Lithosphere, outer part of Earth, consisting of the crust and the upper mantle. The term "lithosphere" is derived from Greek and means rock layer. The lithosphere is approximately 65 to 100 km (40 to 60 mi) thick and lies above the layer known as the asthenosphere, which consists of softer, less rigid rocky material. Geologists regard the lithosphere as the relatively cool, outermost layer of the planet and view it as a rigid shell. Although the rock compositions of the crust and the upper mantle differ, geologists prefer to view the two parts as a single unit because both are brittle and they behave as a single mass in the motion of the rocky plates that make up Earth`s surface layer ( "See "Plate Tectonics).

Earth scientists distinguished the lithosphere

Earth scientists distinguished the lithosphere from the underlying asthenosphere by studying seismic waves. Seismic waves originate from the movement of rock masses during an earthquake. The speed with which seismic waves travel through Earth`s interior depends on the nature and density of the rocks beneath the surface. Scientists have analyzed hundreds of seismic waves and found that such waves abruptly slow down at a depth of about 100 km (60 mi). At this depth lies the boundary between the lithosphere and the underlying, softer asthenosphere. The plastic nature of the asthenosphere causes seismic waves to slow down in this layer ( "See "Seismology: "Seismic Phenomena").

The crust is the outermost part of the

The crust is the outermost part of the lithosphere. Underlying the crust is the upper mantlea relatively brittle part of the earth`s interior. The rocks of the crust include granite and basalt. Granite makes up the crust of the continents, while basalt makes up the crust of the ocean floor. The rocks found in the crust consist mostly of lighter elements such as silicon, potassium, and sodium. The density of these rocks is about three times that of water. The rocks of the upper mantle are denser. Rocks found in the upper mantle include peridotite, a dark-colored rock composed of the minerals olivine and pyroxene. These minerals contain the heavier elements iron and magnesium.

The lithosphere has been at the center

The lithosphere has been at the center of research in plate tectonics, one of the 20th century`s most revolutionary scientific theories. This theory explains that the lithosphere consists of several large and small tectonic plates, or thin, brittle pieces that move against one another. These plates are fragments of the lithosphere that are packed together, similar to pieces of a giant jigsaw puzzle. The tectonic plates move at about the same speed at which fingernails grow. This rate may not seem to be significant, but on a geological time scale of millions of years, such movements may shift entire continents all over the earth`s surface.

The motion of the lithospheric plates results

The motion of the lithospheric plates results from their attachment to the plastic, slowly flowing asthenosphere. Where two plates are in contact, three different types of boundaries (called plate boundaries) are possible. A "convergent boundary" exists between two plates that are moving toward each other. A "divergent boundary" exists between plates that are moving apart from each other (as in the middle of the Atlantic Ocean). A "transform boundary" refers to the boundary between two plates that are sliding past each other (as in the region of the San Andreas Fault in California).

Most violent geologic events

Most violent geologic events, such as earthquakes and volcanic eruptions, occur at plate boundaries where two plates are slowly colliding or rubbing against each other. Indeed, the distribution of most of the major regions of the world that are prone to earthquakes and eruptions closely follows the boundaries between the lithospheric plates ( "See "Earthquake).

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