CURRENT STATUS: Measurements of the total discharge of carbon
dioxide (CO2) gas at the Horseshoe Lake tree kill area range from 50-150
tons per day. Variations are primarily caused by changes in barometric
pressure. There is no obvious trend of either increasing or decreasing
gas flux at this area; we conclude that the total gas flux coming to the
surface at Horseshoe Lake has remained at these relatively high levels since
1996. We do not have enough data from any other gas discharge areas around the
mountain to draw conclusions about changes over time at those locations.
CURRENT HAZARDS: Inhaling high concentrations of carbon dioxide gas can cause dizziness, unconsciousness, and death. It is hazardous to dig holes in and around areas where the trees have been killed by carbon dioxide gas. Natural collapse pits that develop on the northwestern shore of Horseshoe Lake as the lake level declines contain high CO2 concentrations - extreme care should be taken to prevent children and dogs from entering these pits or digging up loose soil that has been placed in the pits. Care should also be taken to avoid a crack 1-2 feet wide that extends from the lake onto the west shore. Do not lie face down on the ground anywhere near Horseshoe Lake or the tree-kill area. As snow levels accumulate in the winter, toxic levels of CO2 can develop in tree wells, around buildings, and immediately below the snow surface in areas of high CO2 emissions. Pay serious attention to signs warning of CO2 hazards. |
Horseshoe Lake treekill (©John
Rogie)
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Aerial view of Horseshoe Lake and Mammoth Mountain. A large area of trees killed by carbon dioxide emissions is visible near the northern (top) shore of the lake. See tree-kill map for a map showing the locations of the carbon dioxide emission sites around Mammoth Mountain. |
High concentrations of carbon dioxide (CO2) in soil gas are killing trees on the flanks of Mammoth Mountain at the southwestern edge of Long Valley Caldera. First noted in 1990, the areas of tree kill now total about 170 acres in six general areas, including the most visually impressive tree-kill area adjacent to Horseshoe Lake on the south side of Mammoth Mountain. The soil gas in the tree-kill areas is composed of 20 to 90 percent CO2; there is less than 1 percent CO2 in soils outside the tree-kill areas.
Carbon dioxide flux (the rate at which CO2 gas comes out of the ground) is monitored periodically at each tree kill 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. See Helium Discharge at the Mammoth Mountain Fumarole for information on helium gas measurements.
The CO2 is released as a cool, diffuse gas from broad areas of soil. Although it quickly dissipates when it leaves the ground, CO2 is heavier than air and can collect in depressions in the land surface, in unvented buildings, and in other confined spaces. Carbon dioxide displaces oxygen and can cause unconsciousness or asphyxiation very quickly at concentrations above 30 percent. The National Institute for Occupational Safety and Health recommends a ceiling limit of 3 percent CO2 for up to 10 minutes for occupational exposure. Summer-time exposure to high levels of CO2 in the Horseshoe Lake area may result from lying directly on the ground or digging pits in the ground. Walking through the area in the summertime is safe for children and dogs, as long as their heads stay above ground level. During the winter, CO2 levels can build up beneath the snowpack and the CO2 gas will preferentially escape around buildings, through tree wells, and through depressions around large rocks. Such areas should obviously be avoided, as should snow camping within tree-kill areas.
McGee and others, 1998, Annual cycle of magmatic CO2 in a tree-kill soil at Mammoth Mountain, California; implications for soil acidification, Geology, vol. 26, no. 5, p. 463-466.
Sorey and others, 1999, Magmatic gas emissions from Mammoth Mountain, California Geology, September/October, p. 4-16.