Lithophysa
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A lithophysa is a felsic volcanic rock with a spherulitic structure and interior cavity with concentric chambers. Its outer shape is spherical or lenticular. They vary in size from very small up to tw
A lithophysa from France
A collection of lithophysae "thundereggs"
Lithophysae from Richardson Ranch near Madras Oregon. 1, 2. Displaying typical plates and "sutures". 3, 4. A cluster with three spheroid centers. 5, 6. Example of slow fill structure layers with a large opal layer at the bottom. Expansion is controlled by where the water was located in the structure. 7. "Dimple" and "pimple" where expansion began on an inner shell. 8. Fortification agate
Spheroids form in high silica magmas
Geode
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A geode is a geological secondary formation within sedimentary and volcanic rocks. Geodes are hollow, vaguely spherical rocks, in which masses of mineral matter are secluded.
Lithophysa
…be caused by expanding gases in tuffs and rhyolitic lavas before solidification. If the cavity becomes lined with crystals it may be referred to as a geode or if filled partially or fully with agate, jasper or opal it is called a thunderegg. The term vug is also used for similar cavities although the meaning…
Quartz-filled geode, shown from inside (top) and outside (bottom)
Reddish chalcedony geodes
Very large amethyst and regular quartz geodes