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At least 1,063 records · Page 59Linked to original sources
USGS2015

Coping with earthquakes induced by fluid injection

Large areas of the United States long considered geologically stable with little or no detected seismicity have recently become seismically active. The increase in earthquake activity began in the mid-continent starting in 2001 ( 1 ) and has continued to rise. In 2014, the rate of occurrence of earthquakes with magnitudes ( M ) of 3 and greater in Oklahoma exceeded that in California (see the figure). This elevated activity includes larger earthquakes, several with M > 5, that have caused significant damage ( 2 , 3 ). To a large extent, the increasing rate of earthquakes in the mid-continent is due to fluid-injection activities used in modern energy production ( 1 , 4 , 5 ). We explore potential avenues for mitigating effects of induced seismicity. Although the United States is our focus here, Canada, China, the UK, and others confront similar problems associated with oil and gas production, whereas quakes induced by geothermal activities affect Switzerland, Germany, and others.

Science↗
USGS2016

Comment on “Reconciliation of the Devils Hole climate record with orbital forcing”

Moseley et al . (Reports, 8 January 2016, p. 165) postulate an increase in dissolved thorium isotope 230 Th with depth below the water table as the explanation for the differing ages of Termination II. Flow of geothermal water through the Devils Hole caverns precludes this explanation. Deposition of younger secondary calcite into the initial porosity of the calcite comprising their cores is a plausible alternate explanation.

Science↗
USGS1964

Relation of temperature distribution to ground-water movement in carbonate rocks of central Israel

The Cenomanian-Turonian formations of central Israel constitute a highly permeable dolomite and limestone aquifer. In this area it is on the west limb of an anticlinorium that trends north-northeast, and it contains water under artesian pressure. A graph of water temperatures and well depths suggests that there is a very small vertical temperature gradient in local segments of the aquifer. The small gradient is believed to result from a large vertical component of flow that tends to equalize the vertical temperature distribution . On a regional scale the apparent horizontal temperature distribution indicates a westward increase with increasing depth of the aquifer, suggesting a manifestation of the regional geothermal gradient. The westward increase in temperature also implies that the lateral component of flow may be in the normal range for artesian carbonate - rock aquifers whose pores consist mainly of solution cavities. Locally, pumping appears to have affected the temperature distribution by modifying the natural flow pattern. In parts of the most intensively developed area, the aquifer is hydraulically connected with overlying coastal-plain deposits, and some cooler water has been induced to move into the aquifer from this source. At three other areas, pumping has resulted in an apparent horizontal shift of the isotherms on a temperature - distribution map. The data suggest that the spatial distribution of temperature may be used to determine some of the flow characteristics of carbonate - rock aquifers.

Geological Society of America Bulletin↗
USGS1970

Radiogenic heat production of contrasting magma series: Bearing on interpretation of heat flow

Variation in radiogenic heat production of rocks of diverse magma series (representative of calcic, calc-alkalic, alkali-calcic, and alkalic petrographic provinces on a worldwide basis) is better correlated with some form of magmatic differentiation index, rather than simply with potassium content alone as commonly supposed. The “lime-alkali” (Peacock) index generally gives the best correlation of the various indices tested. Plots of heat production for igneous suites versus indices of differentiation (“heat production trends”) demonstrate systematic correlations of heat productivity as functions of fractionation trends and/or spatial-temporal associations. In general, heat production trends of calcic suites tend to indicate lower radiogenic heat productivity than those of more alkalic suites for a given silica or potassium content. Because many recent measurements of heat flow in plutons can be linearly related to heat production of surface rocks, these “heat production trends” have important bearing on the interpretation of crustal heat flow. Our data suggest possible alternative interpretations of the heat production-heat flow pattern for the Sierra Nevada which differ from that in Lachen-bruch's (1968) preliminary geothermal model.

