9 MARINE BIOLOGY
Exploration of the ocean would not be completed without a view of sea life. The riot of life in the tropical rain forests is repeated among the animals of the seafloor (Fig.9-1). Some of the strangest creatures on earth live on the deep ocean bottom. The most primitive species, whose ancestors go back several hundred million years, anchor to the ocean floor. The seabed hosts an eerie world comprised of tall chimneys spewing hot, mineral-rich water that supports a variety of unusual animals in the world, dark abyss, with no counterparts found elsewhere in the sea.
Figure 9-1 Marine life at the 100-foot depth at the point Loma kelp beds off San Diego, California. Photo by R. Outwater, courtesy of U.S.Navy
9.1 BIOLOGY DIVERSITY
The oceans have far-reaching effects on the composition and distribution of marine life. Marine biological diversity is influenced by ocean currents, temperature, the nature of seasonal fluctuations, the distribution of nutrients, the patterns of productivity, and many other factors of fundamental importance to living organisms. The vast majority of marine species live on continental shelves or shallow-water portions of islands and subsurface rises at depths less than 600 feet (Fig.9-2). The ecology of shallow-water environments also tends to fluctuate more than habitats father off shore, which affects evolutionary development. The richest faunas live at low latitudes in the tropics, which are crowded with large numbers of highly specialized species.
Figure 9-2 The distribution of shelf faunas.
Progressing to higher latitudes, diversity gradually falls off, until in the polar regions there are less than a tenth as many species as in the tropics. Moreover, twice as much biological diversity occurs in the Arctic Ocean, which is surrounded by continents, as in the Southern Ocean, which surrounds the continent of Antarctica. Species diversity mostly depends on the stability of the food supply. As the seasons become more pronounced in the higher latitudes, food production fluctuates much more than in the lower latitudes. Diversity is also affected by such seasonal changes as variations in surface and upwelling ocean currents that affect the nutrient supply, causing large fluctuations in productivity.
The biggest biological diversity occurs off the shores of small islands or off small continents in large oceans, where fluctuations in nutrient are least affected by the seasonal effects of landmasses. The least amount of diversity is off large continents, particularly when they face small oceans, where shallow water seasonal variations are the greatest. Diversity also increases with distance from large continents. During his visit to the Galapagos Islands in the 1830s (Fig.9-3), Charles Darwin noticed major changes in animals living on islands, compared to their relatives on adjacent continents.
Biological diversity is highly dependent on the stability of food resources, which is largely determined by the shape of the continents, the extent of inland seas, and the presence of coastal mountains. Continental platforms are particularly important, because not only extensive shallow seas provide habitat area for shallow-water faunas, but such area tend to dampen seasonal climatic changes and make the local environment more amenable.
Marine species living in different oceans or on opposite sides of the ocean evolve separately from their overseas counterparts. Even along a continuous coastline, major changes in species occur that generally correspond to changes in climate because latitudinal and climatic changes create barriers to shallow-water organisms. The depth of the seafloor provides another formidable barrier to the dispersal of shallow-water organisms. Furthermore, midocean ridges form a series of barriers to the migration of marine species.
Figure 9-3 Drawing’s journey around the world during his epic exploration.
The barriers partition marine faunas into more than 30 individual “province”. Generally, only a few common species live in each province. The shallow-water marine faunas represent more than 10 times as many species as would be present in a world with only a single province. Such single-province conditions might have occurred 200 million years ago during the existence of a single large continent and a large ocean.
The Indo-Pacific province is the widest ranging of all marine provinces and the most diverse, because of its long chains of volcanic island arcs. When long island chains align east-to-west within the same climatic zone, they are inhabited by highly diverse, wide-ranging faunas. The faunas spill over from these areas onto adjacent tropical continental shelves and islands. However, this vast tropical life is cut off from the western shores of Americas by the East Pacific Rise, which is an effective obstruction to the migration of shallow-water organisms.
Biological diversity depends on the food supply. Small, simple organisms called phytoplankton are responsible for more than 95 percent of all marine photosynthesis. They play a critical role in the marine ecology, which spans 70 percent of the Earth’s surface. Phytoplankton occupy a key position in the marine food chain. They also produce 80 percent of the breathable oxygen and regulate carbon dioxide, which affects the world’s climate.
The surface waters of the ocean vary markedly in color, according to the nature and amount of suspended matter such as phytoplankton, silt, and pollutants. In the open ocean, where the biomass is low, the water has a characteristic deep blue color. In the temperate coastal regions, where the biomass is high, the water has a characteristic greenish color. The temperate waters of the North Atlantic are colored green because they are rich in phytoplankton.
Figure 9-4 The marine food chain, from the simplest plankton to top carnivore.
Upwelling currents off the coasts of continents and near the equator are important sources of bottom nutrients such as nitrates, phosphates, and oxygen. Zones of cold, nutrient-rich upwelling water scattered around the world cover only about 1 percent of the ocean but account for about 40 percent of the ocean’s biological productivity and support prolific booms of phytoplankton and other marine life. These tiny organisms reside at the very bottom of the marine food web are eaten by predators, which are preyed upon by progressively larger predators further up the food chain (Fig.9-4). The phytoplankton-rich areas are also of vital economic importance to the commercial fishing industry.
TABLE 9-1 CLASSIFICATION OF SPECIES
Forams and radiolarians.
The sponges, about 3000 living species.
Tissues composed of three layers of cells.
About 10,000 living species. Jellyfish, hydra, coral.
Moss animals. About 3000 living species.
Two asymmetrical shells. About 120 living species.
Straight, curled, or two symmetrical shells. About
70,000 living species. Snails, clams, squids, ammonites.
Segmented body with well-developed internal organs. About 7000 living species.
Worms and leaches.
Largest phylum of living species, with over one million
known. Insects, spiders, shrimp, lobsters, crabs, trilobites.
Bottom dwellers with radial symmetry. About 5000 living
species. Starfish, sea cucumbers, sand dollars, crinoids.
Spinal column and internal skeleton. About 70,000 living
species. Fish, amphibians, reptiles, birds, mammals.
The most primitive of marine species are sponges. The earliest sponges were giants as much as 10 feet or more across and grew in thickets on the seabed. The sponge’s body consists of three weak tissue layers whose cells can survive independently if separated from the main body. If a sponge is sliced up, individual pieces can grow into new sponges. Certain groups have an internal skeleton of rigid, interlocking spicules composed of calcite or silica. The great success of sponges and other organisms such as diatoms, which extract silica from seawater to construct their skeletons, explains why the ocean is depleted of this mineral.
