英文原版教材选编geology2-n


5 SUBMARINE VOLCANOES
An extraordinary number of volcanoes are hidden under the waves. More than 80 percent of the Earth’ surface above and below the sea is of volcanic origin. The vast majority of the volcanic activity that continually remakes the surface of the Earth takes place at the bottom of the ocean, where the most the world’s volcanoes are located. Oceanic volcanoes also happen to be most explosive in the world, and whole islands have been known to disappear, with new ones popping up to take their places.
Nearly all the world’s islands started out as undersea volcanoes. In volcanic island-building, successive eruption pile up volcanic rocks until the volcano’s peak finally breaks the surface of the sea. Active undersea volcanoes rising tens of thousands of feet off the ocean floor become volcanic islands. Measured from the seabed, some of them are the world’s tallest mountains.
5.1 THE RING OF FIRE
Most of the world’s volcanoes are associated with crustal movements at the margins of lithospheric plates. An almost continuous “ring of fire” runs along the rim of the Pacific, which coincides the circum-Pacific belt because the same tectonic processes that generate earthquakes also produce volcanoes. The greatest activity occurs on plate boundaries associated with deep trenches along volcanic island arcs and the margins of continents.










Figure5-1 The Ring of Fire is a band of subduction zones surrounding the Pacific Ocean

The Ring of Fire corresponds to a band of subduction zones surrounding the Pacific Basin (Fig.5-1), which have devoured almost all the seafloor since the breakup of Pangaea. The oldest oceanic crust lies in a small patch off southeast Japan and is only about 170 million years old, compared with the ocean floor, which is on average about 100 million years. While subducting into the mantle, the oceanic crust melts to provide molten magma for volcanoes that fringe the deep-sea trenches. This is why most of the 600 active volcanoes in the world lie in the Pacific Ocean-nearly half of them in the western Pacific region alone.
Subduction zone volcanism builds volcanic chains on the continents and island arcs in the ocean. At convergent plate boundaries, where one plate subducts under another, magma forms when the lighter constituents of the subducted oceanic crust melt. The upwelling magma creates island arcs, including Indonesia, the Philippines, Japan, the Kuril Islands, and the Aleutians, the longest, extending more than 3,000 miles from Alaska to Asia.
Beginning at the western tip of the Aleutian Islands off Alaska, the Ring of Fire runs along the Aleutian archipelago, a string of volcanic islands (Fig.5-2) created by the subduction of the Pacific plate down the Aleutian Trench. The band of volcanoes turns south across the Cascade Range of British Columbia, Washington, Oregon, and northern California, associated with the subduction of the Juan de Fuca plate down the Cascadia subduction zone. The ring then runs across Baja California and southwest Mexico, where lie the volcanoes Parícutin and EI Chichon.(The latter is perhaps the dirtiest volcano of this century in terms of ash cast up into the atmosphere.)









Figure 5-2 A crater and dome of Great Sitkin Volcano,Great Sitkin lsland,Aleutian , Alaska. Photo by F.S. Simons, courtesy of USGS

TABLE 5-1 COMPARISON BETWEEN TYPES OF VOLCANISM

Characteristic

Subduction

Rift Zone

Hot Spot

Location

Deep ocean trenches

Midocean ridges

Interior of plates

Percent active volcanoes

80%

15%

5%

Topography

Mountains, island arcs

Submarine ridges

Mountains, geysers

Examples

Andes Mts. Japan Is.

Azores Is. Iceland

Hawaiian Is. Yellowstone

Heat source

Plate friction

Convection&nb

Figure 5-3 Locations of killer volcanoes responsible for the deaths of over 1,000 people each since 1700.The volcano belt continues through western Central America, which has numerous active cones, including Nevado del Ruiz of Columbia, whose destructive mudflows killed 25,000 people in November 1985. It is among some 20 other volcanoes that have each killed more than 1,000 people since 1700(Fig.5-3). The Ring of Fire journeys along the course of the Andes Mountains on the western edges of South America, whose highly explosive nature results from the subduction of the Nazca plate down the Chilean Trench.The volcanic band then turns toward Antarctica and the islands of New Zealand, New Guinea, and Indonesia, where the volcanoes Tambora and Krakatoa produced the greatest eruptions in modern history. These eruptions were instigated by the subduction of the Austrialian plate down the Java Trench. The band continues across the Philippines, where Mount Pinatubo spewed a massive eruption cloud in June 1991 that caused dramatic changes in climate; this eruption resulted from the subduction of the Pacific plate down the Philippine Trench. The Ring of Fire runs across Japan (where the Fuji volcano reigns majestically), finally ending on the Kamchatka Peninsula in northeast Asia.Figure 5-4a Location of the great Indonesian volcanoes Krakatoa and Tambora. Subduction zone volcanoes such as those in the western Pacific and Indonesia (Figs. 5-4a&b) are among the most explosive in the world, destroying entire islands when they erupt. One classic example is the near-total destruction of the Indoneasian island of Krakatoa in 1883. The explosive nature of such volcanoes is due to abundant silica and volatiles in the magma, which consists of water and gases derived from sediments on the ocean floor that were subducted into the mantle and melted. When the magma depressurizes as it reaches the surface, the volatiles explode and fracture the molten rock, destroying much of the volcano in the process.Figure 5-4b An active volcano on Andonara Island, Indonesia (center), leaves a 30-mile-long train of ash. Courtesy of NASA On the continents, plate subduction creats long chains of powerful volcanoes. The Cascade Range in the Pacific Northwest is a belt of volcanoes associated with a subduction zone under the North American continent. The Andes Mountains of South America are a chain of volcanoes associated with a subduction zone under the South American continent. As the lithosphere plunges into the mantle, tremendous heat melts the descending plate and the adjancent lithospheric plate, and magma rising to the surface feeds rows of active volcanoes.5.2 THE RISING MAGMASubduction zones created by descending plates accumulate large quantities of sediment from the adjacent continents and volcanic island arcs. When the sediments and seawater become trapped between a subducting ocean plate and an overriding continental plate, they undergo strong deformation, shearing, heating, and metamorphism (recrystallization without melting). The sediments are carried deep into the mantle, where they melt to become the source of new magma for volcanoes fringing the subduction zones. Some magma originates from the partial melting of subducted oceanic crust, with heat supplied by the shearing action at the top of the descending plate. Convective motions in the wedge of asthenosphere caught between the descending oceanic plate and the continental plate forces material upward, where it melts under reduced pressures. The magma rises to the surface in giant plumes called diapirs. Upon reaching the underside of the lithosphere, the diapirs burn holes through the crust as the molten rock melts its way upward. As the diapirs rise toward the surface, they form magma bodies, which become the immediate source for new igneous activity (Fig.5-5). After reaching the ocean floor, the magma erupts to create new volcanic islands. The rock type associated with subduction zone volcanoes is fine-grained gray andesite, which contains abundant silica from deep-seated sources, possibly 70 miles below the surface. The rock derives its named from the Andes Mountains, whose volcanoes are highly explosive because of large amount of volatiles in the magma. As the magma rises toward the surface, the pressure drops and volatiles escape with great force, shooting out of the volcano like pellets fired from a gigantic canon.Figure 5-5 Diapirs supply the magma for volcanoes and spreading ridges on the ocean floor. The mantle material that slowly extrudes onto the surface is black basalt, the most common volcanic rock. The ocean floor is paved with abundant basalt, and most volcanoes are entirely or predominately basaltic. The magma that forms basalt originates in a zone of partial melting in the upper mantle more than 60 miles below the surface. The semimolten rock at this depth is less dense and more buoyant than the surrounding mantle material and rises slowly toward the surface. As the magma ascend, the pressure decreases, allowing more mantle material to melt. Volatiles such as dissolved water and gases make the magma flow easily. The mantle material below spreading ridges that create new oceanic crust consists mostly of peridotite, which is rich in silicates of iron and magnesium. As the peridotite melts on its journey to the surface, a portion becomes highly fluid basalt.The magma’s composition indicates both its source materials and the depth within the mantle from which they originated. The degree of partial melting of mantle rocks, partial crystallization that enriches the melt with silica, and the assimilation of a variety of crustal rocks in the mantle all influence the composition of the magma. When the erupting magma rises toward the surface, it incorporates a variety of rock types along the way, which also changes its composition. The magma’s composition determines its viscosity and the type of eruption that will occur. If the magma is highly fluid and contains little dissolved gas upon reaching the surface, it produces basaltic lava, and the eruption is usually quite mild, as with the Hawaiian volcanoes. If, however, the magma rising toward the surface contains a large quantity of dissolved gases, the eruption can be highly explosive and very destructive. Water is possibly the single most important volatile in magma and affects the explosive nature of some volcanic eruptions by causing a rapid expansion of steam as the magma reaches the surface(Fig.5-6).5.3 ISLAND ARCSAlmost all volcanic activity is confined to the margins of lithospheric plates. As previously noted, deep trenches at edges of continents or along volcanic island arcs mark the seaward boundaries of subduction zones. At convergent plate boundaries, where one plate subducts under another, new magma forms when the lighter constituent of the subducted plate melts and rises to the surface. When the upwelling magma erupts on the ocean floor, it creates island arcs. These occur mostly in the Pacific.

Figure 5-6 A submarine eruption of Myojin-sho Volcano in the Izu Islands, Japan, on September 23, 1952. Courtesy of USGS

TABLE 5-2 CLASSIFICATION OF VOLCANIC ROCKS

Property

Basalt

Andesite

Rhyolite

Silica content

Lowest about 50%, 

basic rock

Intermediate 

about 60%

Highest more 

than 65%, acid rock

Dark mineral content

Highest

Intermediate

Lowest

Typical mineral

Feldspar

Pyroxene

Olivine

Oxides

Feldspar

Amphibole

Pyroxene

Mica

Feldspar

Quartz

Mica

Amphibole

Density

Highest

Intermediate

Lowest

Melting point

Highest

Intermediate

Lowest

Molting rock viscosity 

at the surface

Lowest 

Intermediate

Highest

Tendency to form lavas

Highest

Intermediate

Lowest

Tendency to form pyroclastics

Lowest 

Intermediate

Highest

Figure 5-6 A submarine eruption of Myojin-sho Volcano in the Izu Islands, Japan, on September 23, 1952. Courtesy of USGS

TABLE 5-2 CLASSIFICATION OF VOLCANIC ROCKS

Property

Basalt

Andesite

Rhyolite

Silica content

Lowest about 50%, 

basic rock

Intermediate 

about 60%

Highest more 

than 65%, acid rock

Dark mineral content

Highest

Intermediate

Lowest

Typical mineral

Feldspar

Pyroxene

Olivine

Oxides

Feldspar

Amphibole

Pyroxene

Mica

Feldspar

Quartz

Mica

Amphibole

Density

Highest

Intermediate

Lowest

Melting point

Highest

Intermediate

Lowest

Molting rock viscosity 

at the surface

Lowest 

Intermediate

Highest

Tendency to form lavas

Highest

Intermediate

Lowest

Tendency to form pyroclastics

Lowest 

Intermediate

Highest

The longest island arc is the Aleutians, extending more than 3,000 miles from Alaska to Asia, where the Pacific plate subducts beneath the overriding North American plate. The Kurile Islands to the south form another long arc. The islands of Japan, Philippines, Indonesia, New Hebrides, Tonga, and those from Timor to Sumatra also form island arcs. These island arcs are all similarly curved, have similar geological compositions, and are associated with subduction zones. The curvature of the island arcs results from the curvature of the Earth. Just as an arc forms when a plane cuts a sphere, so does an arc-shaped feature result when a rigid lithospheric plate subducts into the Earth’s spherical mantle.
At deep-sea trenches, created during the subduction process, magma forms when oceanic crust deep into the mantle melts. As the lithospheric plate carrying the oceanic crust descends farther into the Earth’s interior, it slowly breaks up and also melts. Over a period of millions of year, it assimilates into the general circulation of the mantle, possibly descending as deep as the top of the Earth’s core. Eventually, the magma rises to the surface in giant plumes, completing the loop in the convection of the mantle.
The subducted plate is also the immediate supply of molten magma for volcanic island arcs (Fig.5-7). Behind each island arc is a marginal or a back-arc basin, a depression in the ocean crust caused by plate subduction. Steep subduction zones like the Mariana Trench in the western Pacific form back-arc basins, whereas shallow ones like the Chilean Trench off the west coast of South America do not. A classic back-arc basin is the Sea of Japan between China and the Japanese archipelago (Fig.5-8); the archipelago is comprised of ruptured continental fragments. Gradually, the sea will close off entirely as the Japanese islands slam into Asia.
Back-arc basins are regions of high heat flow because they overlie relatively hot material brought up convection currents behind the island arcs or by upwelling from deeper regions in the mantle. The trenches are regions of low heat flow because of the subduction of cool dense lithosphere, while the adjacent island arcs generally are regions of high heat flow because of their high degree of volcanism.
5.4 GUYOTS AND SEAMOUNTS
Marine volcanoes associated with midocean ridges that rise above the sea become volcanic islands. Most of the world’s islands began as undersea volcanoes. Successive volcanic eruptions pile up layers of basalt until the peak finally breaks through the ocean surface. The volcanic ash makes a rich soil; as the island cools, seeds carried by wind, sea, and animals rapidly turn the newly formed land into a lush tropical paradise. Life must still cope with the rumblings deep within the Earth, however, and the island eventually might be destroyed in a single huge convulsion.









Figure 5-7 The formation of volcanic island arcs caused by the subduction of a lithospheric plate.














Figure 5-8 Location of the Sea of Japan.
Most volcanic island, however, end their lives quietly, eroded by the incessant pounding of the sea. Submarine volcanoes called guyots (pronounced “ghee-ohs”) located in the Pacific once towered above the ocean. But the constant wave action eroded them below the sea surface, leaving them as though the tops of the cones had been sawed off. The farther these volcanoes were conveyed from volcanically active regions, the older and flatter they become (Fig.5-9), suggesting that the guyots and the plates they rode on drifted across the ocean floor far from their place of origin. The islands appeared to have formed in assembly-line fashion, each moving in succession away from a magma chamber lying beneath the ocean floor.










