When we discussed the structure of the earth there was a diagram showing the continents floating on top of the rest of the planet like foam on water. This page will describe some of the features of the ocean floor.
The continents lie on the continental plates. The edges of these plates are under water, and they form the continental shelves. These shelves slope outward very gently towards the ocean depths. In some places these shelves go out a long way, up to 900 miles: In other places the shelves are much narrower. These shelves are fairly smooth because debris from the land is falling onto them from the water. This debris contains nutrients washed down from the land, and these nutrients, plus the shallow water, contribute to the abundance of life forms near the continents.
Where the continental shelves end, there is a steep drop downwards. This is called the continental slope. It plunges down into the dark, cold waters of the ocean. At the bottom of the continental slope there is a an area that slopes gradually downward to blend into the abyssal plain. This is the bottom of the ocean as we think of it.
The ocean floor seems like a different world. There is no sun light down here. The water is very cold. The pressure of the weight of miles of water above this area would easily crush a submarine. Tiny flecks of material from the sunlit world float downward and eventually settle on the bottom. There is almost nothing to eat, and the animals that live here are few and far between. There are many different kinds of animals, but they are small and widely scattered. They are able to survive on very little.
However, it is not all silence and darkness here. The earth's crust is thinnest in the ocean basins, and volcanic eruptions are part of this environment. Here and there isolated peaks called sea mounts rise above the flat abyssal plain. However, the most active part of deep ocean geology occurs in the deep sea trenches, where the ocean floor is spreading apart. Lava comes up through the fissures, building up mountains and deep sea vents, called smokers. The constantly up-welling lava pushes the sea floor away from the trenches and out towards the continents. When the sea floor reaches the continental plates, it subducts, or slides underneath them, carrying the cold rocky floor back into the hot mantle of the earth.
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