Press release

A new geologic map of the Long Valley Caldera, Mammoth Mountain, and the Middle Fork canyon of the San Joaquin River including Devils Postpile National Monument, recounts the geologic and volcanic history of the area east of the Sierra Nevada in far greater detail than any previously published report.

Geophysical unrest beneath the Mammoth Mountain volcano and in adjacent parts of the Sierra Nevada and Long Valley caldera has generated concern among residents, stakeholders, and geoscientists since at least 1980, when four magnitude-6 earthquakes shook the area.  Extensive monitoring by the U.S. Geological Survey for three decades has documented numerous earthquake swarms, ground uplift and deformation, changes in hydrothermal systems, and emission of magmatic carbon dioxide at several sites that semi-encircle Mammoth Mountain.

Vertical columns of basaltic volcanic rock at Devils Postpile National Monument are formed when a think lava flows slowly cools.
Mammoth Mountain, U.S. Geological Survey

The Mammoth Lakes area of Mono County, owing to its spectacular alpine landscape, has become one of California’s busiest recreational playgrounds and a regional center of real estate development. Looming nearly 1,000 meters (more than 3,000 feet) above the downtown area, much of the five-kilometer (three mile)-wide Mammoth Mountain volcanic edifice has been laced with chair lifts, gondolas, ski runs and bike paths. In addition to skiing, snowboarding and summer mountain biking have recently become major activities. At the western base of Mammoth Mountain, along the canyon of the Middle Fork San Joaquin River, lies Devils Postpile National Monument. As many as 2,000 visitors per day enter the Monument during the summer season. Surrounded by Inyo National Forest, the area further contains several of the busiest trailheads in the Sierra Nevada, providing wilderness access for hikers, pack animals, mountaineers and fishermen. Any volcanic unrest has the potential to disrupt not only the local area, but could affect lifelines and infrastructure serving much of Southern California.

Because 2016 is the 100th anniversary of the establishment of the National Park Service, and the report is a significant milestone after decades of geological field work and research, USGS and the staff at Devils Postpile have teamed up to offer outdoor educational activities for the public this month as the new map and report are unveiled. The USGS authors, geologists Wes Hildreth and Judy Fierstein, and others will be hosting an interpretive talk at Minaret Vista Overlook near the monument on July 14, and leading an interpretive hike to the Devils Postpile and to nearby Rainbow Falls, both in the monument, on July 15. USGS scientists have a long history of working with the NPS staff including scientific training for the park’s interpretive rangers.

The new USGS map and report describe the comprehensive geologic setting and natural history of the area, including the volcanic history of Mammoth Mountain,Long Valley Caldera, and associated smaller volcanic features, including Devils Postpile, Pumice Buttes and the geologically independent Mono-Inyo chain of lava domes and craters. Also covered in the report, is an estimate of cooling time forformation of the iconic columns in the Devils Postpile basalt flow, and the geologic history of glaciation and faulting that took place before, between, and after the many volcanic eruptions.

With radiometric (argon isotope) ages for the surrounding rock formations, scientists can calibrate the eruption frequency and volcanic history of the area. Four late-Pleistocene (100,000 years old) lava flows inundated the floor of the Middle Fork San Joaquin River canyon. A new magma system developed around 230,000 years ago just west of the 767,000 year-old Long Valley Caldera. That system has had about 30 basaltic eruptions spread over a 10-mile wide volcanic field with Mammoth Mountain at its center. Mammoth Mountain, about 3,362 meters (11,030 feet) in elevation, is a pile of 25 overlapping lava domes, which were much more viscous and explosive than the surrounding basaltic lavas that erupted at nearly the same time. The ski area and part of the Town of Mammoth Lakes are built on top of the volcanic edifice.

The newly released U.S. Geological Survey Professional Paper 1812, by Wes Hildreth and Judy Fierstein, “Eruptive History of Mammoth Mountain and its Mafic Periphery, California,” is available online.

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