continental shelf

continental shelf, a broad, relatively shallow submarine terrace of continental crust forming the edge of a continental landmass. The geology of continental shelves is often similar to that of the adjacent exposed portion of the continent, and most shelves have a gently rolling topography called ridge and swale. Continental shelves make up about 8 percent of the entire area covered by oceans.

Structure

A continental shelf typically extends from the coast to depths of 100–200 metres (330–660 feet). It is gently inclined seaward at an average slope of about 0.1°. In nearly all instances, it ends at its seaward edge with an abrupt drop called the shelf break. Below this lies the continental slope, a much steeper zone that usually merges with a section of the ocean floor called the continental rise at a depth of roughly 4,000 to 5,000 metres (13,000 to 16,500 feet). A few continental margins—such as those off the Mediterranean coast of France and at Porcupine Bank, off the western coast of Ireland—do not have a sharply defined break in slope but rather maintain a generally convex shape to the seafloor.

The average width of continental shelves is about 65 km (40 miles). Almost everywhere the shelves represent simply a continuation of the continental landmass beneath the ocean margins. Accordingly, they are narrow, rough, and steep off mountainous coasts but broad and comparatively level offshore from plains. The shelf along the mountainous western coast of the United States, for example, is narrow, measuring only about 32 km (20 miles) wide, whereas that fringing the eastern coast extends more than 120 km (75 miles) in width. Exceptionally broad shelves occur off northern Australia and Argentina. The world’s largest continental shelf extends 1,500 km (about 930 miles) from the coast of Siberia into the Arctic Ocean.

Continental shelves are usually covered with a layer of sand, silts, and silty muds. Their surfaces exhibit some relief, featuring small hills and ridges that alternate with shallow depressions and valleylike troughs. In a few cases, steep-walled V-shaped submarine canyons cut deeply into both the shelf and the slope below.

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Origin

American oceanographer Donald J.P. Swift has called continental shelves “palimpsests,” parchment writing tablets upon which stories are written after each previous writing has been erased. Each new stand of sea level “writes” a new story of sedimentation on the shelf after the previous episode has been erased by the rise or fall that preceded it, but with some traces of the previous environment of deposition or last erosional event remaining. The “eraser” is the surf, a high-energy force that erodes and reworks everything as it passes over, winnowing out the finer-than-sand-sized sediment and leaving the coarser material behind. An interpreted seismic line shows the complicated array of channels (eroded and then filled), old deltaic deposits, ancient erosional surfaces, and winnowed sand bodies that make up the continental shelf southwest of Cape San Blas on the panhandle of Florida.

How the above processes affect any particular margin depends on its tectonic setting and the size of the rivers that drain into it. On continental shelves backed by high mountain ranges, such as the Pacific coast of North and South America, the difference between high and low sea-level stands may be difficult to detect, being one of degree perhaps noticeable only by marginally increased sedimentation rates during lowstands, or intervals of decreased sea level. In many ways, continental shelves on tectonically active margins at present sea levels approximate lowstands on trailing-edge, or passive, margins.

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When sea level is lowered on a trailing-edge shelf that has no adjacent high mountains, such as the Atlantic coast of North America, rivers are rejuvenated. In other words, their base level is lowered and they begin to erode their beds, carrying sediment from the continent across the former continental shelf that is now exposed and depositing it at the new coast. When sea level falls below the shelf break, the coast lies on the continental slope. As sea level rises again on tectonically stable or sinking shelves, small and medium-sized river mouths drown and estuaries form, trapping the sediment within them and starving the shelves. In these cases, sediment for the shelf is primarily produced by erosion of the coastline as the surf zone advances landward with rising sea level. Fine-grained material is winnowed out, to be either deposited back in the estuaries or carried in steps by advective processes across the shelf to the deeper water beyond. As a result, continental shelf surfaces on trailing-edge margins into which no large rivers flow are veneered with a sand sheet lying over a complex of older deposits, some of which peek through the surface as outcrops—vestiges of an earlier story written on the palimpsest. Large rivers that drain a large, high continent, such as the Mississippi, are able to keep pace with rising sea level and deliver enough sediment to keep an estuary from forming, and, at a high stillstand like that of the present, even fill their entire shelf area. (For a description of modern deposits of this type, see river: Deltas.)

For many years after World War II, the period when many of the world’s continental shelves were first described in detail, it was thought that the sand deposits on continental shelves were “relict,” deposits left stranded by a higher sea level from the higher-energy regime of the surf zone that passed over them perhaps as much as a few thousand years before. Geophysical investigations of the shelf area since the mid-1970s have revealed the presence of many types of sand waves and ripple marks in seafloor sediments that show submerged continental shelf sediments to be constantly undergoing reworking and erosion. As scientific understanding of the physical processes that affect continental shelves increased, it was found that currents set up by large winter storms, monsoons, and hurricanes and typhoons are reworking the bottom by winnowing out the fine-grained materials and carrying them either back into the estuaries or beyond the shelf break, where they are lost from the system.

In short, the kind of sediment that covers the surface of a continental shelf is determined by the interplay among the tectonic setting, the size of the rivers that empty into it (size based on how much sediment they carry), and the wave energy that affects it, just as is the case with continental margins in general. Shelves such as that of western Florida that have been cut off from clastic input (that is, sediments composed chiefly of quartz and clay minerals derived from erosion of the continent) may be covered with carbonate sediments. In some cases, as in the islands of The Bahamas, the carbonate shelf, called a bank, is cut off from a continental source by deep water. Continental shelves with rivers that carry sediments from the continents to the shelf and beyond only at lowstands of sea level and those that drain mountainous areas on high-energy coasts are dominated by quartz sands. In addition, shelves with rivers that drain large continental areas and carry enough sediment to keep abreast of sea-level rise or dominate ambient wave-energy conditions will accumulate muddy sediment deposits out across their surfaces.

Since the 1970s an increasing number of investigators have sought to explain the origin of continental shelves and their related structures in terms of plate tectonics. According to this theory, the shelves of the Pacific Ocean, for example, formed as the leading edges of continental margins on lithospheric plates that terminate either at fracture zones (sites where two such plates slide past each other) or at subduction zones (sites where one of the colliding plates plunges into the underlying partially molten asthenosphere). Shelves of such origin tend to be steep, deformed, and covered by a thin layer of erosional debris. The Atlantic continental shelves, on the other hand, show little or no tectonic deformation and bear a thick veneer of sedimentary material. They are thought to be remnants of the trailing edges of the enormous plates that split apart and receded many millions of years ago to form the Atlantic basin. As the edges of the plates gradually contracted and subsided, large amounts of sand, silts, and mud from the continents settled and accumulated along their seaward side.