Figure 5-9 Guyots were once active volcanoes that moved away from their magma source and have since disappeared beneath the sea.
Beyond the oldest Hawaiian island, Kauai, the persistent pounding of the waves has eroded the ancient volcanoes so that they now lie below sea level. Coral atolls, like Midway Island, and shallow shoals were formed by coral living on the flattened tops of eroded volcanoes. Atolls were rings of coral islands, enclosing a central lagoon (Fig. 5-10). They consist of coral reefs up to several miles across, and many atolls formed on ancient volcanic cone that have subsided beneath the sea, with the rate of coral growth matching the rate of subsidence. Continuing in a northwestward direction is an associated chain of undersea volcanoes, called the Emperor Seamounts (Fig.5-11); these were presumably built by a single hot spot, although how such a plume could persist for over 70 million years remains a mystery.














Figure 5-10 Tarawa (left) and Abaiang Atolls, Gilbert Island. Courtesy of NASA














Figure 5-11 The Emperor Seamounts and Hawaiian Islands in the North Pacific represent motions of the Pacific plate over a volcanic hot spot. Note the sharp northward bend in the seamounts, caused by shifting of the Pacific plate.
Most marine volcanoes never rise above the sea to become islands, but instead become isolated undersea volcanoes called seamounts. Magma upwelling from the upper mantle at depths of more than 60 miles below the surface concentrates in narrow conduits that lead to a main feeder column. The magma erupts on the ocean floor, building seamounts, which are generally isolated and strung out in chains across the interior of a plate. Some seamounts are associated with extended fissures, along which magma wells up through a main conduit, piling successive lava flows on one another.
The crust under the Pacific Ocean is more volcanically active than the Atlantic or Indian oceans, providing a higher density of seamounts. The tallest seamounts rise over 2.5 miles above the seafloor in the western Pacific near the Philippine Trench. The number of undersea volcanoes increases with advanced crustal age and increased thickness. Deep-sea ridges called abyssal hills, which were developed by eruptions along midocean ridges, cover 60 to 70 percent of the Earth’s surface. The average density of Pacific seamounts is 5 to 10 volcanoes per 5,000 square miles of ocean floor, by far outnumbering volcanoes on the continents.
Sometimes the summit of a seamount contains a crater, within which lava extrudes. If the crater exceeds a mile in diameter it is called a caldera, whose depth below the crater rim can be as much as 1,000 feet. Calderas form when the magma reservoir empties, creating a hollow chamber. Without support, the top of the volcanic cone collapses, forming a wide depression similar to calderas of Hawaiian volcanoes (Fig. 5-12). Feeder vents along the periphery of the caldera supply fresh lava that fills the caldera, giving the volcano a flattop appearance. Other undersea volcanoes do not have a collapsed caldera; instead, the summit contains several isolated volcanic peaks rising upward of 1,000 feet high.









Figure 5-12 A broad fountain pit in the cinder cone and large lava rivers draining from it, Halemaumau Volcano, Hawaiian Islands. Photo by G.A.Macdonald, courtesy of USGS
5.5 RIFT VOLCANOES
More than three-quarters of all oceanic volcanism occurs at midocean ridges, where basaltic magma wells up from the mantle and spews out onto the ocean floor in response to seafloor spreading. Lithospheric plates subduct into the mantle like great sheets and arise again in giant cylindrical plumes of hot rock at midocean ridges. A series of plumes miles apart feed separate segments of the spreading ridge.
At the crest of a midocean ridge, the ocean floor consists almost entirely of hard volcanic rock. Along much of its length, the ridge system is divided down the middle by a sharp break or rift that is the center of volcanic activity. The spreading ridges are the sites of frequent earthquakes and volcanic eruptions, as though the entire system were a series of giant cracks in the crust, from which molten magma oozes out onto the ocean floor.
The volcanic activity associated with midocean rift systems is fissure eruption, the most common type. Such eruption built typical conical volcanic structures. Volcanoes formed on or near midocean ridges often develop into isolated peaks when they move away from the ridge axis as the seafloor spreads apart. During fissure eruptions, lava bleeds through 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 splits apart by the separating plates. Magma welling up along the entire length of the fissure forms large lava pools, similar to those of broad shield volcanoes such as the Hawaiian volcano Mauna Loa, the largest of its kind in the world.
Seamounts associated with midocean ridges that grow tall enough to break through the surface of the ocean become volcanic islands.The Galapagos Islands west of Ecuador are volcanic islands associated with the East Pacific Rise. Volcanic islands associated with the Mid-Atlantic Ridge include Iceland, the Azores, the Canary and Cape Verde Islands off West Africa, Ascension Island, and Tristan de Cunha.
The volcanic islands in the middle of the North Atlantic that comprise the Azores were created by a mantle plume or hot spot that once lay beneath Newfoundland, which then drifted westward as the ocean floor spread apart at the Mid-Atlantic Ridge. The Sts. Peter and Paul islands in the mid-Atlantic north of the equator are not volcanic in origin but instead are fragments of the upper mantle uplifted near the intersection of the St. Paul transform fault and the Mid-Atlantic Ridge.
Iceland is a broad volcanic plateau of the Mid-Atlantic Ridge that rose above the sea about 16 million years ago when the ridge assumed its present position (Fig.5-13). The island is unique because it straddles a spreading ridge system, where the two plates of the Atlantic basin and adjacent continents pull apart. Along the ridge, the abnormally elevated topography extends in either direction about 900 miles, and over a third of the plateau lies above level. South of Iceland, the broad plateau tapers off to form a structure more typical of the Mid-Atlantic Ridge.
A steep-sided, V-shaped valley runs northward across the entire length of the Iceland, and is one of the few expressions of a midocean rift on land. Numerous volcanoes flank the rift, making Iceland one of the most volcanically active places on Earth. On other parts of the midocean ridge, volcanic activity is quite prevalent, with perhaps as many as 20 major deep underwater eruptions a year.
Volcanoes formed on or near the midocean ridges often develop into isolated peaks as they move outward from the ridge axis during seafloor spreading. The ocean floor thickens as it moves away from the spreading ridge axis. This thickening of the seafloor influences a volcano’s height, because a thicker crust can support a greater mass. The ocean crust also bends like a rubber mat under the massive weight of a seamount. For instance, the crust beneath Hawaii bulges in a downward concave shape as much as 6 miles.








Figure 5-13 Iceland straddles the Mid-Atlantic Ridge.
A volcano formed at a midocean ridge cannot increase its mass unless it continues to be supplied with magma after it leaves the vicinity of the ridge. Sometimes a volcano formed on or near a midocean ridge develops into an island, only to have its source of magma cut off. Then erosion begins to wear it down until it finally sinks beneath the sea (Fig. 5-14).


















Figure 5-14 The life cycle of an oceanic volcano. From the top: A rift first forms on the ocean floor, lava piles up until the volcano rises above sea level, and the dormant cone sinks below sea level.
5.6 HOT SPOT VOLCANOES
About 100 small regions of isolated volcanic activity known as hot spot volcanoes exist in various parts of the world. The hot spots provide a pipeline for transporting heat from the planet’s core to the surface. The plumes do not rise through the mantle as a continuous stream but, rather, as separate giant bubbles of hot rock. When the bubbles reach the ocean floor at the top of the mantle they create a secession of oceanic islands.
The ascending mantle plumes can lift an entire region. This has happened in a 3,000-mile-wide section of the South Pacific floor where several hot spots have erupted to form the Polynesian island chains. Similar swells occur under the Hawaiian chain in the North Pacific, Iceland in the North Atlantic, and the Kerguelen Islands in the southern Indian Ocean. The most active modern hot spots lie beneath the big island of Hawaii and Reunion Island to the east of Madagascar.
Unlike most other active volcanoes, those created by hot spots are rarely sited at plate boundaries but reside in the interiors of lithospheric plates (Fig. 5-15). Hot spot volcanoes are notable for their very geological isolation far from normal centers of volcanic and earthquake activity. Lavas of hot spot volcanoes differ markedly from those of subduction zones and rifts. The distinctive composition of hot spot magmas suggests that their source is outside the general circulation of the mantle.










Figure 5-15 The world’s hot spots, where mantle plumes rise to the surface.
The lavas comprise basalts that contain larger amounts of alkali mineral such as sodium and potassium, indicating that their source material is not connected with plate margins. Instead, the hot spots derive their source material from deep within mantle, possibly near the top of the core. Hot spot plumes also might arise from stagnant regions in the center of convection cells or from below the region in the mantle stirred by convection currents.
As plumes of mantle material flow upward into the asthenosphere, the portion rich in volatiles rises toward the surface to feed hot spot volcanoes. The plumes exist in a range of sizes that might indicate the depth of their source material. They are not necessarily continuous flows of mantle material but might consist of molten rock, rising in giant blobs or diapirs. If the upwelling plumes stopped feeding the asthenosphere with a continuing flow of mantle material, the plates would grind to a complete halt.
The typical lifespan of a plume is a few hundred million years. Sometimes a hot spot fades away and a new one forms in its place. The position of a hot spot changes slightly as it sways in the convective currents of the mantle. As a result, the hot spot tracks on the surface might not always be linear. However, compared with the motion of the plates, the mantle plumes are virtually stationary. Because the motion of the hot spots is so slight, they provide a reference point for determining the direction and rate of plate travel.
The passage of a plate over a hot spot often results in a trail of volcanic features, whose linear trend reveals the direction of plate motion. This produces volcanic structures aligned in a direction that is oblique to the adjacent midocean ridge system rather than parallel to it like rift volcanoes. The hot spot track might be a continuous volcanic ridge or a chain of volcanic islands and seamounts that rise high above the surrounding seafloor. The hot spot track also can weaken the crust, cutting through the lithosphere like a geologic blowtorch.
The most prominent and easily recognizable hot spot created the Hawaiian Islands, the largest islands of their kind in the world(Fig. 5-16). In effect, Hawaii’s Mauna Kea Volcano, which built most of the main island, is the world’s tallest mountain. It rises 6 miles above the ocean floor, exceeding the height of Mount Everest by more than half a mile.
The youngest and most volcanically active island is Hawaii, at the southeast end of the chain. Some 20 miles south of Hawaii lies a submerged volcano called Loihi, which rises about 8,000 feet above the ocean floor but is still 3,000 feet below the sea surface. Perhaps in another 50,000 years it will rise above the sea and take its place in the Hawaiian chain. The rest of the Hawaiian islands are progressively older, with extinct volcanoes trailing off to the northwest.
The entire Hawaiian chain apparently formed from a single magma source, over which the Pacific plate has passed in a northwesterly direction. The volcanic islands slowly popped out on the ocean floor conveyor belt fashion, with the oldest trailing off to the northwest, and now farthest away from the hot spot. Similar chains of volcanic islands lie in the Pacific and trend in the same general southeast-to-northwest direction as the Hawaiian Island (Fig. 5-17), indicating that the Pacific plate is moving off in this direction. Lying parallel to the Hawaiian chain are the Austral and Tuamotu ridges. The islands and seamounts were formed by the northwestward motion of the Pacific plate over a volcanic hot spot.
















Figure 5-16 Photograph of the Hawaiian Island chain looking south, taken from the space shuttle. The main island Hawaii is the upper portion of the photo. Courtesy of NASA
The plate did not always travel in this direction, however. More than 40 million years ago it followed a more northerly heading. The course change might have resulted from a collision between the Indian and Asian plates, and appears as a distinct bend in the hot spot tracks. A sharp bend in the long Mendocino Fracture zone jutting out from northern California confirms that the Pacific plate abruptly changed direction at the same time as the India-Asia plate convergence. The timing is also coincident with the collision of the North American and Pacific plates. From these observations, geologists conclude that hot spots are generally a reliable means for determining plate activity.












Figure 5-17 Linearity of volcanic islands on the Pacific plate in direction of movement.
The Bermuda Rise in the western Atlantic, however, appears to be a contradiction to this rule. Oriented in roughly northeast direction, parallel to the continental margin off the eastern United States, the Bermuda Rise is nearly 1,000 miles long and rises some 3,000 feet about the surrounding seafloor. The last of its volcanoes ceased erupting about 25 million years ago. A weak hot spot unable to burn a hole through the North American plate apparently was forced to take advantage of previous structure on the ocean floor acting as conduits, which explains why the volcanoes trend almost at right angles to the motion of the plate.
The Bowie seamount is the youngest in a line of submerged volcanoes running toward the northwest off the west coast of Canada. It is fed by a mantle plume more than 400 miles below the ocean floor and nearly 100 miles in diameter. But rather than lying directly beneath the seamount, as plumes usually do, this plume lies about 100 miles east of the volcano. It is believed that the plume might have taken a tilted path upward, or that the seamount somehow moved with respect to the hot spot’ position.
If a midocean ridge passes over a hot spot, the plume augments the flow of molten rock welling up from the asthenosphere to form new crust. The crust is therefore thicker over the hot spot than it is along the rest of the ridge, resulting in a plateau rising above the surrounding seafloor. The Ninety East Ridge, named for its location at 90 degrees east longitude, is a succession of volcanic outcrops that runs 3,000 miles south of the Bay of Bengal and formed when the Indian plate passed over a hot spot on its way to Asia about 120 million years ago, creating an immense lava field on India known as the Rajmahal Traps.
The movement of the continents was more rapid than it is today, with perhaps the most vigorous plate tectonics the world has ever known. About 120 million years ago, an extraordinary burst of submarine volcanism struck the Pacific Basin releasing vast amounts of gas-laden lava onto the ocean floor. The volcanic spasm is evidenced by a collection of massive undersea lava plateaus that formed almost simultaneously. The largest of these plateaus is the Ontong Java, northeast of Australia. Roughly two-thirds the size of the continent, it contains at least 9 million cubic miles of basalt, enough to bury the entire United States under 15 feet of lava.