The coelenterates, which include corals, sea anemones, sea pens and jellyfish (Fig.9-5), are among the most prolific of marine animals. Most coelenterates are radially symmetrical, with body parts radiating outward from a central axis. They have a saclike body with a mouth surrounded by tentacles. The corals possess a large variety of skeletal forms, and successive generations of them have built thick limestone reefs. Corals began constructing reefs about 500 million years ago, forming chains of islands and barrier reefs along the shorelines of the continents.
Figure 9-5 A helmet jellyfish under the ice of MuMurdo sound, Antarctica. Photo by W.R. Curtsinger, courtesy of U.S.Navy
The bryozoans are similar to corals and compromise microscopic individuals living in small colonies up to several inches across, giving the ocean floor a mosslike appearance. Like corals, bryozoans are retractable animals, encased in a calcareous vaselike structure, into which they retreat for safety when threatened. The polyp has a circle of ciliated tentacles, forming a net around the mouth and used for filtering microscopic food particles floating by.
The echinoderms, whose name means “spiny skin”, are perhaps the strangest marine species. Their five-fold radial symmetry make them unique among the more complex animals. They are the only animals possessing a water-vascular system composed of internal canals that operate a series of tube feet or podia used for locomotion, feeding and respiration. The great success of the echinoderms is illustrated by the fact that they have more classes of organisms than any phylum either living or extinct. The major classes of living echinoderms include starfish (Fig. 9-6), brittle stars, sea urchins, sea cucumbers, and crinoids, known as sea lilies because of their plantlike appearance.
Figure 9-6 A starfish off Point Loma, San Diego, California. Photo by R.Outwater, courtesy of U.S.Navy
The brachiopods, also called lamp shells due to their likeness to primitive oil lamps, were once the most abundant and diverse marine organisms. They are similar in appearance to clams, with two saucerlike shells fitted face to face that open and close using simple muscles. More advanced species called articulates have ribbed shells with interlocking teeth that maneuver along a hinge line. The brachiopods are fixed to the ocean floor with a rootlike appendage and filter food particles through opened shells that close to protect the animals against predators.
The mollusks are a highly diverse group of marine animals and finding common features among various members is often difficult. The three major groups are the snails, clams, and cephalopods. Snails and slugs comprise the largest group. The cephalopods, which include the squid, cuttlefish, octopus, and nautilus, travel by jet propulsion. They suck water into a cylindrical cavity through openings on each side of the head and expel it under pressure through a funnel-like appendage.
The nautilus (Fig.9-7) is often referred to as a “living fossil” because it is the only living relative of the swift-swimming ammonoids. It lives in the great depths of South Pacific and India oceans down to 2,000 feet. The octopus, which also live in deep waters, is almost like an alien life form, for it is the only animal with copper-based blood, whereas all other animals have iron-based blood.
Figure 9-7 The nautilus is the only living relative of the ammonoids.
The arthropods are the largest phylum of living organisms, comprising roughly a million species, or about 80 percent of all known animals. The arthropod body is segmented, with paired, jointed limbs generally present on most segments and modified for sensing, feeding, walking, and reproduction. The crustaceans are primary aquatic and include shrimp, lobsters, crabs, and barnacles.
Fish comprise over half the species of vertebrates and include the jawless fish (lampreys and hagfish), the cartilaginous fish (sharks, skates, rays, and ratfish), and the bony fish (salmon, swordfish, pickerel, and bass). The ray-finned fishes are by far the largest group of fish species. Sharks have been highly successful for the last 400 million years. They play a critical role by preying on sick and injured fish and thus help keep the ocean healthy. Closely related to the sharks are the rays, whose pectoral fins are enlarged into wings, allowing them to literally “fly” through the sea.
9.3 LIFE IN THE ABYSS
The deep waters of the open ocean were once thought to be a lifeless desert. While dredging the ocean bottom in the 1870s, the British oceanographic ship Challenger hauled to the surface a large collection of deep-water and bottom-dwelling animals even from the deepest trenches, including hundreds of species never seen before and unknown to science. The catch comprised some of the most bizarre life forms molded by adaptive behavior and natural selection to the cold and dark of the abyss, along with several species thought to have long been extinct.
A century later, a population of large, active animals were discovered thriving in total darkness as deep as 4 miles. These depths were previously thought to be the domain of small, feeble creatures such as sponges, worms, and snails that were specially adapted to live off the debris of dead animals raining down from above. But in fact much of the deep seafloor was teeming with many species of scavengers, including highly aggressive worms, large crustaceans, deep-diving octopuses, and a variety of fishes including giant sharks.
The large physical size of many species is due to an abundance of food, lower levels of competition, and a lack of juveniles, which live in shallower water before descending to deeper depths after they mature. Large numbers of fish from the great depths of the lower latitudes are related to shallow-water varieties of the higher latitudes. Some arctic fishes might represent near-surface expressions of populations that inhabit the cold, deep waters off continental margins.
Figure 9-8 Modern coelacanths have not changed significantly from their 460-million-year-old ancestors.
The coelacanth (Fig.9-8), once thought to have gone extinct along with the dinosaurs and ammonoids, reemerged in 1938 when fishermen caught a 5-foot species in the deep, cold waters of the Indian Ocean off the Comoro Islands, near Madagascar. The fish looked ancient, a castaway from the distant past, with a fleshy tail, a large set of forward fins behind the gills, powerful square toothy jaws, and heavily armored scales. Stout fins enabled the fish to crawl along on the deep ocean floor in much the same manner that its ancestors crawled out of the sea to populate the land.
The oldest species living in the world’s oceans thrive in cold waters. Many arctic species, including certain brachiopods, starfishes, and bivalves, belong to biological orders whose origins extent back hundreds of millions of years. Some 70 species of marine mammals known as cetaceans are among the most adaptable animals and include dolphins, porpoise, and whales, which spend much of their time feeding in the arctic waters of the polar regions.
Figure 9-9 A view of marine life found on the bottom of McMurdo Sound, Antarctica. Photo by W.R.Cursinger, courtesy of U.S.Navy
The Antarctic Sea is the coldest marine habitat in the world (Fig.9-9). It was once thought to be totally barren of life. But in 1899, a British expedition to the southernmost continent found examples of previously unknown fish species related to the perches common in many parts of the world. Upward of 100 species of fish are confined to the Antarctic regions, accounting for about two-thirds of the fish species in the area. Living in subfreezing waters, these fish rely on a chemical antifreeze-like substance in their bodies to prevent freeze-up during the cold Antarctic winters.
