Mammoth Mountain is a lava dome complex partially located within the town of Mammoth Lakes, California, in the Inyo National Forest of Madera and Mono Counties. It is home to a large ski area primarily on the Mono County side.

Mammoth Mountain was formed in a series of eruptions that ended 57,000 years ago. Mammoth still produces hazardous volcanic gases that kill trees.

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 11,059 feet (3,371 m) 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. The magma source for Mammoth Mountain is distinct from those of both the Long Valley Caldera and the Inyo Craters. Mammoth Mountain is composed primarily of dacite and rhyolite, part of which has been altered by hydrothermal activity from fumaroles (steam vents).

Mammoth is outgassing 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 percent CO₂. Measurements of the total discharge of carbon dioxide gas at the Horseshoe Lake tree-kill area range from 50 to 150 short tons (45 to 140 t) per day. This high concentration causes trees to die in six regions that total about 170 acres (0.69 km) 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.

Measurements around the lake found that restrooms and tents had a greater than 1% CO₂ concentration (toxic) and a deadly 25% concentration of CO₂ in a small cabin. CO₂ concentrations of less than 1% are typical and healthy in most soils; however, soil concentrations of CO₂ in the tree-kill areas ranged from 20% to 90%. This overabundance of CO₂ was found to be the cause of the tree-kills because tree roots need to absorb O₂ directly and the high CO₂ level reduced available O₂. Researchers also determined that Mammoth releases about 1,300 short tons (1,200 t) of CO₂ every day. As of 2003, the concentration of carbon dioxide in soil gas at Mammoth Mountain is being monitored on a continuous, year-round basis at four sites—three at Horseshoe Lake and one near the base of the previous Chair 19 at the ski area.

The most likely sources of the CO₂ are degassing of intruded magma and gas release from limestone-rich metasedimentary rocks that are heated by magmatic intrusions. The remarkable uniformity in chemical and isotopic composition of the CO₂ and accompanying gases at different locations around Mammoth Mountain indicates that there may actually 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. There is evidence that the rate of CO₂ discharge has been declining, with emissions peaking in 1991.

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 feet (16 m). As the ski patrollers assessed the fumarole for skier safety, the perimeter snow collapsed, dropping the patrollers 21 feet (6.4 m) 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.

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 (1,000 cm) annually and about 300 out of 365 days of sunshine—due to its location in a low gap in the Sierra crest. The ski area has more than 3,500 acres (1,420 ha) of skiable terrain, with 3,100 ft (940 m) of vertical serviced by 25 lifts. 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.

• Joel Day (28 Jul 2023). "'Explosive' US supervolcano showing 'clues of imminent eruption' with millions at risk".
• Alt, David; Hyndman, Donald (2000). Roadside Geology of Northern and Central California. Missoula, Montana: Mountain Press Publishing Company. ISBN 978-0-87842-409-2.

• Mammoth Mountain Ski Area trail map
• USGS Volcano Hazards Program, Long Valley Observatory
• QTVR of tree kill area
• Mammoth Earthquake 1980 May 25 16:33, 16:49, 19:44 UTC Magnitude 6.2, 5.9, 5.9
• "The Supervolcano in California; Long Valley" (Video (4:52)). GeologyHub. 2021.
• "Secrets of the Underground" (Documentary). Science Channel. 2017.