Parking Area

Point Bonita Lighthouse

Geologic Trips, Marin Headlands

Point Bonita Lighthouse

 

The path to the Point Bonita Lighthouse extends from the parking area on the right and follows the small rugged peninsula through the tunnel to the lighthouse at the end of the Peninsula.

 

The rocks that make up the peninsula are mainly pillow basalt and the lighthouse is built on a foundation of pillow basalt. Look for the pillows as you go along the path to the lighthouse and also in the surf zone near the bridge to the lighthouse where there are fresh exposures of pillow basalt (see below) in the surf zone. This pillow basalt was part of the oceanic crust of the Farallon plate that was carried eastward into the Franciscan subduction zone. 

Related Links

Geologic background:  Marin Headlands (pdf)

Geologic trips:  Hawk Hill   Marin Headlands Terrane    Point Bonita Lighthouse   

 

Exterior Websites

Golden Gate NP Conservancy: Point Bonita

Golden Gate National Recreation Area:  Point Bonita Lighthouse

Pillow basalt (lower) in the surf near the bridge to the lighthouse. Wave action has etched the outlines of the “pillows”. The pillows were formed when basalt magma came into contact with sea water. The outer rim of the magma chilled rapidly—forming a pillow, then the fluid inner part broke through the rim to form another pillow—an so on.