GSA Bulletin↗
USGS1984

The Loma Seca tuff and the Calabozos caldera: a major ash-flow and caldera complex in the southern Andes of central Chile

A 26 × 14-km composite ring-structure caldera of late Pleistocene age has been discovered and mapped near the Andean crest in central Chile (35°30′S). Rhyodacitic to dacitic zoned ash-flow sheets, each representing 150 to 300 km 3 of magma, were emplaced 0.8, 0.3, and 0.15 m.y. ago; the youngest of the associated collapses was closely followed by resurgent doming of the caldera floor and development of a longitudinal graben. Postcaldera eruptions of dacite and andesite have persisted into Holocene time, and active hot springs are abundant along caldera-marginal and resurgent fault systems, suggesting a significant geothermal-energy resource. The Pleistocene eruption rate of this district and the abundance of older Quaternary to Miocene ash-flow remnants in the 33°S to 36°S segment of the glaciated southern Andes indicate that ash-flow magmatism has been no less important here than in the arid central Andes (16°S–28°S), where ash-flow sheets are far better preserved.

Geological Society of America Bulletin↗
USGS1987

Heat transport by fluids during late Cretaceous regional metamorphism in the Big Maria Mountains, southeastern California

The Big Maria Mountains of southeastern California comprise a Late Cretaceous regional metamorphic terrain involving Paleozoic cratonal sediments. Siliceous limestone of the upper Paleozoic Supai Formation has reacted to form massive wollastonite, requiring an enormous fluid flux. The minimum volume ratio of fluid:rock that is necessary to explain the formation of wollastonite may be calculated from the reaction quartz + calcite = wollastonite + CO 2 , using the method of Rice and Ferry. Given average conditions of 3 kbar, 500 °C, an infiltrating fluid of composition X H2O = 1.00, an equilibrium fluid composition of X H2O = 0.97, and 90% wollastonite in the final rock, a fluid:rock ratio of 17:1 may be calculated. Infiltrating fluids of composition 1.00 > X H2O > 0.97 require still higher ratios. Metamorphic reactions which took place in other units of the sequence (metapelites, massive carbonates, metavolcaniclastics) did not record the massive fluid flux, but fluids must have passed through them to have affected structurally isolated masses of Supai Formation. Passage of fluids must have occurred along fractures. Neither magmatism nor radioactive heat sources are adequate to explain the temperatures of metamorphism. If the minimum quantity of fluid that is estimated to have passed through the area was initially at 680 °C, it would result in a 300-degree rise in temperature over that of the stable-craton geotherm. Late metamorphic pegmatite dikes which are most abundant in areas of high metamorphic grade may stem from melts anatectically derived at deeper levels, perhaps as a result of the same fluid flux. Heat that is introduced by large fluid fluxes may be an important cause of anomalously high temperatures which are observed in many cases in regional metamorphic terrains.

California↗
USGS1990

Ages and stable-isotope compositions of secondary calcite and opal in drill cores from Tertiary volcanic rocks of the Yucca Mountain area, Nevada

Stable-isotope compositions of fracture-and cavity-filling calcite from the unsaturated zone of three drill cores at Yucca Mountain Tertiary volcanic complex indicate that the water from which the minerals precipitated was probably meteoric in origin. A decrease in 18 O in the calcite with depth is interpreted as being due to the increase in temperature in drill holes corresponding to an estimated average geothermal gradient of 34° per kilometer. A few of the calcite samples and all of the opal samples yielded uranium-series ages older than 400,000 yr, although most of the calcite samples yielded ages between 26,000 and 310,000 yr. The stable-isotope and uranium- series dates from precipitated calcite and opal of this reconnaissance study suggest a complex history of fluid movement through the volcanic pile, and episodes of fracture filling predominantly from meteoric water during at least the past 400,000 yr.

