Sibley sights


Sibley Regional Volcanic Preserve has some spectacular places like this spot at the north end of the park, where you can visualize when this was a working volcano. Click the view below for a bigger version.

sibley layers

This set of rock layers was laid down flat, but since then it has been tilted counterclockwise to nearly vertical. The red zone appears to be the baked top of a sediment bed that was buried in lava. The sediments were themselves deposited on an earlier lava flow, at right. I didn’t inspect this close up, though, for a couple of reasons. One, I was out for a walk and didn’t have my boots on. For another, this is easy to look at but extremely steep and hard to get close to. Finally, it’s hazardous, being prone to rockfalls:


5 Responses to “Sibley sights”

  1. Sarah Says:

    Hi, I’m new to the East Bay area. Found your blog today after I visited Albany Hill. I thought it looked a little geologically out of place, so I wanted to find out more about it. Ended up being so interested in your writing that I read through the entire archives. Super interesting, though I’m even more freaked out about the earthquake risk now after having read all the Hayward Fault posts!

  2. Andrew Says:

    Mike, the volcano was part of a migrating eruptive center, terrestrial not marine, stretching from the Quien Sabe volcanics near Gilroy up to today’s location at The Geysers. The current theory is that it’s following the Mendocino triple junction north as the descending Farallon plate, cut off by the northward progression of the San Andreas fault system, leaves a gap behind it—a “slab window”—that allows heat and fluids to ascend for a short while. The tilting happened with intermittent transpression across the fault zone, the same forces that raised the East Bay hills. Subduction-related accretion ended in the Miocene around here, but is still going on north of the triple junction (in Cascadia).

    I’m giving you the straight dose because you have a serious interest in geology.

  3. Mike Says:

    Was this volcano a volcanic island that was accreted onto the continent? (collided with & merged with the continent). Or did it erupt on the continent. I’m suspecting the former, and was tilted/folded going through the subduction zone.

  4. Beej Says:

    I love that rock in the park. The unstable bit actually spilled a bunch of rocks across the road a few months ago.

  5. Sheldon White Says:

    This was one of my favorite parks when I lived in the East Bay. There’s such a variety of volcanic features and interesting geomorphology. Plus there’s some great geodes to be found!

    [Sheldon, as you know the park does not allow collecting of any specimens, except by scientists who have permits. If you or I find interesting things, we leave them where they are and take away only pictures. Just saying this so other readers don’t misunderstand. — Andrew]

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