A circum-Antarctic current isolates the Antarctic Sea from the general circulation of the ocean and serves as a thermal barrier, impeding the inflow of warm currents and warm-water fishes along with the outflow of Antarctic fishes. Also because of the extreme cold and low productivity, the Antarctic Sea is less diverse than the Arctic Ocean, which supports almost twice as many species. Biologists of the Smithsonian Institution using a deep-sea submersible made a remarkable discovery off the Bahamas in 1983. A totally new and unexpected algae lived on an uncharted seamount at a depth of about 900 feet, deeper than any previously known marine plant larger than a microbe. The species comprised a variety of purple algae with a unique structure, consisting of heavily calcified lateral walls and very thin upper and lower walls. The cells grow on top of each other, likes cans stacked at a grocery store, for maximum surface exposure to the feeble sunlight. The discovery expanded our understanding of the role algae play in the productivity of the oceans, marine food chains, sedimentary processes, and reef building.
9.4 CORAL REEFS
Figure 9-10 The extent of coral reefs.
Coral reefs are the oldest ecosystems and important land builders in the tropics, forming entire chains of islands and altering the shorelines of the continents. Over geologic time, corals have built massive reefs of limestone. The reefs are limited to clear, warm, sunlit tropical oceans such as the Indo-Pacific and western Atlantic (Fig.9-10). Hundreds of atolls comprising rings of coral islands that enclose a central lagoon dot the Pacific. The islands consists of reefs several thousand feet across, many of which formed on ancient volcanic cones that have vanished below the waves.
The coral-reef environment supports more plant and animal species than any other habitat. The key to this prodigious growth is the unique biology of corals, which plays a vital role in the structure, ecology, and nutrient cycles of the reef community. Coral reef environments have among the highest rates of photosynthesis, nitrogen fixation, and limestone deposition of any ecosystem. The most remarkable feature of coral colonies is their ability to build massive calcareous skeletons, each weighing several hundred tons.
The coral polyp (Fig.9-11) is a soft-bodied, contractible animals, crowded with a ring of tentacles tipped with poisonous stringers that surround a mouthlike opening. The polyp lives in an individual skeletal cup, called a theca, composed of calcium carbonate. It extends its tentacles to feed at night and withdraws into the theca by day or during low tide to avoid drying out in the sun.
Figure 9-11 The coral polyp is a contractible animal that lives in an individual skeletal cup.
The corals exist in symbiosis (living toghter) with zooxanthellae algae within their bodies. The algae ingest the coral’s waste products and produce nutrients that nourish the polyps. Since the algae need sunlight for photosynthesis, corals are restricted to warm ocean waters less than 300 feet deep, with much of the coral growth occurring within the intertidal zone. Widespread coral reef building indicates the presence of warm, shallow seas with little temperature variation. Dense colonies of corals mark times when the temperature, sea level, and climate were to their liking.
Coral reefs forming in shallow water where sunlight can easily penetrate for photosynthesis contain abundant organic material. Over 90 percent of a typical reef consists of fine, sandy detritus, stabilized by plants and animals anchored to the reef surface. Tropical plant and animal communities thrive on the reefs due to the coral’s ability to build massive wave resistant structures. The major structural feature of a living reef is the coral rampart that reaches almost to the water’s surface. It consists of massive rounded coral heads and a variety of branching corals (Fig.9-12).
Figure 9-12 Fire coral growing near High Cay, Andros Island, Bahamas. Photo by R.Hasha, courtesy of U.S. Navy
Living on this framework are smaller, more fragile corals and large communities of green and red calcareous algae. Hundreds of species of encrusting organisms such as barnacles thrive on the coral framework. Large numbers of invertebrates and fishes hide in the nooks and crannies of the reef (Fig.9-13), some of them waiting until night before emerging to feed. Other organisms attach to virtually all available space on the under-side of the coral platform or on dead coral skeletons. Filter feeders such as sponges and sea fans occupy the deeper regions.
Figure 9-13 A species of angelfish swims among the rock and coral off Andros Island, Bahamas. Photo by P.Whitmore, courtesy of U.S.Navy
Fringing reefs grow in shallow seas and hug the coastline or are separated from the shore by a narrow stretch of water. Barrier reefs also parallel the coast but are farther out to sea, are much larger, and extend for longer distances. The best example is the Great Barrier reef, a chain of more than 2,500 coral reefs and small islands off the northeastern coast of Australia. It forms an underwater embankment more than 1,200 miles long, up to 90 miles wide, and as much as 400 feet high. It is one of the great natural wonders of the world and the largest feature built by living organisms. The Great Barrier Reef is a relatively structure, formed largely during the Pleistocene ice ages, when sea levels fluctuated with the growth of continental glaciers during the last 3 million years.
The fore reef is seaward of the reef crest, where coral blankets nearly the entire seafloor. In deeper waters, many corals grow in flat, thin sheets to maximize their light-gathering area for photosynthesis. In other parts of the reef, the corals form massive buttresses separated by narrow sandy channels composed of calcareous debris from dead corals, calcareous algae, and other organisms living on the coral reef. The channels resemble narrow winding canyons with vertical walls of solid coral. They dissipate wave energy, allowing the free flow of sediments to prevent the coral from choking on the debris.
Below the fore reef is the coral terrace, followed by a sandy slope with isolated coral pinnacles, then another terrace, and finally a near-vertical drop off into the darkness of the abyss. The rise and fall of sea levels during the last few million years have produced terraces that resemble a stair-step growth of coral, running up the side of an island or a continent. The drowned coral represent periods of extensive glaciation, when sea levels dropped by as much as 400 feet. In Jamaica, almost 30 feet of reef have built up since the present sea level stabilized some 5,000 years ago following the last ice age.
Coral reefs are centers of high biologic productivity, sustaining fisheries that are a major source of food in the tropical regions. Unfortunately, the spread of tourist resorts along coral coasts in many parts of the world is harming the productivity of these areas due mostly an increase in sedimentation. These developments are usually accompanied by increased sewage dumping, overfishing, and physical damage to the reef from construction, dredging, dumping, and landfills, along with the direct destruction of the reef for souvenirs and curios.
In areas like Bermuda, the Virgin Islands, and Hawaii, development and sewage outflows have caused extensive overgrowth and death of the reef by thick mats of algae. The algae suffocate the coral by supporting the growth of oxygen-consuming bacteria, particularly in the winter, when the algal cover on shallow reefs is high. This action results in the death of the living coral and the eventual destruction of the reef and the biological communities it supports.
Increasing ocean temperatures induced by a possible global warming are bleaching many reefs, causing the corals to turn ghostly white due to the expulsion of algae from their tissues. The algae aid in nourishing the corals, and their loss poses a great danger to the reefs. Foraminifera (Fig.9-14), tiny marine organisms whose skeletons preserve much of the record of behavior of the ocean and climate, exhibit damage similar to that observed in bleached coral reefs. Along with corals, they play an important role in the global ecosystem wherein bleaching could seriously affect the marine food chain.
