Continental Shelf (World of Earth Science)
The continental shelf is a gently sloping and relatively flat extension of a continent that is covered by the oceans. Seaward, the shelf ends abruptly at the shelf break, the boundary that separates the shelf from the continental slope.
The shelf occupies only 7% of the total ocean floor. The average slope of the shelf is about 10 ft per mile (1.9 m per km). That is, for every one kilometer of distance, the shelf drops 1.9 m in elevation until the shelf break is reached. The average depth of the shelf break is 440 ft (135 m). The greatest depth is found off Antarctica (1,150 ft [350 m]), where the great weight of the ice on the Antarctic continent pushes the crust downward. The average width of the shelf is 43 mi (70 km) and varies from tens of meters to approximately 800 mi (1,300 km) depending on location. The widest shelves are in the Arctic Ocean off the northern coasts of Siberia and North America. Some of the narrowest shelves are found off the tectonically active western coasts of North and South America.
The shelf's gentle slope and relatively flat terrain are the result of erosion and sediment deposition during the periodic fall and rise of the sea over the shelf in the last 1.6 million years. The changes in sea level were caused by the advance and retreat of glaciers on land over the same time period. During the last glacial period (approximately 18,000 years ago), sea level was 30000 ft (9020 m) lower than present and the shoreline was much farther offshore, exposing the shelf to the atmosphere. During lowered sea level, land plants and animals, including humans and their ancestors, lived on the shelf. Their remains are often found at the bottom of the ocean. For example, 12,000 year old bones of mastodons, extinct relatives of the elephant, have been recovered off the coast of the northeastern United States.
Continental shelves contain valuable resources, such as oil and gas and minerals. Oil and gas are formed from organic material that accumulates on the continental shelf. Over time the material is buried and transformed to oil and gas by heat and pressure. The oil and gas moves upward and is concentrated beneath geologic traps. Oil and gas is found on the continental shelf off the coasts of California and Louisiana, for example. Minerals come from rocks on land and are carried to the ocean by rivers. The minerals were deposited in river channels and beaches on the exposed continental shelf and sorted (concentrated) by waves and river currents, due to their different densities. Over time as the sea level rose, these minerals were again sorted by waves and ocean currents and finally deposited. The different colored bands of sand that one can see on a beach are an example of density sorting by waves. The concentrated minerals are often in sufficient enough quantities to be mined. Examples of important minerals on the shelf are diamonds, chromite (chromium ore), ilmenite (titanium ore), magnetite (iron ore), platinum, and gold.
See also Glaciation; Ice ages; Sedimentation; Wave motions