The word lithosphere is derived from the word "sphere," combined with the Greek word "lithos" which means rock. The lithosphere is the solid outer section of Earth which includes Earth's crust (the "skin" of rock on the outer layer of planet Earth), as well as the underlying cool, dense, and fairly rigid upper part of the upper mantle. The lithosphere extends from the surface of Earth to a depth of about 44-62 mi (70-100 km). This relatively cool and rigid section of Earth is believed to "float" on top of the warmer, non-rigid, and partially melted material directly below.
Earth is made up of several layers. The outermost layer is called Earth's crust. The thickness of Earth's crust varies. Under the oceans the crust is only about 3-5 mi (5-10 km) thick. Under the continents, however, the crust thickens to about 22 mi (35 km) and reaches depths of up to 37 mi (60 km) under some mountain ranges. Beneath the crust is a layer of rock material that is also solid, rigid, and relatively cool, but is believed to be made up of denser material. This layer is called the upper part of the upper mantle, and varies in depth from about 31 mi (50 km) to 62 mi (100 km) below Earth's surface. The combination of the crust and this upper part of the upper mantle, which are both comprised of relatively cool and rigid rock material, is called the lithosphere.
Below the lithosphere, the temperature is believed to reach 1,832°F (1,000°C) which is warm enough to allow rock material to flow if pressurized. Seismic evidence suggests that there is also some molten material at this depth (perhaps about 10%). This zone which lies directly below the lithosphere is called the asthenosphere, from the Greek word "asthenes," meaning "weak." The lithosphere, including both the solid portion of the upper mantle and Earth's crust, is carried "piggyback" on top of the weaker, less rigid asthenosphere, which seems to be in continual motion. This motion creates stress in the rigid rock layers above it, and the plates of the lithosphere are forced against each other. This motion of the lithospheric plates is known as plate tectonics, and is responsible for many of the movements that we see on Earth's surface today including earthquakes, certain types of volcanic activity, and continental drift.
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