Figure 9-14 Fossil foraminirfera of the North Pacific Ocean. Photo by B.P.Smith, courtesy of USGS
9.5 THE VENT CREATURES
On the crest of the East Pacific Rise south of Baja, California, 8,000 feet beneath the ocean lies an eerie world that time forgot. In volcanically active fields, a habit unlike any other on Earth contains species previously unknown to science that thrive in total darkness near hydrothermal vents. The undersea geysers build forests of exotic chimneys, called black smokers, that spew out hot water blackened with sulfur compounds (Fig.9-15).
The black smokers support some of the world’s most bizarre biology. Flourishing among the hydrothermal vents are perhaps the strangest animals ever encountered. Life might even have originated around such vents, obtaining from the Earth’s hot interior all the necessary nutrients to survive. In such an environment, the evolution of life could have begun as early as 4.2 billion years ago.
Figure 9-15 Hydrothermal vents on the deep ocean floor provide nourishment and heat for bottom-dwellers. Courtesy of USGS
Seawater seeping downward near magma chambers acquires heat and minerals and is expelled through fissures in the ocean floor. The hydrothermal vents not only maintain the bottom waters at livable temperatures, upward of 20 degrees Celsius, they also provide valuable nutrients, making this the only environment totally independent of the sun as a source of energy, which instead comes from the Earth itself.
Clustered around the hydrothermal vents are large communities of unusual animals (Fig. 9-16) as crowded as the tropical rain forests. Large white clams up to a foot long and mussels, having no need for skin pigments, nestle between black pillow lava. Giant white crabs scamper blindly across the volcanic terrain, and long-legged marine spiders and tiny octopuses roam the ocean floor. Living in total darkness, these species have no need for eyes, which have become useless appendages. Clusters of giant tubeworms up to 10 feet tall sway in the hydrothermal currents. The tubeworms are contractable animals that live inside long, white stalks up to 4 inches wide. While feeding, the tubeworm exposes a long bright red plume abundantly supplied with blood that is also a delicacy for the crabs, which climb the stalks to obtain a meal.
Figure 9-16 Tall tube worms, giant clams, and large crabs on the deep-sea floor near hydrothermal vents.
In the Atlantic, the vents are dominated by swarms of small shrimps. They originally were thought to be blind, until it was later discovered that the shrimps had an unusual pair of eyes on their backs instead of in the front of their heads. Apparently they can see using the feeble light emanating from the hot water chimneys. Biologists are intrigued by the deep-sea light because of the possibility that organisms can harness this energy using a type of photosynthesis totally independent of the sun.
Most remarkable is the fact that the vent animals do not obtain their nutrients in the form of detrital material falling from above, but rely on symbiosis with sulfur-metabolize bacteria that live within the host’s tissues. The bacteria metabolize sulfur compounds in the hydrothermal water by chemosynthesis. They harness energy liberated by the oxidation of hydrogen sulfide from the vents to incorporate carbon dioxide for the production of organic compounds such as carbohydrates, proteins, and lipids. The byproducts of the bacteria’s metabolism are absorbed into the host animal and nourish it.
Some animals also feed on bacteria directly. Odd-looking colonies of bacteria – some with long tendrils that sway in the warm currents – become the feeding grounds for complex forms of higher life. Some regions are clouded by drifting bits of whitish bacteria that swirl like falling snow. Occasionally, clumps of waving bacteria break loose from fissures and join the swirl of biologic “snowfall”. The vent animals are so dependent on the bacteria that the mussels have only a rudimentary stomach and the tubeworms lack even a mouth.
The animals live precarious lives. The hydrothermal vent systems turn on and off sporadically, and species can only survive as long as the vents continue to operate – perhaps for only a few years. To illustrate this point, isolated piles of empty clamshells bear witness to local mass fatalities. In new basalt fields, the vent creatures soon establish residency around young hydrothermal vents, and the once-barren abyss its rapidly colonized.
9.6 THE INTERTIDAL ZONE
The constant waxing and waning of the tides is responsible for the prodigious growth in the intertidal zone, the habitat area between high and low tides (Fig.9-17a&b). Most inhabitants of the intertidal zone have biological clocks set to the rhythm of the lunar day. The rhythms are characterized by repetitive behavioral or physiological events, such as feeding, that are synchronized with the tides. Each lunar day, which is about 25 hours long, generally has two tides, producing bimodal lunar-day rhythms, as compared to the unimodal solar-day rhythms of organisms attuned to the 24-hour solar day.
Figure 9-17a Intertidal exposure of chaotic blocks of sandstone near Piller Point, Clallan County, Washington. Photo by W.O.Addicott, courtesy of USGS
Figure 9-17b High and low tide with respect to mean sea level and elevation.
Biological clocks are important aids to survival by giving advance warning of regular changes in the environment, such as nightfall or the return of the tides. Even under constant laboratory conditions without the effects of diurnal or tidal cycles, biological clocks continue to function and the tidal rhythms persist for some time.
Apparently, the tidal rhythms are not learned or impressed on the organisms by the tides themselves. Crabs raised in the laboratory and exposed only to diurnal conditions exhibit a distinct tidal component in their activity after their temperatures are lowered. Also, crabs taken from areas not subject to tides and moved to a tidal flat quickly establish a tidal rhythm. Apparently the clock that measures the tidal frequency is innate, and only needs to be activated by an outsides stimulus.
Rhythmic behavior in organisms is also an expensive of the genetic code. Heredity decides whether an animals is active during high or low tide. The environment also plays an important role in the establishment of a tidal rhythm. The schedule of the tides only determines the setting of the biological clock. Therefore, animals transported to a different ocean synchronize their clocks to the new tidal conditions. Moreover, the pounding surf shapes the activity patterns of inhabitants living on beaches exposed to the open sea.
Intertidal organisms living in protected bays are not as exposed to the vicissitudes of the sea and are controlled by more subtle conditions, such as a drop in temperature or pressure changes introduced by the incoming tides, which help set their tidal rhythms. Even in the absence of outside stimuli, the biological clock continues to run accurately but no longer controls the organism’s activity. It operates independently from tidal influences until the organisms returns to the sea and the clock takes over again. Like all clocks, the accuracy of biological clocks is not altered by changes in the environment, nor do they pertain to intertidal organisms alone but also to the entire spectrum of life.