Figure 5-18 The Deccan Traps flood basalts in India.
About 65 million years ago, a giant rift opened up along the western side of India, and huge volumes of molten lava poured onto the surface, forming the Deccan Traps Flood basalts (Fig.5-18). The rift separated the Seychelles Bank from the mainland, creating the Seychelles Islands. They were followed 40 million years ago by the Kerguelen Islands as India continued to trek northward toward southern Asia.
The Kerguelen plateau is the world’s largest submerged platform. Approximately 50 million years ago, a huge submerged plateau in the Indian Ocean separated into two platforms that now sit about 1,200 miles apart. The plateau grew from the ocean floor more than 90 million years ago, when a series of volcanic eruptions poured out voluminous amounts of molten basalt onto the Antarctic plate as the continent separated from Australia.
During the next several million years, a long rift sliced through the plate and cut off its northern section, which latched onto the Indian plate and started on a long journey northward. Meanwhile, the southern half of the plate continued to move southward. Half of the original platform, called the Broken Ridge, currently lies off the west coast of Australia. The other half, the Kerguelen plateau, sits north of Antarctica. The Exmouth plateau is a submerged feature that sits on a sunken piece of the Australian continent, which itself was attached to India when all continents were assembled into Pangaea.

















6 ABYSSAL CURRENTS
The ocean is continuously in motion, distributing water and heat to all corners of the globe. In effect, the ocean acts as a huge circulating machine that makes the Earth’ s climate equitable. Ocean currents follow well defined courses, transporting tremendous quantities of seawater, serving as a global “conveyor belt” over the planet.
Abyssal storms stir the deep sea ocean floor, shifting sediments on the seabed. El Nino currents, caused by a great sloshing of seawater in the Pacific Basin, generate unusual atmospheric weather patterns throughout the world. Waves and tides are constantly changing and rearranging the shoreline. Tsunamis produced by undersea earthquakes and coastal volcanic eruptions are among the most damaging waves, inflicting death and destruction to many seacoast inhabitants.
6.1 RIVERS IN THE ABYSS
Currents in the upper regions of the ocean (Fig. 6-1) are driven by the winds, which impart their momentum to ocean’s surface. The currents do not flow in the wind direction but are deflected by the Coriolis effect to the right of the wind direction, or to the northwest, in the Northern Hemisphere and to the left of the wind direction, or to the southwest, in the Southern Hemisphere. The currents acquire warm water from the tropics, distribute it to the higher latitudes, and return to the tropics with cold water. This exchange moderates the temperatures of coastal regions, making countries like Japan and northern Europe warmer than they otherwise would be for their latitudes.









Figure 6-1 The major ocean currents
The Gulf Stream snakes 13,000 miles clockwise around the North Atlantic Basin, transporting warm tropical water to the northern regions. Its counterpart in the North Pacific is another strong current called the Japan current. This current bears warm water from the tropics, sweeps northward against Japan, crosses the upper Pacific, and turns southward to warm the western coast of North America. The major current in the South Pacific is the Humboldt or Peru current, which flows northward along the west coast of South America.
Like huge undersea tornadoes, eddies or gyres of swirling warm and cold water accompany the ocean currents. Many eddies are enormous-as much as 100 miles or more across and reaching depths of 3 miles. Most eddies, however, are less than 50 miles across; some, including those in the Arctic Ocean off Alaska, are only 10 miles wide. These small eddies play an important role in mixing the oceans like giant eggbeaters.
The eddies appear to be pinched-off sections of the main ocean currents. Like high pressure systems in the atmosphere, the eddies rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Sea life caught in the eddies is often transported to hostile environments and can only survive as long as the eddies with their more favorable waters continue to operate, perhaps upwards of several months.
The world’s ocean is filled nearly to the top with icy water only a few degrees above freezing that was chilled while at the surface of the polar seas. The sinking of cold, dense water near the poles generates strong, deep currents that flow steadily toward the equator (Fig. 6-2). Associated with these currents are eddies on the western side of ocean basins that are often over 100 times stronger than the main current.









Figure 6-2 The ocean conveyor belt, transporting warm and cold water over the earth.
In the polar regions, the surface water is denser than in other parts of the world because of its low temperature and high salt content. The increased saltiness results from the evaporation of poleward flowing water and the exclusion of salt from ice as it freezes. As seawater increases in density, it sinks to the bottom, then spreads out upon hitting the ocean floor, and heads toward the equator. The Coriolis effect deflects global currents westward because of the earth’s eastward rotation. The distribution of landmasses and the topography of the ocean floor, including ridges and canyons, also affect the path taken by the circulating water.
The Antarctic plays a larger role in global ocean circulation than does the Arctic. Deep cold currents flowing from Antarctica toward the equator trend to the left and press against the western side of the Atlantic, Pacific, and Indian ocean basins. As they sweep against the continents, the currents pick up speed similar to the way in which a stream flows faster in a narrowing channel.
Swift-flowing currents along some parts of the ocean bottom are still much of a mystery. One deep current, after traveling 7,500 miles from its source in the Antarctic, turns and sweeps along the edge of the abyssal plain south of Nova Scotia. The Atlantic Bottom Water, the largest mass of bottom water in the world, sinks from the surface near Antarctica and flows northward along the seafloor into the western North Atlantic.
Before mixing with North Atlantic water and dispersing some of this flow curves to the west (due to the Earth’s eastward rotation) and hugs the lower edge of the continental rise at the border of the abyssal plain. This current, along with the lower reaches of intense eddies pinched off the Gulf Stream, might account for the muddy waters kicked up from the nearly 2,000-foot-deep abyss south of Nova Scotia that extends as far south as the Bahamas. Deep eddies induced by the Gulf Stream might be superimposed on this flow to produce undersea storms of sediment-laden water.
The Indian Ocean is unique because it is not in contact with the north polar region and has only one source of cold bottom water from the Antarctic. By contrast, the Atlantic and Pacific connect with both the Antartic and Arctic oceans. The narrow, shallow Bering Strait that separates Alaska from Asia (Fig. 6-3) blocks the flow of deep cold water from the Arctic Ocean into the Pacific. Seawater freezes more readily in the Arctic regions because the near-surface water is not sufficiently enriched in salt and therefore not dense enough to sink. Consequently the Arctic Ocean is largely a sea of ice.

















Figure 6-3 The Bering Strait between Alaska and Asia.
Seawater in the Atlantic is saltier than in the Pacific because of the greater contribution of river born salts. The Atlantic has two major sources of highly saline water. One is the Gulf of Mexico, whose water is carried northward by the Gulf Stream, and the other is the deep flow from the Mediterranean Sea. The climate of the Mediterranean is so warm that evaporation concentrates salt in that sea, and Mediterranean water spilling westward through the Strait of Gibraltar sinks to a depth of nearly 4,000 feet in the Atlantic. These sources raise the salt content of the surface waters of the North Atlantic to levels much higher than in the North Pacific.
As the surface water of the North Atlantic moves northward, it enters the Norwegian Sea, where it cools below the freezing point of fresh water but, because of it high salt content, does not freeze. The cold, dense water sinks, and upon reaching the bottom it reverses direction and flows back into the Atlantic through a series of deep, narrow troughs in the submarine ridges that connect Greenland, Iceland, and Scotland. This deep-sea current, called the North Atlantic Deep Water, is a subsurface stream with a flow 20 times greater than all the world’s rivers combined.
As this large volume of deep water moves southward, it flows to the right against the continental margin of eastern North America, forming the Western Boundary Undercurrent. This current transports some 20,000 cubic miles of water annually along the east of North America. All these deep-ocean currents travel very slowly, completing the journey from the poles to the equator and back again in upward of 1,000 years, as compared to surface currents, which complete the circuit around an ocean basin in less than a decade.
The volume of rising water in parts of the ocean matches the volume of sinking water in the polar regions. The cold waters from the polar seas rise in upwelling zones in the tropics, creating an efficient heat transport system. Upwelling current off the coasts of continents and near the equator are important sources of bottom nutrients. Modern fishermen track down these areas of upwelling water, which is usually where the fish are.













Figure 6-4 Upwelling and sinking ocean currents are driven by offshore and onshore winds.
The tropical seas are warmed by solar radiation from above and cooled by upwelling water from below. This interaction gives rise to an equator-to-pole cycle of heat transport. Offshore and onshore winds also drive upwelling currents (Fig. 6-4). These processes involve the entire ocean in a gigantic heat engine, transporting a tremendous amount of heat around the globe.
6.2 EL NIÑO
Ocean currents dramatically affect the climate, and major changes in these systems can cause abnormal weather patterns all over the world, (Fig. 6-5). Unusual oceanographic conditions during the 1982-83 EL Niño dramatically affected the Galapagos Islands in the Pacific, off the coast of Ecuador. The ocean current patterns around the islands are complex and are greatly influenced by the equatorial undercurrent, a subsurface, eastward-flowing current about 600 feet thick. During a period when the sea surface temperatures were anomalously high, a major redistribution of phytoplankton around the Galapagos Islands might have contributed to the reproductive failure of seabirds and marine mammals on the islands.











Figure 6-5 Changes in air currents during EL Niño. Dashed lines are normal currents.
About every 3 to 7 years, anomalous atmospheric pressure changes known as EL Niño Southern Oscillation (ENSO) occur in the South Pacific. As atmospheric pressure rises on Easter Island in the eastern Pacific, it falls in Darwin, Australia in the western Pacific. When a major EL Niño occurs, the barometric pressure over the eastern Pacific falls, while the pressure over the western Pacific rises. When EL Niño ends, the pressure difference between these two areas reverses, creating a massive seesaw effect of atmosphere pressure. While the EL Niño is in process, it disrupts the westward-flowing trade winds. Warm water piled up in the western Pacific by the wind force flows back to the east, creating a great sloshing of water in the South Pacific Basin.
The opposite condition occurs during a La Niña, when the surface waters of Pacific cool. In mid-1988, water temperatures in the central Pacific plummeted to abnormally low levels, signaling a climates swing from an EL Niño to a La Niña, and the changing climate affected the precipitation-evaporation balance of the world (Fig. 6-6). Strong monsoons hit India and Bangladesh, and heavy rains visited Australia. The La Niña might also have been responsible for a severe drought in United States and a marked drop in global temperatures a year later.











Figure 6-6 The average precipitation-evaporation balance of the Earth. In positive areas, precipitation exceeds evaporation. In negative areas, evaporation exceeds precipitation.
Along the west coast of South America, the southeast trade winds drive the Peru Current, pushing surface water offshore and allowing cold, nutrient-rich water to well up to the surface. The westward push of the trade winds continues across the eastern and central Pacific, and the resulting stress on the sea surface piles up water in the western Pacific, causing the warm surface layer of the ocean to thicken in the west and thin in the east. The thermocline, the boundary between the cold and warm layers of the ocean, falls to about 600 feet in the western Pacific and rises to about 150 feet in the eastern Pacific. Because the thermocline is near the surface, the upwelling waters off the coast of South America are usually cold.
During an EL Niño event beginning around October, the cessation of the trade winds in the western Pacific causes the thick layer of warm water to collapse. The water flows back toward the east in subsurface waves called Kelvin waves that reach the coast of South America in 2 to 3 months, creating a huge reverse flow of seawater in the South Pacific Basin. The Kelvin waves generate eastward-flowing currents that transport warm water from the west. This lowers the level of the thermocline and prevents the upwelling of cool water from below.
With an increase of warm water from the west and the suppression of cold water from below, the sea surface begins to warm considerably by December or January. The stretch of warm water shifts the position of thunderstorms that pump heat and water into the atmosphere, thus rerouting atmospheric currents around the world. As the EL Niño continues to develop, the trade winds begin blowing from the west, intensifying the Kelvin waves and further depressing the thermocline off South America.
The Peru current, flowing northward along the west coast of South America, is not significantly weakened by the EL Niño and continues to pump water to the surface, though this time the upwelling water is warm, causing a major decline of fisheries. The westward current off equatorial South America is not only weakened by the eastward push of the Kelvin waves but is also much warmer than before. This spreads the warming of the sea surface westward along the equator (Fig. 6-7), and the normal wind pattern reverses, causing a major disruption in global weather patterns.












Figure 6-7 Stippled region shows area affected by increased sea-surface temperature during the 1972 EI Niňo in the Pacific Ocean.

6.3 ABYSSAL STORMS
The dark abyss at the bottom of the ocean was thought to be quiet and almost totally at rest, with sediments slowly raining down and accumulating at a rate of about 1 inch in 20 centuries. Recent discoveries reveal signs that infrequent undersea storms often shift and rearrange the sedimentary material that has rested on the bottom for long periods. Occasionally, the surging bottom currents scoop up the top layer of mud, erasing animal tracks and creating ripple marks in the sediments, much like those produced by wind and river currents.
On the western side of the ocean basins, undersea storms skirt the foot of the continental rise, transporting huge loads of sediment and dramatically modifying the seafloor. The storms scour the ocean bottom in some area and deposit large volumes of silt and clay in others. The energetic currents travel at about 1 mile per hour; because of the high density of seawater, they sweep the ocean floor just as effectively as a gale with winds up to 45 miles per hour erodes shallow areas near shore.
That abyssal storms seem to follow certain well-traveled paths is indicated by long furrows of sediment on the ocean floor (Fig. 6-8). The scouring of the seabed and deposition of thick layers of fine sediment results in much more complex marine geology than that developed simply from a constant rain of sediments. The periodic transport of sediment creates layered sequences that look similar to those created by strong windstorms in shallow seas, with overlapping beds of sediment graded into different grain sizes.









