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What Floats on Top of the Asthenosphere?

The asthenosphere is the upper layer of the Earth's mantle. The entire surface of the Earth to some extent "floats" on it, because the asthenosphere is pliable and yields to pressures from below and above. Most of the asthenosphere is believed to be molten rock maintained under conditions of heat and pressure just below the point of liquefaction. The asthenosphere functions like a lubricated bearing that allows movement of the rigid layers on top of it. Tectonic plates that make up the crust of the Earth glide on the asthenosphere as continental drift gradually reshapes land masses.

Lithosphere
- The lithosphere encompasses everything between the asthenosphere and the surface. It is a rigid layer, brittle in many regions depending upon its age, and an average of 160 miles deep. The processes that result in volcanoes and earthquake faults originate in the lithosphere. The upper portion of the lithosphere comprises the oceanic crust and the continental crust.
Oceanic Crust
- The oceanic crust forms the bottom of the oceans. Composed mainly of dense, heavy basalt, it is four to seven miles thick. The rocks that make up the oceanic crust are less than 200 million years old, significantly younger than the continental crust. The oceanic crust is in a process of continuous renewal as magma exudes through fracture lines at deep ocean ridges.
Continental Crust
- The 29 percent of the Earth's surface above sea level makes up the continental crust, which is divided into seven continents. The continental crust is an average of 25 miles thick and some parts of it are as old as 3.8 billion years. The upper part is mostly granite, while the lower part is basalt and diorite.
Tectonic Plates
- The Earth's crust is broken into segments that fit together like jigsaw pieces. Called tectonic plates, these segments drift on the denser, more pliable asthenosphere at a rate of about two inches per year. Where tectonic plates collide, such as along the Pacific Ocean's "Ring Of Fire," subduction zones create volcanoes. Where the plates slide past each other, earthquake faults are formed.

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