seaboard of South America. The lithospheric plate on which the South America continent rides forces the Nazca plate to buckle under, causing great tensions to build deep within the crust. While some rocks shove downward, others thrust to the surface to raise the Andes Mountains, the fastest growing mountain range on Earth. The resulting forces build great stresses into the entire region. As the strain builds and the crust cracks, great earthquakes roll across the countryside.

4.6 THE DEEP-SEA TRENCHES

TABLE 4-1 THE WORLD’S OCEAN TRENCHES

 

Trench 

Depth

(miles) 

Width 

(miles) 

Length

(miles) 

Peru-Chile

Java

Aleutian 

Middle America 

Mariana 

Kuril-Kamchatka 

Puerto Rico 

South Sandwich 

Philippine 

Tonga 

Japan 

5.0

4.7

4.8

4.2

6.8

6.5

5.2

5.2

6.5

6.7

5.2

62

50

31

25

43

74

74

56

37

64

62

3700

2800

2300

1700

1600

1400

960

900

870

870

500

Figure 4-12 The subduction zones where lithospheric plates enter the mantle are marked by the deepest trenches in the world.
The creation of new lithosphere at midocean ridges is matched by the destruction of old lithosphere at subduction zones (Fig.4-12). Deep trenches lying at the edges of continents or along volcanic island arcs mark the seaward boundaries of the subduction zones. As a lithospheric plate sinks into the mantle, the line of subduction creates a deep-sea trench. While the Pacific plate drifts toward the northwest, its leading edge dives into the mantle, forming the deepest trenches in the world. The Mariana Trench in the western Pacific is the lowest point on Earth. It extends northward from the Island of Guam in the Mariana Islands and reaches a depth of nearly 7 miles below sea level.
Subduction zones, where cool, dense lithospheric plates dive into the mantle, are regions of low heat flow and high gravity (an area where the gravitational pull is strong relative to the average force of gravity on the earth’s surface). Conversely, because of their extensive volcanic activity, the associated island arcs are regions of high heat flow and low gravity. The deep-sea trenches are regions of intense volcanism, producing the most explosive volcanoes in Earth. Volcanic island arcs, which typically share similar curved shapes and similar volcanic origins, fringe the trenches. These island chains, for example the Aleutian Islands and the islands of Japan, are generally arc-shaped because of the geometry of the ocean floor. Any time a plane (in this case a rigid lithospheric plate) cuts into a sphere (here the mantle-encrusted Earth), the point of intersection forms an arc. Think of a knife (plane) slicing into a cantaloupe (sphere) and the arced smile it produces.
The trenches are also sires of almost continuous earthquake activity deep in the bowels of the Earth, about 2 miles down. Plate subduction causes stresses to build into the descending lithosphere, producing deep-seated earthquakes that outline the boundaries of the plate. A band of recent shallow earthquakes clustered in a line running through Microneasia might mark the earliest stages in the birth if a subduction zone and indicate the formation of a trench to the north and west of New Guinea. Gravity in the area is lower than normal-a sign of a trench caused by the sagging of the ocean floor. In addition, a bulge in the crust is beginning to dive into the Earth. The subduction process might not be operating fully for another 5 or 10 million years as the deep-sea trench nibbles away at the Pacific plate.
The seafloor south of New Zealand could also be experiencing the early stages of subduction in the process of creating a deep-sea trench. A geologic scar on the floor of the Pacific known as the Macquarie Ridge is still evolving as part of this process. The ridge is an undersea chain of mountains and troughs that runs south from New Zealand (Fig.4-13) and forms the boundary between the Australian and Pacific plates, which are moving past each other in opposite directions. In 1989, a massive earthquake of 8.2 magnitude struck the ridge.
As the Australian plate slides northwest past the Pacific plate, ruptures occur along vertical faults between the plates, creating large strike-slip earthquakes. As they pass one another, the plates are also pressing together along dipping fault planes, creating smaller compressional earthquakes. This suggests that subduction is just beginning along the Macquarie Ridge. However, the separate dipping faults that flank the sea have not yet connected to form a single large fault plane, a necessary first step before full-fledged subduction commences.









