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Andrew Alden

Amygdules!

By June 9, 2009

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amygdulesLast weekend I visited my local volcanic park (every city should have one) and found myself transfixed by these little blobby things: amygdules. They started out as vesicles—gas bubbles—in the basaltic lava that once flowed there some 10 million years ago, but some time afterward, they filled with minerals. As the lava weathered and eroded away, the mineral-filled former vesicles emerged with their obscure new names. You could call them fossil bubbles.

"Amygdule" is a word first found in the 1878 textbook Elements of Geology by Joseph Le Conte. It's a coinage in scientific Latin, from "amyg-" meaning "almond" and "-dule" from "nodule." British geologists tend to favor the spelling "amygdale," which unfortunately can be confused with the medical term referring to almond-shaped glands and structures, like the uvula, or the French word for tonsils.

Amygdules — Geology Guide photo

Comments

June 9, 2009 at 9:11 pm
(1) Callan Bentley says:
June 9, 2009 at 10:19 pm
(2) Geology Guide says:

Funny that you used an exclamation point too.

June 10, 2009 at 1:40 pm
(3) Eric Logan says:

I would like to ask whether amygdules are likely to exist in submarine lavas. In my local area (Lower Trinity River, California)I have found an outcropping of dark-grey fine-grained rock with rusty weathering. Some parts of the outcropping have apparent gas bubbles. Some bubbles are vacant, some are filled with quartz-like material, others with rusty material, and a few are filled with silvery pyrite. Other rocks in the nearby area are granite, metamorphosed cherty limestone, serpentinite, greenstone, and light-brown slightly porphyritic rhyolite probably contiguous with the outcropping. I am interested whether the outcropping might be from a relatively recent surface eruption.

June 15, 2009 at 6:59 am
(4) Greg McHone says:

I recall an article long ago about how vesicularity varies with pressure in ocean floor lavas, so that depth could be estimated. The minerals that fill vesicles to make amygdules (!) can be interesting. Prehnite “hearts” are sought by collectors in Southbury, Ct. In Jurassic basalts on Grand Manan Island, there are zeolites such as mesolite, chabazite, heulandite, as well as jasper and calcite. Gases also can coalesce into “pipe vesicles” as illustrated at http://earth2geologists.net/grandmanangeology/GMminerals.htm and these can show “up” and sometimes flow directions too.

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