Continental shelf and slope

The continental shelf and slope mark the ocean's continental margins. They are repositories for much of the weathered rock material eroded and transported by rivers and wind. In addition, they serve as major reservoirs for petroleum and various other mineral resources.

Origins of the

The ocean's continental shelf has been called the submerged shoulders of the great landmasses. The shelf, together with the adjacent seaward continental slope, separates the land from the great depths of the sea. Thus, the shelf is part of a dynamic transition zone that has changed markedly over the millennia. It is also a zone contested by several nations eager to maintain what are considered Rights of the Sea. These rights include the free passage of ships, access to valuable submerged minerals, fishing rights, and military intelligence gathering. The shelf is a nearly flat area that marks the submerged edges of continents. It slopes gently toward the ocean basins at approximately 0.5 to 1-degree angles. The width may range from thirty meters in some locations to more than 100 kilometers in others, with an average width of sixty-five kilometers. The nature of the adjacent landmass often dictates the shelf width—broad next to low-lying land and narrow next to rugged, mountainous land.

During the Pleistocene epoch, the massive continental glaciers that covered the Earth some two million years ago helped shape the continental shelf. Wide, deep shelf areas with rugged topography are found in those parts of the world covered by ice sheets during the Pleistocene. As the glacial fronts advanced over the face of the land, they bulldozed vast amounts of the Earth's surface to be deposited many hundreds of kilometers away. The glaciers ground huge boulders into gravel, gravel into sand, and sand into a fine, dustlike “glacial flour.” This debris was deposited on the continental shelf’s surface. The shelf itself was dry land during these glacial excursions. The ice sheets, some three kilometers high at their maximum, had stored so much of the oceans' waters that the sea level was lowered nearly 150 meters below the present level. The immediate impact on the coastal zone was rough, turbulent surf with great waves crashing on the exposed shore. As a result, the beach sediments of these shores were coarse, with abundant gravel, cobbles, and boulders.

The broad expanse of the exposed coastal lowlands encouraged the development of wetlands and ponds. Vegetation that accumulated in the wet areas formed peat, dredged occasionally from depths of fifty meters and sometimes from depths of fifty to one hundred kilometers off the present coast. In addition to the wetlands, the exposed shelf zone supported spruce, fir, pine, and oak forests. Humans lived and hunted various game in these forests some 15,000 to 20,000 years ago. Then, the glaciers began to melt, and the sea level reached its present extent about 5,000 years ago. Fish now swim through marine waters where birds once flew among the towering trees. The nearshore deposits on the present-day continental shelf also contain the fossilized remains of giant mastodons, woolly mammoths, and other huge land mammals that once grazed over the coastal plains in vast numbers. In addition to biological deposits, old sand dunes and the rounded pebbles of ancient beaches have been found on the shelf.

Sediments

The continental shelf is covered with a veneer of sediments that vary greatly in depth. The submerged shelf areas receive most of the weathered rock debris from the erosion of the continents. Indeed, deep exploration has revealed thousands of meters of sediments that have accumulated on the sedimentary rock forming the edges of the continents. Most of the sediments of the shelf are classified as relict. That is, they are not representative of today's environment but were laid down thousands and perhaps millions of years ago. About 70 percent of continental shelf sediment was deposited during the past 15,000 years. During the million-year extent of the Pleistocene epoch, there were four major lowerings of sea level, followed by flooding, as the glaciers waxed and waned. The flooding from the last major stage led to the accumulation of sediments on the continental shelf.

Marine sediments are classified by many criteria. These include origin, particle size, density and shape, mineral composition, and color. Sediments that originate from the erosion of land formations are termed terrigenous. These include sand, gravel, silt, and clay. Sediments that are formed by the accumulation of the shells and skeletons of animals are termed biogenous or biogenic; those that are formed directly from chemicals in seawater are termed hydrogenous or authigenic. Most of the sediments on the continental shelf are terrigenous; a few are biogenic or mixtures of the two.

At one time, it was believed that sediments on the continental shelf showed size gradations—coarse sands and gravels close to shore, with finer particles farther from shore. The finest particles, the silts and clays, formed a “mud line” along the seaward edge of the shelf. Research conducted during World War II revealed that this progression from coarse to fine is rarely found. Samples of the sediments retrieved from the continental shelf reveal that they consist mostly of coarse sand. The sediment particles are stained red by iron deposits. Frequently, the shelf sediments also contain the empty shells of clams, whelks, and other marine animals that live close to the shallow waters of the coastal region. Occasionally, masses of broken shells accumulate along the outer edge of the continental shelf. Fields of cobble and boulders dot some of the shelf areas. These massive structures represent rocks transported by glaciers and deposited when the glaciers melted.

