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Intracaldera tuff on Mount St. Helena, Sonoma Volcanic Field, CA

By California Volcano Observatory
A hiker looks up at a cliff of pockmarked, grungy gray rock which has fractures and multiple shades of white and tan breaking up its surface. The pockmarks resemble honeycomb, and the overall texture of the rock shows that it has numerous chunks of smaller rocks embedded in a white matrix.
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Detailed Description

Ash-flow tuff (ignimbrite) on the fire road leading to the summit of Mount St. Helena, mapped as part of the intracaldera Sonoma volcanics. 

Sources/Usage

Public Domain.

USGS photo by Jessica Ball

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