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September 7, 2023

If you were visiting Long Valley this past week, you might have seen a cavalcade of SUVs periodically disgorging people wielding rock hammers, hand lenses, and brightly colored notebooks.

A stark white wall of pitted and furrowed pumice and volcanic ash rises 3-4 meters above the heads of a large group of brightly-bedecked scientists. Thin layers mark the concave lower half of the wall, with the upper overhang holding fewer layers but more football-sized pumice stones.
USGS geoscientists examine air fall tephra and pyroclastic flow deposits at the base of the Bishop Tuff, in a quarry near Bishop, CA. This pumice-rich tuff was laid down by the caldera-forming eruption of the Long Valley Caldera around 760,000 years ago. USGS photo by Jessica L. Ball

This field excursion treated USGS geoscientists to expansive views of the Long Valley Caldera, Mono-Inyo Craters, Mono Lake and Sierran granite and metamorphic rocks under clear blue skies. On the way, participants examined new geothermal features at Hot Creek, saw the workings of the power plants at Casa Diablo, and checked out the seismic and GPS stations that make up CalVO's extensive monitoring network in the Long Valley region.

The Long Valley Caldera on the eastern side of the Sierra Nevada has a long, complicated eruption history including abundant basalts 4 to 2.6 million years ago, sticky rhyolite domes 2.2 million to 800,000 years ago, an enormous caldera-forming eruption ~760,000 years ago, and other volcanic activity as recent as 600 years ago.  This context provides a geologic framework for understanding current conditions inside and outside the caldera and is critical to characterizing volcanic hazards.

A group of geoscientists in bright t-shirts, hats and vests sit amongst dark volcanic boulders overlooking a desert stream. The banks of the stream are bright green with vegetation, while the slopes above are dotted with sagebrush and short pinon pine trees. In the background, gray rocky peaks hold scattered patches of snow.
At the overlook to the Hot Creek geological site, USGS geoscientists examine the thermal features and rock alteration which characterize the geothermally active area. USGS photo by Jessica L. Ball

This gathering brought together place-based experts with more recent additions to CalVO, CVO, and HVO for an exchange of knowledge, some hands-on time with volcanic rocks, and a healthy dose of discussion and debate. Revisiting, challenging, and revising our scientific assumptions is how geoscientists make advances and test new theories.

Continuing work in the Long Valley region includes installing/maintaining GPS and seismic sensors in Long Valley Caldera and Paoha Island on Mono Lake; monitoring yearly earthquake swarms associated with snowmelt; geochronology of precaldera basaltic lavas; and other geologic, hydrologic, and geophysical studies.

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