Oakland Hills - metamorphic rock (serpentinite)

Serpentinite from the Oakland Hills.

In the hills east of San Francisco Bay, you can find areas made up of serpentinite of the Great Valley complex (shown as Jsp on the geologic map).

Serpentinite is a metamorphic rock. Metamorphic rocks are formed when sedimentary or igneous rocks are exposed to conditions of very high pressure or temperature, or are exposed to large amounts of very hot water. Serpentinite is formed when plutonic rocks rich in iron and magnesium are exposed to very hot water beneath the Earth's surface. Serpentinite in the Sierra Nevada foothills is associated with gold deposits, and so is the California state rock (PDF ).

In the San Francisco Bay region, serpentinite is associated with mercury deposits that were extensively mined from the Gold Rush to the middle of the 20th Century in places like New Almaden in Santa Clara County.