09/30/2026
It's a tuff job being a volcanologist, but someone's gotta do it!
Dad jokes aside, tuff - or ignimbrite, for the international crowd - is a volcanic rock which tells us that a violent eruption once happened. The word "tuff" probably has Latin origins, since it appears as tuf in French and tufo or tufa in Italian. The Latin "tufus" or "tĹŤfus" referred to the porous volcanic rock that the Romans (and later Italians) used for building. Formed by pyroclastic flows containing volcanic ash and pumice, which was later compacted and/or welded into solid masses, these rocks are found throughout southern Italy in the Campanian Volcanic Arc. Usually, tuffs require large explosive eruptions - such as caldera eruptions - to create the extensive pyroclastic deposits that form tuffs.
When geologists began exploring the western United States, they found many examples of calderas and tuffs. One of the largest and most spectacular of these rock units was Long Valley's Bishop Tuff, formed 760,000 years ago during the massive eruption which created the Long Valley Caldera. This tuff can be hundreds of meters thick, and contains many of the features that geologists consider "classic" identifiers: rhyolitic ash and pumice, flow structures, and shattered crystals that grew in the magma before it erupted. It's also huge: more than 170 cubic kilometers (almost 50 cubic miles) of material cover 2,200 square kilometers (850 square miles) of land!
đź“·: A cliffy section of the Bishop Tuff in Long Valley shows that it had enough heat to weld the pyroclastic deposit into rock. USGS photo by M. Mitchell.
California Volcano Observatory Weekly Update for September 21, 2026: All volcanoes in California were at normal background levels of activity for the last week. Earthquakes >M1 were observed at Mammoth Mountain, the Coso Volcanic Field, and Salton Buttes.
