Volcanic Gas Monitoring at Mammoth Mountain
USGS Volcano Hazards Program researchers evaluate changes in volcanic gas concentration at Mammoth Mountain. The concentration of carbon dioxide (CO2) gas in soil is currently being monitored on a continuous, year-round basis at a site at Horseshoe Lake, where an area of trees has been killed due to root suffocation. Bi-annual surveys of the amount of gas being released from the soil (flux) are also conducted at Horseshoe Lake. Gas flux surveys at other locations are also conducted, but on a less regular basis.
Gas flux surveys provide information that allows scientists to develop CO2 emissions maps, like the one to the right produced from data collected in July 2012 around the Horseshoe Lake tree kill. The results shows several areas of very high flow (purple patches, >80% CO2 concentration near the soil surface) surrounded by broader region of lower, but still elevated, CO2 discharge (red to green) that is encircled by an area of normal CO2 emissions (blue). The total CO2 output through the soil when these measurements were made was 240 metric tonnes every day, enough to fill 19 full size blimps (12.5 tonnes per blimp). However, since CO2 is heavier than air these blimps would not float into the sky.
Other than Horseshoe Lake, there have been up to 7 additional areas where elevated CO2 discharge has been observed in the past; four of those have been monitored in the last 10 years. Together these areas contribute more CO2 to the atmosphere than other volcanoes in the Western United States, though the emissions are low compared to erupting volcanoes (typically > 1,000 tonnes per day).
The diffuse emission of CO2 has remained at a fairly low level since degassing in the tree kill areas emerged in 1990, however emission has not remained constant. Long-term monitoring of the degassing shows that CO2 emissions wax and wane and typically peak approximately 2 years following shallow seismic swarms. Experts agree that the most likely cause for this pattern is that the earthquakes mark the time when gas that was released from magma below moves its way into a subsurface reservoir in the shallow crust. The lag in gas detection at the surface is caused by the time required for the gas to travel upward from the reservoir and reach the surface.
Mammoth Mountain Fumarole
Mammoth Mountain fumarole (MMF), is a thermal gas vent located on the north side of Mammoth Mountain. CO2 makes up over 98% of the fumarole gas with lesser concentrations of other gases including helium. The isotopic compositions of carbon dioxide and helium gas discharging at MMF are similar to those in the soil gas at the Mammoth Mountain tree kill areas. These similarities and the persistent high rate of CO2 emissions from these areas, strongly indicates that they are derived from the same long-lived reservoir of magmatic and crustal gas trapped at depth beneath the mountain.