The shelf usually ends at a point of increasing slope (called the shelf break). The sea floor below the break is the continental slope. Belowthe slope is the continental rise, which finally merges into the deepocean floor, the abyssal plain. The continental shelf and the slope arepart of the continental margin.
Continental margins
The region that lies beneath the neritic zone is called the continental margin. Beyond the continental margin lies the ocean basin. The continental margin is composed of the continental shelf and the continental slope (figure 1). Continental shelves are the shallow, submerged extension of continents. They are composed of granite that is covered by sediments and have physical features similar to the edge of the nearby continent. Continental shelves are generally flat areas, averaging 68 kilometers (40 miles) wide and 130 metres (430 feet) deep, which slope gently toward the bottom of the ocean basin. The width of a continental shelf is frequently related to the slope of the land it borders. Mountainous coasts such as the western coast of the United States such as the continental shelf, whereas low-lying land such as the eastern coast of the United States usually has a wide one.
The transition between the continental shelf and the deep floor of the ocean is called the continental slope (fig 1). At the point where the continental shelf ends and the continental slope begins there is an abrupt change in the shelf called the shelf break. From this point there is an increase in the slope and a rapid change in depth to the sea floor. The extent of the sloping can very from a gradual decline to a steep drop into an ocean trench, such as the slope that occurs off the western coast of South America. Because of its steep angle, the continental slope usually has less sediment.
Submarine Canyons
Some continental slopes have submarine canyons that are similar to canyons found on land (fig. 1). Many of these submarine canyons are aligned with river systems on land and were probably formed by these rivers during periods of low sea level. Other sub-marine canyons have ripple marks on the floor, and at the ends of the canyons sediments fan out, suggesting that they were formed by moving sediments and water. Oceanographers believe that these canyons were formed by turbidity currents.
Continental rise
At the base of the steep slope there may be a gentle slope called a continental rise (fig. 1 and 2). A continental rise is produced by processes such as landslides that carry sediments to the bottom of the continental lope. Most continental rises are located in the Atlantic and Indian Ocean and around the continent of Antarctica. In the Pacific Ocean, it is more common to find trenches located at the bottom of the continental slopes.
Figure 2: Landscape of the ocean floor (Gross, 1972).
Ocean basin
The floor of the ocean is referred to as the ocean basin, and it covers slightly more of the earth’s surface than the continents. They are four main ocean basins: the Pacific, Atlantic, Indian, and Artic basins. Ocean basins are composed of basaltic rocks that are covered by a thick blanket of sediments that have either been washed down from the continental shelves or settled there from the surface waters. Some of the most spectacular geological features are found in the ocean basins. The bottom of many ocean basins is a flat expanse called the abyssal plain (fig 2). There plains extend from the seaward side of the continental slope and are formed by sediments deposited by turbidity currents, as well as sediments falling from above the water column. Dotting the ocean floor are abyssal hills rising as high as 1,000 meters (fig. 2). Abyssal hills cover as much as 50% of the Atlantic seafloor and as much as 80% of the seafloor of the Pacific and Indian Oceans. Another feature of ocean basins is the sea mount, a steep sided formation that rises sharply from the bottom (fig. 2). All sea-mounts are formed from underwater volcanoes and are most prevalent in the Pacific Ocean. Some seamounts show evidence of coral reefs and surface erosion, suggesting that at one time they were above the surface. Plate tectonic movement of the ocean floor, the natural process of compaction that volcanic materials undergoes, subsidence due to cooling of the top floor, erosion, and the increased weight of sediments at the top may be the reasons for the sinking of these structures. A continuous series of large, underwater, volcanic mountains run through every ocean of the world and stretches some 69, 000 kilometers around the earth. The ridges and rises of these mountain ranges separate the ocean basin into a series of smaller, deep-water sub-basins.
Other impressive features of the ocean floor are the trenches (fig. 2). Deep water trenches are more common and more spectacular in the Pacific Ocean and are usually associated with chains of volcanic Islands called island arcs. The deepest off all ocean trenches is the Mariana trench.