Nevada↗
USGS1994

The volcanic, sedimentologic, and paleolimnologic history of the Crater Lake caldera floor, Oregon:Evidence for small caldera evolution

Apparent phreatic explosion craters, caldera-floor volcanic cones, and geothermal features outline a ring fracture zone along which Mount Mazama collapsed to form the Crater Lake caldera during its climactic eruption about 6,850 yr B.P. Within a few years, subaerial deposits infilled the phreatic craters and then formed a thick wedge (10-20 m) of mass flow deposits shed from caldera walls. Intense volcanic activity (phreatic explosions, subaerial flows, and hydrothermal venting) occurred during this early postcaldera stage, and a central platform of subaerial andesite flows and scoria formed on the caldera floor. Radiocarbon ages suggest that deposition of Iacustrine hemipelagic sediment began on the central platform about 150 yr after the caldera collapse. This is the minimum time to fill the lake halfway with water and cover the platform assuming present hydrologic conditions of precipitation and evaporation but with negligible leakage of lake water. Wizard Island formed during the final part of the 300-yr lake-filling period as shown by its (1) upper subaerial lava flows from 0 to -70 m below present water level and lower subaqueous lava flows from -70 to -500 m and by (2) lacustrine turbidite sand derived from Wizard Island that was deposited on the central platform about 350 yr after the caldera collapse. Pollen stratigraphy indicates that the warm and dry climate of middle Holocene time correlates with the early lake deposits. Diatom stratigraphy also suggests a more thermally stratified and phosphate-rich environment associated respectively with this climate and greater hydrothermal activity during the early lake history. Apparent coarse-grained and thick-bedded turbidites of the early lake beds were deposited throughout northwest, southwest, and eastern basins during the time that volcanic and seismic activity formed the subaqueous Wizard Island, Merriam Cone, and rhyodacite dome. The last known postcaldera volcanic activity produced a subaqueous rhyodacite ash bed and dome about 4,240 yr B.P. The late lake beds with base-of-slope aprons and thin, fine-grained basin-plain turbidites were deposited during the volcanically quiescent period of the past 4,000 yr. Deposits in Crater Lake and on similar caldera floors suggest that four stages characterize the postcaldera evolution of smaller (≤10 km in diameter) terrestrial caldera lake floors: (1) initial-stage caldera collapse forms the ring fracture zone that controls location of the main volcanic eruptive centers and sedimentary basin depocenters on the caldera floor; (2) early-stage subaerial sedimentation rapidly fills ring-fracture depressions and constructs basin-floor debris fans from calderawall landslides; (3) first-stage subaqueous sedimentation deposits thick flat-lying lake turbidites throughout basins, while a thin blanket of hemipelagic sediment covers volcanic edifices that continue to form concurrently with lake sedimentation; and (4) second-stage subaqueous sedimentation after the waning of major volcanic activity and the earlier periods of most rapid sedimentation develops small sili-ciclastic basin base-of-slope turbidite aprons and central basin plains. Renewed volcanic activity or lake destruction could cause part or all of the cycle to repeat.

Oregon↗
USGS1998

Tectonic controls on magmatism in the Geysers-Clear Lake region: Evidence from new geophysical models

