Commonly known as thundereggs, lithophysae (Latin for "rock bubbles") originate as spheroids drawn from the rhyolite into the immiscible components of co-eruptive rhyolite-perlite lava flows and domes. Portions of rhyolite are pulled by turbulence or buoyancy from the parent rhyolite into the perlite, forming spheres which, as temperatures drop to a critical point, crystallize, driving dissolved gases out of solution. 
The progressive stage in the opening of a cavity in lithophysae is captured in these specimens, as controlled by the pressure of depth in the deposit from which they came.
From above left, at 8 feet deep, no cavities can develop. To its right, the specimen at 7' degassed to form a small crescent. From there, the specimens were dug from 6', 5', 4', 3', 2', and at the surface.
Over millions of years, the deposit is exposed to weathering and diagenesis-- the breaking down of the perlite to clay and silica. The silica is transported to the cavities within the waiting lithophysae, and is deposited as opal, agate, and/or quartz. If these nodules fill only partially, leaving a cavity, they-- like any hollow nodule-- are called geodes.
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Basin Range Volcanics Geolapidary Museum
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This web page was created for the Geode Kid by Carlton J Donaghe and Bill Boomhower
All contents copyright © 1997 Robert Colburn. All rights reserved.