The mantle is the thickest layer of the Earth, located between the crust and the outer core. It is composed primarily of silicate rocks and is characterized by its high temperature and pressure. The upper mantle, which is closest to the crust, is relatively solid and rigid, while the lower mantle is partially molten and can flow over long periods. The mantle is responsible for generating and transporting heat from the Earth's interior to the surface, as well as driving plate tectonics, which shapes the Earth's surface features.