In order to study magmatism and geothermal systems in The Geysers–Clear Lake region, we developed a detailed three-dimensional tomographic velocity model based on local earthquakes. This high-resolution model resolves the velocity structure of the crust in the region to depths of approximately 12 km. The most significant velocity contrasts in The Geysers–Clear Lake region occur in the steam production area, where high velocities are associated with a Quaternary granitic pluton, and in the Mount Hannah region, where low velocities occur in a 5-km-thick section of Mesozoic argillites. In addition, a more regional tomographic model was developed using traveltimes from earthquakes covering most of northern California. This regional model sampled the whole crust, but at a lower resolution than the local model. The regional model outlines low velocities at depths of 8–12 km in The Geysers–Clear Lake area, which extend eastward to the Coast Range thrust. These low velocities are inferred to be related to unmetamorphosed Mesozoic sedimentary rocks. In addition, the regional velocity model indicates high velocities in the lower crust beneath the Clear Lake volcanic field, which we interpret to be associated with mafic underplating. No large silicic magma chamber is noted in either the local or regional tomographic models. A three-dimensional gravity model also has been developed in the area of the tomographic imaging. Our gravity model demonstrates that all density contrasts can be accounted for in the upper 5–7 km of the crust. Two-dimensional magnetotelluric models of data from a regional, east-west profile indicate high resistivities associated with the granitic pluton in The Geysers production area and low resistivities in the low-velocity section of Mesozoic argillites near Mount Hannah. No indication of midcrustal magma bodies is present in the magnetotelluric data. On the basis of heat flow and geologic evidence, Holocene intrusive activity is thought to have occurred near the Northwest Geysers, Mount Hannah, Sulphur Bank Mine, and perhaps other areas. The geophysical data provide no conclusive evidence for such activity, but the detailed velocity model is suggestive of intrusive activity near Mount Hannah similar to that in the “felsite” of The Geysers production area. The geophysical models, seismicity patterns, distribution of volcanic vents, heat flow, and other data indicate that small, young intrusive bodies that were injected along a northeast trend from The Geysers to Clear Lake probably control the thermal regime.

Geological Society of America Bulletin↗
USGS1975

K-Ar ages of Pleistocene rhyolitic volcanism in the Coso Range, California

Rhyolite domes and rhyolite and basalt flows near Coso Hot Springs in the Coso Range, California, give K-Ar ages that range from about 0.04 ± 0.02 m.y. to 0.96 ± 0.19 m.y. Most of the units appear to be between 0.05 and 0.15 m.y. old. The total volume of erupted rhyolitic magma is about 2.4 km 3 . The age, volume, and composition of these volcanic rocks indicate that the Coso volcanic field is a potential geothermal resource area.

California↗
USGS1984

Casto Ring Zone: A 4,500-km2 fossil hydrothermal system in the Challis Volcanic Field, central Idaho

δ 18 O analyses of granitic and volcanic rocks reveal the largest hydrothermal system ever documented around a single granite pluton, occupying a 4,500-km 2 area in central Idaho. The remains of this meteoric-hydrothermal system are principally preserved within a sharply bounded, 15-km-wide, 70-km-diameter annulus of low δ 18 O rock (+2.0 to −8.8‰) termed the Casto Ring Zone. The zone centered on a less depleted (+4.5) core zone consisting of granitic rocks of the Casto pluton. This 700-km 2 Eocene subvolcanic batholith has intruded, domed, and hydrothermally metamorphosed a thick sequence of Challis Volcanics, the stratigraphically low rocks in the 2,000-km 2 Van Horn Peak and the 1,000-km 2 Thunder Mountain cauldron complexes being most strongly altered. Less extreme 18 O depletions occur in the youngest major ash-flow sheets of these complexes, indicating a vertical 18 O gradient. Water/rock ratios of geothermal systems are surprisingly insensitive to the circulation scale.

Geology↗
USGS1985

Gas hydrates on the northern California continental margin

The inner continental margin of northern California is underlain by a well-defined and extensive acoustic reflector that crosses other reflectors and mimics the surface of the sea floor. This bottom-simulating reflector (BSR) lies at a typical subsurface depth of about 250 m and has been mapped continuously beneath the Klamath Plateau and upper slope (water depths of 800 to 1200 m) for a distance of more than 130 km; it covers an area of at least 3000 km 2 . Limited data show that the BSR extends northward into Oregon and seaward at least to the base of the slope (3000 m). The water depths, subsurface depths, and pervasive nature of the BSR all suggest that it represents the base of a natural-gas hydrate. Using standard phase boundary diagrams for hydrate stability, we estimate the local geothermal gradient within the gas hydrate to be about 55 °C/km. This value is higher than that of most subduction margins and may be a result of the youthfulness of the subducting oceanic crust. This acoustically inferred gas hydrate is the first to be mapped on the western conterminous United States continental margin.

