Mammoth Mountain

Mammoth Mountain is a lava dome complex west of the town of Mammoth Lakes; California in the Inyo National Forest of Madera County and Mono County.3 It is home to a large ski area on the Mono County side. The mountain formed from a long series of eruptions that started about 110,000 years ago and lasted to perhaps 57,000 years ago. The volcano is still active with minor eruptions; the largest of which was a minor phreatic eruption 700 years ago. Mammoth Mountain is composed primarily of dacite and rhyolite; some of which altered by hydrothermal activity from steam vents. Mammoth lies on the South end of the Mono-Inyo chain of volcanic craters, some of which erupted as recently as 250 years ago. Mammoth Mountain is on the southwestern edge of Long Valley Caldera, a large area that subsided after an enormous eruption 760,000 years ago. The Mammoth Mountain magmatic system considered distinct from that of Long Valley caldera and Inyo Craters. Mammoth is outgassing large amounts of carbon dioxide out of its south flank, near Horseshoe Lake. 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 per day. This high concentration causes trees to die in six regions that total about 170 acres in size. Camping prohibited in the tree kill area since 1995, to prevent asphyxiation of campers due to accumulation of carbon dioxide in tents and restrooms. The tree kills originally 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 killed regardless of age or health. Then in March 1990, a United States 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 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 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 Chair 19 at the ski area.