New Releases by Robert Baer

This volume describes the story of the volcanic eruptions and earthquakes that formed the landscape of two great national parks. It also reveals the dynamic processes within.

Overview of the Intermountain seismic belt, a first order feature of the seismicity map of North America.

Using explosion source, seismic refraction data, recorded in the 1978 and 1980 Yellowstone-Snake River Plain seismic experiments, a three-dimensional inversion of differential P wave attenuation was used to assess the relative variations in Q?1 in and around the volcanically active, 45 km by 70 km, Yellowstone caldera, northwestern Wyoming. Differential attenuation was derived from spectral decay of upper crustal Pg phases, observed from six explosions and recorded at 90 temporary stations. Because of the relatively short time windows used to determine the spectral content, a maximum entropy technique was employed to estimate the spectra that yielded an optimally small variance. Differential P wave attenuation was calculated from least squares determinations of the spectral ratios corrected for source and path effects. The observed differential attenuation parameters were then inverted using a weighted least squares technique for a discretized, 70?105 km, three-dimensional surface and upper crustal Q?1 model of the Yellowstone caldera and surrounding region. Results showed that the surface layer, to depths of 2 km within the Yellowstone caldera, is characterized by relatively high attenuation with low Q values less than 30, compared to values of 38 to 50 outside the caldera. The higher attenuation in the caldera''s surface layer is thought to be associated with Quaternary lake sediments, highly altered rhyolites, and the possible influence of steam in areas of hydrothermal activity. In the crystalline upper crust, at depths of 2 km to 12 km, Q values of 40 to 70 were observed in areas of thick sedimentary fill northwest of the caldera and in areas of hydrothermal activity. Within the caldera, upper crustal attenuation generally corresponded to Q of 200 in areas that are interpreted to be associated with hot but now solidified granitic material. In comparison, relatively high attenuation, Q = 40, was observed in the upper crust of the northeastern Yellowstone caldera in an area that also corresponds to a 20% reduction in Pg velocity, a prominent negative Bouguer gravity low of ?20 mGal, near Yellowstone''s largest area of hydrothermal activity and near a Quaternary, volcanic resurgent dome. On the basis of constitutive models of velocity and density, this zone of high attenuation is likely a product of unusual hydrothermal activity, highly altered upper-crustal rocks, a shallow magmatic source or a highly porous, steam-saturated body. Outside the caldera, the upper crust is relatively unaffected by high temperatures and thermal alteration, and the corresponding attenuation is low with Q values greater than 200.

The proposed trench will cut across the Teton Fault near the mouth of Granite Canyon.

Large volumes of Quaternary silicic volcanics (~6700km 3), associated explosive caldera-forming eruptions, and high heat flow (in excess of 1800 mW m -2 infer the presence of silicic magmas within the crust and upper mantle beneath the Yellowstone Plateau. Seismic refraction-relections data, analyses of earthquake hypocenters, and seismic attenuation have revealed a laterally inhomogeneous upper crust with lowP-wave velocities but a more seismically homogeneous lower crust. The upper crust beneath the Yellowstone caldera is characterized by P-wave velocities of the surrounding thermally undisturbed crystalline basement of 6.0 km/sec. The 5.7 km/sec body generally underlies the Yellowstone caldera (35 km x 65 km) and coincides with a regional -60 mgal gravity low, suggesting concomitant low density and low velocity. The 5.7 km/sec low-velocity body is interpreted to represent a hot but relatively solid body approximately 8 to 10 km thick that was probably the reservoir for the silicic magmas. A 4.0 km/sec low-velocity body located beneath the northeast boundary of the caldera coincides with a local -20 mgal gravity low and has a tenfold increase in seismic attenuation - properties that can be interpreted to result from a steam-water-dominated system to a body of 10-50 percent silicic partial melt. The P-wave velocity of the upper 100 to 250 km of the mantle beneath the Yellowstone region, analyzed from teleseismic arrivals, is reduced by ~5 percent, suggesting the presence of a basaltic partial melt that is probably the source of heat that drives the Yellowstone hydrothermal system. In comparison, the lower crust of the Yellowstone region appears seismically homogeneous to the horizontally propagating refracted rays and similar to that of the thermally undisturbed lower crust of the surrounding Rocky Mountains. This suggests that the seismic properties of the lower crust were relatively unaffected by the ascension of the parental basaltic magmas that are hypothesized to have intruded and partially melted the crust producing the voluminous rhyolite and ash flow tuffs of the Yellowstone Quaternary volcanic system. Maximum focal depth of earthquakes in Yellowstone systematically shallow from ~20 km outside the caldera to ~5 km beneath the caldera, suggesting the influence of high-temperature abnormal pore pressure, compositional changes that restrict brittle failure to the upper crust. Orthometrically corrected reobservations of level lines across the Yellowstone caldera show an area of crustal uplift, up to 15 mm/yr, that generally coincides with the outline of the 5.7 km/sec low velocity layer. These data are consistent with the model in which an upper-crustal low-velocity/low-density layer, 75 km x 25 km, appears to be plastically deforming. Taken together with the geologic data this crustal model is interpreted to reflect the structure and properties of a thermally deforming Archean crust and the initial stages of the bimodal rhyolitic/basaltic volcanism of the Yellowstone-Snake River Plain volcano-tectonic system. While the interpretations are not uniquie; the youthfulness and volume of Quaternary volcanism, the high heat flow, the high rates of contemporary uplift, and the upper-crustal low-velocity layers infer the presence of hot crustal material and possible partial melts that underlie the Yellowstone Plateau. These properties cannot yet be evalulated to indicate temporal variations in volcanism, but the geologic record and the new geophysical models suggest future volcanic activity in the Yellowstone Plateau. -Abstract.