Sediments are deepest on those shelf areas that received masses of sand and gravel deposited during glacial meltback. Georges Bank, off Massachusetts, and the shallow North Sea between the British Isles and northeastern Europe feature much glacially deposited debris. The varied topography of these glaciated shelf areas includes banks, channels, and deep basins. An example of a broad but unglaciated shelf, however, is found off the coasts of New York and New Jersey. The deep shelf sediments there are generally smoother, although they may be sculpted by strong currents to form low-relief ridges and, occasionally, deep submarine canyons.

The West Coast of the United States, in contrast, is relatively clear of sediment on the shelf. There is only a thin veneer of materials, and the shelf shows little evidence of glaciation. Strong ocean currents can also sweep the shelf clear of sediments. These currents also contribute to the formation of narrow, or even absent, continental shelf areas. The East Coast of southern Florida is a good example of this phenomenon. There, the Gulf Stream, often flowing at eleven kilometers per hour, sweeps close to the mainland. This strong current has prevented normal shelf development. The Gulf Stream is credited with sweeping sediments from the Blake Plateau, located several hundred meters below the sea surface.

Continental Shelf Dams

The sediments that carpet most of the continental shelf are kept in place by various dams along the shelf break. The break marks the boundary between the gently sloping continental shelf and the steeper continental slope. The dams, thus, trap the sediments and hold them against the continents, preventing them from spilling downslope into the deep-sea basins. Continental shelf dams may be formed by volcanoes, coral reefs, or salt domes. Often, the dams result when massive blocks of basement rock are thrust up by powerful forces in the Earth's interior. The dams and the sediments behind them are often eroded by waves, glacial ice, or powerful local currents. The waves and currents ridge the sediments in a series of terraces that are parallel to the shore. Cutting across the terraces are channels that were eroded by streams during the glacial periods when the shelves were exposed.

Major modern river systems, including the Congo and Hudson Rivers, cut deep channels in the exposed shelf sediments. These channels are easily traced across the continental shelf off the east coast of North America and the west coast of Africa. The Hudson River channel, for example, is so large that it has not yet filled with sediment despite the active rate of erosion by the riverine system. The great scar of the canyon extends across the broad continental shelf, cuts through the dam at the shelf edge, and plunges down the continental slope. This canyon is a major feature of the shelf and the adjacent slope.

Continental Slope

The continental slope is a well-delineated geologic separation between the flat shelf and the moderate grade of the continental rise. Of the three features, the slope is the steepest. It has an average angle of 4 degrees (with a range of between 3 degrees and 20 degrees); in many oceans, this steep boundary extends all the way to the floor of the deep ocean basin. It is one of the largest topographic features of the Earth's surface and may extend nearly 4,000 meters from the depths of the seabed to the shelf edge. Perhaps the most dramatic continental slope area is below the narrow continental shelf off the West Coast of South America. There, the slope wall drops precipitously for 8,000 meters into the Peru-Chile Trench. This slope is similar to others in that it features craggy outcroppings and is relatively bare of sediments because of the steepness.

The continental slope is steepest in the Pacific Ocean basin. There, it averages more than 5 degrees, while in the Atlantic and Indian Ocean basins, it averages about 3 degrees. The Pacific Ocean continental slope is associated with the geologic processes that form the coastal mountain ranges and the deep ocean trenches. The word “slope” does not adequately describe these oceanographic regions' great and varied topography. Underwater landslides, submarine earthquakes, and subsurface erosion processes have produced various topographic features. The continental slope is incised by numerous valleys and canyons, some rivaling the Grand Canyon in size. They are long and short, straight and branched, and may be cut through solid rock and the sediments that may carpet the slope. Many of the canyon walls are cut by side canyons in a variety of sizes.

Most canyons that cut across and down the slope are continuations of the canyons of the continental shelf. These are easily traceable to continental landforms and are believed to have been eroded by rivers or glaciers during the Pleistocene epoch when the shelves were exposed. Many canyons on the slope, however, are nowhere connected to shelf canyons and may be far removed. It is believed that these canyons may have been (and continue to be) eroded by fast-moving, powerful turbidity currents. The currents may be triggered by earthquakes or by an excessive buildup of sediment on steep areas of the slope. Moving rapidly downslope, the currents—fast-moving submarine avalanches of mud, sand, fine gravel, and water—erode the walls of the slope and carve out the submarine canyons. In 1929, an earthquake caused a turbidity current on the shelf and slope off Newfoundland estimated to have moved 700 kilometers at speeds of 40 to 55 kilometers per hour. The millions of tons of abrasive sediments that periodically sweep through existing canyons effectively erode the walls and floors. As the currents move downslope, the velocity decreases, and the sediments spread out to be deposited at the base of the slope as deep-sea fans.

