Geology topics

Research about Long Valley caldera

Source-linked reports with geographic coverage including Long Valley caldera.

11 recordsLinked to original sources

USGS2021

A new analysis of caldera unrest through the integration of geophysical data and FEM modeling: The Long Valley caldera case study

The Long Valley Caldera, located at the eastern edge of the Sierra Nevada range in California, has been in a state of unrest since the late 1970s. Seismic, gravity and geodetic data strongly suggest that the source of unrest is an intrusion beneath the caldera resurgent dome. However, it is not clear yet if the main contribution to the deformation comes from pulses of ascending high-pressure hydrothermal fluids or low viscosity magmatic melts. To characterize the nature of the intrusion, we developed a 3D finite element model which includes topography and crust heterogeneities. We first performed joint numerical inversions of uplift and Electronic Distance Measurement baseline length change data, collected during the period 1985–1999, to infer the deformation-source size, position, and overpressure. Successively, we used this information to refine the source overpressure estimation, compute the gravity potential and infer the intrusion density from the inversion of deformation and gravity data collected in 1982–1998. The deformation source is located beneath the resurgent dome, at a depth of 7.5 ± 0.5 km and a volume change of 0.21 ± 0.04 km 3 . We assumed a rhyolite compressibility of 0.026 ± 0.0011 GPa −1 (volume fraction of water between 0% and 30%) and estimated a reservoir compressibility of 0.147 ± 0.037 GPa −1 . We obtained a density of 1856 ± 72 kg/m 3 . This density is consistent with a rhyolite melt, with 20% to 30% of dissolved hydrothermal fluids.

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USGS2017

Graphite in the Bishop Tuff and its effect on postcaldera oxygen fugacity

Several cubic kilometers of Paleozoic graphite-bearing argillitic country rocks are present as lithic fragments in Bishop Tuff ignimbrite and fallout. The lithics were entrained by the 650 km 3 of rhyolite magma that vented during the 5- to 6-day-long, caldera-forming eruption at Long Valley, California. The caldera is floored by a 350 km 2 roof plate that collapsed during the eruption and consists in large part of the Paleozoic strata that provided the abundant hornfelsed metapelitic lithic clasts in the tuff. Graphite has been identified by Raman spectroscopy, electron-dispersive spectroscopy, and X-ray diffraction as an irregularly dispersed component in the small fraction of Bishop Tuff pumice that is dark-colored. Carbon concentration has been determined in pumice, lithics, and wall rocks. Values of δ 13 C range from –21‰ to –29‰ Vienna Peedee Belemnite (VPDB) for pumice, lithics, and argillitic wall rocks, reflecting the biogenic origin of the reduced carbon in oxygen-limited black Paleozoic marine mudrocks. Carbonate contents, measured separately, are negligible in fresh pumice and lithics. Microprobe analyses of titanomagnetite-ilmenite pairs show that oxygen-fugacity values of numerous batches of postcaldera Early Rhyolite (750–640 ka; ~100 km 3 ) are up to one log unit more reduced than those of the temperature–oxygen fugacity (T-fO 2 ) array of the Bishop Tuff (767 ka), despite similar major-element compositions and Fe-Ti–oxide temperature ranges. All of the many batches of Early Rhyolite, which erupted episodically over an interval of ~125,000 years, yield the reduced fO 2 values, indicating that reaction with graphite lowered magmatic fO 2 after the caldera-forming eruption but before the first eruption of Early Rhyolite. It is inferred that reaction of postcaldera rhyolite magma with the reduced carbon in a great mass of subsided roof rocks lowered its fO 2 . It is suggested that comparable effects could have attended caldera collapse of other magma chambers hosted in continental sedimentary rocks.

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USGS2012

Deformation in the White Mountain seismic gap, California-Nevada, 1972-1982

A 100×40 km trilateration network extending from Bishop, California, to near Hawthorne, Nevada, crosses the east end of the Long Valley caldera, site of renewed magma inflation in the 1979–1980 interval, and spans most of the White Mountain seismic gap. The network was surveyed in 1972, 1973, 1976, 1979, 1980, and 1982. The 1980 survey may be contaminated by a scale error. In addition, leveling surveys across the caldera have been run in 1932, 1957, 1975, 1980, 1982, and 1983. Interpretation of the deformation is complicated by the occurrence of the May 1980 Mammoth Lakes earthquake sequence (four earthquakes M L ≥6) at the south edge of the caldera as well as other moderate earthquakes within the White Mountain seismic gap. The vertical deformation is largely accounted for by 0.10‐ to 0.15‐km 3 expansion of a spherical magma chamber 8–10 km beneath the resurgent dome within the Long Valley caldera sometime between July 1979 and September 1980 with an additional expansion of perhaps 0.05 km 3 between September 1980 and July 1982. Some additional sources of deformation within the aftershock zone of the Mammoth Lakes earthquakes seem to be required to explain the horizontal deformation. We show that right‐lateral slip on vertical faults extending WNW from each of the three largest earthquakes in the Mammoth Lakes sequence provides the required additional deformation, but this solution is by no means unique. There are simply too few data to define the rather complex deformation that apparently occurred within the aftershock zone. There is little doubt, however, that inflation of a magma chamber beneath the resurgent dome within the Long Valley caldera was involved in the deformation.