Figure 4-13 Location of the Macquarie Ridge south of New Zealand.
A plate extending away from its place of origin at a midocean spreading ridge thickens and becomes denser as additional material from the asthenosphere adheres to its underside in a process called underplating. The depth at which a lithospheric plate sinks as it moves away from a spreading ridge increases with age. Thus, the older the lithosphere, the more basalt that underplates it, making the plate thicker, denser, and deeper.
Eventually, the plate becomes so dense that it loses buoyancy and sinks into the mantle, and the subduction creates a deep-sea trench at clear defined subducton zones. As the subduction portion of the plate dives into the Earth’s interior, the rest of the plate, which might carry a continent on its back, is pulled along with it like a freight train hauled by a locomotive. The force of the pull on the sinking slab depends on the length of the subduction zone, the rate of subduction, and the amount of trench suction produced by mantle convection. Plate subduction is the main driving force behind plate tectonics, and pull at subduction zones is more active than push at spreading ridges to move the continents around the surface of the globe.
New oceanic crust generated by seafloor spreading in the Atlantic and the eastern Pacific is offset by the subduction of old oceanic crust along the rim of the Pacific to make more room. Because the seafloor spreading rate is not always the same as the rate of subduction, associated midocean ridges often move laterally. Most of the subduction zones are in the western Pacific, which accounts for the fact that oceanic crust is not older than 170 million years.
Subduction zone are the sites of almost continuous seismic activity, with a band of earthquakes marking the boundaries of a sinking lithospheric plate (Fig.4-14). As plates slide past each other along subduction zones, they create highly destructive earthquakes, such as those that have always plagued Japan, the Philippines, and other islands connected with subduction zones.









Figure 4-14 Cross-section of a descending lithospheric plate. Each O denotes a shallow earthquake. Each X denotes a deep-seated earthquake. The Benioff zone is a plane of seismic activity, marking the outline of a descending plate.
The subduction zones are also regions of intense volcanic activity, producing the most explosive volcanoes on the planet. Magma reaching the surface of the oceanic crust erupts on the ocean floor, creating new volcanic islands. Most volcanoes do not rise above sea level; rather, they become isolated undersea volcanic structures called seamounts. The Pacific Basin is more volcanically active and has a higher density of seamounts than the Atlantic or Indian basins. Subduction zone volcanoes are so explosive because their magmas contain large quantities of volatiles and gases that escape violently when reaching the surface. The type of volcanic rock erupted in this manner is andesite, named for the Andes Mountains that form the spine of South America and that are well known for their violent eruptions.

4.7 PLATE SUBDUCTION
As the lithospheric plate carrying the crust into the Earth’s interior, it slowly breaks up and melts. Over a period of millions of years, it is absorbed into the general circulation of the mantle. When the plate dives into the interior, most of its trapped water goes down with it becoming an important volatile in magma. The subducted plate also supplies molten magma for volcanoes, most of which ring the Pacific Ocean and recycle chemical elements to the Earth.
The amount of subducted plate material is vast. When the Atlantic and Indian oceans opened up and began forming new oceanic crust some 125 million years ago, an equal area of oceanic crust disappeared into the mantle. This meant that 5 billion cubic miles of crust and lithospheric material was destroyed. At the present rate of subduction, the mantle will consume an area equal to the entire surface of the plant in 160 million years.
The convergence of lithospheric plates forces the thinner, more dense oceanic plate under the thicker, more buoyant continental plate. When oceanic plates collide, the older and denser plate dives under the younger plate (Fig. 4-15). A deep-ocean trench marks the line of initial subduction. At first the plate’s angle of decent is low, but it gradually steepens to about 45 degrees, with the rate of vertical descent (typically 2 to 3 inches per year) less than the rate of horizontal motion of the plate.









Figure 4-15 Collision between two continental plates (top), a continental plate and an oceanic plate (middle), and two oceanic plates (bottom).
If continental crust moves into a subduction zone, its greater buoyancy prevents it from being dragged down into the trench. When two continental plates converge, the crust is scraped off the subducting plate and fastens onto the overriding plate, welding the two pieces of continental crust together. Meanwhile, the subducted lithospheric plate, now without its overlying crust, continues to dive into the mantle, squeezing the continental crust together and forcing up mountain ranges. In many subduction zones, such as the Lesser Antilles, sediments and their contained fluids are removed by offscraping and underplating in accretionary prisms, wedges of sediment that form on the overriding plate adjacent to the trench. In other subduction zones, such as the Mariana and Japan trenches, little or no sediment accretion occurs. Thus, subduction zones differ markedly from one another in the amount of sedimentary material removed at the accretionary prism. In most cases at least some sediment and bound fluids appear to be subducted to deeper levels.
The underthrusting of continental crust by additional crustal material increases continental buoyancy and pushes up mountain ranges. A similar process occurred when India collided with Asia about 45 million years ago, pushing up the Himalayas. A strange series of east-west wrinkles in the ocean crust just south of India verifies that the India plate is still pushing northward, shrinking the Asian continent as much as 3 inches a year. Further compression and deformation might eventually take place beyond the line of collision, producing a high plateau with surface volcanoes, similar to the Tibetan Plateau, the largest in the world.
When continental and oceanic plates converge, the denser oceanic plate dives beneath the lighter continental plate and is forced farther downward. The sedimentary layers of both plates are squeezed like an accordion, swelling the leading edge of the continental crust to create folded mountain belts such as the Appalachians. As the descending plate dives farther under the continental, it reaches depths where the temperatures are extremely high. The upper part of the plate melts to form magma that rises toward the surface to provide volcanoes with a new supply of molten rock.