10RARE SEAFLOOR FORMATIONS
The floor of the ocean is host to a myriad unique geologic structures found nowhere else on Earth. Unusual seamounts associated with deep-sea trenches erupt cold mud instead of hot lava. Scattered around the midocean ridges are remarkable volcanic deposits, including piles of pillow lavas, forests of black smokers, and undersea geysers that eject vast quantities of hot water rising toward the surface in massive plumes.
Submarine slides larger than any landslide gouge out deep chasms in the ocean floor and deposit enormous heap of sediment on the bottom of the sea. The active seafloor sports a variety of depressions, including sea caves, blue holes, gas blowouts, calderas, and numerous craters formed by undersea explosions. Large meteorites or comets falling into the sea blast deep craters in the ocean floor, many of which are better preserved than their landward counterparts.
10.1 MUD VOLCANOES
In the Pacific Ocean, about 50 miles west of the Mariana Trench, the world’s deepest depression, lies a cluster of large seamounts 2.5 miles below the surface of the sea in a zone about 600 miles long and 60 miles wide. The undersea mountains were built not by hot volcanic rock like most Pacific seamounts but by cold, plastic serpentine, which is a soft, mottled green rock similar to the color of a serpent, hence its name. Serpentine is a low-grade metamorphic rock and the main mineral is asbestos. It originates from the reaction of water with olivine, an olive-green, iron and magnesium-rich silicate and a major constituent of the upper mantle.
The erupting serpentine rock flows down the flanks of the seamounts like lava from a volcano and forms gently sloping structures. Many of these seamounts rises more than a mile above the ocean floor and measures as much as 20 miles across at the base, resembling broad shield volcanoes such as Mauna Loa, which built the island of Hawaii. Drill cores taken during the international Ocean Drilling Program in 1989 showed that serpentine not only covered the tops of the seamounts but also filled the interiors.
Several smaller seamounts only a few hundred feet high are mud volcanoes, looking something like those in hydrothermal areas on land (Fig.10-1). They are composed of mounds of remobilized sediments formed in association with hydrocarbon seeps, where petroleumlike substances ooze out of the ocean floor. Apparently, sediments rich in planktonic carbon are “cracked” into hydrocarbons by the heat of the Earth’s interior. Even cores recovered around hydrothermal fields smell strongly of diesel fuel.
Figure 10-1 Mud volcanoes and acidulated ponds northwest of Imperial Junction, Imperial County, California. Photo by Mendenhall, courtesy of USGS
The mud comprise peridotite that is converted into serpentine and ground down into rock floor flour by movement along underlying faults called fault gouge. The mud volcanoes appear to undergo pulses of activity, interspersed with long dormant periods. Many seamounts formed recently (in geologic parlance), probably within the last million years or so.
The Mariana seamounts might be diapirs similar to the salt diapirs of the Gulf of Mexico, which trap oil and gas. The diapirs appear to be composed of the mantle rock peridotite altered by interaction with fluids distilled from the subducted portion of the Pacific plate as it descends into the Mariana Trench and slides under the Philippine plate. Fluids expelled from the subducting plate react with the mantle rock, transforming portions of the mantle into low-density minerals that rise slowly through the subduction zone to the seafloor.
About 90 million years ago, the Mariana region forward of the island arc consisted of midocean ridge and island arc basalts that have been eroding away by as much as 400 miles by plate subduction over the last 50 million years. The seamount-forming process has been proceeding for perhaps 45 million years, as oceanic lithosphere vanishes into the subduction zone, distilling enormous quantities of fluids from the descending plate. The fluids reacting with the surrounding mantle produce blobs of serpentine that rise to the surface through fractures in the ocean floor.
The fluid temperatures in subduction zones are cool, compared to those associated with midocean ridges where hydrothermal vents eject high-temperature black effluent (Fig.10-2). But instead of comprising heavy minerals like the black smokers of the East Pacific Rise and other midocean ridges, the ghostly white chimneys of the Mariana seamounts are composed of aragonite, a calcium carbonate mineral that normally dissolves in seawater at these great depths. Hundreds of corroded and dead carbonate chimneys are strewn across the ocean floor in wide “graveyards”.
Figure 10-2 A black smoker on the East Pacific Rise. Photo by R.D.Ballard, courtesy of WHOI
Apparently, cool water emanating from beneath the seafloor surface allows the carbonate chimneys to grow and avoid dissolution by seawater. Many carbonate chimneys are thin, and they are generally less than 6 feet high. Other chimney structures are thicker and taller and occasionally coalesce to form ramparts encrusted with a black manganese deposit. Small manganese nodules are also scattered atop many of the mountains of mud.
Exotic terranes are fragments of oceanic lithosphere originating from distant sources and exposed on the continents and islands in zones where plates collide. Many terranes contain large serpentine bodies that are similar in structure to the Mariana seamounts. Their presence is a constant reminder that the ocean floor was highly dynamic in the past and continues to be so today.
10.2 SUB-SEA GEYSERS
Perhaps the strangest environment on Earth lies on the ocean floor in deep water near seafloor spreading centers such as the crests of the East Pacific Rise and the Mid-Atlantic Ridge, which are portions of the Earth’s largest volcanic system. Solidified lava lakes hundreds of feet long and up to 20 or more feet deep probably formed by rapid outpourings of lava. In places, the surface of a lava lake has caved in, forming a collapse pit (Fig. 10-3).
Figure 10-3 The rim of a lava lake collapse pit on the Juan de Fuca Ridge in the east Pacific. Courtesy of USGS
Seafloor spreading is often described as a wound that never heals as magma slowly bleeds from the mantle in response to diverging lithospheric plates. During seafloor spreading, magma rising out of the mantle solidifies on the ocean floor, producing new oceanic crust. At the base jagged basalt cliffs is evidence of active lava flows and fields strewn with pillow lava formed when molten rock ejects from fractures in the crust and quickly cools by the deep cold water.
Lava erupting from undersea volcanoes continuously forms new crust along the midocean ridges, as lithospheric plates on the sides of the rift inch apart and molten basalt from the mantle slowly rises to fill the gap. Occasionally, a colossal eruption of lava along the ridge crest flows downslope for more than 10 miles. Most of the time, however, the basalt just oozes out of the spreading ridges, forming a variety of lava structures on the ocean floor.
The ridge system exhibits many uncommon features, including massive peaks, sawtooth ridges, earthquake fractured cliffs, deep valleys, and lava formations of every description (Fig.10-4). Lava formations associated with midocean ridges consist of sheet flows and pillow, or tube flows. Sheet flows are more common in the active volcanic zone of fast spreading ridge segments like those of the East Pacific Rise, where the plates are separating at a rate of 4 to 6 inches a year.
