Abyssal plain

The abyssal plains of the oceans lie beyond the continental margins at depths greater than 2,000 meters. They are thought to be the flattest areas on Earth and are carpeted with thick layers of sediment. Their greatest economic value lies in the metallic minerals that form part of these sediments.

Deep Ocean Floor

The abyssal plains of the deep ocean floor represent the flattest surface areas on Earth. They are far flatter than any plain on land. Geologists define an abyssal plain as having a slope ratio of less than 1:1,000. Abyssal plains occupy about 40 percent of the ocean basin floor and are widespread in the major ocean basins, the Gulf of Mexico, and the Mediterranean Sea. The peculiar topography of the abyssal plains results from deep sediments deposited by turbidity currents. Additional sediments are derived from the rain of biological material from the surface. Mariners and researchers once believed that the entire ocean basin beyond the continental margin was a flat, featureless plain. Subsequent studies using sonic devices, deep-sea cameras, submersibles, and other instruments revealed a rugged, varied topography over most of the ocean floor. The research also supported the conclusion that abyssal plains represent less than one-half the area of all the ocean basins.

It was, however, not always thus. As the continents drifted apart and the ocean basins were formed following the breakup of the supercontinent Pangaea about 150 million years ago, the ocean floor was well-contoured. The dominant feature of what are now the abyssal plains was broad areas of low hills. Weathering of the continental landmasses produced abundant sedimentary material that eroded into the oceans. The coarse material settled on the continental shelves and partly on the continental slopes. The fine material drifted farther offshore and settled on the continental rise and the adjacent abyssal hills. Over time, the hills were covered by sediment to become the abyssal plains. Remnants of the ancient terrain exist beyond the abyssal plains in the form of abyssal hills less than 1,000 meters high, steep-sided seamounts, and flat-topped seamounts. Seamounts frequently jut above the ocean surface as islands. Guyots are seamounts that ocean waves have eroded. Also called table mounts, guyots may be 1,000 to 1,500 meters below the sea surface. In some areas of the ocean, the abyssal plains are cut by deep, narrow trenches whose bottoms may lie many kilometers below the sea's surface. The deepest is the Mariana Trench in the southwest Pacific. At approximately 11,000 meters below the surface, it is the deepest place on Earth.

Abyssal plains are most abundant in the Atlantic Ocean and Indian Ocean basins. They usually form next to the edges of the continental margins rather than near the centers of the basins. The Pacific Ocean basin features a few abyssal plains, but for the most part, the Pacific basin exhibits the relict, rough topography. There, the abyssal hills, thinly covered with sediment, rise 200 to 400 meters above the basin floor. Deep-sea drilling in the northeast Pacific has revealed basalt as the major rock type of the abyssal hills.

Several explanations have been offered for the thin sedimentation of the Pacific abyssal plains. One theory suggests that since there are relatively few large rivers that drain into the Pacific Ocean, transport of sediments is reduced. Therefore, the sediments deposited since the Pacific was formed were too sparse to bury the hilly topography. In addition, magmatic arcs formed in some areas, creating marginal seas where sediments discharged by rivers were trapped. Another explanation points to the submarine trenches as possible traps for the sediments where the trenches lie between the continental margins and the abyssal plains. Sediment-laden turbidity currents, flowing down the steep sides of the continental slopes, plunge into the trenches and dump the sediment load. With much of the sediment going into the trenches, little of it flows out over the plains. A third possibility is the powerful “storms” that sweep across the ocean floor in places, scouring it in some regions and reforming the sediments in others. Oceanographic instruments moored on the ocean floor at depths of 4,800 meters have detected massive bottom currents flowing at more than 0.5 meter per second. Labeled as storms by researchers, the turbulent conditions can rage for about one week, lifting loads of sediment and moving them elsewhere.