Figure 6-8 A wide, flat furrow on the seabed of the Atlantic Ocean from the deep submersible Alvin. Photo by N.P. Edgar, courtesy of USGS
Sedimentary material deposited on the ocean floor consists of detritus, which is terrestrial sediment and decaying vegetation, along with shells and skeletons of dead microscopic organisms that flourish in the sunlit waters of the top 300 feet of the ocean. The ocean depth influences the rate of marine-life sedimentation. The farther the shells descend, the greater their chance of dissolving in the cold, high-pressure waters of the abyss before reaching the bottom. Preservation also depends on rapid burial land protection from the corrosive action of the deep-sea water.
Rivers carry detritus to the edge of the continent and out onto the continental shelf, where marine currents pick up the material. When the detritus reaches the edge of the shelf, it falls to the base of the continental rise under the pull of gravity. Approximately 15 billion tons of continental material reaches the mouths of rivers and streams annually (Fig.6-9). Most of this detritus is deposited near the river outlets and on continental shelves; only a few billion tons falls into the deep sea. In addition to the river-borne sediments, strong desert winds in subtropical regions sweep out to sea a significant amount of terrestrial material. The windblown sediment also contains significant amounts of iron, an important nutrient that supports prolific blooms of plankton. In iron-deficient parts of the ocean, there are “deserts” where “jungles” should have been even though plenty of other nutrients exist.












Figure 6-9 The Yahtse River delta, Icy Bay, Alaska. Photo by J.H.Hartshorn, courtesy of USGS
The biological material in the sea contributes about 3 billion tons of sediment to the ocean floor each year. The biologic productivity, controlled in large part by the ocean currents, governs the rates of accumulation. Nutrient-rich water up wells from the ocean depths to the sunlit zone, where microorganisms ingest the nutrients. Areas of high productivity and high rates of accumulation normally occur near major oceanic fronts, such as the region around Antarctica, and along the edges of major currents, such as the Gulf Stream and the Kuroshio or Japan current that circles clockwise around the North Pacific Basin.
The greatest volume of silt and mud and the strongest bottom currents are in the high latitudes of the western side of the North and South Atlantic. These areas have the largest potential for generating abyssal storms that form and shape the seafloor. They also have the biggest drifts of sediment on Earth, covering an area more than 600 miles long, 100 miles wide, and 1 mile thick. Abyssal currents at depths of 2 to 3 miles play a major role in shaping the entire continental rise off North and South America. Elsewhere in the world, bottom currents shape the distribution of fine-grained material along the edges of Africa, Antarctica, Australia, New Zealand, and India.
Instruments lowered to the ocean floor measure water dynamics and their effects on sediment mobilization (Fig. 6-10). During abyssal storms, the velocity of bottom currents increases from about one-tenth to over 1 mile per hour. The storms in the Atlantic seem to derive their energy from eddies that emerge from the Gulf Stream. While the storm is in progress, the suspended sediment load increases tenfold, and the current is able to carry about 1 ton of sediment per minute for long distances. The moving clouds of suspended sediment appear as coherent patches of turbid water with a residence time of about 20 minutes. The storm itself might last from several days to a few weeks, at the end of which the current velocity slows to normal and the sediment drops out of suspension.
Not all drifts are directly attributable to abyssal storms. Material carried by deep currents has modified vast areas of the ocean as well. The storm’s main effect is to stir sediment that bottom currents then pick up and carry downstream for long distances. The circulation of the deep ocean does not show a strong seasonal pattern; therefore, the onset of abyssal storms is unpredictable. Abyssal storms are likely to strike an area every 2 to 3 months.





















Figure 6-10 An instrument that measures water dynamics and sediment mobilization on the ocean floor. Photo by N.P.Edgar, courtesy of USGS
6.4 TIDAL CURRENTS
Tides result from the pull of gravity of the moon and sun on the ocean. The moon revolves around the Earth in an elliptical orbit and exerts a stronger pull on the side facing the moon than on the opposite side. The difference between the moon’ gravitational attraction on both sides is about 13 percent, which elongates the center of gravity of the Earth-moon system. The pull of gravity creates two tidal bulges on the Earth. As the Earth revolves, the ocean flow into the two tidal bulges, one facing toward the moon and the other facing away from it. Between the tidal bugles, the ocean is shallower, giving it an overall egg-shaped appearance. The middle of the ocean only rises about 2.5 feet at maximum high tide; but due to a sloshing-over effect and the configuration of the coastline, the tides on the coasts often rise several times higher.
The daily rotation of the Earth causes every point on the surface to go into and out of the two tidal bulges once every day. Thus, as the Earth spins into and out of each tidal bulge, the tides appear to rise and fall twice daily. The moon also orbits the Earth in the same direction that it rotates, only faster. By the time a point on surface has rotated halfway around, the tidal bugles have moved forward with the moon, and the point must travel farther each day to catch up with the bulge. Therefore, the actual period between high tides is 12 hours 25 minutes.
If continents did not impede the motion of the tides, all coasts would have two high tides and two low tides of nearly equal magnitudes and durations each day. These are called semidiurnal tides, and occur at places such as along the Atlantic coasts of North America and Europe. However, different tidal patterns form when the tide wave is deflected and broken up by the continents. Because of this action, the tidal ocean forms a complicated series of crests and troughs thousands of miles apart. In some regions, the tides are coupled with the motion of large nearby bodies of water. As a result, some areas-for example, the coast of the Gulf of Mexico-have only one tide a day, called a diurnal tide, with a period of 24 hours 50 minutes.
The sun also raises tides with semidiurnal and diurnal periods of 12 and 24 hours. Because the sun is much farther away from the Earth, its tides are only about half the magnitude of lunar tides. The overall tidal amplitude, which is the difference between the high-water level and the low-water level, depends on the relation of the solar tide to the lunar tide and is controlled by the relative positions of the Earth, moon, and sun.
The tidal amplitude is at its maximum twice a month during the time of new and full moon, when the Earth, moon, and sun align in a nearly straight-line configuration (known as “syzygy” from the Greek word syzygos, meaning “yoked together”). This is the time of spring tides-from the Saxon word springan, meaning “a rising or swelling of water.” Neap tides occur when the amplitude is at a minimum during the first and third quarters of the moon, when the relative positions of the Earth, moon, and sun form a right angle and the solar and lunar tides oppose each other.
A tidal basin near the mouth of a river can actually resonate with the incoming tide. The oscillation makes the water at one side of the basin high at the beginning of the tidal period, low in the middle, and high again at the end of the tidal period. The incoming tide sets the water in the basin oscillating, sloshing back and forth. The motion of the tide moving in toward the mouth of the river and the motion of the oscillation are synchronized, which reinforces the tide in the bay and makes the high tides higher and the low tides lower than they would be otherwise.

TABLE 6-1 MAJOR TIDAL BORES

Country 

Tidal Basin

Tidal body

Known Bore Location

Bangladesh

Ganges

Bay of Bengal

 

Brazil

Amazon

Capim

Canal do Norte

Guama

Tocantuns

Araguari

Atlantic Ocean

 

Capim

Canada

Petitcodiac

Salmon

Bay of Fundy

Moncton

Truro

China

Tsientang

East China Sea

Haining to Hangchow

England

Severn

Parret

Wye

 

Mersey

Dee

 

Trent

Bristol Channel

 

 

 

Irish Sea

 

 

North Sea

Framilode to 

Gloucester

Bridgwater

 

Liverpool to

Warrington

 

Gunness to

Gainsborough

France

Seine

Orne

Coueson

Vilaine

Loire

Gironde

Dordogne

Garonne

English Channel

 

Gulf of St. Malo

Bay of Biscay

Gaudebec

 

 

 

 

Iies de Margaux

La Caune to Brunne

Bordeaux to Cadillac

India

Narmada

Hooghly

Arabian Sea

Bay of Bengal

 

Hooghly Pt. To Calcutta

Mexico

Colorado

California Gulf

Colorada Delta

Pakistan

Indus

Arabian Sea

 

Scotland

Solway Firth

Forth

Irish Sea

 

United States

Turnagain Arm Knik Arm

Cook Inlet

Anchorage to Portage

Tidal bores are a special feature of this type of oscillation within a tidal basin. They are solitary waves that carry tides upstream, usually during a new or full moon. One of the largest tidal bores sweeps up the Amazon River, with waves up to 25 feet high and several miles wide reaching 500 miles upstream. Although any body of water with high tides can generate a tidal bore, only half of all tidal bores are associated with resonance in a tidal basin.
The seaward ends of many rivers experience tides; in such cases, at the river mouth the tides are symmetrical, with ebb and flood tide lasting about 6 hours each. Ebb and flood tides refer to the currents associated with the tides. Ebb currents flow out to sea, while flood currents flow into an inlet. Upstream, the tides become increasingly asymmetrical, with less time elapsing between low water and high water than between high water and low water, as the tide comes in quickly but goes out gradually with the river current. A tidal bore exaggerates this asymmetry because the tide comes up the river very rapidly in a single wave. As the tidal bore moves upstream, it must continue to travel faster than the river current or else it will be swept downstream and out to sea.
6.5 OCEAN WAVES
Ocean waves form by large storms at sea when strong winds blow across the water’s surface (Fig.6-11). The wave fetch is distance over which the wind blows on the surface of the ocean and is dependent on the size of the storm and the width of the body of water. For waves to reach a fully developed sea state, the fetch must be at least 200 miles for a wind of 20 knots, 500 miles for a wind of 40 knots, and 800 miles for a wind of 60 knots.
The wind speed and duration determine the wave height. With a wind speed of 30 miles per hour, for example, a fully developed sea is attained in 24 hours, with wave heights up to 20 feet. The maximum sea state occurs when waves reach their maximum height, usually after 3 to 5 days of strong, steady storm winds blowing across the surface of the ocean. However, if the sustained wind blew at 60 miles per hour, a fully developed sea would have wave heights averaging over 60 feet.
The wave height, measured from the top of the crest to the bottom of the trough, is generally less than 20 feet. Occasionally, storm waves of 30 to 50 feet high have been reported, but these are not very frequent. Exceptionally large ocean waves are rare. One such wave reported in the Pacific by a U.S. Navy tanker in 1933 was over 100 feet high. Another large wave buckled the flight deck of the aircraft carrier USS Bennington during a typhoon in the western Pacific in 1945(Fig.6-12).














Figure 6-11 Open ocean waves and a mysterious weather phenomenon known as sea smoke, 150 miles east of Norfolk, Virginia. Courtesy of U.S. Navy











Figure 6-12 The buckled flight deck of the USS Bennington after a typhoon in the western Pacific in June 1945. Courtesy of U.S. Navy
The wave shape varies with the water depth (Fig. 6-13). In deep water, a wave is symmetrical, with a smooth crest and trough. In shallow water, a wave is asymmetrical, with a peaked crest and a broad trough. If the water depth is more than one-half the wave length, the wave are considered deep-water waves. If the water depth is less than one-half the wave length, the waves are called shallow-water waves.








Figure 6-13 The mechanics of a breaker.
The wave length (Fig.6-14) is measured from crest to crest and depends on the location and intensity of the storm at sea. The average lengths of storm waves vary from 300 to 800 feet. As waves move away from a storm area, the longer waves move ahead of the storm and form swells that travel great distances. In the open ocean, swells of 1,000-foot wave-lengths are common, with a maximum of about 2,500 feet in the Atlantic and about 3,000 feet in the Pacific.









Figure 6-14 Properties of waves. L=wave length, H=wave height, D=wave depth.
The wave period is the time a wave takes to pass a certain point and is measured from one wave crest to the next. Wave periods in the ocean vary from less than a second for small ripples to more than 24 hours. Waves with periods of less than 5 minutes are called gravity waves and include the wind driven waves that break against the coastline, most of which have periods between 5 and 20 seconds. A seismic sea wave from an undersea earthquake or landslide usually has a period of 15 minutes or more and a wave length of up to several hundred miles.
Waves with periods of between 5 minutes and 12 hours are called long waves, and are generated by storms. Other long waves result from seasonal differences in barometric over various parts of the ocean such as the Southern Oscillation discussed in a previous section. Waves with longer periods travel faster than shorter-period waves, and the speed is proportional to the square root of the wave length. Short-period waves are relatively steep and are particularly dangerous to small boats because the bow might be on a crest while the stern is in a trough, causing it to capsize or be swamped.
6.6 SEISMIC SEA WAVES










Figure 6-15 Seismic sea wave damage at a railroad marshaling yard, Seward district, Alaska, from the March 27, 1964, earthquake. Courtesy of USGS
Destructive waves also result from undersea and near-shore earthquakes (Fig. 6-15). They are called seismic sea waves or tsunamis, a Japanese word meaning “tidal waves”-so named because of their common occurrence in this region. The waves really have nothing to do with the tides, however. The vertical displacement of the ocean floor during earthquakes causes the most destructive tsunamis, whose wave energy is proportional to the intensity of the quake.
In the open ocean, the wave crests are up to 300 miles long and usually less than 3 feet high. However, the waves extend downward for thousands of feet, all the way to the ocean bottom. The distance between crests, or the wave length, is 60 to 120 miles, giving the tsunami a very gentle slope, which allows it to pass beneath ships practically unnoticed. Tsunamis travel at speeds of between 300 and 600 miles per hour. Upon entering shallow coastal waters, tsunamis have been known to grow into a wall of water up to 200 feet high, although most tsunamis are only a few tens of feet high.
















Figure 6-16 A fishing boat beached several hundreds feet inland from the head of Resurrection Bay, Seward district, by seismic sea waves from the March 27, 1964, Alaskan earthquake. Courtesy of USGS
When a tsunami touches bottom in a harbor or narrow inlet, its speed rapidly diminishes to about 100 miles per hour. The sudden breaking action causes the water to pile up, magnifying the wave height tremendously. The destructive force of the wave is immense, and the damage it causes as it crashes into the shore is considerable. Large buildings are crushed with ease, and sizable ships are tossed up and carried well inland like toys (Fig. 6-16).
Explosive eruptions associated with the birth or the death of a volcanic island also set up large tsunamis that are highly destructive. Volcanic eruptions that develop tsunamis are responsible for about a quarter of all deaths from tsunamis. The powerful waves transmit the volcano’s energy to areas outside the reach of the volcano itself. Large pyroclastic (volcanic fragment) flows into the sea or landslides triggered by volcanic eruptions produce tsunamis as well. Coastal and submarine slides also generate large tsunamis that can overrun portions of the adjacent coast (Fig.6-17).