Figure 10-4 Cluster of blue worms and sulfide deposits around hydrothermal vents near the Juan de Fuca Ridge. Courtesy of USGS
Pillow lavas (Fig.10-5) erupt as though basalt were squeezed out of a giant tooth paste tube. They generally arise from slow spreading centers, such as those of the Mid-Atlantic Ridge, where plates spread apart at a rate of only about an inch per year, and the lava is much more viscous. The surface of the pillows often contains corrugations or small ridges, indicating the direction of flow. The pillow lavas typically form small, elongated hillocks, pointing downslope.
In rapidly spreading rift systems such as the East Pacific Rise south of Baja, California, hydrothermal vents built prodigious forests of exotic chimneys up to 30 feet tall.
Seawater seeping through the oceanic crust acquires heat near magma chambers below the rifts and is expelled with considerable force through vents as though they were undersea geyser. (The term geyser originates from the Icelandic word geysir, meaning “gusher”.)
Figure 10-5 Pillow lava on the ocean floor. Courtesy of WHOL
The hydrothermal water is up to 400 degrees Celsius but does not boil because, at this great depth, it is under a pressure of 200 to 400 atmospheres. The superhot water is rich in dissolved minerals, such as iron, copper, and zinc, that precipitate out upon contact with the cold water of the abyss. The sulfide minerals ejected from hydrothermal vents build tall chimney structures, some with branching pipes. The black sulfide minerals drift along in the ocean currents like thick effluent from factory smokestacks.
About 750 miles southwest of the Galapagos islands, along the undersea mountain chain that comprises the East Pacific Rise (Fig.10-6), lies an immense lava field that recently erupted. The eruption appears to have started near the crest and flowed downslope over cliffs and valleys for more than 12 miles. The volume of erupted material was nearly 4 cubic miles spreaded over an area of some 50,000 acres, which is about half the total annual production of new basalt on the seafloor worldwide. It is enough lava to pave the entire U.S. interstate highway system to a depth of 30 feet. Although not the greatest eruption in geologic history, this could well be the largest basalt flow in historic times. Associated with these huge bursts of basalt are megaplumes of warm, mineral-laden water, measuring up to 10 miles or more across and thousands of feet deep.
Figure 10-6 Location of the East Pacific Rise.
The submersible Alvin (Fig.10-7), launched from the oceanographic research vessel AtlantisⅡ,is the workforce for exploring the ocean floor. In April 1991, oceanographers aboard Alvin witnessed an actual eruption or its immediate aftermath on the East Pacific Rise about 600 miles southwest of Acapulco, Mexico. The scientists realized that the seafloor had recently erupted because the scenery did not match photographs taken at the same location 15 months earlier.
The scene showed recent lava eruptions that sizzled a community of tubeworms and other animals living on the deep ocean floor 1.5 miles below the sea. Suspended particles turned seawater near the seafloor extremely murky, and prodigious streams of superhot water poured from the volcanic rocks, where lava flows scorched tubeworms that had not yet decayed. A few partially covered animal colonies still clung to life, while hordes of crabs fed on the carcasses of dead animals.
Figure 10-7 An artist’s rendition of the deep-submersible Alvin. Courtesy of U.S.Navy
A huge undersea eruption on the Juan de Fuca Ridge about 250 miles off the Oregon coast poured out batches of lava, creating new oceanic crust in a single convulsion. The ridge forms a border between the huge Pacific plate to the west and the tiny Juan de Fuca plate to the east (Fig. 10-8). Eruptions along the ridge occur when the two plates separate, allowing molten rock from the mantle to rise to the seafloor surface, forming new crust. Over time, the process of seafloor spreading carries older oceanic crust away from the ridge.
The young volcanic rocks included pillow lavas and shiny, bare basalt lacking any sediment cover. Water warmed to 50 degrees Celsius seeped out of cracks in the freshly hardened basalt. In some places, tubeworms had already established residency around thermal vents. The eruption appeared to be related to two megaplumes discovered in the late 1980s. A string of basaltic mounds more than 10 miles long erupted on a fracture running between the sites of the two megaplums. The hot hydrothermal fluids, along with fresh basalt, gush out of the ocean floor when the ridge system cracks open and churns out more new crust.
A field of seafloor geysers off the coast of Washington State expels into the near-freezing ocean hot brine at temperatures approaching 400 degrees Celsius. Massive undersea volcanic eruptions from fissures on the ocean floor at spreading centers along the East Pacific Rise create large megaplumes of hot water. The megaplumes result from bursts of intense volcanic activity and can measure up to several tens of miles wide.
Figure 10-8 Location of volcanic site on the Juan de Fuca Ridge.
The ridge apparently splits open and spills out hot water while lava erupts in an act of catastrophic seafloor spreading. In a matter of a few hours, or at most a few days, more than 100 million cubic yards of superheated water gushes from a large fracture in the oceanic crust up to several miles long. When the seafloor ruptures, vast quantities of hot water held under great pressure beneath the surface violently rush out, creating colossal plumes. The release of massive amounts of superheated water beneath the sea might explain why the ocean remains salty.
10.3 SUBMARINE SLIDES
The deep sea is not nearly as quiet as it seems. The constant tumbling of seafloor sediments down steep banks churns the ocean bottom into a murky mire. The largest slides occur on the sea floor, and as many as 40 giant submarine slides are located near U.S. coasts. Submarine slides moving down steep continental slopes have buried undersea telephone cables under a thick layer of rubble. Sediments eroding out from beneath the cable leave it dangling between uneroded areas of the seabed, causing the cable to fail. A modern slide that broke a submarine cable near Grand Banks, south of Newfoundland, moved downslope at a speed of about 50 miles per hour – comparable to large terrestrial slides that can devastate the landscape.
Undersea flow failures also generate large tsunamis overrun parts of the coast. In 1929, an earthquake on the coast of Newfoundland set off a large undersea slide that triggered a tsunami, killing 27 people. On July 3, 1992, apparently a large submarine slide sent a 25-mile-long, 18-foot-high wave crashing down on Daytona Beach, Florida, overturning automobiles and injuring 75 people.
Submarine slides carve out deep canyons in continental slopes. They consist of dense, sediment-laden water that moves sediments swiftly along the ocean floor. These muddy waters, called turbidity current (Fig.10-9), travel down continental slopes and transport immensely large blocks. Turbidity currents are also initiated by river discharge, coast storms, or other currents. They deposit huge amounts of sediment that build up the continental slopes and the flat-lying abyss below.