In 1978 a major seismic profiling experiment was conducted in the Yellowstone-eastern Snake River Plain region of Idaho and Wyoming. Fifteen shots were recorded that provided coverage to distances of 300 km. In this paper, travel time and synthetic seismogram modeling was used to evaluate an average P wave velocity and apparent Q structure of the crust from two seismic profiles (reversed) across the Yellowstone National Park region. This area includes the well-known hydrothermal features of Yellowstone National Park (geysers, fumeroles, etc.), a large collapse caldera, and extensive silicic volcanism of Quaternary age?features attributed to shallow crustal sources of magma. The averaged crustal structure for this region as interpreted from the seismic data consists of (1) a highly variable, near-surface layer approximately 2 km thick with variable velocities of 3.0 to 4.8 km/s and a low apparent Q of 30 that is interpreted to be composed of weathered rhyolites and sedimentary infill, (2) an upper crustal layer 3 to 4 km thick with variable velocities of 4.9 to 5.5 km/s and apparent Q of 50 to 200 that is thought to represent the accumulation of the Pleistocene-Quaternary rhyolite flows, ash flow tuffs, and possible Paleozoic and Precambrian metamorphic equivalents, (3) the crystalline, upper crust that is characterized by a laterally inhomogeneous layer that varies in velocity from 4.0 to 6.1 km/s, averaging 5 km thick with a Q of 300. This layer appears to be a cooling but still hot body of granitic composition beneath the Yellowstone caldera. It is thought to be a remnant of the magma chambers that produced the Quaternary silicic volcanic rocks of the Yellowstone Plateau and may still be a major contributor to the high heat flow, (4) a laterally homogeneous intermediate crustal layer 8 to 10 km thick with a velocity of 6.5 km/s and apparent Q of 100 to 300, (5) a homogeneous 25-km-thick lower crust with a velocity of 6.7 to 6.8 km/s and an apparent Q of 300, and (6) a total crustal thickness of ?43 km. The upper crustal layer, 5.5 to 6.0 km/s, is thought to be the thermally altered equivalent of the continental crystalline basement that is normally 15 to 20 km thick in the surrounding thermally undisturbed Archean crust. An interpretation from these results suggests that mafic melts from the mantle have penetrated the lower crust without significant variations in the velocity structure but produce the main source of heat that drives the volcanic and hydrothermal systems of Yellowstone. The high apparent attenuation and large lateral velocity variations in the upper crust are consistent with a model in which partial fractionation, partial melting, and metamorphism differentiate the original upper crust to produce silicic melts that were extruded as rhyolites and ash flow tuffs across the Yellowstone Plateau. This seismic model is consistent with the evidence for a systematic northeastward propogation of silicic volcanic centers along the eastern Snake River Plain to their present location beneath the Yellowstone hydrothermal system. While these findings do not bear directly on the origin and source of the heat, i.e., mantle lumes, lithospheric fractures, mantle radiogenetic heat, basal lithospheric shearing, etc., they provide a constraint on the configuration and lateral extent of crustal layers that reflect thermal and compositional boundaries.

The geology of Utah as studies in relation to geothermal exploration areas in Utah.