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 on my site. Amygdules were named by a 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.

8 Responses to “Amygdules”

  1. Silver Fox Says:

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

  2. hypocentre Says:

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

  3. 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

  4. 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!

  5. 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. :(

  6. Thurston Says:

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

  7. Andrew Says:

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

  8. 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.

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