Fault gouge from California, USA. (~9.6 centimeters across at its widest)

Fault gouge is a relatively soft, unlithified, mud-like to clayey material that occupies some fault zones or fills matrix between fault breccia clasts. It forms from extreme pulverization and grinding during multiple fault movements over geologic time.

This sample is from the San Andreas Fault Zone, which is a transform plate boundary between the Pacific Plate and the North American Plate. These two tectonic plates are sliding past each other. Despite the word "sliding", movement along the fault zone is usually in the form of sudden jolts - these movements send out shock waves, which cause earthquakes.

The San Andreas fault gouge specimen seen here is principally composed of montmorillonite (also known as smectite), a silicate mineral often called "swelling clay". When wet, it expands. When dry, it shrinks. Surface outcrops of this fault gouge display popcorn weathering, which forms by repeated wetting and drying of swelling clays. Minor amounts of kaolinite clay are also present.