Fig. 6-17 Wave damage shown in bare areas on Cenotaph Island and south shore of Lituya Bay, Alaska, resulting from a massive rock slide in 1958. Photo by D.J.Miller, courtesy of USGS
Large parts of Alaska’s Mount St. Augustine have collapsed and fallen into the sea, generating large tsunamis. Massive landslides have ripped out the flanks of the volcano ten or more times during the past 2,000 years. The last slide occurred during the October 6, 1883, eruption, when debris from the flanks of the volcano crashed into the Cook Inlet. The impact sent a 30-foot tsunami to Port Graham 54 miles away destroying boats and flooding houses.
Until about 40 years ago, earthquakes on the ocean floor went largely undetected, and the only warning people had of a tsunami was a rapid withdrawal of water from the shore. Residents of coastal areas frequently stricken by tsunamis heed this warning and immediately head for higher ground. Several minutes after sea retreats, a tremendous surge of water extends hundreds of feet inland. Often a succession of surges occurs, each followed by a rapid retreat of water back to the sea. On coasts and islands protected by barrier reefs or where the seafloor rises gradually, much of the tsunami’s energy is spent before reaching the shore. On volcanic islands, which lie in very deep water, like the Hawaiian Islands, or where deep submarine trenches lie immediately outside harbors, an oncoming tsunami can build to prodigious heights.
The most tsunami-prone area in the world is the Pacific rim, which has the most earthquakes as well as the most volcanoes. Destructive tsunamis from submarine earthquakes can travel clear across the Pacific and reverberate through the ocean for days. A tsunami originating in Alaska could reach Hawaii in 6 hours, Japan in 9 hours, and the Philippines in 14 hours. A tsunami originating off the Chilean coast could reach Hawaii in 15 hours and Japan in 22 hours. Fortunately, this gives people in the coastal areas enough time to take the necessary safety precautions to protect life and property.












7 COASTAL GEOLOGY
The constant shifting of sediments on the surface and the accumulation of deposits on the ocean floor assures that the face of the Earth continues to change over time. Seawater lapping against the shore during a severe storm causes coastal erosion. Steep waves that accompany storms at sea erode sand dunes and sea cliffs. The continuous pounding of the surf also tears down most artificial barriers against the rising sea.
America’s once sandy beaches are sinking beneath the waves. Barrier islands and sand bars running the American East Coast and the coast of Texas are disappearing at alarming rates. Sea cliffs are eroding farther inland in California, often destroying expensive homes. Most defenses, such as seawalls erected to stop beach erosion, usually end in defeat as waves relentlessly batter the shoreline (Fig.7-1).











Figure 7-1 Damage to a beach area caused by Storms and high tides at Virginia Beach, Virginia. Photo by K. Rice, courtesy of USDA-Soil Conservation Service
7.1 SEDIMENTATION
Most sedimentary processes take place very slowly on the bottom of the ocean. The continents are mainly the sites of erosion, whereas the oceans are mostly the sites of sedimentation. Marine sediments consist of material washed off the continents, and most sedimentary rocks form along continental margins and in the basins of inland seas. Such seas invaded the interiors of North and South America, Europe, and Asia during the Mesozoic era. Areas with high sedimentation rates form deposits thousands of feet thick; where they are exposed to the surface, individual sedimentary beds can be traced for hundreds of miles.
The formation of sedimentary rock begins when erosion wears down mountain ranges and rivers carry the debris into the sea. The sediments originate from the weathering of surface rocks. The products of weathering include a wide range of materials, from very fine-grained sediments to huge boulders. Exposed rocks on the surface chemically break down into clays and carbonates and mechanically break down into silts, sands, and gravels.
Erosion by wind, rain, or glacial ice brings the sediments to streams, and the loose sediment grains travel downstream to the sea. Angular sediment grains indicate a short time spent in transit. Rounded sediment grains indicate severe abrasion from long-distance travel or from reworking by fast-flowing streams or by pounding waves on the beach. Indeed, many sandstone formations were once beach deposits.
Annually, some 25 billion tons of sediment are carried by stream runoff into the ocean and settle onto the continental shelf. The towering landform of the Himalayas is the greatest single source of sediment. Rivers draining the region-notably the Ganges and the Brahmaputra-discharge about 40 percent of the world’s total amount of sediment into the Bay of Bengal where sedimentary layers stack up miles thick.
River like the Amazon and the Mississippi transport enormous quantities of sediment from their respective continental interiors. Largescale deforestation and severe soil erosion at its headwaters force the Amazon of South America, the world’s largest river, to carry heavier sediment loads. The Mississippi River and its tributaries drain a major section of the central United States, from the Rockies to the Appalachian Mountains. All tributaries emptying into the Mississippi have their own drainage area, forming a part of a larger basin.
Every year, the Mississippi River dumps hundreds of millions of tons of sediment into the Gulf of Mexico, widening the Mississippi Delta (Fig.7-2) and slowly building up the land area of Louisiana and nearby states. The Gulf coastal states, from eastern Texas to the Florida panhandle, were built up with sediments eroded from the interior of the continent and hauled in by the Mississippi and other rivers. Streams, heavily laden with sediments, overflow their beds and are forced to detour as they meander toward the sea. When the streams reach the ocean, their velocity falls off sharply, and the sediment load drops out of suspension. In addition, chemical solutions carried by rivers mix thoroughly with seawater through the action of ocean waves and currents.

1930 Conditions                             1956 Conditions

Figure 7-2 Sediment deposition in the Mississippi River delta:1930 conditions (left), 1956 conditions (right). Photo by H. P. Guy, courtesy of USGS

TABLE 7-1 MAJOR CHANGES IN SEA LEVEL

Date

Sea Level

Historical Event

2200 B. C.

Low

 

1600 B. C

High

Coastal forest in Britain inundated by the sea

1400 B. C

Low

 

1200 B. C

High

Egyptian ruler Ramses Ⅱ builds first Suez canal

500 B. C

Low

Many Greek and Phoenician ports built around this time are 

now under water

200 B. C

Normal

 

A.D. 100

High

Port constructed well inland of present-day Haifa, Israel

A.D. 200

Normal

 

A.D. 400

High

 

A.D. 600

Low

Port of Ravenna, Italy becomes landlocked Venice is built; 

presently being inundated by the Adriatic Sea

A.D. 800

High

 

A.D. 1200

Low

Europeans exploit low-lying salt marshes

A.D. 1400

High

Extensive flooding in low-lying countries along the North Sea. The Dutch begin 

building dikes

Upon reaching the ocean, the riverborne sediments settle out of suspension by grain size. The coarse-grained sediments deposit near the turbulent shore and the fine-grained sediments deposit in calmer waters farther out to sea. As the shoreline advances toward the sea because of the buildup of coastal sediments or a falling sea level, finer sediments are covered by progressively courser ones. As the shoreline recedes through the lowering of the land surface or a rising sea level, coarser sediments are covered by progressively finer ones.
The difference in sedimentation rates as the sea transgresses and recedes produces a recurring sequence of sands, silts, and muds. The sands comprise quartz grains about the size of beach sands, and marine sandstones exposed in the American West were deposited along the shores of ancient seas. Gravels are rare in the ocean and move mainly from the coast to the deep abyssal plains by submarine slides. In dry regions where dust storms are prevalent, the wind airlifts fine sediment out of the region. Windblown sediments landing in the ocean slowly build deposits of red clay, whose color signifies its terrestrial origin, whereas green or gray sediments indicate a marine environment.
The weight of the overlying sedimentary layers pressing down on the lower strata lithifies the sediments into solid rock, providing a geologic column of alternating beds of limestone, shales, siltstones, and sandstones (Fig.7-3). Abrasion eventually grinds down all rocks to clay-size particles. Because clay particles are small and sink slowly, they normally settle out in calm, deep waters far from shore. Compaction from the weight of the overlying strata squeezes out water between sediment grains, lithifying the clay into mudstone or shale.










Figure 7-3 A Stratigraphic cross-section showing a sequence of sandstones, siltstones, and shales overlying a basement rock composed of limestone.
The varying thicknesses of sediment layers reflects the different depositional environments at the time they were laid down. Thick sandstone beds might be interspersed with thin beds of shale, indicating periods of coarse sediment deposition punctuated by periods of fine sediment deposition. Graded bedding occurs when particles in a sedimentary bed vary from coarse at the bottom to fine at the top. This type of bedding indicates the rapid deposition of sediments of differing sizes by a fast-flowing stream emptying into the sea. The largest particles settle out first and, because of the difference in settling rates, are covered by progressively finer material. Beds also grade laterally, producing a horizontal gradation of sediments from coarse to fine.
The sediments settle onto the continental shelf, which extends up to 100 or more miles and reaches a depth of roughly 600 feet. In most places, the continental shelf is nearly flat, with an average slope of only about 10 feet per mile. Beyond the continental shelf lies the continental slope, which extends to an average depth of more than 2 miles (Fig. 7-4). It has a steep angle, comparable to the slopes of many mountain ranges.

















Figure 7-4 Profile of the ocean floor. Vertical profile is highly exaggerated.
Sediments reaching the edge of the continental shelf slide down the continental slope under the pull of gravity. Often, huge masses of sediment cascade down the continental slope by gravity slides that can gouge out steep submarine canyons. They play an important role in building up the continental slope and the smooth ocean bottom below.
7.2 STORM SURGES
Storm at sea produces pressure changes and strong winds that pile up seawater and cause flooding when occurring at high tides. Waves generated by high winds superimposed on regular tides produce the most severe tidal floods, especially when the moon, sun, and Earth are in alignment. While the tide is in, high waves raise the tide’s maximum level. Strong onshore wind blowing toward the coast push seawater onto the shore. The opposite condition occur when strong offshore winds blow toward the ocean during low tide, lowering the sea significantly and sometimes grounding vessels in port.
Most high waves and beach erosion occur during coastal storms. Thunderstorms and squalls are the most violent storms. They are most frequent in the mid-latitudes and produce gusty winds, hail, lightning, and a rapid buildup of seas. The life cycle of a single thunderstorm cell is usually less than half an hour. When the cell dies, a new one develops in its place.
Frontal storms form at the leading edge of a cold front. A squall line often precedes a cold front, with a distinctive dark gray, cylindrical-shaped cloud that appears to roll across the sky from one end of the horizon to the other (Fig.7-5). Squall lines travel about 25 miles per hour, with winds in the squall reaching 60 miles per hour. However, they are generally short-lived, usually lasting less than 15 minutes. When a squall arrive it produces wave several feet high, but since the winds do not last long the waves die down almost as rapidly as they build up.
Hurricanes and typhoons produce the most dramatic storm surges (Fig.7-6). Hurricane-force winds caused by the rotation and forward motion of the storm reach 100 miles per hour or more, pushing water out in front of the storm. The low pressure in the eye of the hurricane draws water up into a mound several feet high. As the hurricane moves across the ocean and its speed matches the speed of the waves, it often sets up a resonance with the swells it generates. This adds to the height of the swells, which have been reported as more than 60 feet high in some hurricanes.

Figure 7-5 The leading edge of a roll cloud formation as a prefrontal squall line passes Jacksonville, Florida. Courtesy of U.S. Navy

TABLE 7-2 THE BEAUFORT WIND SCALE

Beaufort number

description

Miles per Hour

Indication

0

Calm

 

Smoke rises vertically

1

Light air

1-3

Direction of wind shown by smoke drift 

but not by wind vane

2

Light breeze

4-7

Wind felt on face; leaves rustle

3

Gentle breeze

8-12

Leaves and small twigs in constant 

motion; wind extends light flag

4

Moderate breeze

13-18

Raises dust and loose vapor; move small 

branches

5

Fresh breeze

19-24

Small trees begin to sway; crested 

wavelets form on inland water

6

Strong breeze

25-31

Large branches in motion; telephone wires 

whistle

7

Near gale

32-38

Whole trees in motion; resistance when 

walking against the wind

8

Gale

39-46

Breaks twigs off trees; large waves form 

on open ocean

9

Strong gale

47-54

Breaks large limbs off trees; slight 

structural damage occurs

10

Storm

55-63

Uproots trees; considerable structural 

damage occurs

11

Violent storm

64-75

Widespread damage; beach erosion occurs 

in coastal areas

12-17

Hurricane

>75

Devastation occurs; storm surge damages 

coastal areas

When a hurricane approaches the coast, the water piled up by the wind, the mounding of water by the low pressure, and the generation of swells and the possible resonance of swell waves can make a most deadly combination, especially when superimposed on the regular cycle of incoming tides. The result is massive flooding, devastation of property, and the loss of life.




















