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Magmatic unrest beneath Mammoth Mountain, California

January 1, 2005

Mammoth Mountain, which stands on the southwest rim of Long Valley caldera in eastern California, last erupted ∼57,000 years BP. Episodic volcanic unrest detected beneath the mountain since late 1979, however, emphasizes that the underlying volcanic system is still active and capable of producing future volcanic eruptions. The unrest symptoms include swarms of small (M ≤ 3) earthquakes, spasmodic bursts (rapid-fire sequences of brittle-failure earthquakes with overlapping coda), long-period (LP) and very-long-period (VLP) volcanic earthquakes, ground deformation, diffuse emission of magmatic CO2, and fumarole gases with elevated 3He/4He ratios. Spatial-temporal relations defined by the multi-parameter monitoring data together with earthquake source mechanisms suggest that this Mammoth Mountain unrest is driven by the episodic release of a volume of CO2-rich hydrous magmatic fluid derived from the upper reaches of a plexus of basaltic dikes and sills at mid-crustal depths (10–20 km). As the mobilized fluid ascends through the brittle–plastic transition zone and into overlying brittle crust, it triggers earthquake swarm activity and, in the case of the prolonged, 11-month-long earthquake swarm of 1989, crustal deformation and the onset of diffuse COemissions. Future volcanic activity from this system would most likely involve steam explosions or small-volume, basaltic, strombolian or Hawaiian style eruptions. The impact of such an event would depend critically on vent location and season.

Citation Information

Publication Year 2005
Title Magmatic unrest beneath Mammoth Mountain, California
DOI 10.1016/j.jvolgeores.2005.03.002
Authors D. P. Hill, S. Prejean
Publication Type Article
Publication Subtype Journal Article
Series Title Journal of Volcanology and Geothermal Research
Series Number
Index ID 70027809
Record Source USGS Publications Warehouse
USGS Organization Volcano Hazards Program