THE CONTINENTAL SHELF is the submerged outer edge of a continent. The shelf begins at the shoreline’s low tide mark and slants gently beneath the ocean. At the outer edge of the shelf, a continental slope breaks downward to the great ocean depths. The maximum width and the depth of the shelf vary. The depth is generally less than 330 feet (100 meters) to 660 feet (200 meters) deep. The width varies from less than one mile (1.6 kilometers) to several hundred miles.
A continent’s position relative to tectonic plate boundaries influences the width and depth of its shelf. A continent has a narrow shelf where it sits on the leading edge of a plate that collides with an oceanic plate. Jolting earthquakes and erupting volcanoes accompany the convergence. The crunch also causes downward movement (subduction) of the oceanic plate and creates a deep oceanic trench. The shelf is narrow because the collision causes the edge of the continent to rise and the continental slope to plunge sharply into the deep trench. River-borne sediments do not ordinarily accumulate in thick layers on the narrow shelf; they quickly slough off into oceanic trench. The trailing edge of the same continent is too distant to experience earthquakes and volcanic activity. As a result, its continental shelf slopes gently beneath the sea, where it accumulates thick layers of sediments carried to the ocean by eroding streams and glaciers.
The North American continent is a good example of a continent with both types of continental shelves. The west coast has a narrow continental shelf due to ongoing or relatively recent tectonic plate convergence. Conversely, North America’s east coast is thousands of miles from the violent edges of conflicting tectonic plates, so its continental shelf sinks gently beneath the sea, and in some areas, it extends hundreds of miles offshore.
Sedimentary rocks of some continental shelves contain biogenous components (parts of dead plants and animals) that form hydrocarbon compounds, such as oil and natural gas. These fuels are the most valuable geological resources of the continental shelf. The offshore dredging of loose aggregates (gravel and sand) and the extracting of salt from evaporation ponds generate incomes locally. Additionally, coastal winds cause phosphorus and nitrogen-based nutrients to upwell from the shelf floor. As a result, some of the best commercial fisheries in the world occur in zones of coastal upwelling. Herring, sardines, and anchovies are the main commercial fish harvested from these areas. Oysters, clams, scallops, and mussels are the main shellfish varieties. Seaweed, which is an important food item in Asia and an additive in other foods, is the most important commercial plant harvested on continental shelves.
The management of resources of the continental shelf is vital to the security of national economies and to the global economy. Thus, most nations are signatories to the 1982 United Nations Convention of the Law of the Sea (or Law of the Sea Treaty). This treaty allows coastal nations to extend their control of the ocean’s resources from their coastlines to 200 nautical miles (370 kilometers) and to as far as 350 nautical miles (649 kilometers), if the shelf extends beyond 200 nautical miles.
Law of the Sea; Oceanography; Oceans.
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