Sources of Sediment

Nearly fifteen billion tons of weathered rock is eroded annually from the land and carried by rivers and streams to the sea. Some of this material is deposited in vast deltas, such as those found at the mouths of the Mississippi and Amazon Rivers. A large proportion is trapped on the continental shelves. A few billion tons are transported over the great depths to settle on the abyssal plains. They are joined by a rain of calcareous or siliceous skeletons of microscopic drifting organisms called plankton, which are abundant in the upper, sunlit portion of the water. These biological remains add about three billion tons of sediment annually.

In addition, the sediment includes particles swept up by strong winds blowing over the world's deserts, like the Sahara. These particles travel great distances through the atmosphere and settle over the vast expanse of the oceans. They settle slowly to the bottom to form part of the carpet of sediment. Sediments may also be transported from the continental shelf onto the abyssal plain. This process has been detected in the Gulf of Mexico, where underwater landslides break off masses of sediment deposited by the Mississippi River. During times of low sea level, ocean waves breaking at the shelf edge cause the sediments to collapse and slide down the steep continental slope. From there, the sediments fan out in the deep ocean over the abyssal plain.

The thin carpet on the ocean floor includes mineral matter derived from various sources. It includes the ash erupted by volcanoes thousands of kilometers away and extraterrestrial material in the form of meteorites. Some minerals precipitate directly out of the seawater and accumulate as crystals or nodules, including manganese nodules. These valuable, mineral-rich nodules in places obscure the sediments beneath them. Because of the biological skeletons in them, these sediments have a very fine texture and are classified as oozes, which are named after the dominant organism represented: globigerina ooze, radiolarian ooze, and so on. The oozes are calcareous if the dominant organisms were foraminifera and pteropods (animals with chalky skeletons); they are siliceous if the remains were derived from radiolarians or diatoms.

Red clay is a very common sediment on the abyssal plains. It consists of the finest-grained particles eroded into the sea. It is the most durable of the sediment types on the ocean floor. Calcareous skeletons dissolve rapidly as they sink toward the floor and are scarce in the sediments. Siliceous skeletons are limited in their distribution. They are found mainly under the productive surface waters of the polar and equatorial zones.

Sediments accumulate very slowly on the abyssal plains. The amount and time involved ranges from a few centimeters to only a fraction of a millimeter per one thousand years. Clay particles accumulate at less than two millimeters every one thousand years; shells may accumulate at a rate of 20 millimeters every one thousand years.

Sediment Classification

Abyssal plain sediments are classified by geologists as terrigenous, biogenic, hydrogenous (or authigenic), and cosmogenic. The latter include meteorites and tektites, small rounded objects composed almost entirely of glass. Although their origin is unknown, tektites are believed to be meteoric.

Terrigenous sediments are the clays, sands, and gravels derived from the land. Most of them are lacustrine sediments; that is, they were transported by water as material eroded by rivers and streams. Some, however, were transported by glaciers during the Pleistocene epoch. Many sites of such sediments, including Georges Bank, off New England, and the Grand Bank, off Newfoundland, are the terminal moraines of such glaciers. During the thousands of years since they were deposited, these sandy moraines have been worked and reworked by ocean currents and waves and deposited farther out onto the abyssal plains. Some glacial deposits have been moved thousands of kilometers by floating icebergs. As the continental glaciers, such as on Greenland and Antarctica, travel toward the sea, they erode and transport sand, silt, and gravel. These are incorporated in the ice and, as the glacial ice breaks off into the sea as icebergs, the sediments are carried off by ocean currents. Ultimately, when the icebergs melt, they drop their mineral burdens into the sea to become part of the seafloor sediments. Many of the fine components of the terrigenous sediments, including the silts and clays, are aeolian, or windblown, sediments.

The biogenic sediments owe their origin entirely to materials derived from organisms in the water column. They include the tests, or shells and external skeletons, of phytoplankton and zooplankton. As the organisms die in the sunlit, or photic, zone, their remains fall to the ocean floor as “snow.” Many biogenic sediments are highly fossiliferous, or likely to contain fossils. These fossil-rich deposits enable scientists to date the sediments and to interpret ocean temperatures in the geologic past. Carbon-14 dating has also been used to establish the relative age of biogenic sediments where carbonaceous minerals are present.

