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Magma intrusion beneath long valley caldera confirmed by temporal changes in gravity
1Department of Geophysics, Stanford University, Stanford, CA 94305, USA. U.S. Geological Survey, Menlo Park, CA 94025, USA.
Science (New York, N.Y.)
|September 25, 1999
Summary
Gravity measurements in Long Valley, California, show a significant decrease, indicating magma intrusion. This finding excludes thermal expansion or hydrothermal pressure as causes for uplift and seismic activity.
Area of Science:
- Geophysics
- Volcanology
Background:
- Long Valley Caldera, California, exhibits surface uplift and seismicity.
- Previous studies suggested various causes for these phenomena, including thermal expansion and hydrothermal activity.
Purpose of the Study:
- To investigate the subsurface processes driving uplift and seismicity in Long Valley Caldera.
- To determine the nature of the intruding material responsible for gravity changes.
Main Methods:
- Precise relative gravity measurements were conducted in 1982 and 1998.
- Gravity data were analyzed to detect changes and correct for known effects like uplift and water table fluctuations.
Main Results:
- A gravity decrease of up to -107 +/- 6 microgals was observed, centered on the resurgent dome.
- A positive residual gravity change of up to 64 +/- 15 microgals was detected after corrections.
- The density of the intruding material was estimated to be between 2.7 x 10^3 and 4.1 x 10^3 kg/m^3.
Conclusions:
- The gravity data strongly support the intrusion of silicate magma as the cause of uplift and seismicity.
- In situ thermal expansion or hydrothermal system pressurization are excluded as primary drivers.
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