Specimen Measures approx: 2" x 2" This particular mine is no longer being mined for these geodes.
Composed of rhyolite, agate and quartz, these beautiful hollow geodes were formed from volcanic activity... They began as hollow 'Lithophysae' (Rock Bubbles) produced within the flow of molten rhyolite rock as it surfaced and began to flow above ground. As the flowing molten rhyolite began to cool. Cristobalite and Perthite from within, crystallized into ball or button-shaped "Spherulites'. Some of these spherulites, upon reaching maturity, expelled a flash of hot gas creating a star-shaped bubble. The size and state of development of the bubble are related to the size and depth of the spherulite at the time of its firing. Those closer to the surface face less pressure from the overburden weight of the molten rock above, and produce larger gas cavities. Those deeper down may only manage to develop a small arch-shaped cavity. Some are duds and never de-gas. The rock bubble becomes buoyant and rises out of the main body of rhyolite, up into the layer of rhyolitic ash above. Some float up even further and into the layer of obsidian-like 'Perlite which accompanies and flows above the heavier rhyolite.
Over millions of years, trapped gas inside the geode and moisture seepage (containing dissolved silica) caused layers of chalcedony "fortification" agate to line the inside of the cavity. Later the rate of moisture seepage decreased, changing the water chemistry to grow quartz crystals.
All Dugways start out as hollow rock bubbles, but end up as unique and beautiful objects coming in various colors of crystals, agate and rhyolite. Few Dugways are found having the classic un disturbed exterior shape and star-shaped cavity common in Oregon Geodes. The irregular shapes indicate that the rhyolite flow at Dugway moved over an uneven surface tumbling the rock bubbles.
Utah Dugways occur from 1 inch to 12 inches in diameter. The geode bed is confined to an area approximately 1 1/2 square miles in size, and is located in the desert of western Juab County, Utah.
Rhyolite, Agate and Quartz