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Pyroclastic flow and carbon dioxide hazards at Mammoth Mountain.

Pyroclastic Flow and Surge Hazard Zones from Potential Vents in the Mammoth Mountain Area, Long Valley Caldera, California

This hazard zone is based on explosive eruptions from vents located along the chain in the past 10,000 years that are known to have ejected <1 km3 (0.25 mi3) of magma and generated pyroclastic flows or surges. The zone is centered along the south moat of the caldera, which is the location of epicenters of many swarms of earthquakes since 1980 and the area of most intense ground movement (deformation). Thus, scientists suggest that future eruptions might occur from this restless zone, and pyroclastic flows and surges could travel as far as 15 km (10 mi) from a new vent. Future pyroclastic flows and surges from a single eruption in the south moat would affect only a part of the total hazard zone shown on the map.

Pyroclastic-flow hazard zones

Potential vents from both the Mono-Inyo chain and Mammoth Mountain area of the caldera.

Carbon dioxide at Mammoth Mountain

High concentrations of carbon dioxide (CO2) gas in soil are killing trees on the flanks of Mammoth Mountain. First noted in 1990, the areas of tree kill 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.

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 CO2hazards.

The ultimate source of much of the CO2 is magma beneath Mammoth Mountain. The onset of CO2 emissions is closely linked to a major seismic swarm beneath the mountain in 1989. Scientific studies indicates that there may be a large reservoir of gas deep below the mountain.

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.