🔎 UNIDENTIFIED What are these? Most have stone sphere center, some with

Rgk50

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Mar 20, 2015
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Crystal like center
 

In an earlier post it was indicated that the outcrop with the Spheres was located near Bruneau Idaho. So I contacted the Idaho Geological Survey and showed them the above photos.
😀Here is their response :
It looks like you have some well-developed spherulites that formed in what is almost certainly a rhyolite. These form when the original glass recrystallizes as feldspar and quartz. When hollow, the term lithophysae is used, and that is how you get thunder eggs, the Spencer opal deposits, and geodes. Those with the crystals in the center are geodes. Hot fluids percolating through the rhyolite after the spherulites form carry the silica needed to form opal, agate, and quartz crystals.
These spherulites are probably from one of the many rhyolitic ash-flow tuffs in the region that formed in massive eruptions as a result of the passage of the Yellowstone hot spot. Rhyolite tuffs form when hot ash lands and welds back together. If that happens quickly enough, glass forms. It is that glass that will sometimes recrystallize.
An old USGS geologic map that shows the distribution of the various rhyolites is available here: https://ngmdb.usgs.gov/Prodesc/proddesc_9085.htm
CHEERS!🍻
 

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I've come across similar formations in area's where there was volcanic activity. Never had the explaination though. Good stuff.
 

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