A continental shelf is the edge of a continent that lies under the ocean. Continents are the seven main divisions of land on Earth. A continental shelf extends from the coastline of a continent to a drop-off point called the shelf break. From the break, the shelf descends toward the deep ocean floor in what is called the continental slope.
Even though they are underwater, continental shelves are part of the continent. The actual boundary of a continent is not its coastline, but the edge of the continental shelf. The widths of the continental shelves vary. Along parts of the U.S. state of California, for example, the continental shelf extends less than a kilometer (.62 miles). But along the northern coast of Siberia, the shelf extends about 1,290 kilometers (800 miles). The average width of a continental shelf is 65 kilometers (40 miles).
Most continental shelves are broad, gently sloping plains covered by relatively shallow water. Water depth over the continental shelves averages about 60 meters (200 feet). Sunlight penetrates the shallow waters, and many kinds of organisms flourish—from microscopic shrimp to giant seaweed called kelp. Ocean currents and runoff from rivers bring nutrients to organisms that live on continental shelves.
Plants and algae make continental shelves rich feeding grounds for sea creatures. The shelves make up less than 10 percent of the total area of the oceans. Yet all of the ocean’s plants and many types of algae live in the sunny waters.
In some places, deep canyons and channels cut through the continental shelves. Little light penetrates these submarine canyons, and they are sometimes the least-explored areas of continents. Often, submarine canyons are formed near the mouths of rivers. Strong river currents cut deeply into the soft material of the continental shelf, just like they erode rocks above ground. The Congo Canyon, extending from the mouth of the Congo River, is 800 kilometers (497 miles) long and 1,200 meters (3,900 feet) deep. The Congo Canyon is part of Africa.
Formation of a Continental Shelf
Over many millions of years, organic and inorganic materials formed continental shelves. Inorganic material built up as rivers carried sediment—bits of rock, soil, and gravel—to the edges of the continents and into the ocean. These sediments gradually accumulated in layers at the edges of continents. Organic material, such as the remains of plants and animals, also accumulated.
Many continental shelves were once dry land. Some 18,000 years ago, at the peak of the most recent ice age, much of the Earth’s water was frozen into huge masses of ice called glaciers. The sea level dropped, exposing continental shelves. During this glacial period, scientists say that sea levels were perhaps 100 meters (330 feet) lower than they are today.
The continental shelves between North America and Asia were probably exposed during the Ice Age. Some scientists say that the shelves provided a “land bridge” between the two continents. People may have used this land bridge—now the Bering Strait—to migrate from Siberia to what is now Alaska, becoming the first human beings in North America.
Biologists have also found the remains of land-based plants and animals on shelves that are now underwater. For example, scientists have discovered 11,000-year-old mastodon teeth and spruce pollen off the coast of the northeastern United States. Scientific instruments can show that the mastodon and pollen lived during the time of the last ice age.
When the shelves were above water, glaciers moved over them and changed their surfaces. As huge alpine glaciers moved quickly downhill, they gouged deep, narrow valleys. Now, the valleys are filled with seawater. These narrow, flooded valleys that descend into the continental shelf are known as fjords.
