I visited Sibley Volcanic Reserve on Sunday and was transfixed by this:

The light-colored blobs are amygdules, or fossil bubbles. Many of the lava flows that issued from a small volcano here about 10 million years ago were full of gas bubbles. Later those filled with minerals, and today the amygdules are weathering out. The minerals involved include quartz, its noncrystalline variety chalcedony, and various zeolite minerals. Read a little more about them in this page from my old site.

This view shows a set of stretched-out amygdules that represent bubbles deformed by flowing lava.

And this closeup shows two of them sticking out of the rock surface.

A reminder that collecting rocks is not permitted in any of the regional parks, although you can collect rocks and (non-vertebrate) fossils in East Bay MUD watershed lands and Forest Service lands for personal enjoyment.

Amygdules were given their name by UC Berkeley geology professor Joseph Le Conte in 1878. He surely saw them in these very hills.

I also sought out the labyrinths and found three localities. The first one is on the north side of Round Top, hidden in a sweet spot surrounded by vegetation and butterflies, including this Chalcedon checkerspot (Euphydryas chalcedona).

8 Responses to “Amygdules”

  1. Cynthia Adkisson Says:

    Hi, Andrew, this is great info. We are doing habitat restoration on the Northside of the Caldecott (–you can see we’ve cleared out the top of Barberry Peak, which you have visited), and a lot of these amygdules have weathered out, along with the zeolites. Lots of “Berkeley Agates” and then some small pieces that have the blue green look of copper bearing rocks. Do you have any idea when the mineral bearing hot water came through the Moraga Formation, or was it from one eruption to the other? Happy to share our restoration work with you.

  2. Mike Caton Says:

    I clicked through to your previous post about the labyrinth up there. You begin one paragraph “What amazes me more satisfyingly…” Sir, this phrase needs to make its way into science and philosophy texts. That’s a great post.

  3. Thurston Says:

    What is the mechanism(s) by which material infiltrates congealed lava and forms amygdules?

    [Thurston, groundwater is everywhere, even in lava beds once they cool. It brings in dissolved minerals and leaves them behind. — Andrew]

  4. SteveN Says:

    I’ve heard the Sibley amygdules describes as opaline, but in my experience it seems straight quartz, not even crypto-crystalline. Maybe most of the good ones have already been dug out. :(

  5. Sheldon White Says:

    Go down below the lower end of the deep quarry at Sibley Park and you’ll find amygdules larger than your fist!

  6. Andrew Says:

    But UK geologists are confused, probably because they’re all taught French, in which amygdale means tonsils. Amygdule is based on sound scientific Greek as I explain on

  7. hypocentre Says:

    You say amygdules, I say amygdales – US & UK: two nations divided by a common language

  8. Silver Fox Says:

    Those are great amygdules, I was first introduced to amygdules in field camp at Sunset Crater, Arizona.

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