We recently revisited the Coast Range Ophiolite, which is found on Mount Diablo in Contra Costa County.

An ophiolite is a section of oceanic crustal rocks, a sequence that typifies ocean crust wherever it exists. One might be led, then, to ask what ocean crust is doing on Mount Diablo (40 km outside of San Francisco), 300 meters or so above sea level. Good question. It happens that when a terrane docks with a continent some of the ocean floor is often included in the suture. This particular parcel of ocean floor was formed about 165 million years ago, in the mid to late Jurassic. Subsequently it was buried under 10,000 meters of sediment, and then was faulted to the surface as the Franciscan subduction (~145 - 30 million years before present) of the Farallon plate came to a close, and the right lateral faulting associated with the San Andreas system began.

A cross section of sea floor is typically ordered like this, from top down: chert, pillow lavas, sheeted dikes, gabbro, peridotite. One can see in this set of photos chert, pillow lavas, a diabase (gabbro) quarry, and a manzanita ‘barrens’ (which usually grow on serpentine sourced soils; serpentinite being hydrothermally altered peridotite).

Clicking on any of the images below takes you to a set of images for that day, and away from this web log.

Chert:

Chert (closeup):

Chert (even closer):

Pillow lava:

Diabase (Gabbro) quarry (the next photo is from an earlier visit):

Manzanita barrens: