Oakland Rock Gallery 1
Geologic Tour of Oakland, California
(c) 2003 Andrew Alden, licensed to About.com, Inc.
Oakland bedrock isn't always easy to find because the hills are vegetated (often guarded by poison oak) and fenced off by housing or watershed lands. The greenschist (left) and serpentine (middle) come from a roadcut along Butters Road near the Butters Land Trust land, above the fault near Joaquin Miller Park. They're in a belt of Jurassic serpentinite, part of the Coast Range Ophiolite. They are deep seafloor mantle rocks altered long ago by chemical action with seawater, then baked and squeezed against the buttress of the North American continent by plate interactions in the 150 million years or so since then.
The other rock is pumice from the old sulfur mine property at the end of McDonell Road (see some photos of the property by Jef Poskanzer). The area is mapped as keratophyre of Late Jurassic age (about 160-145 million years), the same rock found in the great Leona Quarry. Keratophyre is an altered igneous rock high in silica, alumina and sodium. Anyway, the pumice is undoubtedly younger, related to Miocene volcanism around 10 million years ago.
