Geologic Trips, San Francisco
The point of land that guards the south side of the Golden Gate is a large block of serpentine that lies within the Hunters Point melange of the Franciscan. The fort at Fort Point was built on this large block of serpentine. Another large block of serpentine supports the south tower of the Golden Gate Bridge. Serpentine consists of ultramafic rocks from the earth’s mantle that were altered by hot seawater. The hot seawater reached the ultramafic rocks through cracks formed in the earth’s oceanic crust during the spreading process. The Hunters Point mélange was formed in the Franciscan subduction zone as the Farallon Plate was thrust under the North American plate. Wherever you see serpentine on land, you are looking at a piece of oceanic crust that is out of place and was somehow injected into or squeezed onto the land.