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 and caused ski patroller fatalities in 2006. 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 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). Volcanic gas discharge 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 . Measurements of the total discharge of carbon dioxide gas at the Horseshoe Lake tree-kill area range from per day. This high concentration causes trees to die in six regions that total about 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% concentration (toxic) and a deadly 25% concentration of in a small cabin. concentrations of less than 1% are typical and healthy