Though beneath the water, continental shelves are still part of their respective continents. Therefore, their edges define the actual boundaries of the earth's continents, not the visible coastlines.
According to the U. S Bureau of Ocean Energy Management, waters in continental shelves are rarely over feet deep, compared to open, deeper ocean waters. Widths of continental shelves vary, but their average width is 40 miles before giving way to deeper open ocean waters. Initially, continental shelves were dry land. However, over several millions of years, organic and inorganic materials combine to form them as sea levels rise.
There was a buildup of inorganic material when rivers carried sediments like rocks, soil, and gravel to the edges of the continents and then into the oceans. The inorganic sediments accumulated in layers at the continental edges, as well as organic materials like plant and animal remains.
According to scientists, over the course of 18, years during the peak Ice Age, when much of water on the earth was frozen, sea levels dipped and exposed the continental shelves. Wedges of sediment that can be several kilometers thick and several hundred kilometers wide developed because of the seaward transport of sediment from the more shallow water continental shelves and slopes.
Overall, the gradients of the continental rises are typically less than 1 degree. Skip to main content. Continental Margins Before we get too far along in a discussion of plate tectonics and coastal zones, we need to address the characteristics and form of continental margins because this is where the coastal zones that we will be referring to are located.
Continental Shelf The continental shelf of a continental margin includes the seafloor that extends from the subaerial part of the continent, or shoreline, seaward to what is known as the shelf break.
Cross-sectional profile illustrating a continental margin and the physiographic provinces of the continental shelf, continental slope, and continental rise, which ultimately transitions into the nearly flat, deep-water abyssal plain of the ocean basins.
These transitions are also accompanied by a change in thickness of the underlying crust and a change in composition from granitic continental crust under the continental shelves to basaltic oceanic crust toward the open ocean basins. Credit: Wikipedia, Public Domain. Global map of the continents showing the transition from subaerial continents to the abyssal plains of the deep ocean basins. The light blue coloration along the edges of the continents represents the relatively shallow bathymetry of the continental shelves.
Notice the wide range in the width of the continental shelves and that some of the narrowest continental shelves are associated with collisional tectonic margins, whereas the wider continental shelves are associated with passive tectonic margins. The average width of a continental shelf is 65 kilometers 40 miles. Most continental shelves are broad, gently sloping plain s covered by relatively shallow water. Water depth over the continental shelves averages about 60 meters feet.
Sunlight penetrate s the shallow waters, and many kinds of organisms flourish—from microscopic shrimp to giant seaweed called kelp. Ocean currents and runoff from rivers bring nutrient s 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. In some places, deep canyon s and channel s cut through the continental shelves. Little light penetrates these submarine canyon s, and they are sometimes the least-explored areas of continents. Often, submarine canyons are formed near the mouth s 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 kilometers miles long and 1, meters 3, 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 accumulate d 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. The sea level dropped, exposing continental shelves. During this glacial period , scientists say that sea levels were perhaps meters feet lower than they are today.
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,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 glacier s moved quickly downhill, they gouged deep, narrow valleys. Now, the valleys are filled with seawater.
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