Small Fault at Bodega Head

Provided and copyright by: David Lynch, Thule Scientific
Summary authors & editors: David Lynch

All plate boundaries are faults, but not all faults are plate boundaries. In fact, most are not. A fault is defined as a crack in a rock along which slippage has taken place. Most faults are actually quite small and have no names because they're so numerous. One such fault is particularly well-exposed in the Bodega Head diorite near the city of Bodega Bay, California. Having been transported hundreds of miles northwest from southern California by the San Andreas Fault, the Salinian Block of granite and diorite is heavily sheared. More recent granitic intrusions have marbled the rock.

Here a small fault is easily seen thanks to a light colored granite pegmatite intruded into the darker diorite. A small backpack is shown for scale. The obvious fault plane runs from lower left to upper right, and like most faults is curved a little. The offset is no more than about 6 inches (15 cm) and along the fault plane is a thin layer of brownish gouge that is soft enough to dig out with a fingernail. An even smaller fault cutting the pegmatite is evident at the lower right.

Related Links:

• Field Guide to the San Andreas Fault
• Faults and Earthquakes
• More About Diorite
• Plate Boundaries