Figure 1: A section of the continental shelf showing submarine canyons (Wikipedia.org)
The widths of the continental shelves vary. Theaverage width of continental shelves is about 80 km (50 mi), the depth of the shelf also varies, but is generally limited to water shallower than 150 m (490 ft). The average slope of the continental shelves varies from 0.2% to 1°. But depending on the nature of the coastline and the variation in the form of the adjoining continent the slope of the shelf may also vary.
Though the continental shelf is treated as a physio graphic province of the ocean, it is not part of the deep ocean basin proper, but the flooded margins of the continent.Passive continental margins such as most of the Atlantic coasts have wide and shallow shelves, made of thick sedimentary wedges derived from long erosion of a neighboring continent. Active continental margins have narrow, relatively steep shelves, due to frequent earthquakes that move sediment to the deep sea.
The shelf area is commonly subdivided into the inner continental shelf,mid continental shelf, and outer continental shelf, each with their specific geomorphology and marine biology.
The continental shelves are covered by terrigenous sediments; that is, those derived from erosion of the continents. However, little of the sediment is from current rivers; some 60-70% of the sediment on the world's shelves is relict sediment, deposited during the last ice age, when sea level was 100–120 m lower than it is now.
Sediments usually become increasingly fine with distance from the coast; sand is limited to shallow, wave-agitated waters, while silt and clays are deposited in quieter, deep water far offshore. These shelf sediments accumulate at an average rate of 30 cm/1000 years, with a range from 15–40 cm.Though slow by human standards, this rate is much faster than that for deep-sea pelagic sediments.