Figure 7-6 Overwash and storm-surge penetration near Cape Hatteras, North Carolina, in 1984. Photo by R. Dolan, Courtesy of USGS
Tidal floods are overflows on coastal areas bordering the ocean, an estuary, or a large lake. Coastal lands, including bars, spits, and deltas, offer the same protection from the sea that floodplains do from rivers. Coastal flooding is primarily a result of high tides, waves from high winds, storm surges of tsunamis, or any combination of these. Tidal floods also occur when waves generated by hurricane winds combine with flood runoff due to heavy rains that accompany the storms.
The flooding can extend over large distances along a coastline. The duration is usually short and depends on the elevation of the tide, which usually rises sand falls twice daily. If the tide is in, other forces that produce high waves can raise the maximum level of the prevailing high tide. The most severe tidal floods result when waves produced by high winds combine with the regular tides, causing a tremendous amount of damage as well as severe beach erosion that continues to move the coastline inland.
7.3 COASTAL EROSION
Coastal landslides occur when a sea cliff is undercut by wave action and falls into the ocean (Fig.7-7). Sea cliff retreat is caused by marine and nonmarine agents, including wave attack, wind-driven salt spray, and mineral solution. The nonmarine agents responsible for sea cliff erosion include chemical and mechanical processes, surface drainage water, and rainfall. Mechanical erosion processes include cycles of freezing and thawing of water in crevices, which forces existing fractures to split even farther apart. Weathering agents break down rocks or cause the outer layers to peel or spall off. Animal trails weaken soft rock also affect sea cliff erosion, as do burrows that intersect cracks in the soil.
Surface water runoff and wind-driven rain further erode the sea cliff. Excessive rainfall along the coast also can lubricate sediments, causing huge blocks of land to slide into the ocean. Water running over the cliff edge and wind-driven rain produce the fluting often seen on cliff faces. Groundwater seeping from a cliff can create indentations on the cliff face, which undermine and weaken the overlying strata. The addition of water also increases pore pressure within sediments, reducing the shear strength (internal resistance to stress) that holds the rock together. If bedding planes, fractures, or jointing dip seaward, water moving along these areas of weakness might produce rock slides. This process has excavated large valleys on the windward parts of the Hawaiian Islands, where springs emerge from porous lava flows.












Figure 7-7 Highway 1 at the Devils Slide, San Mateo County, California. Photo by R. D. Brown, courtesy of USGS
The main type of marine erosion is direct wave attack at the base of the sea cliff, which quarries out weak beds and undercuts the cliff until the overlying unsupported material collapses onto the beach. Wave also work along joint or fault planes to loosen blocks of rock or soil. In addition, the wind carries salt spray from breaking waves into the air and drives it against the sea cliff. Porous sedimentary rocks absorb the salty water, which evaporates, forming salt crystals that weaken rocks. The surface of the cliff slowly flakes off and falls to the beach below. The material falling to the base of the cliff piles up, forming a talas cone, a steep-sided pile of rock fragment.
Solution erosion attacks limestone cliffs, where chemical processes dissolve soluble minerals from the rocks. The seawater dissolves lime in the rocks, forming deep notches in the sea cliffs (Fig.7-8). Chemical erosion also removes cementing agents in rocks, causing sediment grains to separate. Limestone erosion is prevalent on coral islands in the South Pacific and on the limestone coasts of the Mediterranean and Adriatic seas.
The erosion of sea cliffs and dunes that mark the coastline causes the shoreline to retreat a considerable distance. Between 1888 and 1958, the Atlantic coastline between Nauset Spit and Highland Light on Cape Cod, Massachusetts, retreated at an average rate of more than 3 feet per year. In England, the soft cliffs of the Suffolk coast, on the North Sea, erode at an average rate of 10 to 15 feet per year. At the town of Lowestoft, a single storm eroded a 40-foot-high cliff of unconsolidated rock some 40 feet. Where the cliff stood only 6 feet high, it eroded 90 feet inland.











Figure 7-8 A tidal terrace at low tide, Puerto Rico. Photo by C.A. Kaye, courtesy of USGS
Beach erosion is difficult to predict and almost impossible to stop. It depends on the strength of beach dunes or sea cliffs, the intensity and frequency of coastal storms, and the exposure of the coast. Most attempts to prevent beach erosion are defeated because the waves constantly batter and erode man-made defenses to keep out the sea. Jetties and seawalls erected to halt the tides increase erosion dramatically (Fig.7-9). In their attempts to stabilize the seashores, developers are destroying the very beaches upon which they intend to build.
The rate of retreat varies with the shape of the shape of the shoreline and the prevailing wind and tides. More than half of the 72-mile-long south shore of Long Island, New York, is considered a high-risk zone for development, with the sea reclaiming some locations at a rate of 6 feet per year. The barrier island, running from Cape Henry, Virginia, to Cape Hatteras, North Carolina, has narrowed from both the seaward and landward sides. The rest of the North Carolina coast is moving back at 3 to 6 feet annually, and most of eastern Texas is vanishing even faster. In California, homes are falling because of the undercutting of sea cliffs (Fig.7-10), causing considerable property damage.













Figure 7-9 A system of groins to trap sand moving laterally along the beach at Lake Michigan. Photo by P. W. Koch, courtesy of USDA-Soil Conservation Service













Figure 7-10 The erosion of these bluffs at point Montara, California, will eventually deliver buildings, roads, and other structures to the sea. Photo by R. D. Brown, courtesy of USGS
About 80 percent of America’s once-sandy beaches are sinking beneath the waves. Most of the problem stems from the methods that engineers use to stabilize the beaches. Jetties cut off the natural supply of sand to beaches, and seawalls increase erosion by bouncing waves back without absorbing much of their energy. The rebounding waves carry sand out to sea, undermining the beach and destroying the shorefront property the seawall was designed to protect.
In an effort to protect houses on eroding bluffs overlooking the sea, coastline residents often erect expensive seawalls. Yet these structures actually hasten the erosion of sand from the beaches in front of the wall. In effect, the seawalls are saving the bluffs to the detriment of the beaches. Barriers erected at the bottom of sea cliffs might deter wave erosion but have no effect on sea spray and other erosional processes. While beaches in front of the seawalls might lose sand naturally during certain seasons, waves return sand at other times.
Natural processes will not replenish the disappearing sand along beaches on the East Coast until the next ice age. Most of the sand currently along the coast and continental shelf originated in the north from sources such as the Hudson River. For sand to move as far south as the Carolina coast, it must progress in stages possibly taking millions of years.
As sand moves along a coast, ocean currents push it into large bays or estuaries. The embayment continues to fill with sand until sea levels drop and the accumulated sediment flushes down onto the continental shelf. In a single glacial cycle, however, the sand travels only as far as the next bay. Therefore, most beaches will not receive a major restocking of sand until the next ice age.
7.4 WAVE IMPACTS
Large storms at sea generate most ocean waves when strong winds blow across the surface of the water. Wave breaking along the coast dissipate energy and are responsible for generating along-shore currents, which in turn transport sand along the beach. Waves also cause coastal erosion, a serious problem in areas where the shoreline is steadily receding (Fig.7-11).
Most beach erosion from high waves occurs during coastal storms. On large lakes and bays, sudden barometric pressure changes cause the water to slosh back and forth, producing a wave called a seiche. Seiches are common on Lake Michigan and on occasions can be quite destructive. Hurricanes produce the most dramatic storm surges, which are responsible for destroying entire beaches. As a wave approaches the shore, it touches bottom and slows. The shoaling of the wave distorts its shape, causing it to break upon the beach. The breaking wave dissipates its energy along the coast and causing beach erosion.













Figure 7-11 Beach wave erosion at Grand Isle, Louisiana. Courtesy of U.S. Army Corps of Engineers
Wave reflection bounces wave energy off of steep beaches or seawalls and is responsible for the formation of sand bars. When waves approach the shore at an angle to the beach, the wave crests bend by refraction. When waves pass the end of a point of land or the tip of a breakwater, a circular wave pattern generates behind the breakwater. The refracted waves intersect other incoming waves, increasing the wave height.
Wave steepness is the ratio of wave length to wave height and is one of the most important aspects of waves. Storm waves with high steepness have short wave lengths and high wave heights and produce choppy seas. Steep waves accompanying storms at sea cause erosion of sea cliffs and sand dunes along the coast. Swells with low steepness generally result in the shoreward transport of sediment. Therefore, much of the sediment carried offshore by storm waves returns by swells during the interval between storms.
As waves leave the storm area, they develop into swells that travel great distances, sometimes halfway around the world before dying out or intercepting a coastline. As the waves spread outward from the storm area, the longer-period waves move out in front while the shorter-period waves trail behind. As swells move across the ocean toward distant shores, the low, long-period waves are the first to arrive, followed by higher swells with shorter periods.
Waves expanding outward from a storm center form rings similar to those produced by tossing a rock into a quiet pond. As the rings enlarge, the wave spreads out along a greater length, expanding the circumference of the circle. This increases the wave height as it moves away from the storm area. When swells arrive at the coast, they generate a uniform succession of waves, each with about the same period and height. The period and height change when the slower swells begin to arrive.
The wave motion changes as waves travel from deep water toward the shore. The waves transport energy but not the water itself. As the wave crest approaches, an object floating on the surface first rises and moves forward with the crest, drops into the trough, and then moves backward. Thus, a floating object describes a circular path, with the diameter equal to the wave height, and return to its original position after the wave passes.














Figure 7-12 Types of breakers: spilling breaker (top), plunging breaker (middle), and surging breaker (bottom).
When swells reach a coast, they form various types of breakers (Fig.7-12), depending on the wave steepness and bottom slope conditions near the beach. If the slope is relatively flat-less than 3 degrees- the wave forms a spilling breaker, the most common type. This is an over-steepened wave that starts to break at the crest and continues breaking as the wave travels toward the beach, providing good waves for surfing.
A plunging breaker forms when the bottom slope is between 3 and 11 degrees, and the crest curls over, forming a tube of water. As the wave breaks, the tube moves toward the shore bottom and stirs up sediments. Plunging waves are the most dramatic breakers and do the most beach damage because the energy concentrates at the point where the wave breaks.
A collapsing breaker forms when the bottom slope between 11 and 15 degrees. The breaker is confined to the lower half of the wave, but as the wave moves toward the coast most of it reflects off the beach.
A surging breaker develops on a steep bottom where the slope is greater than 15 degrees. The wave does not break but surges up the beach face and reflects off the coast, generating standing waves near the shore. Standing waves are important for the development of offshore structures such as bars, sand spits, bench cusps, and rip tides.
7.5 COASTAL SUBSIDENCE
Coastal subsidence often occurs during large earthquakes that cause one block of crust to drop below another. Vegetated lowlands along the coast are elevated by influx of sediments to avoid inundation by the sea. When an earthquake strikes, these lowlands sink far enough to be submerged regularly and become barren tidal mud flats. Between earthquakes, sediments fill the tidal flats and raise them to the level where vegetation can grow once again. Therefore, repeated earthquakes produce alternating layers of lowland soil and tidal flat mud.
Earthquake-induced subsidence in the United States has occurred mainly in California, Alaska, and Hawaii. The subsidence results from vertical displacements along faults that can effect broad areas. During the March 27,9164, Alaska earthquake, over 70,000 square miles of land tilted downward more than 3 feet, causing extensive flooding in coastal areas of southern Alaska.
Flow failures usually develop in loose saturated sands and silts on slopes with grades greater than 6 percent and originate both on land and on the seafloor near coastal areas. The Alaskan earthquake produced submarine floor failures that destroyed seaport facilities at Valdez, Whittier, and Seward. The floor failures also generated large tsunamis that overran coastal areas and caused additional damage and casualties.
Some of the most dramatic examples of nonseismic subsidence in the United States are along coasts (Fig.7-13). The Houston-Galveston area in Texas has experienced local subsidence of as much as 7.5 feet and a foot or more over an area of 2,500 miles, mostly as a result of the with drawal of groundwater. In Galveston Bay, the ground subsided 3 feet or more over an area of several square miles following oil extraction from the underlying strata. Subsidence in some coastal towns has increased susceptibility to flooding during severe coastal storms.













Figure 7-13 Submergent coastline north of Portland, Maine. Photo by J. R. Balsley, courtesy of USGS
The pumping of large quantities of oil at Long Beach, California caused the ground to subside, forming a huge bowl up to 25 feet deep over an area of about 20 square miles. In some parts of the oil field, land subsided at a rate of 2 feet per year. In the downtown area, the subsidence was upward of 6 feet, causing severe damage to the city’s infrastructure. The injection of seawater under high pressure into the underground reservoir halted most of the subsidence, with the added benefit of increasing the production of the oil field.
Many coastal cities subside because of a combination of rising sea levels and withdrawal of groundwater, which causes compaction of the aquifer beneath the city. During the last 50 years, the cumulative subsidence of Venice, Italy, has been about 5 inches. The Adriatic Sea has risen about 3.5 inches during this century, resulting in a relative sea level rise of more than 8 inches. The severe subsidence causes Venice to flood during high tides, heavy spring runoffs, and storm surges.
The overdrawing of groundwater has caused the land to sink around building foundations in the northeastern section of Tokyo, Japan. The subsidence progressed at a rate of half a foot a year over an area of about 40 square miles, a third of which sank below sea level, prompting the construction of dikes to keep out the sea from certain sections of the city. A threat of catastrophe hangs over Tokyo from inundation by floodwaters during earthquakes and typhoons that plague the region.
7.6 MARINE TRANSGRESSION
The level of the sea is rising at a rate approaching 10 times faster than only a half century ago. In most temperate and tropical regions, the sea level is rising as much as 1 inch every 5 years. Most of the increase might be due to the melting of the West Antarctic and Greenland ice sheets from two decades of apparent global warming. Ice streams flowing into the ocean calve off to form icebergs (Fig. 7-14), whose number and size seem to be increasing. In March 1995, an extremely large iceberg broke off the Antarctic ice sheet and drifted into the Pacific Ocean. In addition, alphine glaciers, which contain substantial amounts of the world’s ice, appear to be melting as well.
The increased temperature also causes a thermal expansion of the ocean, increasing its overall volume. Over the last century, thermal expansion has raised the level of the sea about 2 inches. Surface waters off the coast of southern California have warmed nearly 1 degree Celsius over the last half century, causing the water to expand and raise the sea level by about 1.5 inches. The additional rise in global sea levels alters the shapes of the continents and submerges low-lying barrier islands and atolls. For every foot of sea level rise, from 100 to 1,000 feet of shoreline disappears, depending on the slope of the coast.











