The Pillows of Point Bonita EarthCache
The Pillows of Point Bonita
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Point Bonita in the Marin Headlands offers stunning views, rich geology, and a historic lighthouse. Through its geology we can learn about how the Earth has changed and how this Point has endured storms, earthquakes and pounding waves. To log this cache, you will need to visit the area, learn about its geology, and answer the logging questions below.
The Geology
As you approach the area, you will find incredible views of the Golden Gate. Try to imagine the area 12,000 years ago when a glacially-fed river flowed through a gorge before it crossed a coastal plain and flowed into the Pacific Ocean. But the Headlands are far older and contain the geologic record of the forces that created California.
Around 160 millions years ago (give or take a few years), the North American plate moved westward, away from what is now Europe and Africa. At that time, California did not exist. The North American Plate collided with the Farallon Plate, and the Farallon in turn moved under the North American Plate. This subduction zone resulted in oceanic rock being scraped off the Farallon Plate and piling up against the edge of the North American continent. The pillow basalts of the Point Bonita Lighthouse, which were formed in the middle of the Pacific basin, ended up here on the edge of North America as a result of this process. They are among the last remnants of the Farallon Plate.
When subjected to constant wave action, basalt forms the hard, erosion-resistant black to dark green sea cliffs that can be seen at the point. In particular, the pillow basalts were formed when volcanic magma erupted into cold ocean water -- the outer layer of basalt hardened forming a round, pillow shaped shell. As magma pushed from behind, part of the shell burst and more magma rushed out. This pattern of magma release led to a formation similar to pillows stacked one on top of another. Pillow basalts therefore indicate that the area was once underwater.
The chemistry of the rock here suggests that it was created at a volcanic oceanic island or seamount - in this regard, the basalt rocks at Point Bonita appear to be somewhat different than others in the Headlands, which formed along a mid-ocean ridge near a hot spot.
Seeing the Pillows
The most scenic place to see the pillow basalt formations is near the Lighthouse on the left side of the suspension bridge beyond the tunnel. Look down and you will see a beautiful stack of pillow basalt. It is one of the best examples of this formation in California.The tunnel and the bridge to the Lighthouse is currently open (as of August 201`8) on Sundays and Mondays from 12:30-3:30, but it is best to check on current hours and conditions if you want to go to the lighthouse itself.
Even if the tunnel is closed, pillow basalt can be seen to the right of the path just before the tunnel entrance. These pillows have been cut in half and may be seen exposed on the cliff as weathered oval-shaped pockets. Just before you get to the tunnel entrance, see if you can find the red chert formations on the cliff among the basalt. Cherts are formed by clay and the skeletons of microscopic plankton which fell to the sea floor and accumulated there.
To Log the Cache
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The listed coordinates will take you to a spot just before the tunnel entrance. You can complete this cache even if the tunnel is locked and the lighthouse is closed. To log you find, email me through my profile:
- Some pieces of basalt look like they have polka dots -- white dots on the rock that are minerals filled with gas bubbles in the lava. Pillow basalt with these dots erupted at a shallower depth. In very deep water, such as at a seafloor spreading center, the weight of the water squeezes out the gas in the lava, and the basalt that forms has no vesicles (gas bubble holes). What does the basalt you see at Point Bonits say about its formation?
- Look at the red chert near the tunnel entrance and think about how it was deposited. What does the chert indicate about the sequence of the volcanic eruptions? Were the eruptions continuous or intermittent? Why?
- Photo Requirement: Post a picture showing the tunnel, basalt, or the Lighthouse with your log. Under the June 2019 guidelines, you do not need to be in the photo but it should otherwise include your gpsr, a personal item, or an informational sign.
Please submit your answers before logging this as a "find." Logs that have not fulfilled the logging requirements will be deleted.
For further information
You can learn more about the area by visiting the Marin Headlands Visitor Center, which is is open daily from 9:30 a.m. to 4:30 pm.
The geology of the area is explained in depth in the excellent guide by William Elder of the National Park Service, The Geology of the Golden Gate Headlands.
Additional Hints
(Decrypt)
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