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 About.com 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.
9 June 2009 at 11:00 pm
Those are great amygdules, I was first introduced to amygdules in field camp at Sunset Crater, Arizona.
9 June 2009 at 11:28 pm
You say amygdules, I say amygdales – US & UK: two nations divided by a common language
10 June 2009 at 9:13 am
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 About.com.
10 June 2009 at 9:21 am
Go down below the lower end of the deep quarry at Sibley Park and you’ll find amygdules larger than your fist!
10 June 2009 at 10:51 am
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. :(
10 June 2009 at 12:44 pm
What is the mechanism(s) by which material infiltrates congealed lava and forms amygdules?
10 June 2009 at 4:12 pm
Thurston, groundwater is everywhere, even in lava beds once they cool. It brings in dissolved minerals and leaves them behind.
10 June 2009 at 4:14 pm
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.
12 June 2009 at 2:14 pm
According to the sidebar at The Accretionary Wedge, you have participated in this geology carnival in the past. We have a new edition going up soon, around the theme of “When and where would you most like to visit in person to witness something first-hand?” Are you going to be able to join our time warp? (more details at the link) We’re trying to get The AW back on it’s feet, and would love to have you participate. Sorry for the short notice, but late submissions will be added on as they arrive.