Continental Slope and Rise

The continental slope and rise marks the edge of the continent. The slope begins at the edge of the shelf, where there is an increase in the downward dip of the surface. Slopes typically have an angle of 4 or 5 degrees, but on charts and cross-sections of the seafloor they are exaggerated to make them clearer to the viewer. Farther down the slope the surface inclination becomes gentler, a decrease that marks the beginning of the continental rise, which typically slopes at an angle of 1/2 degree. The slope and rise are among the least known parts of the ocean basin, but beneath their surface lies the transition between continents and oceans.

Some 8.5 percent of the ocean floor is covered by the continental slopes and rises. Usually a thick wedge of sediments covers them, but occasionally rock outcrops appear on the surface of the slope. Slumping (that is, a down-slope composed of sediment and organic debris) and turbidity currents carry sediments beyond the shelf and dump them on the slope, where they stay temporarily until gravity and occasionally earthquakes move the material to the rise and farther, to the deeper seafloor. Additional material, composed both of clastic and biogenic particles, settles down through the water column.

Many of the great rivers of the world, including the Hudson, Amazon, Ganges, and Congo, have submarine extensions on the continental shelf that extend to the slope, where deep canyons are incised into the surface. These submarine canyons are among the most spectacular features on earth. Although they are seen in a variety of sizes and shapes, they usually have steep walls, curving courses, and a tributary system something like the Grand Canyon of Arizona. At the base of the canyon great aprons of fan-shaped deposits are found on the rise, built of sediments that traveled down the canyons. The origins of submarine canyons are not entirely clear, but lowered sea level, faulting, and turbidity currents play a role in their cutting and shape.

—Sidney Horenstein

See also: Abyssal Floor; Beaches; Continental Shelf; Oceans

Bibliography

Hamblin, W. Kenneth, and Eric H. Christiansen. 2000. The Earth's Dynamic Systems. Upper Saddle River, NJ: Prentice Hall; Plummer, Charles C., David McGeary, and Diane Carlson. 2002. Physical Geology, 9th ed. New York: McGraw-Hill; Press, Frank, and Raymond Siever. 2000. Understanding Earth. New York: W. H. Freeman; Siebold, E., and W. H. Beyer. 1993. The Sea Floor: An Introduction to Marine Geology. Berlin: Springer Verlag.

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