Final answer:
The lithosphere is the solid part of the Earth's crust, including both the crust and uppermost mantle, and is responsible for tectonic activities such as earthquakes.
Step-by-step explanation:
The lithosphere is the solid part of the Earth's crust, encompassing both the crust and the uppermost mantle. These layers behave as a brittle, rigid solid, which is fundamental to the concept of plate tectonics. The lithosphere's rigidity is what causes it to break under stress, leading to phenomena such as earthquakes.
The crust itself is thin and rock-like, varying in composition and part of the more extensive lithosphere. Moreover, the mantle beneath the crust, which also forms part of the lithosphere, can deform and flow slowly, although it is considered solid at the temperatures and pressures found within the Earth. The behavior of the lithosphere as a rigid layer is in contrast to that of the asthenosphere, which is partially molten and flows plastically.