Carbon dioxide (CO₂) of magmatic origin is seeping out of the ground in unusual quantities at several locations around the flanks of Mammoth Mountain, a dormant volcano in Eastern California. The most recent volcanic activity on Mammoth Mountain was steam eruptions about 600 years ago, but seismic swarms and long-period earthquakes over the past decade are evidence of an active magmatic system at depth. The CO₂ emission probably began in 1990 but was not recognized until 1994. Seismic swarms and minor ground deformation during 1989, believed to be results of a shallow intrusion of magma beneath Mammoth Mountain, probably triggered the release of C0₁, which persists in 1998.

The CO₂ gas is at ambient temperatures and emanates diffusely from the soil surface rather than flowing from distinct vents. The CO₂ has collected in the soil by displacing air in the pore spaces and reaches concentrations of greater than 95 percent by volume in places. The total area affected by high CO₂ concentrations and high CO₂ flux from the soil surface was estimated at 60 hectares in 1997. Coniferous forest covering about 40 hectares has been killed by high CO₂, concentrations in the root zone.

In more than 300 soil-gas samples collected from depths of 0.5 to 2 m in 1995, CO₂ concentrations ranged from background levels (less than 1 percent) to greater than 95 percent by volume. At 250 locations, CO₂ flux was measured using a closed chamber in 1996; values, in grams per square meter per day, ranged from background (less than 25) to more than 30,000. On the basis of these data, the total emission of magmatic CO₂, in 1996 is estimated to be about 530 megagrams per day.

Concentrations of CO₂, exceeding Occupational Safety and Health Administration standards have been measured in pits dug in soil and snow, in poorly ventilated buildings, and in below-ground valve-boxes around Mammoth Mountain. CO₂, concentrations greater than 10 percent in poorly ventilated spaces are not uncommon on some parts of Mammoth Mountain. Humans and other animals exposed to CO₂ concentrations greater than 10 percent could lose consciousness and die rapidly. With knowledge of the problem and reasonable caution, however, the health hazard to humans can be avoided.

As noted earlier, the CO₂ emission is related to magmatic activity at depth, but at present (1998) it does not portend an imminent volcanic eruption.