Reducing the Risk From Volcano Hazards
Reducing the Risk From Volcano Hazards
Mammoth Mountain is a young volcano on the southwestern rim of Long Valley Caldera, a large volcanic depression in eastern California. The Long Valley area, well known for its superb skiing, hiking, and camping, has been volcanically active for about 4 million years. The most recent volcanic eruptions in the region occurred about 200 years ago, and earthquakes frequently shake the area. Because of this, the U.S. Geological Survey (USGS) operates an extensive network of instruments to monitor the continuing unrest in the Long Valley area.
(Click on image for larger version - 105k)Numerous small earthquakes occurred beneath Mammoth Mountain from May to November 1989. Data collected from monitoring instruments during those months indicated that a small body of magma (molten rock) was rising through a fissure beneath the mountain. In the following year, U.S. Forest Service rangers noticed areas of dead and dying trees on the mountain. After drought and insect infestations were eliminated as causes, a geologic explanation was suspected. USGS scientists then made measurements and discovered that the roots of the trees are being killed by exceptionally high concentrations of CO2 gas in the soil. Today areas of dead and dying trees at Mammoth Mountain total more than 100 acres. The town of Mammoth Lakes, just east of this volcano, has not been affected.
Although leaves of plants produce oxygen (O2) from CO2 during photosynthesis, their roots need to absorb O2 directly. The high CO2 concentrations in the soil on Mammoth Mountain are killing trees by denying their roots O2 and by interfering with nutrient uptake. In the areas of tree kill, CO2 makes up about 20 to 95% of the gas content of the soil; soil gas normally contains 1% or less CO2.
(Click on image for larger version - 48k)When CO2 from soil leaves the ground, it normally mixes with the air and dissipates rapidly. CO2 is heavier than air, however, and it can collect at high concentrations in the lower parts of depressions and enclosures, posing a potential danger to people. Breathing air with more than 30% CO2 can very quickly cause unconsciousness and death. Therefore, poorly ventilated areas above and below ground can be dangerous in areas of CO2 seepage. Where thick snowpacks accumulate in winter, the CO2 can be trapped within and beneath the snow. Dangerous levels of CO2 have been measured in pits dug in the snowpack in tree-kill areas on Mammoth Mountain, and snow-cave camping in such areas is not advised.
Geologists have detected CO2 emissions, like those at Mammoth Mountain, on the flanks of other volcanoes, including Kilauea in Hawaii and Mount Etna in Sicily. Measuring the rate of such gas emissions on the flanks of volcanoes or within calderas is difficult and labor intensive. Readings must be made at many locations using small gas-collection instruments placed on the soil.
A preliminary estimate of the current rate of CO2 gas emission at Mammoth Mountain is 1,300 tons per day. Similar rates of CO2 emission have been measured from the craters of Mt. St. Helens (Washington) and Kilauea (Hawaii) volcanoes during periods of low-level eruptive activity. Past eruptions at Mammoth Mountain, such as the phreatic (steam-blast) eruptions that occurred about 600 years ago on the volcanoÕs north flank, may have been accompanied by CO2 emissions. Scientists think that the current episode of high CO2 emission is the first large-scale release of the gas on the mountain for at least 250 years, because the oldest trees in the active tree-kill areas are about that age.
(Click on image for larger version - 162k)
In 1989Ð90, trees in this area on the south side of Mammoth Mountain volcano began dying from high concentrations of CO2 gas in the soil. Although leaves of plants produce oxygen (O2) from CO2 during photosynthesis, their roots need to absorb O2 directly. High CO2 concentrations in the soil kill plants by denying their roots O2 and by interfering with nutrient uptake. In the areas of tree kill at Mammoth Mountain, CO2 makes up about 20 to 95% of the gas content of the soil. Inset shows U.S. Geological Survey scientists taking samples of soil gas in this tree kill area.
The characteristics of CO2 and other gases seeping from Mammoth Mountain indicate that they were originally derived from magma. Large amounts of these gases probably were trapped beneath the volcano until 1989. In that year the magma rising through a fault may have opened cracks, allowing the gases to leak upward. Although infrequent small earthquakes continue to occur below the mountain, there is no evidence of current magma movement.
Earthquakes and CO2 seepage beneath Mammoth Mountain are only two signs of volcanic unrest in the Long Valley area. Mammoth Mountain is the southernmost volcano in the Mono-Inyo Craters volcanic chain, and over the past 4,000 years, small eruptions have occurred somewhere along this chain every few hundred years. Scientists with the USGS Volcano Hazards Program are closely monitoring CO2 emissions and other geologic hazards at Mammoth Mountain. Their continued studies in the Long Valley area of eastern California and in other volcanic regions of the United States, including Hawaii, the Pacific Northwest, Wyoming, and Alaska, are helping to protect the citizens of our Nation from geologic hazards.
Michael L. Sorey, Christopher D. Farrar, William C. Evans,
David P. Hill, Roy A. Bailey, James W. Hendley II, and Peter H. Stauffer
Graphic design by
Susan Mayfield and Sara Boore
COOPERATING ORGANIZATIONS
Mammoth Mountain Ski Area
Town of Mammoth Lakes
U.S. Department of Agriculture
U.S. Forest Service
For more information contact:
Earthquake Information Hotline (415) 329-4085
U.S. Geological Survey, MS 977
345 Middlefield Road, Menlo Park, CA 94025
See also Living With a Restless Caldera--Long Valley, California (USGS Fact Sheet 108-96).
U.S. Geological Survey Fact Sheet-172-96
Web page by Will Prescott
(wprescott@isdmnl.wr.usgs.gov).
http://quake.wr.usgs.gov/QUAKES/VOLCANOES/LongValley/
