The layer of the Earth composed of molten rock is known as the mantle. This thick layer lies between the Earth's crust and the outer core and is made up of solid rock that behaves like a viscous liquid over long periods. The mantle's convection currents are primarily responsible for the movement of tectonic plates, which shapes the planet's surface. While the upper mantle is more rigid, the lower mantle is composed of hotter and denser molten rock that can flow. Understanding the mantle's composition and behavior is crucial for studying geological processes, such as volcanic activity and earthquakes.