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Parking Area |
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Point Bonita Lighthouse |
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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. Exterior Websites Golden Gate NP Conservancy: Golden Gate National Recreation
Area: _____ |
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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. |