Final answer:
The brittle, rigid solid comprised of the crust and uppermost mantle is called the lithosphere. When stresses act on the lithosphere, it breaks, causing earthquakes.
Step-by-step explanation:
The brittle, rigid solid comprised of the crust and uppermost mantle is called the lithosphere. The lithosphere is the outermost mechanical layer of the Earth, about 100 kilometers thick. It behaves as a solid and includes both the crust and the uppermost mantle, which are both brittle. When stresses act on the lithosphere, it can break, resulting in earthquakes.