Figure 10-9 An underwater river of sediment-laden water, called a turbidity current. Photo by R.F.Dill, courtesy of U.S.Navy
The continental slopes plunge thousands of feet to the ocean floor, inclined at steep angles of 60 to 70 degrees. Sediments reaching the edge of the continental shelf slide off the continental slope by the pull of gravity. Huge masses of sediment cascade down the continental slope by gravity slides, gouging out steep submarine canyons and depositing great heaps of sediment. Such undersea slides are often as catastrophic as landslides and move massive quantities of sediment downslope in a matter of hours.
Submerged deposits near the base of the main island of Hawaii rank among the greatest slides on Earth. On the southeast coast of Hawaii, on Kilauea Volcano’s south flank, about 1,200 cubic miles of rock is slumping toward the sea at a breakneck speed in geologic terms – up to 10 inches per year. It is the biggest mass on Earth that is moving in this fashion. Ultimately, catastrophic collapse will occur, far more destructive than any of the volcano’s eruptions.
Figure 10-10 Devastation from the 1980 eruption of Mount St. Helens, showing extensive ice and rock debris in the foreground. Courtesy of NASA
On the ocean floor lies evidence that great chunks of the Hawaiian islands had once slid into the sea. By far the largest examples of an undersea rock slide is along the flank of a Hawaii volcano. The slide measured roughly 1,000 cubic miles in volume and spread some 125 miles from its point of origin. The collapse of the island of Oahu sent debris 150 miles across the deep-ocean floor, churning the sea into gargantuan waves. When part of Mauna Loa Volcano collapsed and fell into the sea about 100,000 years ago, it created a tsunami 1,200 feet high that was not only catastrophic to Hawaii but might even have caused damage along the Pacific coast of North America.
The bottom of the rift valley of the Mid-Atlantic Ridge holds the remnants of a vast undersea slide at a depth of 10,000 feet, surpassing in magnitude any landslide in recorded history. A large scar on one side of the submarine volcanic range indicates that the mountainside gave way and slid downhill at a tremendous speed, running up and over a smaller ridge farther downslope in a matter of minutes. The slide carried nearly 5 cubic miles of rock debris, or six times more than the 1980 Mount St. Helens landslide, the largest in modern history (Fig.10-10). The slide appears to have occurred about 450,000 years ago, possibly creating a gigantic sea wave 2,000 feet high.
10.4 SEA CAVES
Caves are pounded into existence by ocean waves, plowed open by flowing ice, or arise out of lava flows. They are the most spectacular examples of the dissolving power of groungwater. Over time, acidic water flowing underground dissolves large quantities of limestone, forming a system of large chambers and tunnels. Caves develop from underground channels that carry out water that seeps in from the water table. This creates an underground stream similar in structure to streams on the surface that flow from a breached water table. The limestone landforms resulting from this process are known as karst terrains, after a region in Slovenia famous for its caves.
Caves also develop in sea cliffs (Fig.10-11) by the ceaseless pounding of the surf or by groundwater flow through an undersea limestone formation hollowed out as the water empties into the ocean. Wave action on limestone promontories with zones of different hardness creates sea arches. A major storm at sea erodes the tall cliffs landward several tens of feet. Sometimes the pounding of the surf punches a hole in the chalk to form a sea arch.
In the jungle on Mexico’s Yucatan Peninsula is a bizarre realm of giant caverns links by miles of twisting passage a hundred feet below the sea. The karst terrain gives birth to underwater caves and sinkholes. The sinkholes form when the upper surface of a limestone formation collapses, exposing the watery world below. The underlying limestone is honeycombed with long tunnels, some several miles in length, and huge caverns that could easily hold several houses.
Figure 10-11 Sea cave cut into siltstone, Chinitna district, Cook lnlet region, Alaska. Photo by A.Grantz, courtesy of USGS
Just like surface caves, the Yucatan caverns contain a profusion of icicle-shaped formations of stalactites hanging from the ceiling and stalagmites rising from the floor. The formation also include delicate, hollow stalactites called soda straws that took millions of years to create. Fish, crustaceans, and other small, primitive creatures, blind as a result of generations without light, marking eyes useless, live in the darkness recesses of the caves.
Blue holes are submerged sinkholes in the sea that appear dark blue because of their great depth. Many blue holes dot the shallow waters surrounding the Bahama Islands southwest of Florida. They formed during the last ice age, when the ocean fell by several hundred feet, exposing parts of the ocean floor well above sea level. The sea lowered in response to the growing ice sheets that covered the northern regions of the world, locking up huge quantities of the world’s water.
During its exposure on dry land, acidic rainwater seeping into the seabed dissolved the limestone bedrock, creating immense subterranean caverns. Under the weight of the overlying rocks, the roofs of the caverns collapsed, forming huge gaping pits (Fig.10-12). At the end of ice age, when the ice sheets melted, the sea inundated the area and submerged the sinkholes. Blue holes can be very treacherous because they often have strong eddy currents or whirlpools that are particularly hazardous to small boats.
Figure 10-12 Possibly the nation’s largest sinkhole, measuring 425 feet long, 350 feet wide, and 150 feet deep, in Shelby County, central Alabama. Courtesy of USGS
10.5 SEAFLOOR CRATERS
Because water covers over 70 percent of the Earth’s surface, most meteorites land in the ocean, and several sites on the seafloor are possible marine impact craters. An asteroid or comet landing in the ocean would produce a conical-shaped curtain of water as billions of tons of seawater splashes high into the air. The atmosphere would become over saturated with water vapor, and thick cloud banks would shroud the planet, blocking out the sun. Massive tsunamis would race outward from the impact site and traverse clear around the world. When striking seashores, they would travel hundreds of miles inland, devastating everything in their paths. About 65 million years ago, a large meteorite supposedly struck the Earth, creating a crater at least 100 miles wide whose debris sent the planet into environmental chaos. This catastrophe might have caused the demise of the dinosaurs along with 70 percent of all other species. The actual crater has yet to be found, suggesting that the meteorite landed in the ocean. If so, millions of years of seafloor subduction would have erased all sighs of it.
Much of the search for the dinosaur-killer impact site has been concentrated around the Caribbean area (Fig.10-13), where thick accumulations of wave-deposited rubble exist along with melted and crusted rock apparently ejected from a crater. The most suitable site for the crater is the 110-mile-wide Chicxulub structure, the largest known on Earth. It lies beneath 600 feet of sedimentary rock on the northern coast of the Yucatan Peninsula. If the meteorite landed on the seabed just offshore, 65 million years of sedimentation would have long since buried it under thick deposits of sand and mud. Furthermore, a splashdown in the ocean would have created an enormous sea wave, or tsunami, that would have scoured the seafloor and deposited its rubble on nearly shores.
Figure 10-13 Locations of possible impact structures in the Caribbean area that might have ended the Cretaceous period.