Exploration of the Sea Floor

Early mariners' curiosity about the sea floor was limited to practical matters—for example, finding the channel for entering a harbor. The earliest instrument that yielded any information about the sea floor was the sounding lead. This club-shaped device, about twenty to thirty centimeters long, was attached to a long hemp or cotton line marked in fathoms. (One fathom is approximately two meters long.) The base of the lead, the part that touched the ocean floor, had a shallow, cup-shaped indentation packed with lard or tallow. When the lead was dropped over the side of the vessel and touched bottom, markers on the retrieving line indicated the depth of water. When it was hauled back on deck, the fat in the cup usually brought back a small sample of the seafloor sediment. With no other information available to them, mariners (and even the first curious scientists) believed the ocean basin to be a smooth, unrelieved depression whose bottom dropped off to unmeasurable and unimaginable depths.

Early research was limited to mapping the coast and making soundings in water less than 200 meters. The principal concern was safe navigation. Toward the end of the nineteenth century, however, laying transoceanic telegraphic cables, including those from North America to Europe, spurred greater interest in making more accurate and detailed surveys. The pioneering global voyage of the British research vessel Challenger, from 1873 to 1876, included many soundings in areas previously not studied, but research methods were tedious and progress was slow. The soundings were made with hemp rope, which frequently broke under the strain of its own weight. The line and its weight—often a cannonball—were laboriously hauled up with a capstan turned by the crew. Later, twisted wire rope and single-strand wire, often graduated in diameter, reduced the time required for lowering and reeling in the line and weight. An innovation that involved releasing the weight after it had touched bottom, which lessened the load, further reduced the retrieval time. By the early twentieth century, there were fewer than 10,000 soundings in waters deeper than 2,000 meters and only about 500 in waters deeper than 5,500 meters. Such data provided information about the depths involved but gave no indication of the submarine topography.

Technological Advances

In the 1920s, sonic devices for making depth soundings revolutionized the procedure. These devices called sound navigation and ranging (sonar) or depth sounders, transmit a sound impulse from the ship to the ocean floor. The impulse “bounced” off the sea floor and was received by an instrument translating the round-trip time to depth. Later refinements of the device provided paper traces with near-photographic representation of the ocean floor over the entire span of the basins. For the first time, the grandeur of the seafloor topography was revealed. It showed that the land beneath the sea surface is nearly as rugged and sculpted as any part of the dry land. The sonic devices could also penetrate the surface sediments to reveal their depth and complexity. Further refinements made it possible to probe the basement rock beneath the veneering sediments.

As valuable as the sonic data are, nothing is more revealing than samples of the ocean floor over the continental margins. The earliest specimens were collected with rugged iron dredges dragged over the bottom. As with the early hemp sounding lines, the dredges took hours to drop, tow, and retrieve. Often, the dredge closed up before it reached the bottom. Other times, powerful underwater currents twisted the line into an impossible tangle, and again, the dredge failed to gather bottom samples. Despite these hazards, dredges did collect much valuable material and are still widely used. Various grabs lowered from vessels collect bottom samples in specific locations. As with the dredges, the grabs often retrieve biological specimens with the bottom sediments. Corers also collect bottom sediments. These devices are dropped or thrust into the bottom sediment to collect a cylinder of sample sediment.

Perhaps the most dramatic sampling utilizes deep-drilling equipment. Special vessels drill into the bottom sediments, penetrating more than 2,000 meters deep. The vessel Glomar Challenger (1968-1983) successfully drilled into the continental slope in water depths of nearly 3,000 meters. Human-occupied submersibles such as Alvin (DSV-2), which began service in 1964, are fitted with maneuverable arms, collecting baskets, still and video cameras, viewing ports, computer systems, and motor controls. Thus, scientists' observations aboard the submersibles are supplemented by photographs and specimens of the ocean floor. In 2022, Alvin was certified to operate at 6,500 meters.

Other technological advancements that have supported continental shelf and slope research include sonar mapping, remotely operated underwater vehicles (ROUV), and the Smith-McIntyre grab sampler, which can retrieve sediment from any ocean depth.

Principal Terms

sediment: solid matter that settles on a surface; sediments may be transported by wind, water, and glaciers

shelf dams: geologic formations that hold back sediments on the continental shelf

submarine canyons: channels cut deep in the sediments by rivers or submarine currents

trench: a long, narrow, and deep depression in the ocean floor, usually with steep sides and often adjacent to island arc systems and continental landmasses

turbidity currents: fast-moving submarine avalanches of inorganic sediment

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