Which term describes the block of rock that forms the lower half of a fault?

The term that describes the block of rock that forms the lower half of a fault is the footwall. In the context of faulting, a fault is a fracture or zone of fractures in rock along which there has been displacement. When a fault occurs, two blocks of rock are created: the hanging wall and the footwall. The hanging wall is the block that lies above the fault plane, while the footwall lies below it.

To visualize this, imagine a slanted fault line; the block of rock you would "stand" on if you were at the top of the fault is the footwall, while the block that hangs above it would be the hanging wall. Understanding this terminology is crucial in geology, as it helps in discussing the mechanics of earthquakes and the movement of tectonic plates.