During what geological process does a normal fault typically occur?

A normal fault is a type of fault that occurs when the Earth's crust is subjected to tensile stress. This type of stress stretches the crust and causes it to break, resulting in one block of rock moving downward relative to another block. This mechanism is primarily associated with divergent boundaries where tectonic plates are moving apart, leading to an extension of the crust.

In the context of the other choices, the expansion of the Earth's crust relates to processes that can result in features like rift valleys, which are indeed areas where normal faults can occur. However, the fundamental driving force is the tensile stress itself. The sinking of the crust might suggest a different type of faulting or geological process, like subsidence, which is less about the faulting mechanism. Lastly, compression of layers is associated with reverse faults, where the layers are pushed together rather than pulled apart.

Thus, understanding that normal faults are characterized by their formation under tensile stress clarifies why this answer is the most accurate.