Are Dugway Geodes "Thundereggs"????

I bought a small piece of 'thunderegg' rough at fullmoon on facebook. I buy anything that looks interesting and made two cabs.



I listed the one on the left on Etsy as a thunderegg cab and quickly a message that it was a Dugway. I did a search on Dugway and nothing I saw looked exactly like what I bought but I was given the correct id.

pubs.usgs.gov/pp/0415/report.pdf

The Answer is YES!

94 GEOLOGY AND MINERAL DEPOSITS, THOMAS AND DUGWAY RANGES, UTAH

Lithology.- The rhyolite varies from place to place in color, texture, abundance of phenocrysts, degree of layering, and presence or absence of spherulites. The variations are gradational, and there may be as many differences between parts of a single flow as between two separate flows; it is rarely possible to determine, by inspection or even by microscopic study, what flow a given specimen of rhyolite came from. Two rhyolite flows in the northwestern part of the Thomas Range, however, can be distinguished in places by the presence of copper-colored biotite flakes in the older flow.

The rhyolite is divisible into three facies: obsidian, red spherulitic rhyolite, and gray rhyolite. A thin layer of obsidian, formed by rapid cooling, is generally formed at the base of a thick flow of rhyolite (fig. 28).



Obsidian has also been found within or beneath layers of volcanic breccia in the northwestern part of the Thomas Range and about a mile northwest of the north end of Antelope Ridge. Discontinuous layers of obsidian occur, also, within other bodies of rhyolitic lava south of Dugway Pass, where they probably mark chilled surfaces of flows. The red spherulitic rhyolite is almost always found just above the obsidian near the base of a flow, but in a few places, notably in the northeastern part of the Thomas Range, it occurs considerably above the base. Whether in these places the red layers mark pauses in the outpouring of the lava is not known. Red spherulitic rhyolite resting on obsidian forms extremely irregular layers, which may vary laterally in thickness from less than an inch to 200 feet within a distance of 600 or 700 feet. The spherulitic rhyolite was found in all the subgroups except the lowest, in which only the top of the flow is exposed. The red rhyolite usually grades into gray rhyolite, through rock consisting of red spherulites in a gray matrix, but in ·some places the contact between the two facies is fairly sharp. The gray rhyolite makes up about 95 percent of all the rhyolite in the younger volcanic group.

Because the obsidian facies has sharp boundaries and is useful in distinguishing main subgroups from one another where pyroclastics are absent, it was mapped separately, but the gray and red rhyolites are represented by a single color symbol.

This one stands upon the flat on the right and my mother in law couldn't resist placing a little miner in the grotto.

RWA3006 This is the way it is going in the book, posted here for your records.

Utah’s Dugway Geodes are different from most lithophysae in that the central spherulites are often nearly microscopic, and the surrounding layers are of quartz and partial tridymite sheaves.



Almost like blowing up a balloon, this specimen from Dugway Utah preserved 3 stages of expansion. Above, 1. the moisture in the center began expanding, 2. Moisture in a layer halfway out barely began to expand, when 3. The area just above the center exploded as much as space would allow.

6-8 million years ago there were voluminous spherulite rhyolite eruptions in the Thomas Mt. Range.

Between 32,000 and 14,000 thousand years ago Lake Bonneville covered most of western Utah. The lake’s wave activity eroded the geode-bearing rhyolite out of Dugway Valley and redeposited the geodes several miles away in the Dugway geode bed area as lake sediments.







Having dug there, beneath the geodes in sediments are some of the original rhyolite flows.

geology.utah.gov/popular/places-to-go/rock-mineral-collecting-sites/the-rockhounder-dugway-geode-beds-juab-county/