California↗
USGS1999

Geological implications of a permeability-depth curve for the continental crust

The decrease in permeability ( k ) of the continental crust with depth ( z ), as constrained by geothermal data and calculated fluid flux during metamorphism, is given by log k = −14 − 3.2 log z, where k is in meters squared and z is in kilometers. At moderate to great crustal depths (>∼5 km), this curve is defined mainly by data from prograde metamorphic systems, and is thus applicable to orogenic belts where the crust is being thickened and/or heated; lower permeabilities may occur in stable cratonic regions. This k-z relation implies that typical metamorphic fluid flux values of ∼10 −11 m/s are consistent with fluid pressures significantly above hydrostatic values. The k-z curve also predicts that metamorphic CO 2 flux from large orogens may be sufficient to cause significant climatic effects, if retrograde carbonation reactions are minimal, and suggests a significant capacity for diffuse degassing of Earth (10 15 –10 16 g/yr) in tectonically active regions.

Geology↗
USGS2007

Remotely triggered earthquakes following moderate main shocks

Since 1992, remotely triggered earthquakes have been identified following large (M > 7) earthquakes in California as well as in other regions. These events, which occur at much greater distances than classic aftershocks, occur predominantly in active geothermal or volcanic regions, leading to theories that the earthquakes are triggered when passing seismic waves cause disruptions in magmatic or other fluid systems. In this paper, I focus on observations of remotely triggered earthquakes following moderate main shocks in diverse tectonic settings. I summarize evidence that remotely triggered earthquakes occur commonly in mid-continent and collisional zones. This evidence is derived from analysis of both historic earthquake sequences and from instrumentally recorded M5–6 earthquakes in eastern Canada. The latter analysis suggests that, while remotely triggered earthquakes do not occur pervasively following moderate earthquakes in eastern North America, a low level of triggering often does occur at distances beyond conventional aftershock zones. The inferred triggered events occur at the distances at which SmS waves are known to significantly increase ground motions. A similar result was found for 28 recent M5.3–7.1 earthquakes in California. In California, seismicity is found to increase on average to a distance of at least 200 km following moderate main shocks. This supports the conclusion that, even at distances of ∼100 km, dynamic stress changes control the occurrence of triggered events. There are two explanations that can account for the occurrence of remotely triggered earthquakes in intraplate settings: (1) they occur at local zones of weakness, or (2) they occur in zones of local stress concentration.

Special Paper of the Geological Society of America↗
USGS2009

Petrology and geochronology of crustal xenoliths from the Bering Strait region: Linking deep and shallow processes in extending continental crust

