Friday, August 26, 2011

Mystery Solved... "Variolites"

Variolite cobble, Crescent Formation, Olympic Peninsula WA

For several years I have puzzled over the origin of this stone type, which is a remarkably common component of beach gravels around the Olympic Peninsula. (See my earlier post of July 19) It is so common, in fact, that it argues for a local source.  I haven't found that source yet - but I think I have discovered what it is.
The answer came from Scott Babcock, Professor of Geology at Western Washington State University, who has studied the Crescent Formation extensively.  He proposed that they were something called a "variolite" said to be a metamorphosed basalt (possibly pillow lava).  He sent me Googling for the quite-famous (though not to me) variolites of Durance, France.  Bingo.
A quick look at some photos of the Durance rocks was all I needed - these are clearly related.  (And there is a LOT of  basalt in the Crescent Formation here.)
As I said, the outcrop source for this rock is not known, at least that I've been able to discover, but maybe one of these days...
Variolite from Maguelon, France (www.variolite.fr)
Further note : I was also able to confirm the name, and provenance of this rock with Rowland Tabor, Geologist Emeritus at the US Geological Survey and author of the Guide to the Geology of Olympic National Park. He refers to it in his book (page 67-68) as "globular devitrification structures."  Wow, that's a mouthful.  However, he wasn't able to confirm a bedrock source. That, I guess, I am going to have to find for myself!




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