A continuous range of mountains called the mid-ocean ridge D winds around Earth. There are mountains on the abyssal plain, too. Some reach above the ocean surface to form volcanic islands E. Others, called seamounts B , are completely under water. To understand how ocean-floor features are formed, it is important to understand the Earth's structure.
The Earth consists of layers around its center, or core. The outer layer, or crust, is thin and rocky. The thick layer between the crust and the core is the mantle. It contains hot, liquid rock called magma. Magma flows out of the mantle and onto the surface through cracks in the crust. Magma on the surface of the Earth is called lava, which hardens to form new crust.
Earth's crust is broken into large plates that float on the mantle. As these plates slowly move, they create different landforms. The mountain ranges of the mid-ocean ridge, trenches, and underwater volcanoes are all formed by interactions of Earth's plates.
At the mid-ocean ridge, plates are diverging or moving apart. 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. These narrow, flooded valleys that descend into the continental shelf are known as fjord s. Oil on the Shelf A lot of fuel we use is collected from beneath the continental shelves. For example, 30 percent of all the oil and 20 percent of the natural gas produced in the U. Most of these sites are on the North American continental shelf off of Alaska and the Gulf of Mexico.
The last ice age peaked about 20, years ago. Also called glacial age. The last mastodons became extinct about 11, years ago. Usually rivers enter another body of water at their mouths.
Sea level is determined by measurements taken over a year cycle. Seaweed can be composed of brown, green, or red algae, as well as "blue-green algae," which is actually bacteria. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited. Caryl-Sue, National Geographic Society.
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