Continental shelves were formed in between glacial periods as the ocean flowed over the continents forming shallow areas along the coasts. About 18,000 years ago, during the height of the Pleistocene ice ages, much of what is now a continental shelf was actually above water.
The continental shelves that exist today drop off around 130 m off the coast at a steep embankment called the shelf break, which descends to the abyssal plain. The continental margin is a combination of the continental shelf and slope, a varied seascape with underwater canyons carved out by turbidity currents. Turbidity currents are also responsible for the continental rise or the gradual slope of the continental shelf into the abyssal plain. The largest shelf in the world is the Siberian Shelf in the Arctic, which measures in places up to 1,500 km in width. Some regions like Chile and Sumatra have no shelf because of their location near a subduction zone. Normal continental shelves are found in the South China Sea, the North Sea, and the Persian Gulf and are usually about 80 km in width with a depth of 30-600 m.
Continental shelves are an oasis in the ocean for plants and animals due to the abundance of sunlight, shallow waters, and nutrient packed sediment that washes in from rivers, wave action, and in some areas, upwelling. In addition to the vast amount of benthic life, the continental shelf is home to a variety of species such as tuna, menhaden, mackerel, and cod. People have depended on the continental shelf for thousands of years to provide 90% of the fisheries production in the world and, more recently, for petroleum. According to the abiogenic theory, fossil fuels were formed when ancient plants and animals fell to sedimentary rock in the ocean floor.
In 1930, many nations began to realize the value of their continental shelves, and requested new laws to replace older ones that limited their territory to only three miles from the coastline. In 1945, the U.S. was the first to gain control of the continental shelf. The international laws evolved, and in 1982 the UN Convention on the Law of the Sea was enacted to regulate marine resources and pollution worldwide. The international laws are still heavily debated and undergoing changes to allow fair access while protecting the resources belonging to individual nations.
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