A continental shelf is a portion of a continent that extends far out into the sea shore. It usually extends from the line of a coast to a number of miles into the sea and to a point called the shelf break, where the shelf takes a steeper slope that leads into the deep ocean floor. Even though the continental shelf is covered by sea water, it is part of a continent, the limits of which have been set up by the U.N. as the stretch of sea bed belonging to the countries of which they’re part. The continental shelf varies in width from 1 to more than one thousand km in different parts of the world, although its average width is of about 65 km (40 miles).
Continental shelves have been formed over millions of years. Many continental shelves were once land when the sea level dropped down to about 100 meters (328 ft.) below from the present sea level. During this time, inorganic materials in the form of rock, dust and gravel were deposited on the continental shelf and organic material, such as animal and plant remains also were deposited. 18,000 years ago much of the Earth’s oceans were frozen with huge masses of ice known as glaciers. It is believed that the shelves provided walking ground for people to migrate from Siberia to Alaska. Although a continental shelf is thought to be part of an ocean, it is not, but only the flooded portion of a continent.
The continental shelf ends at the shelf break, which is a point of increasing slope. The shelf changes dramatically at the shelf break, where the sea bed begins a sloping descent, with an average sloping angle of 3º, but this angle can vary from 1º to 10º. The shelf break lies at a region of uniform depths of about 140 meters (460 ft.). Further below the slope is the continental rise, which merges into the deep ocean floor. The area covering the continental shelf is typically divided into three distinct shelves; the inner continental shelf, the mid continental shelf, and the outer continental shelf. Each of these shelves has its own geographical characteristics and biological life forms.
The inner part of the continental shelf is periodically disturbed by waves and tidal currents. Sediments deposited in this part of the shelf are mostly sand grains, while silt and clays are deposited in more calmed deep sea waters in the outer continental shelf. At the transition between the inner and outer shelf, layers of mud and sand are found. Most sediment in the continental shelf come from the continents, with 60-70% deposited during the last ice age, when sea levels were 100 meters (328 ft.) lower than in the present. Continental shelf sediments are deposited at an average rate of 30 cm (12 inches) every 1,000 years.
Continental shelves in the world
Continental shelves are broad, gently slopes, which vary considerably in width. Some shores lack a continental shelf, such as off the coast of Chile and the west coast of Sumatra, which lie in subduction zones. In most other coastal regions, the continental shelf may extend less than one km (0.62 miles) such as the coast of California, while the largest continental shelf, the Siberian Shelf, stretches 1,290 km (800 miles) off the northern coast of Siberia. In some coastal places, deep canyons are formed where strong river currents cut deeply into the smooth sediment material of continental shelves.
Plankton and fish turn continental shelves in rich feeding grounds for small and larger marine organisms. The shallow waters of continental shelves allow the penetration of sunlight, which plankton species use for the process of photosynthesis. Phytoplankton provides the main source of food for bottom dwelling organisms, while zoo-plankton is the principal source of food for other marine organisms, including large whales. Upwelling of rich nutrient water provides an abundance of marine life in the continental shelves. Upwelling regions throughout the world, which lie in continental shelves, constitute one percent of the ocean´s surface, yet, they account for 50% of the world’s fish catch.
The term ‘continental shelf’ is used by marine geologists to refer to the region between the shoreline and the shelf break; however, this term is used as an official term, since some regions in the world lack a continental shelf. According the U.N. Conference on the Law of the Sea, the continental shelf of a country includes the submerged prolongation to a distance of 200 nautical miles (230 miles). This allows countries that lack an official continental shelf to own a portion of the sea bed and subsoil in front of their territories.