Mammoth Mountain - Volcanic Gas Discharge

Volcanic Gas Discharge

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 (45 to 140 t) per day. This high concentration causes trees to die in six regions that total about 170 acres (0.69 km2) in size (see photo, below).

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 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 (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 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.