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New geochronology reveals the volcanic history of Mono Craters

The Mono Craters, a line of volcanic domes and craters south of Mono Lake in eastern California, represent the youngest rhyolitic volcanoes in the western United States.

Rhyolite is a magma that is viscous and prone to explosive eruption. Consequently, these volcanoes pose a significant volcanic hazard to the region. Volcanic ash from past eruptions of Mono Craters covered large areas of California, and fell as far as Utah and Nevada. Up to now, the chronology of volcanism at Mono Craters has only been partly understood. The timing of the youngest eruptions has been known from carbon-14 dating of plants that were buried by ash; however, the chronology of the older eruptions has been uncertain.

A new study using tiny mineral crystals and the radioactive-decay series of uranium has revealed the early eruption history of Mono Craters. Marcaida et al. (2019) used an ion-shooting mass spectrometer to measure uranium and its daughter isotopes in zircon and allanite crystals in the rhyolites, and calculated the ages of their crystallization immediately before eruption. The results reveal that about 20 eruptions occurred between 10,000 and 65,000 years ago. In addition, the researchers used the new data to correlate ash beds around Mono Lake to their source volcanoes, and were able to identify ash expelled by explosive eruptions at nearby Mammoth Mountain.

Marcaida et al., 2019, Constraining the early eruptive history of the Mono Craters rhyolites, California, based on 238U–230Th isochron dating of their explosive and effusive products: Geochemistry, Geophysics, Geosystems.

View of the Mono Lake-Long Valley volcanic region
An aerial view looking south of the Long Valley volcanic region in the area of Mono Lake, showing a line of rhyolite lava domes and explosion craters. An inset map shows the locations of silcic magmatic centers in the Long Valley area.