What best defines a tectonic plate?

• A. A small river
• B. A sheet of ice
• C. A part of the crust that moves slowly
• D. A mountain range

Answer: (C)

Tectonic plates are large, rigid pieces of Earth's outer shell that move slowly over the mantle. They make up the lithosphere, which includes the crust and the uppermost part of the mantle, and their motion is driven by convection currents beneath. Saying a tectonic plate is a part of the crust that moves slowly gets at the heart of the idea: a plate is a substantial chunk of the crust that has its own gradual motion, rather than a surface feature or a moving body of ice or water. The other options describe things that are not movable slabs of the Earth's outer shell: a river is liquid water, a sheet of ice is frozen water, and a mountain range is a raised landform built by tectonic forces but not a moving plate itself.