Petrologic, geochemical, and metamorphic data on gneissic xenoliths derived from the middle and lower crust in the Neogene Bering Sea basalt province, coupled with U-Pb geochronology of their zircons using sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG), yield a detailed comparison between the P-T-t and magmatic history of the lower crust and magmatic, metamorphic, and deformational history of the upper crust. Our results provide unique insights into the nature of lithospheric processes that accompany the extension of continental crust. The gneissic, mostly maficxenoliths (constituting less than two percent of the total xenolith population) from lavas in the Enmelen, RU, St. Lawrence, Nunivak, and Seward Peninsula fields most likely originated through magmatic fractionation processes with continued residence at granulite-facies conditions. Zircon single-grain ages (n ??? 125) are interpreted as both magmatic and metamorphic and are entirely Cretaceous to Paleocene in age (ca. 138-60 Ma). Their age distributions correspond to the main ages of magmatism in two belts of supracrustal volcanic and plutonic rocks in the Bering Sea region. Oscillatory-zoned igneous zircons, Late Cretaceous to Paleocene metamorphic zircons and overgrowths, and lack of any older inheritance in zircons from the xenoliths provide strong evidence for juvenile addition of material to the crust at this time. Surface exposures of Precambrian and Paleozoic rocks locally reached upper amphibolite-facies (sillimanite grade) to granulite-facies conditions within a series of extension-related metamorphic culminations or gneiss domes, which developed within the Cretaceous magmatic belt. Metamorphic gradients and inferred geotherms (??30-50 ??C/km) from both the gneiss domes and xenoliths aretoo high to be explained by crustal thickening alone. Magmatic heat input from the mantle is necessary to explain both the petrology of the magmas and elevated metamorphic temperatures. Deep-crustal seismic-reflection and refraction data reveal a 30-35-km-thick crust, a sharp Moho and refl ective lower and middle crust. Velocities do not support a largely mafic (underplated) lower crust, but together with xenolith data suggest that Late Cretaceous to early Paleocene maficintrusions are likely increasingly important with depth in the crust and that the elevated temperatures during granulite-facies metamorphism led to large-scale flow of crustal rocks to produce gneiss domes and the observed subhorizontal refl ectivity of the crust. This unique combined data set for the Bering Shelf region provides compelling evidence for the complete reconstitution/re-equilibration of continental crust from the bottom up during mantle-driven magmatic events associated with crustal extension. Thus, despite Precambrian and Paleozoic rocks at the surface and Alaska's accretionary tectonic history, it is likely that a significant portion of the Bering Sea region lower crust is much younger and related to post-accretionary tectonic and magmatic events. ?? 2009 The Geological Society of America.

Special Paper of the Geological Society of America↗
USGS2009

Deep drilling in the Chesapeake Bay impact structure - An overview

The late Eocene Chesapeake Bay impact structure lies buried at moderate depths below Chesapeake Bay and surrounding landmasses in southeastern Virginia, USA. Numerous characteristics made this impact structure an inviting target for scientific drilling, including the location of the impact on the Eocene continental shelf, its threelayer target structure, its large size (??85 km diameter), its status as the source of the North American tektite strewn field, its temporal association with other late Eocene terrestrial impacts, its documented effects on the regional groundwater system, and its previously unstudied effects on the deep microbial biosphere. The Chesapeake Bay Impact Structure Deep Drilling Project was designed to drill a deep, continuously cored test hole into the central part of the structure. A project workshop, funding proposals, and the acceptance of those proposals occurred during 2003-2005. Initial drilling funds were provided by the International Continental Scientific Drilling Program (ICDP) and the U.S. Geological Survey (USGS). Supplementary funds were provided by the National Aeronautics and Space Administration (NASA) Science Mission Directorate, ICDP, and USGS. Field operations were conducted at Eyreville Farm, Northampton County, Virginia, by Drilling, Observation, and Sampling of the Earth's Continental Crust (DOSECC) and the project staff during September-December 2005, resulting in two continuously cored, deep holes. The USGS and Rutgers University cored a shallow hole to 140 m in April-May 2006 to complete the recovered section from land surface to 1766 m depth. The recovered section consists of 1322 m of crater materials and 444 m of overlying postimpact Eocene to Pleistocene sediments. The crater section consists of, from base to top: basement-derived blocks of crystalline rocks (215 m); a section of suevite, impact melt rock, lithic impact breccia, and cataclasites (154 m); a thin interval of quartz sand and lithic blocks (26 m); a granite megablock (275 m); and sediment blocks and boulders, polymict, sediment-clast-dominated sedimentary breccias, and a thin upper section of stratified sediments (652 m). The cored postimpact sediments provide insight into the effects of a large continental-margin impact on subsequent coastal-plain sedimentation. This volume contains the first results of multidisciplinary studies of the Eyreville cores and related topics. The volume is divided into these sections: geologic column; borehole geophysical studies; regional geophysical studies; crystalline rocks, impactites, and impact models; sedimentary breccias; postimpact sediments; hydrologic and geothermal studies; and microbiologic studies. ?? 2009 The Geological Society of America.