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USGS2011

Monitoring CO2 emissions in tree kill areas near the resurgent dome at Long Valley Caldera, California

We report results of yearly measurements of the diffuse CO 2 flux and shallow soil temperatures collected since 2006 across two sets of tree-kill areas at Long Valley Caldera, California. These data provide background information about CO 2 discharge during a period with moderate seismicity, but little to no deformation. The tree kills are located at long-recognized areas of weak thermal fluid upflow, but have expanded in recent years, possibly in response to geothermal fluid production at Casa Diablo. The amount of CO 2 discharged from the older kill area at Basalt Canyon is fairly constant and is around 3-5 tonnes of CO 2 per day from an area of about 15,000 m 2 . The presence of isobutane in gas samples from sites in and around Basalt Canyon suggests that geothermal fluid production directly effects fluid upflow in the region close to the power plant. The average fluxes at Shady Rest are lower than average fluxes at Basalt Canyon, but the area affected by fluid upflow is larger. Total CO 2 discharged from the central portion of the kill area at Shady Rest has been variable, ranging from 6 to11 tonnes per day across 61,000 m 2 . Gas collected at Shady Rest contains no detectable isobutane to link emissions chemically to geothermal fluid production, but two samples from 2009-10 have detectable H 2 S and suggest an increasing geothermal character of emitted gas. The appearance of this gas at the surface may signal increased drawdown of water levels near the geothermal productions wells.

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USGS2007

Boiling water at Hot Creek— The dangerous and dynamic thermal springs in California’s Long Valley Caldera

The beautiful blue pools and impressive boiling fountains along Hot Creek in east-central California have provided enjoyment to generations of visitors, but they have also been the cause of injury or death to some who disregarded warnings and fences. The springs and geysers in the stream bed and along its banks change location, temperature, and flow rates frequently and unpredictably. The hot springs and geysers of Hot Creek are visible signs of dynamic geologic processes in this volcanic region, where underground heat drives thermal spring activity.

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USGS2004

Non-double-couple microearthquakes at Long Valley caldera, California, provide evidence for hydraulic fracturing

Most of 26 small (0.4≲ M ≲3.1) microearthquakes at Long Valley caldera in mid-1997, analyzed using data from a dense temporary network of 69 digital three-component seismometers, have significantly non-double-couple focal mechanisms, inconsistent with simple shear faulting. We determined their mechanisms by inverting P - and S -wave polarities and amplitude ratios using linear-programming methods, and tracing rays through a three-dimensional Earth model derived using tomography. More than 80% of the mechanisms have positive (volume increase) isotropic components and most have compensated linear-vector dipole components with outward-directed major dipoles. The simplest interpretation of these mechanisms is combined shear and extensional faulting with a volume-compensating process, such as rapid flow of water, steam, or CO 2 into opening tensile cracks. Source orientations of earthquakes in the south moat suggest extensional faulting on ESE-striking subvertical planes, an orientation consistent with planes defined by earthquake hypocenters. The focal mechanisms show that clearly defined hypocentral planes in different locations result from different source processes. One such plane in the eastern south moat is consistent with extensional faulting, while one near Casa Diablo Hot Springs reflects en echelon right-lateral shear faulting. Source orientations at Mammoth Mountain vary systematically with location, indicating that the volcano influences the local stress field. Events in a ‘spasmodic burst’ at Mammoth Mountain have practically identical mechanisms that indicate nearly pure compensated tensile failure and high fluid mobility. Five earthquakes had mechanisms involving small volume decreases, but these may not be significant. No mechanisms have volumetric moment fractions larger than that of a force dipole, but the reason for this fact is unknown.

