Brittle deformation results in the breaking of rock along fractures. Joints are fractures along which no movement has occurred. These may be tectonic structures formed in response to regional stresses or formed by other processes such as cooling of igneous rocks. Columnar joints are common in igneous rocks, forming six-sided columns when the magma cools and shrinks.
Fractures along which relative displacement has occurred are known as faults. Most faults are inclined surfaces. The block of rock above the fault is the hanging wall, and the block beneath the fault is the footwall, after old mining terms. Faults are classified according to their dip and the direction of relative movement across the fault. Normal faults are faults along which the hanging wall has moved down relative to the footwall. Reverse faults are faults along which the hanging wall moves up relative to the footwall. Thrust faults are a special class of reverse faults that dip fewer than 45°. Strike-slip faults are steeply dipping (nearly vertical) faults along which the principal movement is horizontal. The sense of movement on strike-slip faults may be right lateral or left lateral, determined by standing on one block and describing whether the block across has moved to the right or to the left.
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