The most pronounced undersea impact crater known is the 35-mile-wide Montagnais structure 125 miles off the southeast coast of Nova Scotia (Fig.10-14). Oil companies exploring for petroleum in the area discovered the circular formation. The crater is 50 million years old and closely resembles craters on dry land, except that its rim is 375 feet beneath the sea and the crater bottom is 9,000 feet deep. A large meteorite up to 2 miles wide excavated the crater. The impact raised a central peak similar to those seen inside craters on the moon.
Figure 10-14 Location of the Montagnais crater off Nova Scotia, Canada.
The impact structure also contained rocks melted by a sudden shock. Such an impact would have sent a tremendous sea wave crashing down on nearly shores. Because of its size and location, the crater was thought to be a likely candidate for the source of tektites (small, glassy bodies) strewn across the American West. Unfortunately, its age is several million years too young to have created the North American tektites. However, the ocean is vast, and better candidates might some day reveal themselves.
A meteorite slamming into the Atlantic Ocean along the Virginia coast about 40 million years ago released a huge wave that pounded the adjacent shoreline. Apparently, the tsunami gouged out of the seafloor a 5,000-square-mile region about the size of Connecticut. When the meteorite crashed into the submerged continental shelf, it created a wave that ripped the seafloor into an enormous number of large boulders. A 200-foot-thick layer of 3-foot boulders was deposited in three locations, buried under 1,200 feet of sediment. Within the boulder layer are mineral grains showing shock features and glassy rocks called tektites that formed when a meteorite blasted the seafloor and flung the molten rock in all directions.
A large meteorite impact might have created the Everglades at the southern tip of Florida. The Everglades is a swamp and forested area surrounded by an oval-shaped system of ridges upon which rests most of southern Florida’s cities. A giant coral reef, dating about 6 million years old, lies beneath the rim surrounding the Everglades. The coral reef probably formed around the circular basin gouged out by the meteorite impact. A thick layer of limestone surrounding the area and laid down about 40 million years ago is suspiciously missing over most of the southern part of the Everglades. Apparently, a large meteorite slammed into limestones submersed under 600 feet of water and fractured the rocks. The impact also would have generated an enormous tsunami and swept the debris far out to sea.
About 2.3 million years ago, a major asteroid appears to have impacted on the ocean floor in the Pacific Ocean roughly 700 miles west of the tip of South America. Although no crater has been found, an excess of iridium (a rare isotope of platinum found in abundance on meteorites) in sand-size bits of glassy rock existed in the area, suggesting an extraterrestrial origin. The impact created at least 300 million tons of debris, consistent with an object about a half mile in diameter. The blast from the impact would have been equal to that of all the nuclear arsenals in the world today, with devastating consequences for the local ecology. Moreover, geologic evidence suggests that the Earth’s climate cooled dramatically between 2.2 and 2.5 million years ago, when glaciers covered large parts of the Northern Hemisphere.
10.6 UNDERSEA EXPLOSIONS
Figure 10-15 Caldera formed by the explosion of Thera.
The most explosive volcanic eruption in recorded history occurred during the 17th century B.C. on the island of Thera, 75 miles north of Crete in the Mediterranean Sea. The magma chamber beneath the island apparently flooded with seawater, and like a gigantic pressure cooker the volcano blew its lid. The volcanic island collapsed into the emptied magma chamber, forming a deep water-filled caldera that covered an area of 30 square miles (Fig.10-15). The collapse of Thera also created an immense sea wave that battered the shores of the eastern Mediterranean.
Krakatoa lies in the Sundra Strait between Java and Sumatra, Indonesia. On August 27, 1883, a series of four powerful explosions ripped the island apart. The explosions were probably powered by the rapid expansion of steam, generated when seawater entered a breach in the magma chamber. Following the last convulsion, most of the island caved into the emptied magma chamber and created a large undersea caldera more than 1,000 feet below sea level, resembling a broken bowl of water with jagged edges protruding above the surface of the sea.
Figure 10-16 The test of the first hydrogen bomb, “Mike,” on Elugelab Atoll on November 1, 1955. Courtesy of Defense Nuclear Agency
The first hydrogen bomb test was conducted on November 1,1952, on Elugelab atoll, in the Eniwetok lagoon in the South Pacific. The nuclear device was named “Mike” and measured 22 feet long, 5 feet wide and weighed about 65 tons, with an explosive force estimated at 10 megatons of TNT. When Mike was detonated (Fig.10-16), the fireball expanded to more than 3 miles in diameter in less than a second. Millions of gallons of seawater instantly boiled into steam. After the clouds cleared, Elugelab was no more. A huge crater was blown in the ocean floor 1 mile wide and 1,500 feet deep.
Another type of crater on the bottom of the ocean forms by a natural seafloor explosion. In 1906, sailors in the Gulf of Mexico witnessed a massive gas blowout that sent mounds of bubbles to the surface. The area is known for its reservoirs of hydrocarbons that might have caused the explosion. Pockets of gases lie trapped under high pressure deep beneath the floor of the ocean. As the pressure increases, the gases explode undersea, spreading debris in all directions and producing huge craters on the ocean floor. The gases rush to the surface in great masses of bubbles that burst in the open air, resulting in a thick foamy froth on the surface of the ocean.
Further exploration of the site yielded a large crater on the ocean floor, lying in 7,000 feet of water southeast of the Mississippi River Delta. The elliptical hole measured 1,300 feet long, 900 feet wide, and 200 feet deep and sat atop a small hill. Downslope lay more than 2 million cubic yards of ejected sediment. Apparently, gases seeped upward along cracks in the seafloor and collected under an impermeable barrier. Eventually, the pressure forced the gas to blow off its cover, forming a huge blowout crater.
In the Gulf of Mexico, as well as in other parts of the world, the seabed overlies thick salt deposits formed when the sea evaporated during a warmer climate. The bottom of the Gulf is lined with a layer of anhydrite, an anhydrous (water-saturated) calcium sulfate common in evaporite deposits. The anhydrite forms impervious stratum to the buildup of gas beneath the surface.
When the building gas pressure overcomes the barrier, gases rush toward the surface, forming a froth on the open ocean. A ship sailing into such a foamy sea would suddenly lose all buoyancy because it would no longer be supported by seawater, and would immediately sink to the bottom. An airplane flying overhead might stall out, its engines choking on the pall of poisonous gases. Perhaps these phenomena might explain the strange disappearances of ships and aircraft in the Caribbean around a region known as the Bermuda Triangle (Fig.10-17), one of many unsolved mysteries of the sea.
Figure 10-17 The Bermuda Triangle, connecting Miami, Florida, Puerto Rico, and Bermuda, has been blamed for mysterious disappearances of ships and planes.