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USGS2000

Spherical and ellipsoidal volcanic sources at Long Valley caldera, California, using a genetic algorithm inversion technique

We model the second inflation period at Long Valley caldera, California using a genetic algorithm technique and high quality geodetic measurements of elevation changes and baseline extensions. We compare two source inversions for both spherical Mogi point sources and the finite prolate ellipsoid of Yang and Davis. A sensitivity analysis for the genetic algorithm is performed based upon synthetic data set inversions on similar sources in order to better constrain the areal location, orientation, and volume of the potential sources. The spherical sources are well constrained, the larger located at 9.9 km beneath the resurgent dome, with a volume of 0.036 km 3 , while the second, at only 0.008 km 3 , is located at a depth of 7.3 km beneath the south moat. The depths to the ellipsoidal sources are switched, with the larger source at a depth of 9.6 km and the smaller at 11.8 km, with volumes of 0.037 and 0.002 km 3 , respectively.

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USGS1990

Monitoring the hydrologic system for potential effects of geothermal and ground-water development in the Long Valley caldera, Mono County, California, U.S.A.

In the early 1980's, renewed interest in the geothermal potential of the Long Valley caldera, California, highlighted the need to balance the benefits of energy development with the established recreational activities of the area. The Long Valley Hydrologic Advisory Committee, formed in 1987, instituted a monitoring program to collect data during the early stages of resource utilization to evaluate potential effects on the hydrologic system. Early data show declines in streamflow, spring flow, and ground-water levels caused by 6 years of below-average precipitation. Springs in the Hot Creek State Fish Hatchery area discharge water that is a mixture of nonthermal and hydrothermal components. Possible sources of nonthermal water have been identified by comparing deuterium concentrations in streams and springs. The equivalent amount of undiluted thermal water discharged from the springs was calculated on the basis of boron and chloride concentrations. Quantifying the thermal and nonthermal fractions of the total flow may allow researchers to assess changes in flow volume or temperature of the springs caused by groundwater or geothermal development.

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USGS1985

Hydrologic and geochemical monitoring in Long Valley caldera, Mono County, California, 1982-1984

The Long Valley caldera is a potentially active volcanic area on the eastern side of the Sierra Nevada in east-central California. Hydrologic and geochemical monitoring of surface and subsurface features began in July 1982 to determine if changes were occurring in response to processes causing earthquakes and crustal deformation. Differences since 1982 in fluid chemistry of springs has been minor except at Casa Diablo, where rapid fluctuations in chemistry result from near surface boiling and mixing. Ratios of 3 He/ 4 He and 13 C/ 12 C in hot springs and fumaroles are consistent with a magnetic source for some of the carbon and helium discharged in thermal areas, and observed changes in 3 He/ 4 He between 1978 and 1984 suggest changes in the magmatic component. Significant fluctuations in hot spring discharge recorded at several sites since 1982 closely followed earthquake activity. Water levels in wells have been used as strain meters to detect rock deformation associated with magmatic and tectonic activity and to construct a water table contour map. Coseismic water level fluctuations of as much as 0.6 ft have been observed but no clear evidence of deformation caused by magmatic intrusions can be seen in the well records through 1984. Temperature profiles in wells, which can be used to delineate regionally continuous zones of lateral flow of hot water across parts of the caldera, have remained constant at all but two sites.

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USGS1985

Monitoring the hydrothermal system in Long Valley caldera, California

An ongoing program to monitor the hydrothermal system in Long Valley for changes caused by volcanic or tectonic processes has produced considerable data on the water chemistry and discharge of springs and fluid temperatures and pressures in wells. Chemical and isotopic data collected under this program have greatly expanded the knowledge of chemical variability both in space and time. Although no chemical or isotopic changes in hot spring waters can be attributed directly to volcanic or tectonic processes, changes in hot spring chemistry that have been recorded probably relate to interactions between and variations in the quantity of liquid and gas discharged. Stable carbon isotope data are consistent with a carbon source either perform the mantle or from metamorphosed carbonate rocks. Continuous and periodic measurements of hot spring discharge at several sites show significant co seismic and a seismic changes since 1980.

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USGS1982

Tilt measurements at Long Valley caldera, California, May-August 1982

The Mammoth Lakes area in east-central California has experienced unusual seismicity and ground deformation since 1978, highlighted by four M>6 earthquakes in May 1980 and by the discovery soon thereafter of a broad uplift within Long Valley caldera. Recurrent seismic swarms during June 1980-May 1982 raised concern over the possibility of renewed volcanic activity in the foreseeable future, prompting a USGS Notice of Potential Volcanic Hazard on 28 May 1982. As part of an intensified Long Valley monitoring effort sponsored by the USGS Volcanic Hazards Program, a network of nine tilt sites was established near Mammoth Lakes in May and July 1982. This report describes those stations and presents results from three tilt resurveys during July-August 1982.

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