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USGS Geology in the Parks



Pyroclastics or tephra is a general term for all sizes of particles ejected into the air during volcanic eruptions. Includes particles as tiny as volcanic ash and as large as bombs and blocks.


The image at left shows a very close-up view of volcanic ash. Each one of the ash particles in this image is smaller than a single pixel on your computer screen! These ash particles are pumice, volcanic glass that is full of gas bubbles. The bubbles, called vesicles, form as gas bubbled out from molten rock that spewed out during a huge eruption of Brokeoff Mountain in Lassen Volcanic National Park.

The chart on the right shows the names we use to describe different sizes of tephra. Bombs and blocks can be enormous! Some of the blocks thrown out of the Woods Mountains volcanic center in Mojave National Preserve are 15 meters across!

Metric vs. English system chart

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