Gas Releases At Mammoth Mountain More Complex Than Expected

"In the past, magmatic carbon dioxide degassing from the areas of tree kill was measured once a year," says John Rogie, Penn State graduate student in geosciences. "If the gas flux was lower than the previous year, it was thought that the carbon dioxide degassing rate was continually decreasing."

However, last year, Rogie monitored the area for a 24-hour period and found that the flux of carbon dioxide varied by up to a factor of three throughout the day.

"The 24-hour variation suggested that the system was very dynamic and that more than a single measurement was needed to tell us if the carbon dioxide rates were going up, staying the same or going down," says Rogie.

"We would really like to know what the carbon dioxide flux is at any time," he told attendees at the fall meeting of the American Geophysical Union, today, (Dec. 6) in San Francisco.

Working with Derrill M. Kerrick, professor of geosciences, Penn State; Michael Sorey, United States Geological Survey, Menlo Park; Giovanni Chiodini, Osservatorio Vesuviano, Naples, Italy; and Giorgio Virgili, WEST Systems, Pisa Italy, Rogie designed a plan to continuously monitor the rate of carbon dioxide degassing in the 35-acre tree-kill area near Horseshoe Lake.

Using specially manufactured instrumentation produced by WEST Systems, Rogie took continuous measurements of carbon dioxide flux and other environmental variables from August until the equipment was removed in early November, when snow levels reached more than two feet.

"As the snow accumulates, the carbon dioxide concentration at the snow/ground interface, and in the snowpack itself, builds up enormously and overwhelms the sensors on the instrument," says Rogie. "This also increases the risk to visitors in the tree kill area during the winter."

The tree kill areas on Mammoth Mountain are well posted, indicating they are not to be entered during the winter and are closed for camping even in the summer. These areas will also be off limits to skiing because of the dangers of carbon dioxide poisoning.

Rogie found that the average carbon dioxide flux at the instrument was 2,000 grams of carbon dioxide per square meter per day, but that levels could fall as low as 50 grams or go as high as 5,000 grams. The carbon dioxide flux showed a daily, cyclic pattern.

"The flux was typically high in the afternoons and low in the mornings which would seem to follow the 24-hour temperature cycle, but we have ruled out temperature as the cause of the variation," says Rogie. "There was also a 12-hour cycle which may be linked to barometric pressure oscillations ."

Kerrick adds, "Hopefully these measurements will help us determine how environmental factors like barometric pressure influence the degassing rate. We want to identify changes in the rate of gas ascent from depth by filtering out variation caused by weather related phenomena."

"This instrumentation and research will allow a better understanding of the dynamics of gas emission from volcanoes and provide an immediate warning device should the carbon dioxide emission rate suddenly increase," says Rogie.

Besides measuring carbon dioxide flux, the monitoring equipment measured barometric pressure, air temperature, humidity, wind speed, wind direction, soil temperature and soil moisture. The data was sent by Freewave ultrahigh frequency radio-modem to the village of Mammoth Lakes where it was recorded. A 12-volt battery, recharged by solar energy, powers the instrumentation.

"Determining which variables are associated with the 12-hour and 24-hour cycles is difficult," says Rogie. "One possibility is barometric pressure and another is Earth tides."

Earth tides are the effects of the moon's gravity on the solid portion of the Earth in the same way that ocean tides are the effects of the moon's gravity on the liquid parts of the Earth. Earth tides are very small, but do effect the width of fractures and faults, which could affect carbon dioxide degassing rates.

"In all likelihood, the variations in gas flux depend on a combination of barometric pressure, wind speed, Earth tides and soil moisture, but we have not yet developed a model to simplify this complex system," says Rogie.

Mammoth Mountain is considered to be a dormant volcano with its last eruption some 200 years ago, but may produce as much carbon dioxide as the active volcano Kilauea in Hawaii. Because the daily variability of carbon dioxide degassing is large, it is currently impossible to know exactly how much carbon dioxide is being emitted from Mammoth Mountain, but it is thought to be in the range of 400 to 1,200 metric tons per day.

Safety concerns in the area are based on the lethal effects of high concentrations of carbon dioxide.

"Normally, the concentration of carbon dioxide in the air is very low, well below 1 percent," says Rogie. "The levels seen in the ground and snow within tree kill areas are much higher, up to 99 percent, and very dangerous."

Rogie plans to reinstall his instrumentation before all the snow melts in the spring so that he can record the changes as the snow pack rapidly disappears.

This research is supported by the National Science Foundation, Penn State and the U.S.G.S.