Measuring volcanic gases: soil efflux

Scientists measuring carbon dioxide gas flux in soil, Mammoth Mountain, California

Measuring CO2 efflux in soil
Photograph by R. Kessler
on September 6, 1997

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

Map of carbon dioxide contration in soil near Horseshoe Lake, California

Map of CO2 concentration near
Horseshoe Lake and Mammoth
Mountain, California

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

Aerial view of Horseshoe Lake from atop Mammoth Mountain, California

Tree-kill area near
Horseshoe Lake