I’ve heard, and you may have heard that Mindego Hill is an extinct volcano. It certainly looks like one from a distance.
Currently, geologists believe the hill was formed by a different scenario that goes like this:
Well, it is Volcanic
Mindego Hill is primarily basalt, a dark-colored igneous rock. Basalt can form in 1 of 3 ways.
1. Most of the earth’s basalt is formed at oceanic divergent boundaries. Hot mantle is pulled up from below by convection. When the plates pull apart, the mantle rises to the surface, melts and erupts onto the ocean floor. 2. Basalt can also be formed at geological hot spots, such as the big island of Hawaii. Eruptions start on the ocean floor and build up into a giant cone. 3. Finally, basalt can form in plumes and hot spots below continents, such as Yellowstone National Park, where an enormous amount of basalt is delivered through the earth’s crust up onto the continent.
About 17-20 million years ago, and 100s of kilometers to the south, there was an accumulation of lava flows, probably on the ocean floor. The lava was of low viscosity, and formed broad, nearly flat layers. The lava flows occurred over long periods of time as the San Andreas Fault System was developing.
Nothing Stays Put
The hardened basaltic rock then moved north as the Pacific Plate slid past the North American Plate along the San Andreas Fault. As they moved north, the lava flows that were once horizontal, were then folded and tilted, and even in many places are vertical or even over turned, with the top of the flow facing down. Mindego Basalt is also found nearby in Pilarcitos Creek, Tunitas Creek, and in Portola Redwoods State Park. The rock is in a northwest/southeast orientation, consistent with being moved up the coast. Another chunk of Mindego basalt moved 125 mi further northward along the San Gregorio fault to our west and is now located in Mendocino County (where it known as Iverson Basalt).
Erosion Resistance
Mindego Hill is still here not just because of lava eruptions or tectonic plate activity. It is also here because it is capped with Monterey mudstone. Monterey mudstone formed after the lava flows, 11.5 to 16 million years ago. It formed from fine-grained mud and the bodies of plankton that accumulated on the ocean floor. Many of these plankton were diatoms – organisms high in silica. Unlike basalt, which is relatively soft, the silica-rich Monterey mudstone is hard and resistant to erosion. Where the hard Monterey mudstone caps the softer basalt, hills remain.
Mindego Hill is here today, not because of a volcano, but because of planar lava flows, followed by folding, northward, movement, and erosion-resistant capping by of millions of tiny algae fossils. And the views from the summit are still just as lovely.
To log this find, please email or message me the answers to the following questions:
1. How many members are in your party?
2. Is Mindego Hill an extinct volcano?
3. Which of the 3 basalt formation processes listed above formed the basalt found at Mindego Hill?
4. Why is this basaltic hill still here and not eroded away?
5. Continue on to Waypoint MS - Monterey Shale. What colors do you see in the hard Monterey mudstone at GZ?
6. (Not required) Post a picture of yourself or your gizmo at the Mindego Hill summit sign post.
7. (Not required) Mindego Hill is a local landmark. Loggers are encouraged to post pictures of Mindego Hill from anywhere.
Sources
1. Chris Detwiller memo to POST, July 27, 2007
2. Sloan, Doris (2006) Geology of the Bay Area
3. Best, Timothy C. Road and Trail Erosion Inventory: Mindego Ranch Area Russian Ridge Open Space Preserve (2012)
4. http://ngmdb.usgs.gov/ngm-bin/pdp/zui_viewer.pl?id=34649 Dibblee, T.W., and Minch, J.A., 2007, Geologic map of the Mindego Hill quadrangle, San Mateo, Santa Clara, and Santa Cruz Counties, California: Dibblee Geological Foundation, Dibblee Foundation Map DF-349, scale 1:24,000
5. http://geology.com/rocks/basalt.shtml Basalt - What Is Basalt, How Does It Form and How Is It Used?
Congratulations to Damy2011 on FTF!