SERPENTINITE

Serpentinite is California's state rock. This 1-1/2 meter serpentinite boulder sits in a place of honor in the roof garden of the Oakland Museum of California, its polished surface gleaming blue and jade-green.

This rock is composed almost exclusively of minerals of the serpentine group, which forms from the alteration of oceanic mantle rocks.  It happens that northern California's Coast Ranges are full of just that kind of material, carried by plate motions against the North American plate and subducted onto the continent, where the delicate rock type is preserved.

These seafloor rocks were kneaded by both the horizontal compression due to subduction and the horizontal elongation due to movement along the San Andreas fault system. The Franciscan Complex is what resulted, a challenging problem for generations of western geologists. Only with the advent of plate tectonics did the story of this rock become clear.

Serpentinite can contain chrysotile the serpentine mineral that crystallizes in long, thin fibers. This is the mineral commonly known as asbestos.

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