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USGS Geology in the Parks

Visual Glossary

Close-up of tuff at Hole in the Wall, Mojave National Preserve


Tuff is a volcanic rock made up of a mixture of volcanic rock and mineral fragments in a volcanic ash matrix. Wherever there are explosive volcanic eruptions you can expect to find tuff.

Tuff forms when some combination of ash, rock and mineral fragments (pyroclastics or tephra) are blasted into the air, then fall to the ground as a mixed deposit. Most of the rock fragments tend to be volcanic rocks that were once solidified parts of the volcano that erupted to produce the tuff, but sometimes other types of rock are blasted out and incorporated into the tuff as well. Sometimes erupted material is so hot when it reaches the ground that it fuses together to produce a welded tuff.

The image above shows a typical tuff. This tuff is one of several layers deposited during violent volcanic eruptions that shook the Mojave Desert about 18 million years ago. You can see these colorful rocks at Hole in the Wall in Mojave National Preserve.

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