The Angel's Serpent EarthCache
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As an earthcache, there is no physical container. Instead you are invited to learn more about Angel Island and its geology. The Island has a remarkable history, from its use by Native Americans to roles as an army outpost, an immigration and quarantine station, and a park. It also has an important geological history.
Angel Island is the largest island in San Francisco Bay, rising from a valley that was formed about 2 to 3 million years ago when geologic forces started to push up the surrounding mountains and hills. Sediments were deposited by glacial runoff from the Sierras, which formed sandstone that was uplifted from the bay floor. As the glaciers of the last ice age started to melt 14,000 to 16,000 years ago, the rising ocean slowly covered the coastal plain, which is now the continental shelf. About 8,000 years ago, the ocean covered the bay, flooding through the Golden Gate, and leaving this island.
In addition to the sandstone, Angel Island has numerous outcrops of metamorphic rock that were altered by extreme high pressure in the subduction zone where the earth's plates collide and at spreading centers where the plates were pulled apart. These rocks, including serpentine, were thrust over the sandstone and became part of the Angel Island terrane.
Serpentinite
Serpentinite, composed from serpentine minerals, was named California's "State Rock" in 1965, the first rock to be designated by a state in this way. It is found in 42 of the state's counties and was closely associated with deposits during the Gold Rush.
The name is taken from the Latin word serpentinus, meaning serpent rock, for its color and surface quality. Its chemical composition allows only uniquely adapted native plants grow in its soil. One of the minerals associated with it is chrysotile asbestos, which was mined extensively for using in construction and household appliances. Lobbyists for the makers of asbestos products were among those who urged state legislators to recognize serpentinite as the state rock. However, asbestos was found to cause cancer when fibers are inhaled and bills have been introduced to remove serpentinite as the state rock because of this association.
As a metamorphic rock, it formed underneath the ocean and the colliding continental plates forced it upward. It often resulted from a spreading center, such as that pictured here. When plates move apart, the hot rocks in the earth's mantle are pulled upward. Sea water enters fractures at the spreading center and reacts with the mantle rocks. Softer, more slippery, and more buoyant, the resulting rock squeezes upward along fault lines. When you see serpentinite, then, you are looking at part of the oceanic crust that was injected or squeezed onto the land.
The Earthcache Location
The serpentine at the earthcache coordinates is part of a belt that is several hundred feet thick. The rocks appear to have been almost unchanged by the metamorphic process so that the original rock type is still apparent.
A quarry was established at this location and a rock crusher was built here. It was used primarily to create road building material in the 1930s. Some of the serpentinite gravel can still be seen below the pavement of the perimeter road. However, using it in a rock crusher may not have been the best idea because of the asbestos content.
Logging Requirements
Please email me through my profile rather than use the message center.
Identify the name of this cache and email the earthcache owner with the answers to the following questions:
- What qualities and characteristics of the rock that your find here remind you of a serpent? In other words, what is the color, the texture, and the feel of the rock?
- Do the qualities of serpentinite vary depending on its location in the quarry? Does weathering affect the luster of the rock?
- Why do you think these rocks were chosen for the quarry?
- Photo Requirement: Post a picture showing the Quarry or the old equipment 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. No online research is needed to answer these questionsbr
Park Information
Angel Island is part of the California State Parks. It is a perfect place to hike or bike. For more information on visiting the island please contact the Angel Island Association or the California State Parks.
Visitors generally take a ferry from Tiburon, Alameda, or San Francisco. Please be aware of their schedule and fairs.
Resources
- Geological Trips: San Francisco and the Bay Area, Ted Konigsmark, GeoPress, 1998
- Geology of the San Francisco Bay Region, Doris Sloan, University of California Press, 2006.
- Angel Island State Park: An Island In History, Greenbelt Alliance.
Additional Hints
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Treasures
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