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Measuring volcanic gases: soil efflux

Soil efflux measurements can be made in areas where volcanic gases, typically CO₂, rise from depth and discharge into the soil just beneath the surface. Because the gases escape from the ground over a broad area, a small accumulation chamber must be used to collect and measure the gas at dozens to hundreds of separate sites in order to calculate the total soil efflux of gas.

Soon after a batch of magma rose beneath Mammoth Mountain, California to within a few kilometers of the surface in 1989-90, several zones of high CO₂ soil efflux developed around the volcano. To determine the extent and amount of CO₂ gas escaping from the soil, scientists (left) have made thousands of soil efflux measurements in the area in the past few years.

Surveys of soil-gas efflux use accumulation chamber & analyzer

To measure the rate at which these gases are released into the atmosphere, an accumulation chamber connected to a infrared CO₂ analyzer is placed on the soil surface. As the gas enters the chamber from the soil, the LI-COR analyzer (on a backpack in the image) then records the increasing concentration of CO₂ gas in the chamber.

The rate of increase of CO₂ in the chamber is then used along with ambient temperature, pressure, and other parameters to calculate a soil CO₂ efflux for that location. The measuring apparatus is then moved to other nearby locations and additional efflux values are measured.

Detailed surveys yield soil-CO₂ concentration maps

If enough points are measured, a map (top left) of the soil CO₂ anomaly can be constructed and a total gas emission can be calculated. Although individual measurements can be influenced by soil and meteorological conditions, such soil gas surveys, if repeated often enough, can provide insight into any trends that may develop.

First noticed in 1990, several zones of high CO₂ soil efflux have developed around Mammoth Mountain volcano in California. The gas is associated with a batch of magma that rose to within a few kilometers of the surface in 1989-90. These zones generally occur where faults and other structures intersect the surface and provide conduits for the flow of gas to the surface.

The cores of these high CO₂ zones are easily recognized by the presence of dead trees. One of the best known tree-kill areas is at Horseshoe Lake on the south side of Mammoth Mountain (bottom left). Since 1995, USGS scientists have been tracking the CO₂ anomaly there with periodic soil CO₂ efflux surveys.

More information about carbon dioxide gas at Horseshoe Lake

• Photographs of the tree-kill area
• Fact Sheet: CO₂ gas killing trees at Mammoth Mountain | PDF | HTML |
• More information from the USGS Long Valley Observatory