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 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.