![]() Over time, as the sea level rose, these minerals were again sorted by waves and ocean currents and finally deposited. 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. Minerals come from rocks on land and are carried to the ocean by rivers. Oil and gas is found on the continental shelf off the coasts of California and Louisiana, for example. The oil and gas moves upward and is concentrated beneath geologic traps. Over time the material is buried and transformed into oil and gas by heat and pressure. Oil and gas are formed from organic material that accumulates on the continental shelf. 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.Ĭontinental shelves contain valuable resources, such as oil and gas and minerals, as well as fisheries. Their remains are often found at the bottom of the ocean. During lowered sea level, land plants and animals, including humans and their ancestors, lived on the shelf. During the last glacial period (approximately 18, 000 years ago), sea level was 300-400 ft (90 –120 m) lower than present and the shoreline was much farther offshore, exposing the shelf to the atmosphere. The changes in sea level were caused by the advance and retreat of glaciers on land over the same time period. 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. Some of the narrowest shelves are found off the tectonically active western coasts of North and South America. The widest shelves are in the Arctic Ocean off the northern coasts of Siberia and North America. 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 greatest depth is found off Antarctica, where the great weight of the ice on the Antarctic continent pushes the crust downward. The average depth of the shelf break is 440 ft (135 m). That is, for every one kilometer of distance, the shelf drops 1.9 m in elevation until the shelf break is reached. The average slope of the shelf is about 10 ft/mi (1.9 m/km). The shelf occupies only 7% of the total ocean floor. Seaward, the shelf ends abruptly at the shelf break, the boundary that separates the shelf from the continental slope. The continental shelf is a gently sloping and relatively flat extension of a continent that is covered by the oceans.
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