Final answer:
The lower mantle moves due to convection, with heat driving the slow ascent of warmer material and descent of cooler material, while the rocks remain mostly solid due to high pressures, even under extreme heat. Melting can occur through pressure decrease or volatile addition, contributing to volcanic and tectonic processes.
Step-by-step explanation:
The movement of the lower mantle is primarily driven by the process of convection, where heat from Earth's interior causes warmer material to rise upwards, while cooler material sinks. Despite the extreme heat, the rocks within the mantle remain solid due to the incredibly high pressures that counteract the melting process. However, when conditions allow, such as the decrease in pressure or the addition of volatiles like water that lower the melting point, parts of the mantle can melt, forming magma. This melting can occur at various tectonic settings, including subducting plate boundaries and hotspots, creating a dynamic environment responsible for volcanic and tectonic activity.