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Geologic
Background
San
Francisco (from Geologic Trips, San Francisco and
the Bay Area, by Ted Konigsmark) Selected Websites Aerial Photographs of California
Coastline:
Fort Point Golden Gate National Parks
Conservancy:
Fort Point Golden Gate National Recreation
Area:
Fort
Point |
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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.
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