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Daniel O'Hara, Mendenhall Fellow at the USGS California Volcano Observatory. 

These spectrograms of seismic stations near Long Valley's Mammoth Mountain show the brightly-colored punctuated spasms of small earthquakes happening within a 10 minute period.

USGS Mendenhall Postdoctoral Scholar Genna Chiaro specializes in petrology, geochronology, and volcanology. 

The Burdell Mountain volcanics are flow-banded porphyritic andesite, volcanic breccia, volcanic mudflow deposits, and minor flow-banded dacite which can be found west of the Petaluma Valley. Dated at about 11 million years old, they are related to the Quien Sabe Volcanics to the southeast and were displaced by the Hayward-Calaveras fault system.

A view of the southern Quien Sabe Range from high on Basalt Hill in Merced County. The Quien Sabe range is comprised of igneous intrusions, from >9 Ma to >11 Ma, predecessors to the volcanics of the Berkeley Hills and the southeastern equivalent of the Burdell Mountain lavas near Petaluma. Photo courtesy of Stephen W. Edwards

This exposure of volcanic breccia is a volcanic rock comprised of broken pieces of vitrophyre, a welded volcanic glass. The breccia is found in the Tolay Volcanics, a sequence of rhyolite, andesite, and basalt at least 1220 m thick. The 9 Ma Tolay Volcanics are equivalent to the Berkeley Hills Volcanics. Photo courtesy of Ross Wagner.