Mammoth Mountain

Geology

Mammoth Mountain is a lava dome complex in Mono County, California. It lies in the southwestern corner of the Long Valley Caldera and consists of about 12 rhyodacite and dacite overlapping domes. These domes formed in a long series of eruptions from 110,000 to 57,000 years ago, building a volcano that reaches 11059 ft in elevation. During this time, massive dacite eruptions occurred roughly every 5000 years. The volcano is still active with minor eruptions, the largest of which was a minor phreatic (steam) eruption 700 years ago.

Mammoth Mountain also lies on the south end of the Mono-Inyo chain of volcanic craters. Mammoth Mountain is composed primarily of dacite and rhyolite, part of which has been altered by hydrothermal activity from fumaroles (steam vents).

Volcanic gas discharge

Volcanic gas emissions at Mammoth Mountain have been extensively studied due to their environmental and safety impacts.

Mammoth is degassing large amounts of carbon dioxide out of its south flank, near Horseshoe Lake, causing mazuku in that area. The concentration of carbon dioxide in the ground ranges from 20% to 90% . Measurements in the 1990s of the total discharge of carbon dioxide gas at the Horseshoe Lake tree-kill area ranged from 50 to per day; this high concentration caused trees to die in six regions that total about 170 acre in size (see photo).

The tree-kills originally were attributed to a severe drought that affected California in the late 1980s and early 1990s. Another idea was that the kills were the result of a pathogen or other biological infestation. However, neither idea explained why all trees in the affected areas were killed regardless of age or health. Then, in March 1990, a U.S. Forest Service ranger became ill with suffocation symptoms after being in a snow-covered cabin near Horseshoe Lake. Unusual rates were not identified until 1994, but through analysis of 14C quantity in tree rings, it was later inferred to have begun in 1990.

measurements of the Horseshoe Lake area recorded concentrations of 2% in the visitor restroom, 25% in the snow cabin, and up to 60% in tree-wells within the dead-zone. In 1995, it was estimated that 1200 metric tons of were released per day. There is evidence that the rate of discharge has been declining, down to ~10 metric tons per day in 2010, with emissions peaking in 1991.

As of 2023, the concentration of carbon dioxide in soil gas at Mammoth Mountain was being monitored on a continuous, year-round basis at Horseshoe Lake, according to the U.S. Geological Survey, at a site where tree root suffocation was observed.

The originates from the degassing of magmatic carbonate metasedimentary rocks below the caldera. The uniformity in chemical and isotopic composition of the and accompanying gases at different locations around Mammoth Mountain indicates that there may be a large reservoir of gas deep below the mountain, from which gas escapes along faults to the surface. Measurements of helium emissions support the theory that the gases emitted in the tree kill area have the same source as those discharged from Mammoth Mountain Fumarole.

Ski patrol fatality incidents

In April 2006, three members of the Mammoth Mountain Ski Area ski patrol died while on duty when a fumarole caused a snow bridge to collapse under the patrollers. The 2005/2006 winter season delivered significant snow depth of 52 ft. As the ski patrollers assessed the fumarole for skier safety, the perimeter snow collapsed, dropping the patrollers 21 ft into the bottom of the fumarole, exposing them to extremely high levels of carbon dioxide. It is unclear if they died as a result of the fall or as a result of the gas.

On February 14th 2025, two Mammoth Mountain ski patrollers, including Claire Murphy, were caught in an avalanche while attempting avalanche mitigation. The avalanche occurred due to an atmospheric river storm that caused around 6 feet of snow to fall within 36 hours. One patroller was recovered at the scene uninjured, while Claire Murphy was hospitalized with serious injuries and later died. An official statement on the Mammoth Mountain website read: "It is with the heaviest of hearts that we share the Mammoth Mountain Ski Patroller, Claire Murphy, who was hospitalized last Friday, passed away as a result of her injuries."

Climate

Mammoth Mountain experiences an alpine climate, with cold winters and significant snowfall.{{Weather box

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Recreational use

Mammoth Mountain is home to the Mammoth Mountain Ski Area, founded by Dave McCoy in 1953. Mammoth is a ski, snowboard, and snowmobile mountain during the winter months. Mammoth is the highest ski resort in California and is notable for the unusually large amount of snowfall it receives compared to other Eastern Sierra peaks—about 400 in annually and about 300 out of 365 days of sunshine—due to its location in a low gap in the Sierra crest. In the summer months the ski gondolas are used by mountain bikers and tourists who wish to get a summit view of Long Valley Caldera directly to the east and Sierra peaks to the west, south and north. To the south of the mountain, there are a number of lakes that serve as tourist attractions in the summer.

References

References

1. {{cite ngs
2. {{cite peakbagger
3. Hildreth, Wes. (2004-09-14). "Volcanological perspectives on Long Valley, Mammoth Mountain, and Mono Craters: several contiguous but discrete systems". Elsevier B.V..
4. Pease, Robert C.. "Hydrothermal System of the Long Valley Caldera". Volcanoes of the Eastern Sierra Nevada: Geology and Natural Heritage of the Long Valley Caldera.
5. (2015-01-01). "Hazards of Volcanic Gases". Academic Press.
6. Farrar, C. D.. (December 2009). "Mammoth Mountain Magmatic Gas Emissions Persist 20 Years after the 1989 Geologic Unrest". AGU Fall Meeting Abstracts.
7. Farrar, Christopher D.. (1999). "Magmatic carbon dioxide emissions at Mammoth Mountain, California". U.S. Geological Survey.
8. "Volcanic Gas Monitoring at Mammoth Mountain {{!}} U.S. Geological Survey".
9. (2003-07-30). "Helium Discharge at Mammoth Mountain Fumarole (MMF)". [[United States Geological Survey]].
10. Covarrubias, Amanda. (2006-06-07). "3 Die in Mammoth Ski Patrol Accident". [[Los Angeles Times]].
11. "Mammoth Mountain Snowfall History @ MammothSnowman.com".
12. (7 April 2006). "3 ski patrollers killed at Mammoth". Orange County Register.
13. "Mammoth Mountain Avalanche Incident {{!}} February 14, 2025 {{!}} Operations Update".
14. Uranga, Rachel. (2025-02-23). "Ski patroller caught in Mammoth Mountain avalanche dies of her injuries, officials say".
15. "Mammoth Mountain Fact Sheet 22/23 Winter Season". Mammoth Mountain Ski Area.