Volcanic Hazards and CO2 Emissions:
Mammoth Mountain Long Valley Caldera, California
(1) Prof. Don DePaolo, Center for Isotope Geochemistry, Lawrence
Berkeley National Laboratory and Dept. of Geology and Geophysics,
University of California, Berkeley.
(2) Mike Sorey, Bill Evans, and Chris Farrar, U.S. Geological Survey,
Menlo Park, CA.
(3) Andrea Cook and Laura Hainsworth, Lawrence Livermore National
(4) John Rogie, Pennsylvania State University.
Crustal Unrest and CO2 and He Emissions From a Reservoir
of Magmatic Gas Beneath Mammoth Mountain
The Mammoth Mountain volcano, host of one of the largest recreational
ski areas in California, has a history of volcanism over the past
50,000-200,000 years. The volcano is located on the southwestern
rim of the 760,000-year-old Long Valley Caldera in eastern California
and forms the southern end of the Inyo Craters volcanic chain that
has produced intermittent rhyolitic and phreatic eruptions over
the last 40,000 years, most recently about 600 years ago.
Carbon dioxide and helium with isotopic compositions indicative
of a magmatic source are discharging at anomalous rates from Mammoth
Mountain. The gas is released mainly as diffuse emissions from normal-temperature
soils, but some gas issues from steam vents or leaves the mountain
dissolved in cold groundwater. The rate of gas discharge increased
significantly following a 6-month period of persistent earthquake
swarms and associated strain and ground deformation that has been
attributed to dike emplacement beneath the mountain (Figure 4).
Figure 4: Temporal variations in the helium isotopic composition,
normalized to the ratio in air (R/Ra), in gas discharged from a
fumarole near the summit of Mammoth Mountain demonstrating the dramatic
increase in magmatic helium following the onset of seismic activity
and dike intrusion beneath Mammoth Mountain in 1989. The high rate
of magmatic helium discharging from the fumarole has persisted until
the present time. For a continuos update of our monitoring of gas
discharges from this site visit: http://quake.wr.usgs.gov/VOLCANOES/LongValley/HeliumDischarge.html
Soil Gas CO2 and Helium Emissions
Anomalous discharge of CO2 and magmatic helium from
soils first occurred during the winter of 1990 and was followed
by observations of several areas of tree kill and/or heavier than
normal needlecast the following summer. Subsequent measurements
have confirmed that the tree kill areas are associated with CO2
concentrations of 30-90% in soil gas and gas flow rates of up to
31,000 g m-2 d-1 at the soil surface. The
carbon and helium isotopic compositions are indistinguishable from
that in the Mammoth Mountain fumarole. We estimate that the total
diffuse soil gas CO2 flux from the mountain is approximately
520 tonnes/day. For additional information visit: http://quake.wr.usgs.gov/VOLCANOES/LongValley/gasses.html
Groundwater CO2 and Helium Emissions
Numerous cold springs and wells around the flanks of Mammoth Mountain
contain significant quantities of dissolved CO2. The
isotopic composition of the carbon and the helium associated with
the dissolved CO2 is similar to that found in the tree-kill
areas and the Mammoth Mountain fumarole (Figure 5).
Figure 5: The dissolved helium and neon in the CO2-rich
cold groundwaters appear to be a mixture of water in equilibrium
with air (air-saturated water ,ASW) and a gas component that is
similar in composition to the soil gas in the tree-kill zones. This
suggests that some of the cold groundwaters may recharge through
the zones of diffuse soil CO2 flux.
Some of the CO2-rich groundwaters yield apparent 14C
ages of 30,000-40,000 years. The very old apparent ages reflect
a component of deep-sourced carbon that is depleted (i.e. dead)
in radioactive 14C. Wells operated by the Mammoth Mountain
ski area that were drilled prior to 1989 also contain high concentrations
of dissolved CO2. This suggests that anomalous concentrations
of CO2 may have been discharging from Mammoth Mountain
prior to the 1989 period of unrest and dike intrusion, but the overall
rate of discharge has increased significantly since 1989. We estimate
that presently 30-50 tonnes/day of CO2 dissolved in cold
groundwater flows off the flanks of the mountain. For further information
Isotopic and chemical analyses of soil, fumarolic and dissolved
gas demonstrate a remarkable homogeneity in composition, suggesting
that the CO2 and associated helium and an excess nitrogen component
may be derived from a common gas reservoir whose source is associated
with some combination of magmatic degassing related to the 1989
dike intrusion and thermal metamorphism of metasedimentary rocks.
Furthermore, N2/Ar ratios and nitrogen isotopic values
indicate that the Mammoth Mountain gases are derived from sources
separate from those that supply gas to the hydrothermal system within
the Long Valley caldera. Various data suggest that the Mammoth Mountain
gas reservoir is a large, low-temperature cap over an isolated hydrothermal
system, that it predates the 1989 intrusion, and that it could remain
a source of gas discharge for some time.
Sorey, M.L., Kennedy, B.M., Evans, W.C., Farrar, C.D., and Suemnicht,
G.A., Helium isotope and gas discharge variations associated with
crustal unrest in Long Valley caldera, California, J. Geophys. Res.,
98, 15,871-15,889, 1993.
Farrar, C.D., Sorey, M.L., Evans, W.C., Howle, J.F., Kerr, B.D.,
Kennedy, B.M., King, C-Y, and Southon, J.R., Forest-killing diffuse
CO2 emissions at Mammoth Mountain as a sign of magmatic
unrest, Nature, 376, 675-678, 1995.
Sorey, M.L., Evans, W.C., Kennedy, B.M., Farrar, C.D., Hainsworth,
L.J., and Hausback, B., Carbon dioxide and helium emissions from
a reservoir of magmatic gas beneath Mammoth Mountain, California,
J. Geophys. Res., 103, 15,303-15,323, 1998.
Sorey, M.L., Ebans, W.C., Kennedy, B.M., Rogie, J., and Cook,
A., Magmatic gas emissions from Mammoth Mountain, California Geology,
52, 4-16, 1999.
This project was supported by the Director, Office of Energy Research,
Office of Basic Energy Sciences, Engineering, and Geosciences Division
of the U.S. Department of Energy (http://www.er.doe.gov/production/bes/bes.html).
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