1 January 2011

GoSF10: The San Andreas Fault & Mussel Rock

Posted by Callan Bentley

“GoSF” = Geology of San Francisco

Happy new year. Take it from me, nothing cures a hangover like traipsing across a plate boundary and thinking about one of the best geology books ever written. Mountain Beltway has been reviewing three field trips I took the week before last to check out the geology of San Francisco and neighboring areas. The overall plan (now complete) was to cover:

Introduction and overview

Seafloor basalt

Deep sea chert

Kirby Cove, Marin Headlands

(wildlife interlude)

Graywacke turbidites

Serpentinite and mélange

Fractures and the chemistry along them

Pleistocene dunes

The San Andreas Fault and Mussel Rock (today)

Our final stop on the two-year-college field trip was to the town of Pacifica, California, where the San Andreas Fault runs off the edge of the land and into the Pacific Ocean, running parallel to the Golden Gate Bridge and eventually coming ashore again adjacent to Point Reyes. (It runs underneath preternaturally-straight Tomales Bay.)

Here’s an animated view over the area where the San Andreas drops into the sea. Perspective shifts from the north to the east:

The viewpoint is looking from the Pacific Plate, across the transform (right lateral) plate boundary at the North American Plate. The land surface jumps up roughly along that boundary. Along that high bluff (landslide scarp) you will see a bunch of houses perched:

…Except where you don’t.

The area of trees at top center-right is an area where the houses have already tipped over the edge. As this bluff retreats, the other houses will disappear too. Nice view while it lasts, though.

The houses on the bluff look west over an icon of the San Andreas Fault and geological folklore, Mussel Rock:

Mussel Rock is the site of the opening vignette of John McPhee’s Assembling California. He states (enigmatically to the non-geologists):

Mussel Rock is a horse.

As any geologist will tell you, a horse is a displaced rock mass that has been caught between the walls of a fault. This one appeared to have got away. It seemed to have strained successfully to jump out of the continent.

It was a nice homage to McPhee, I thought, for me to make a pilgrimage (however brief) to this spot which I first read about so many years ago.

Thanks for joining me on this journey through a few of the rocks that San Francisco and its surrounding areas have to offer. What a great place to learn about geology!