Hydrogenous, or authigenic, sediments are those that have precipitated out of the seawater solution. These sediments are particularly common on abyssal plains in the Pacific. They include phosphorite nodules and manganese nodules. Some hydrogenous sediments are found near hot springs near mid-ocean ridges. The minerals in the metal-rich water spewing from the springs are carried by deep ocean currents across the seafloor. The metals precipitate out and add to the layering on the plain. The nodules grow by accretion around some hard nucleus, usually a pebble or a fossilized shark's tooth. The water temperature from the hot springs generally is about 10 to 15 degrees Celsius (compared to the ambient deepwater temperature of 2 degrees Celsius). Several extremely hot springs have been found with vent waters of about 300 degrees Celsius. Hot springs on the ocean floor near the Galápagos Islands are believed to be above a massive magma chamber. Here, Earth's molten mantle is estimated to be between 1,200 and 1,400 degrees Celsius. Some springs, really more like seeps, have been found oozing water at ambient temperature. These so-called cold seeps also release metal-rich water. Seamounts on the abyssal plain in the southwest Pacific Ocean basin feature cold springs spewing out mineral-laden water.

For decades, scientists considered the abyssal plains to be among the most unchanging environments on Earth. The water temperature is a near-constant 2 or 3 degrees Celsius year-round. The salinity is unvarying, and the darkness is constant and total. The plains are generally tectonically stable as well. They were, therefore, frequently looked on as a safe repository for various wastes. Nations dumped quantities of obsolete chemical and biological weapons and some nuclear materials on the abyssal plains, certain that they would remain there forever and do no harm to humans. As land dumps became unavailable, the abyss was even considered as a place to dump industrial and domestic wastes and high-level nuclear wastes. Research has demonstrated, however, that the abyss is not as unchanging as it was once believed to be.

Contributing to these changes is the increase in underwater drilling for rare metals. Dark oxygen, or oxygen produced without the use of sunlight, is critical for areas like abyssal plains that lie deep underwater on the dark ocean floor. When saltwater comes into contact with particular rare minerals, dark oxygen is created. However, because these rare elements are essential in the production of many twenty-first-century low-carbon energy technologies like solar panels and electric car batteries, mining has increased. In the Pacific Ocean's Clarion–Clipperton Zone between Hawaii and Mexico, researchers found this oxygen production process produces more oxygen than deep-sea creatures consume. Because about half of the oxygen on Earth comes from its oceans, this oxygen surplus is critical for marine ecosystems as well as all life on land. Mining the minerals that are critical in dark oxygen production from abyssal plains may irreversibly damage the oceans and have severe implications for climate change.

Ocean-Floor Exploration

The abyssal plain, of all the ocean's benthic features, is the only one that has borne out the theory that the ocean floor was a flat, featureless expanse. The few scattered soundings made toward the end of the nineteenth century for transoceanic telegraphic cables yielded small amounts of data. Later, the British oceanographic vessel Challenger made many soundings in areas not previously probed. During a global voyage, the Challenger's crew laboriously lowered long hemp ropes and, later, single-strand wire to great depths. The ropes and wires were retrieved slowly with capstans turned by the crew. Individual soundings of a few thousand meters often required several hours to complete. Sometimes, the hemp rope broke under its own weight, and the entire effort was wasted. Despite the obstacles encountered by researchers, however, data about the seafloor accumulated. As the nineteenth century ended, some ten thousand soundings had been made in water deeper than 2,000 meters and about five hundred in water deeper than 5,500 meters.

Information about the creatures of the abyssal plain was being gathered as well, although slowly. Retrieval of damaged submarine cables brought back a host of organisms attached to the cable sheathing. This evidence dismissed forever the belief in an “azoic zone,” a depth limit in the ocean below which no life existed. The Challenger also had deployed dredges and trawls that dragged over the ocean floor. Bizarre-looking fish, worms, and clamlike animals came up in the collecting gear, as did intriguing samples of the rocks and minerals that carpeted the abyssal plain. Further research revealed an abundance of lifeforms on the abyssal plains, most of which were new scientific discoveries. These collections, however, were made at great expense of time and equipment. The dredges and trawls, like the sounding lines, took hours to drop and retrieve. Frequently, the gear turned upside down and never collected anything. Sometimes, the lines and cables were twisted by swift currents and left in a hopeless tangle that, again, collected nothing. Modern oceanographic geologists continue to use dredges, although the hazards are much the same.

Further research on the Clarion Clipperton Zone using remotely operated vehicles (ROVs) photographed a never-before-seen cup-shaped glass sponge that scientists believe may live on the plain for up to 15,000 years. Similar findings include transparent unicumbers with visible intestines and long tails and pink or Barbie sea pigs with small feet for collecting food. The abyssal plains are also home to cactus urchins, squat lobsters, ancient coral, translucent squids and sea anemones, and cockatoo cucumber sea anemones. Many of these deep-water regions have yet to be fully explored, and research is ongoing.

Measurement of Sediment

Several different kinds of “grab” are used to scoop large samples of the sediments, but more intensive sampling is done with “corers.” These long pipes are dropped or pushed into the sediment to collect a cylinder of the material. Stretched out in the laboratory and cut lengthwise, the core samples expose the millennia-long history of the sediment. The texture, particle size, color, and chemistry of the sample can be measured and correlated with specific dates. Drills operated from surface ships have penetrated the rock under the abyssal plains to depths of more than 2,000 meters.

The depth of the sediments on the floor of the abyssal plains has been measured using various techniques. The oldest technique involved tossing sticks of dynamite into the ocean behind a moving vessel. Special instruments measured the time it took for the vibrations set off by the explosion to reach the ocean floor and bounce back. Since rock and sediment of differing materials reflected the vibrations differently, careful analysis of the echoes could reveal the nature of the sediments and their depth. The dangerous practice of using dynamite was replaced by the bouncing of harmless subsonic signals off the ocean floor. This same technique had long been used to measure the water's depth over the ocean floor. Remarkably detailed soundings of the ocean floor, with printouts of the surface features, are made with multiple scanning devices towed by research vessels. These devices include a multibeam system called Sea Beam and a special side-scanning device called Gloria. Satellites can survey the oceans in just a few days using a radar altimeter to produce maps of the ocean surface from which the seafloor topography can be deduced. This has made it possible to map features in areas not covered previously by ships.

Cameras—still, motion-picture, and video—loaded with black-and-white or color film have captured the features of the abyssal plains. They show ripple marks from submarine currents and the remains of ancient volcanic eruptions. This pictorial record is a valuable adjunct to the physical specimens collected.

Rock and sediment samples and photographs offer dramatic evidence of the nature of the abyss, but no technique surpasses sending humans to the bottom to make on-the-spot assessments of what is being collected or photographed. Several manned submersibles, such as the U.S. mini-submarine Alvin and its French counterpart Cyana, have carried researchers many kilometers into the depths of the sea. These craft have enabled their human passengers to view the ocean floor firsthand and to collect and photograph materials systematically. In addition, ROVs are being used extensively and may eventually supersede crewed vehicles.

Principal Terms

manganese nodules: lumps of minerals consisting mostly of iron, manganese, nickel, and copper that form on deeper parts of continental shelves

sediment: solid matter, either organic or inorganic in origin, that settles on a surface; it may be transported by wind, water, or glaciers

submarine canyon: a channel cut deep in the seafloor sediments by rivers or submarine currents

terrigenous: originating from the weathering and erosion of mountains and other land formations

trench: a long, narrow, and very deep depression in the ocean floor

turbidity current: a current resulting from a density increase brought about by increased water turbidity; the turbid mass continues under the force of gravity down a submarine slope

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