Figure 7-14 A large iceberg along the coast of Antarctica. Courtesy of U.S. Maritime Administration








Figure 7-15a U.S. Coast Guard icebreaker Polar Star near Palmer Peninsula, Antarctica. Photo by E. Moreth, courtesy of U.S. Navy
Sea level trends are estimated from tidal gage records at stations dispersed around the world’s seacoasts. In some areas, such as Louisiana, the relative level of the sea has risen as much as 3 feet per century. Louisiana is losing about 6,000 acres of land each year to the encroaching sea. The beaches along North Carolina are retreating at a rate of 4 to 5 feet per year. The higher sea levels are due in part to the sinking of the land by the increased weight of water pressing down on the continental shelf.



















Figure 7-15b Dashed lines indicate the normal extent of sea ice around Antarctica.
During the last 100 years, the global sea level, has risen upward of 6 inches, due mainly to the melting of the polar ice caps. Sea ice forms a frozen band around Antarctica (Figs. 7-15a&b) and covers most of the Arctic Ocean during the winter season in each hemisphere. The total surface area of the ice appears not to have changed significantly in the last 100 years. However, the maximum extent that the ice pack reaches outward from the poles during the winter season has diminished. The ice obtains its maximum extent during the spring in the Southern Hemisphere, from October through December, when the Antarctic ice is breaking up and Arctic ice is starting to expand. As the ocean continues to warm, the ice melts closer to the poles, further reducing the perimeter of sea ice.
During the Antarctic winter, from June through September, sea ice covers nearly 8 million square miles of ocean that surrounds the continent, with an average thickness of less than 3 feet. Because of this great expanse of ice, Antarctica plays a more significant role in atmospheric and oceanic circulation than does the Arctic. The sea ice is punctured in various places by coastal and ocean polynyas, large open-water areas that are kept from freezing by upwelling warm water currents. The coastal polynayas are like sea ice factories because they expose portions of open ocean that later freeze, continuing the ice-making process.
Most countries would feel the adverse affects of rising sea levels as rising sea temperatures cause the ice caps to melt. If the melting continues at its present rate, the sea could rise 6 feet by the middle of the next century. Large tracks of coastal land would disappear along with shallow barrier islands and coral reefs. Low-lying fertile delta that support millions of people would drown. Delicate wetlands, where many species of marine life hatch their young, would be reclaimed by the ocean. Vulnerable coastal cities would have to relocate farther inland or build costly seawalls to protect against the rising waters.

















8 SEA RICHES
The world is fortunate to have such an abundance of natural resources, which has dramatically advanced civilization. Much of this wealth comes from the sea, which holds the key to unheard of riches. Hidden in the world’s oceans are untouched reserves of petroleum and minerals, along with huge fisheries that provide half the dietary protein requirements for the human race.
The capacity of the oceans to generate surpasses that of all available fossil fuels combined, and the harnessing of this vast energy source could meet the demand for centuries to come. New frontiers for future exploration include the continental shelves and the ocean depths. Improved exploration techniques will ensure, with proper management, a continued supply of ocean resources well into the future.
8.1 LAW OF THE SEA
The United States initiated the expansion of national claims to the ocean and its resources with the Truman Proclamations on the Continental Shelf and the Extended Fisheries Zone of 1945. Other nations followed this expansion of national boundaries and began carving up the world’s oceans in a manner similar to the colonial division of Africa a century earlier. On December 6, 1982, 119 countries signed the United Nations Convention on the Law of the Sea. The declaration was a kind of constitution for the sea and put 49 percent of the ocean and its bottom adjacent the coasts of continents and islands under the management of the states in possession of these regions. The other 60 percent of the ocean surface and the water below it was reserved for the traditional freedom of the seas.

TABLE 8-1 THE FUTURE OF SOME NATURAL RESOURCES

Commodity

Consumption Rate in years

Reserves

Resources 

Aluminum 

250

800

Coal

200

3000

Platinum

225

400

Cobalt

100

400

Molybdenum

65

250

Nickel

65

160

Copper

40

270

Petroleum

35

80

* Reserves are recoverable resources with today’s mining technology

The remaining wealth of the ocean floor, or about 40 percent of the Earth’s surface, was deeded to the Common Heritage of Mankind. The convention placed that heritage under the management of an International Seabed Authority, with the capacity to generate income, the power of taxation, and an eminent domainlike authority over ocean-exploiting technology. The convention also provided a comprehensive global framework for protecting the marine environment, a new regime for marine scientific research, and a comprehensive legal system for settling disputes. It ensured freedom of navigation and free passage through straits used for international maritime activities, a right that cannot be suspended under any circumstances. In essence, the Law of the Sea provided a new order more responsive to the real needs of the world.
Coastal states were accorded a 12-miles limit of territorial sea and a 12-mile contiguous zone. Beyond these limits, each state was granted a 200-mile economic zone (Fig.8-1) that includes fishing rights and rights over all resources. In cases where the continental shelf extends beyond the 200-mile limit, the economic zone with respect to resources on the seabed is expanded to 350 miles. The economic zone concept also has been described as the greatest territorial grab in history, giving coastal nations unfair advantage over landlocked ones and increasing inequality among nations.








Figure 8-1 The zone of marine resources.
In march 1983, the United States added more than 3 million square miles to its jurisdiction by declaring the waters 200 miles offshore as the nation’s Exclusive Economic Zone (EEC); this area is slightly larger than the country itself. In 1984, the British oceanographic ship Farnella began a 6-year comprehensive mapping project of the ocean floor in the United States’ EEC for future resources of petroleum and minerals. The maps revealed feature possibly overlooked by smaller-scale studies. Along the West Coast were dozens of newly discovered seamounts and earthquake faults. On the western side of the Gulf of Mexico were oil-trapping salt domes, submarine slides, and undersea channels. In addition, large sand-dune fields similar to those found in the deep Pacific lay in the Gulf under 10,000 feet of water. The American research vessel Samuel P. Lee (Fig.8-2) went on a similar mission in the Bearing Sea to explore for oil and gas.












Figure 8-2 The research vessel Samuel P. Lee carried out geophysical surveys in the Pacific Ocean and Alaskan waters. Courtesy of USGS
While diving along a midocean spreading center called the Gorda Ridge about 125 miles off the coast of Oregon, the U.S. Navy deep submersible Sea Cliff discovered in September 1988 a lush community of exotic animals in a field of hot springs. Similar hot spring oases have been found on other spreading centers, where molten rock from the mantle rises to create new oceanic crust as two adjoining crustal plates pull apart. However, this was the first hydrothermal vent system existing within the United States’ EEC. Moreover, the site might be a source for such strategic minerals as manganese and cobalt, used for strengthening steel. The hydrothermal water up to 400 degrees Celsius often carriers dissolved minerals that form deposits on the ocean floor when the hot water mixes with near-freezing bottom water.
The discovery of a significant resource anywhere in the world’s ocean could invite a claim from the nearest coastal or island state even if that resource lies beyond the limits of national jurisdiction. Such a dispute has occurred over a splattering of semisubmerged coral reefs in the South China Sea for their oil potential. Disputes over the ownership of midocean ore deposits have diminished the interests of western industrial nations, leaving the future of undersea mining and refining of manganese nodules and other metallic ores in the hands of many Asian countries, including Japan, China, South Korea, and India, which needed these resources in order to reduce their dependence on foreign raw materials.
The expansion of national jurisdictions into the oceans also constrains the freedom of the seas for scientific research (Fig.8-3). Under present law, other nations must apply for consent from coastal state in order to conduct research in waters that were once open to all. Opposition to such a scientific project by a coastal nation that controls the waters in question might undermine the cooperative atmosphere among nations that the Law of the Sea was supposed to foster.


















Figure 8-3 The seafloor drillship Paul Langevin III was used to obtain rock cores of the Juan de Fuca Ridge. Courtesy of USGS
8.2 OIL AND GAS
Of all the mineral wealth lying beneath the waves, only oil and natural gas fields in shallow coastal waters are profitable under present economic conditions. Petroleum provides nearly half the world’s energy, with about 20 percent of the oil and about 5 percent of the natural gas production offshore. In the future, perhaps half the petroleum will be extracted from under the seas. Unfortunately, much offshore oil-up to 2 million tons each year-leaks into the oceans. Such pollution could become an enormous environmental problem as production increases to keep up with demand.
















Figure 8-4 An oil tanker approaches the Valdez terminal of the trans-Alaskan pipeline, bringing North Slope petroleum to the lower 48 States. Courtesy of U.S. Maritime Administration
Over the last 2 decades, offshore drilling for oil and natural gas in shallow coastal waters has become extremely profitable. Interest in offshore oil began in the mid-1960s, with a considerable increase in drilling a decade later following the 1973 Arab oil embargo, when American motorists stood in long lines at gas stations. New important finds such as at Prudhoe bay on Alaska’s North Slope (Fig.8-4) and on the North Sea off Great Britain resulted from intensive exploration for new reserves of offshore oil.
The desire for energy independence encouraged oil companies to explore for petroleum in the deep oceans, where they encountered many difficulties, including storms at sea and the loss of personnel and equipment. Such difficulties and problems could not justify the few discoveries that were made. Futuristic plans foresee drilling equipment and workrooms being built on the seafloor, where they are not affected by storms. This would make some deep-sea oil and gas fields available for the first time.
The creation of reservoirs of oil and natural gas requires a special set of geologic conditions, including a sedimentary source for the oil, a porous rock to serve as a reservoir, and a confining structure to trap the oil. The source material is organic carbon trapped in fine-grained, carbon-rich sediments. Porous and permeable sedimentary rock such as sandstones and limestones form the reservoir. Geologic structures produced by folding or faulting of sedimentary beds trap or pool the oil. Petroleum often associates with thick beds of salt, and because salt is lighter than the overlying sediments, it rises toward the surface, creating salt domes that help trap oil and natural gas.
The organic material that forms petroleum originates from microscopic organisms living primarily in the surface waters of the ocean and concentrated in fine particulate matter on the ocean floor. The transformation of organic material into oil requires a high rate of accumulation or a low oxygen content in the bottom water to prevent oxidation of organic material before burial under layers of sediment. Oxidation causes decay, which destroys organic matter. Therefore, areas with high rates of accumulation of sediments rich in organic material are the most favorable sites for the formation of oil-bearing rock. Deep burial in a sedimentary basin heats the organic material under high temperature and pressures and chemically alter it. Essentially, the organic material is “cracked” into hydrocarbons by the heat generated in the Earth’s interior. If the hydrocarbon are overcooked, natural gas results.
The hydrocarbon volatiles locked up in the sediments along with seawater migrate upward through permeable rock layers and accumulate in traps formed by sedimentary structures that provide a barrier to further migration. In the absence of such a cap rock, the volatiles continue rising to the surface and escape into the ocean from natural seeps, amounting to about 1.5 million barrels of oil yearly. (This amount is minuscule compared to the approximately 25 million barrels of oil accidentally spilled into the ocean each year (Fig.8-5. Depending mainly on the temperature and pressure conditions within the sedimentary basin, it takes anywhere from several tens of millions to a few hundred million years to produce petroleum.











Figure 8-5 The December 19,1976, Argo Merchant oil spill off Nantucket, Massachusetts. Courtesy of NOAA.
Reservoirs of hot gas-charged seawater called geopressured deposits lying beneath the Gulf Coast off Texas and Louisiana are a hybrid form of natural gas and geothermal energy. The gas deposits formed millions of years ago when seawater permeated porous beds of sandstone between impermeable clay layers. The seawater captured heat building up from below and dissolved methane from decaying organic matter. As more sediments piled on top of this formation, the hot-gas charged seawater became highly pressurized. Wells drilled into this formation would tap both geothermal energy and natural gas, providing an energy potential equal to about one third of all coal deposits in the United States.
The geology of the ocean floor determines whether the proper conditions exist for trapping oil and gas and greatly aids oil companies in their exploration activities. Petroleum exploration begins with a search for sedimentary structures conductive to the formation of oil traps. Seismic surveys delineate these structures by using explosions from air guns that generate waves similar to sound waves and received by hydrophones towed behind a ship (Fig.8-6). The seismic waves reflect and refract off various sedimentary layers, providing a sort of geological CAT scan of the oceanic crust.









Figure 8-6 A seismic survey of the ocean’ s crust.
After choosing a suitable site, the oil company brings in a drilling rig, which stands on the ocean floor in shallow water or free-floats anchored to the bottom in deep water (Fig.8-7). While drilling through the bottom sediments, workers lines the well with steel casing to prevent cave-ins and to act as a conduit for the oil. A blowout preventer placed on top of the casing prevents the oil from gushing out under tremendous pressure once the drill bit penetrates the cap rock. If the oil well is successful, additional wells are drilled in the area to fully developed the field.
8.3 MINERAL DEPOSITS
Hydrothermal ores deposited by hot water are associated with volcanically active zones on the ocean floor, including midocean ridges that create new oceanic crust and island arcs on the margins of subduction zones that destroy old oceanic crust. Hydrothermal deposits exist on young seafloors along active spreading centers of the major oceans as well as in regions that are rifting apart and forming new bodies of water, such as the Red Sea, the Afar Rift, and the Gulf of Aden (Fig.8-8). In addition, deep-sea drilling has uncovered identical deposits in older ocean floors far from modern spreading centers, which suggests that the process responsible for the creation of metal deposits has operated throughout the history of the major oceans.
Rich ores, including copper, zinc, gold, and silver, lie hidden among the midocean rifts. The hydrothermal deposits form by the precipitation of minerals in hot water solutions rich in silica and metals discharged from hydrothermal springs. Silica and other minerals build prodigious chimneys, from which turbulent black clouds of fluid (black smokers) billow out. Metal-rich particles precipitated from the effluent fill depression on the seafloor and eventually form an ore body.














