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Serpentinite Boulder

Gallery of Pet Rocks and Favorite Stones

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Photos can't capture all the color and textural appeal of this serpentinite specimen, polished by deep movements in the California Coast Ranges. (more below)
A glossy green pal
(c) 2007 Andrew Alden. This rock comes from near stop 4 of the California Subduction Rocks Tour
Countless times as a child, I would walk a rocky beach and pick up pretty stones, discarding one as another more attractive caught my eye. The agates of Pebble Beach, south of San Francisco, were particularly hard to choose from. (My free wallpaper image of Rodeo Beach pebbles may stir similar memories.) Each pebble was unique and promising. Today I feel that same way about serpentinite.

Serpentine rocks are born fighting. The hydration of deep seafloor rocks—peridotite—creates serpentinite, which is not only a softer material but one that occupies something like 30 percent greater space. It pushes back. Serpentinization and deserpentinization make subducting plates very energetic places, and huge mud volcanoes made of serpentinite can punch their way through kilometers of cover to erupt on the deep sea floor, often to be recycled again into the subduction zone. Serpentinite is "rode hard and put away wet."

California is full of serpentinite because it was a Japan-style subduction zone for hundreds of millions of years, and many scraps of ocean crust got swept up and plastered onto it. The resulting bodies of serpentinite are slippery and mobile, fighting their way to the surface all over the state especially after the slice-and-dicing brought about by the San Andreas fault zone. This boulder comes from a former seafloor mud volcano near Lake Berryessa. It has been kneaded, as a pitcher kneads a fresh baseball, by movements in the Coast Ranges, and almost every surface is glossy. Lots of serpentinite is fragile, but this chunk feels pretty solid.

Serpentine is named for its resemblance to snakeskin. Also like snakes, it is beautiful and hard to kill.

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