Continental Margin (Encyclopedia of Science)
The continental margin is that underwater plain connected to continents, separating them from the deep ocean floor. The continental margin is usually divided into three major sections: the continental shelf, the continental slope, and the continental rise.
Continental shelves are the underwater, gradually sloping ledges of continents. They tend to be quite flat, with an average seaward slope of less than 10 feet per mile (about 3 meters per kilometer). They vary in width from almost zero to more than 930 miles (1,500 kilometers), with a worldwide average of about 50 miles (80 kilometers). The widest shelves are in the Arctic Ocean off the northern coasts of Siberia and North America. Narrow shelves are found off the western coasts of North and South America. The average depth at which the continental shelf begins to fall off toward the ocean floor (the beginning of the continental slope) is about 430 feet (130 meters).
Changes in sea level during Earth's history have alternatingly exposed and then covered portions of the continental shelf. During lowered sea level, land plants and animals, including humans and their ancestors, lived on the shelf. Today, their remains are often found there. For example, 12,000-year-old bones of mastodons, extinct relatives of the elephant, have been recovered off the coast of the northeastern...
(The entire section is 667 words.)
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