The layer of the Earth that lies beneath the crust is called the mantle. The mantle is composed of semi-solid rock that flows slowly over geological time. It plays a critical role in plate tectonics, as convective currents within the mantle drive the movement of the Earth's tectonic plates. This movement can lead to various geological phenomena, including earthquakes and volcanic activity. The mantle is significantly thicker than the crust and is fundamental to the geological processes that shape the Earth's surface over millions of years.