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. 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 about 57,000 years ago. The volcano is still active, having minor eruptions; the largest eruption 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 Mountain 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 whose activity subsided after an enormous eruption 760,000 years ago. Volcanologists consider the Mammoth Mountain magmatic system distinct from that of Long Valley caldera and Inyo Craters. Mammoth emits 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. 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 of carbon dioxide emissions causes trees to die in six regions totaling about 170 acres in size. Authorities have prohibited camping prohibited in the tree kill area since 1995, to prevent asphyxiation of campers due to accumulation of carbon dioxide in tents and restrooms. Experts originally attributed the death of so many trees to a severe drought that affected California in the late 1980s and early 1990s. Others assumed that the barren areas were the result of a pathogen or other biological infestation. However, neither idea explained why all trees in the affected areas died 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 uncovered that restrooms and tents had greater than 1 percent carbon dioxide (CO2) concentration and a deadly 25 percent concentration of carbon dioxide (CO2) in thee small cabin. Carbon dioxide (CO2) concentrations of less than 1 percent are typical and present in most healthy soil; however, soil concentrations of carbon dioxide (CO2) in the tree kill areas ranged from 20 percent to 90 percent. Researchers discovered that this overabundance of carbon dioxide (CO2) caused the trees in the area to die because tree roots need to absorb oxygen (O2) directly and the high carbon dioxide (CO2) level reduced available oxygen. Researchers also determined that Mammoth releases about 1,300 short tons (1,200 t) of carbon dioxide (CO2) every day. Since 2003, experts have monitored the concentration of carbon dioxide in the soil at Mammoth Mountain 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. Mammoth press articles describe events in Mammoth and the surround communities. Mammoth Lakes is a premiere resort town nestled in the Sierra Nevada Mountains. Home of Mammoth Mountain Ski Area, the town boasts skiing and snowboarding activities for winter sports lovers. Summer brings fishing, hiking and biking. Nearby attractions, include Yosemite National Park, Mono Lake and Benton Hot Springs.