What is a scientific fault?

A fault is a crack in a rock along which slippage has occurred. If there’s no movement, the crack is called a joint. Most faults are small, even microscopic. Only the large ones get names, like the San Andreas Fault. The upper picture shows a small fault in fine-grained basaltic rock that was intruded by molten quartz. After the quartz cooled and hardened, tectonic stress caused the rock to crack and shift, offsetting the quartz veins along the fault. The lower picture shows a digital reconstruction of the rock before faulting occurred.