Faults

When stress builds up in Earth's crust, rocks can break, forming fractures called faults. Faults occur when rocks on either side of the break move.

If there is no movement along the break, it is called a joint, like the cracks found in cooled basalt rocks.

Faults are classified by the type of movement between rocks. There are three main types: normal faults, reverse faults, and strike-slip faults.

In a normal fault, the hanging wall—the rock above the fault—drops relative to the footwall beneath the fault. This happens when tension pulls rocks apart, often creating mountain ranges and rift valleys.

A reverse or thrust fault forms when compression pushes the hanging wall over the footwall, producing folded landforms like ridges and cliffs.

Strike-slip faults occur when rocks slide past each other horizontally due to stress pushing in opposite, parallel directions. An example is the San Andreas Fault, where the Pacific Plate moves past the North American Plate.

Fault zones, where multiple faults are clustered, can trigger earthquakes as rocks break and release energy.