THE CRUST The crust is actually very thin – just 100 kilometers thick on average. The oceanic part of the crust is denser and not as thick as the continental crust. Both float on the mantle but the ocean's crust sits lower than continental crust. This is a good thing because otherwise it would be too high to be called the ocean floor. Crustal density does not equate with thickness; the ocean floor is very thin in some places – so thin that the mantle seeps up there to create new crust. This is especially true in areas like the midPacific near where the Hawaiian Islands are still being built. Other areas where the crust is thicker, mountains build up or there are large inland plates of land lying far inland. These are spots where the crust is most likely to have crumpled up. Inside the crust, the temperature changes from what we experience on the surface to as high as 870 degrees Celsius deep inside. Rock will start to melt in these regions and turn to mantle. The earth has the same area around its surface, which means you cannot make more crust in the deep ocean without removing crust from somewhere else. Mantle gets inserted along the mid-oceanic ridges but is also sinking or subducting below the continental crustal areas. You will learn more about the different plates that float like a jigsaw puzzle on earth and what happens at their edges. The rocks you'll see at the oceanic crust will be basalt, which is mostly just lava that has hardened. The rocks at the continental crust will be like granite. Below the crust, the mantle is like plastic and is somewhat cool. These two parts make up the lithosphere. The asthenosphere is below the lithosphere. It is semisolid like gelatin and hot. It is like the lubricant that helps the lithosphere travel over the mantle. This layer goes to about 700 kilometers deep to earth's surface.
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