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Large spherulites in Hot Creek rhyolite lava in the Long Valley Caldera

By California Volcano Observatory
A geologist crouches next to a rhyolite outcrop and points to an egg-shaped mass of radiating pink minerals about the size of a cantaloupe. The rest of the rock is pocked with other egg to grapefruit sized pink crystal masses, interspersed with chunky fragments of black glassy lava.
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Detailed Description

The presence of spherulites indicates that a lava flow cooled quickly, and their mineralogy holds clues to its precise cooling history. USGS photo by Jessica Ball

Sources/Usage

Public Domain.