Figure 8-7 A semisubmersible drilling rig in the Mid-Atlantic outer continental shelf. Courtesy of USGS
The minerals that contribute to hydrothermal systems originate from the mantle at depths of 20 to 30 miles below the seafloor. Magma upwelling from the mantle penetrates the oceanic crust and provides new crustal material at spreading centers. Seawater seeping into fractures in the basaltic rock on the ocean floor penetrates below the crust near the magma chamber, where it circulates within the zone of young, highly fractured rock and heats to a temperature of several hundred degrees Celsius.
The hot water kept from boiling by the pressure of several hundred atmospheres dissolves silica and mineral from the basalt, which are carried in solution to the surface by convection and discharged through fissures in the seafloor (Fig.8-9). In addition, metal-rich fluids derived directly from the magma and volatile elements from the mantle also travel along with the hydrothermal waters to the surface. When the hot metal-rich solution emerges from a vent into cold, oxygen-rich seawater, metal such as iron and manganese are oxidized and deposit along with silica. Some deposits on the Mid-Atlantic Ridge contain as much as 35 percent manganese, an important metal used in steel alloys.
The hydrothermal deposits are generally poor in copper, nickel, cobalt, lead, and zinc because these elements remain in solution longer than iron and manganese. Under oxygen-free conditions, such as those in stagnant pools of brine, copper and zinc tend to concentrate along with iron and manganese. These deposits occur in the Red Sea, where the concentrations of copper and zinc reach ore grades sufficiently high to make mining economical.



















Figure 8-8 Location of the Red Sea and the Gulf of Aden.
Another type of ore deposit exists in ophiolites, which are fragment of ancient oceanic crust uplifted and exposed on land by continental collisions. The grounded oceanic crust consists of an upper layer of marine sediments, a layer of pillow lava (basalts erupted undersea), and a layer of dark, dense ultramafic (iron-magnesium-rich) rocks possibly derived from the upper mantle. The metal ore deposits exist at the base of the sedimentary layer just above the area where it contacts the basalt.
Ophiolite ore deposits are scattered throughout many parts of the world (Fig.8-10). They include the 100-million-year-old ophiolite complexes exposed on the Apennines of northern Italy, the northeastern margins of the Himalayas in southern Tibet, the Ural Mountains in Russia, the eastern Mediterranean (including Cyprus), the Afar Desert of Northeastern Africa, the Andes of South America, on islands of the western Pacific such as the Philippines, uppermost Newfoundland, and Point Sol along the Big Surcoast of central California.


















Figure 8-9 The operation of hydrothermal vents on the seafloor.










Figure 8-10 Location of ore deposits originally formed by seafloor hot springs.
Massive sulfides are metal ore deposits formed at midocean spreading centers. The deposits contain sulfides of iron, copper, lead, and zinc, and occur in most ophiolite complexes, mined extensively throughout the world for their rich ores. The circulating seawater below the ocean floor acquires sulfate ions and become strongly acidic. This reaction promotes the combination of sulfur with certain metals leached from the basalt and extracted from the hydrothermal solution to form insoluble sulfide minerals.
The sulfide metals deposited by hydrothermal systems on the ocean floor form large mounds (Figs.8-11a&b). They also occur as disseminated inclusions or veins in the rock below the seafloor in ophiolites (Fig.8-12). Another deposit forms only when a ridge axis is near a landmass, which is a source of large amounts of erosional debris. The massive sulfide ore body lies in the midst of a sediment layer, usually shale derived from fine-grained clay. Some of the world’s most important deposits of copper, lead, zinc, chromium, nickel, and platinum that are critical to modern industry originally formed several miles below the seafloor and upthrusted onto dry land during continental collisions.











Figure 8-11a A weathered sulfide mound on the Juan de Fuca Ridge. Courtesy of USGS
Ore deposits also associated with hot brines resulting from the opening of a new ocean basin by a slow spreading center such as the one bisecting the Red Sea. Hot, metal-rich brines fill basin along the spreading zone. The cold, dense seawater percolating down through volcanic rocks becomes unusually salty because it passes through thick beds of halite (sodium chloride) buried in the crust. These salt beds are formed under dry climatic conditions when evaporation exceeds the inflow of seawater in a nearly enclosed basin.
When salinity levels reached the saturation point, salt crystals precipitated out of solution and settled on the ocean floor, accumulating in thick beds. The highly salinity of hot solutions circulating through these salt beds enhanced their ability to transport dissolved metals by forming complexes with the chlorine in the salt. When they discharged from the floors of the basins, the heated solutions collected as hot brines. Metals precipitate from the hot brines and settled in basins, where they formed layered deposits of metalliferous sediments up to 6 miles thick in places.
















Figure 8-11b Formation of a massive sulfide deposit by hydrothermal fluids.












Figure 8-12 Metal-rich massive sulfide vein deposit in ophiolite. Courtesy of USGS
The most promising mineral deposits on the ocean floor are manganese nodules (Fig. 8-13). They are hydrogenous deposits named from the Greek words meaning “water-generated.” They form on the ocean floor by the slow accumulation of metallic elements extracted directly from seawater, which contains metals such as iron and manganese in solution at concentrations of less than one part per million by weight. The metals enter the oceans from streams that transport minerals derived from the weathering and decomposition of rocks on the continents and through hydrothermal vents on the ocean floor that acquire minerals from active volcanic zones beneath the crust.
Most metallic elements have a limited solubility in an alkaline, oxygen-rich environment such as seawater. Dissolved metals such as iron and manganese are oxidized by the presence of oxygen in the seawater, forming insoluble oxides and hydroxides. The metals then deposit on the ocean floor as tiny particles or as films or crusts covering any solid material on the seafloor. Living organisms also extracted certain metals from seawater; when these organisms die, their remains collected on the ocean floor, where the metals incorporate with the bottom sediments.
The growth rates of hydrogenous deposits are generally less than 1 inch in 10 million years. Most of the seafloor concretions such as manganese nodules are particularly well developed in deep, quiet waters far from continental margins and active volcanic spreading ridges where the steady rain of clay and other mineral particles prevents the metals from growing into concentrated deposits. The deposits occur in basins that receive a minimal inflow of sediments that would otherwise bury them. Depositional areas include abyssal plains and elevated areas on the ocean floor such as seamounts and isolated shallow banks.
The manganese nodules grow around a small, solid nucleus, or seed, such as a grain of sand, a piece of shell, or a shark’s tooth. The seed acts as a catalyst, allowing the metals to accrete to it in a manner similar to the growth of a pearl. Concentric layers accumulate until the nodules reach about the size of a potato, giving the ocean floor a cobblestone appearance.
A ton of manganese nodules contains about 600 pounds of manganese, 29 pounds of nickel, 26 pounds of copper, and 7 pounds of cobalt. But the location of these nodules at depths approaching 4 miles makes extraction on large scale extremely difficult. About 100 square yards of bottom ooze must be sifted to extract a single ton of nodules. One mining method would use a dredge to scoop up the nodules. Another approach would employ a gigantic vacuum cleaner to suck up the nodules. A yet more exotic scheme envisions using television-guided robots to rake up the nodules, which are crushed into a slurry and pumped to the surface.
8.4 ENERGY FROM THE SEA
The world’s oceans are a large solar collector. Daily, 30 million square miles of tropical seas absorb the equivalent heat content of 250 billion barrels of oil-greater than the world’s total reserves of recoverable petroleum. If only a tiny fraction of this vast store of energy is converted into electricity, it could substantially enhance the world’s future energy supply. The conversion of less than a tenth of 1 percent of the heat energy stored in the surface waters of the tropics could generate roughly 15 million megawatts of electricity, or more than 20 times the current generating capacity of the United States.
Ocean thermal-energy conversion, or OTEC (Fig. 8-14), takes advantage of the temperature difference between the surface and abyssal waters. Where a significant temperature difference exists between the warm surface water and the cold deep water, efficient electrical energy can be generated. In a closed-cycle OTEC system, warm seawater evaporates a working fluid with a very low boiling point, such as freon or ammonia. The working fluid enclosed in the system recycles continuously like that in a refrigerator.















Figure 8-14 The ocean energy program at the National Renewable Energy Laboratory, Hawaii. Courtesy of U.S. Department of Energy
In an open-cycle OTEC system, also known as the Claude cycle after its inventor, the French biophysicist Georges Claude, the working fluid is a constantly changing supply of seawater. The warm seawater boils in a vacuum chamber, which dramatically lowers the boiling point. This system has the added benefit of producing desalinated water for irrigation in arid regions. In both systems, the resulting vapor drives a turbine to generate electricity. Cold water drawn up from depths of 2,000 to 3,000 feet condenses the gas back to a fluid to complete the cycle.
The nutrient-rich cold water also could be used for aquaculture, the commercial raising of fish, and serve nearby buildings with refrigeration and air conditioning. The power plant could be located onshore, offshore, or on a mobile platform out to sea. The electricity could supply a utility grid system or be used on site to synthesize substitute fuels such as methanol and hydrogen, to refine metals brought up from the seabed, or to manufacture ammonia for fertilizer.
The open-cycle system offers several advantages over the closed-cycle system. By using seawater as the working fluid, it eliminates the possibility of contaminating the marine environment with toxic chemicals. The heat exchangers of an open-cycle system are cheaper and more effective than those used in a closed-cycle system. Therefore, open-cycle plants would more efficiently convert ocean heat into electricity and would be less expensive to build.
The breaking of a large wave on the coast is a vivid example of the sizable amount of energy that ocean waves produce. The intertidal zones of rocky weather coasts receive much more energy per unit area from waves than from the sun. The waves form by strong winds from distant storms blowing across large areas of the open ocean. Local storms near the coasts provide the strongest waves, especially when superimposed on the rising and falling tides. Many hydroelectric schemes have been developed to utilize this abundant form of energy, which is economical and efficient. A crashing wave at the base of wave-powered generator (Fig.8-15) compresses the air at the bottom of a chamber and forces it into a vertical tower, where the compressed air spins a turbine that drives an electrical generator.
Gulfs and embayments along the coast in most parts of the world have tides exceeding 12 feet, called macrotides. Such tides are dependent on the shapes of bays and estuaries, which channel the wavelike progression of the tide and increase its amplitude. The development of exceptionally high tidal ranges in certain embayments is due to the combination of convergence and resonance effects within the tidal basin. As the tide flows into a narrowing channel, the water movement constricts and augments the tide height.
Generating electricity using tidal power involves damming an embayment, letting it fill with water at high tide, then closing the sluice gates at the tidal maximum when a sufficient head of water can drive the water turbines (Fig.8-16). Many locations with macrotides also experience strong tidal currents, which could be used to drive turbines that rotate with both the incoming and outgoing seawater to generate electricity.
8.5 HARVESTING THE SEA
The world’s fisheries are in danger of collapsing from overfishing. The once relative abundance of various species has fallen dramatically in many parts of the world. The dangers result from a constant harvest rate of a dwindling resource caused by fluctuating environmental conditions, resulting in a major decline in fish catches. The composition of the catch is also changing toward smaller fish species, and even the average size of fish within the same species is becoming smaller.


















Figure 8-15 Artist’s rendering of the Norsk wave-power generator on the rock coast of Bergen, Norway.









Figure 8-16 Crosss-section of the La Rance tidal power station.
Overfishing drives populations below levels needed for competition to regulate population densities of desired species. Therefore, under heavy exploitation, species that produce offspring quickly and copiously have a relative advantage. The extent to which these changes are due to shifts in fish populations, changes in patterns of commercial fishing, or environmental effects is uncertain. What is apparent is that if present trend continues, the world’s fishery could become smaller and composed of increasing less desirable species. The world’s annual fish catch is about 100 million tons, with the northwest Pacific and the northeast Atlantic yielding nearly half the total. A pronounced decline in heavily exploited fleshy fish is compensated by increased yields of so-called “trash” fish along with other small fishes. The systematic removal of large predator fish might increase annual catches of other fish species by several million tons. However, such catches would consist of smaller fish that could eventually dominate the northern latitudes, where population changes tend to be more variable and unpredictable than in the tropical regions.
Many changes in the world’s fisheries are due to the strongly seasonal behavioral patterns of the fishes as well as significant differences in climate and other environmental conditions from one season to the next. Climate influence fisheries by altering ocean surface temperatures, global circulation patterns, upwelling currents, salinity, pH balance, turbulence, storms, and the distribution of sea ice, all of which affect the primary production of the sea. Climatic conditions could cause a shift in species distribution of the sea. Climatic conditions could cause a shift in species distribution and loss of species diversity and quantity.
To compensate for the shortfall in marine fisheries, a variety of sea animals are raised commercially for human consumption. The shrimp, lobster, eel, and salmon raised by aquaculture account for less than 2 percent of the world’s annual seafood harvest. But their total economic value is estimated at 5 to10 times greater. The development of aquaculture and mariculture could help meet the world’s growing need for food. The Chinese lead the world with more than 25 million acres of impounded water in canals, ponds, reservoirs and natural and artificial lakes that are stocked with fish.
The food requirements of the world also might be met by cultivating seaweed and algae, which are becoming important sources of nourishment rich in vitamins. The Japanese gather about 20 edible kinds of seaweed and weekly consume about a pound of dried algae preparations per person as appetizers or deserts; they are becoming the world’s leaders in the production of sea plants. The seaweed is harvested wild, and many varieties are also cultivated. When algae grows under controlled conditions, it multiplies rapidly and produces large quantities of plant material for food.
Algae crops can be harvested every few days, whereas agricultural crops grown on land require 2 to 3 months between planting and harvesting. An acre of seabed could yield 30 tons of algae a year, compared with an average of 1 ton of wheat per acre of land. The algae can be artificially flavored to taste like meat or vegetables and are highly nutritious. The ocean farm is immensely rich and can meet human nutritional needs far into the future, provided people do not turn it into a desert as they have done with so much of the land.

TABLE 8-2 PRODUCTIVITY OF THE OCEANS

Location

Primary Production 

(tons per year of 

organic carbon)

Percent

Total Avaliable Fish 

(tons per year of 

fresh fish)

Percent

Oceanic 

16.3 billion 

81.5

0.16 million

0.07

Coastal Seas

3.6 billion

18.0

120.00 million

49.97

Upwelling Areas

o.1 billion

0.5

120.00 million

49.97

Total

20.0 billion

 

240.16 million