Special Paper of the Geological Society of America↗
USGS2009

Hydrothermal processes above the Yellowstone magma chamber: Large hydrothermal systems and large hydrothermal explosions

Hydrothermal explosions are violent and dramatic events resulting in the rapid ejection of boiling water, steam, mud, and rock fragments from source craters that range from a few meters up to more than 2 km in diameter; associated breccia can be emplaced as much as 3 to 4 km from the largest craters. Hydrothermal explosions occur where shallow interconnected reservoirs of steam- and liquid-saturated fluids with temperatures at or near the boiling curve underlie thermal fields. Sudden reduction in confining pressure causes fluids to flash to steam, resulting in significant expansion, rock fragmentation, and debris ejection. In Yellowstone, hydrothermal explosions are a potentially significant hazard for visitors and facilities and can damage or even destroy thermal features. The breccia deposits and associated craters formed from hydrothermal explosions are mapped as mostly Holocene (the Mary Bay deposit is older) units throughout Yellowstone National Park (YNP) and are spatially related to within the 0.64-Ma Yellowstone caldera and along the active Norris-Mammoth tectonic corridor. In Yellowstone, at least 20 large (>100 m in diameter) hydrothermal explosion craters have been identified; the scale of the individual associated events dwarfs similar features in geothermal areas elsewhere in the world. Large hydrothermal explosions in Yellowstone have occurred over the past 16 ka averaging ~1 every 700 yr; similar events are likely in the future. Our studies of large hydrothermal explosion events indicate: (1) none are directly associated with eruptive volcanic or shallow intrusive events; (2) several historical explosions have been triggered by seismic events; (3) lithic clasts and comingled matrix material that form hydrothermal explosion deposits are extensively altered, indicating that explosions occur in areas subjected to intense hydrothermal processes; (4) many lithic clasts contained in explosion breccia deposits preserve evidence of repeated fracturing and vein-filling; and (5) areal dimensions of many large hydrothermal explosion craters in Yellowstone are similar to those of its active geyser basins and thermal areas. For Yellowstone, our knowledge of hydrothermal craters and ejecta is generally limited to after the Yellowstone Plateau emerged from beneath a late Pleistocene icecap that was roughly a kilometer thick. Large hydrothermal explosions may have occurred earlier as indicated by multiple episodes of cementation and brecciation commonly observed in hydrothermal ejecta clasts. Critical components for large, explosive hydrothermal systems include a water-saturated system at or near boiling temperatures and an interconnected system of well-developed joints and fractures along which hydrothermal fluids flow. Active deformation of the Yellowstone caldera, active faulting and moderate local seismicity, high heat flow, rapid changes in climate, and regional stresses are factors that have strong influences on the type of hydrothermal system developed. Ascending hydrothermal fluids flow along fractures that have developed in response to active caldera deformation and along edges of low-permeability rhyolitic lava flows. Alteration of the area affected, self-sealing leading to development of a caprock for the hydrothermal system, and dissolution of silica-rich rocks are additional factors that may constrain the distribution and development of hydrothermal fields. A partial low-permeability layer that acts as a cap to the hydrothermal system may produce some over-pressurization, thought to be small in most systems. Any abrupt drop in pressure initiates steam flashing and is rapidly transmitted through interconnected fractures that result in a series of multiple large-scale explosions contributing to the excavation of a larger explosion crater. Similarities between the size and dimensions of large hydrothermal explosion craters and thermal fields in Yellowstone may indicate that catastrophic events which result in large hydrothermal explosions are an end phase in geyser basin evolution. The Mary Bay hydrothermal explosion crater complex is the largest such complex in Yellowstone, and possibly in the world, with a diameter of 2.8 km in length and 2.4 km in width. It is nested in Mary Bay in the northern basin of Yellowstone Lake, an area of high heat flow and active deformation within the Yellowstone caldera. A sedimentary sequence exposed in wave-cut cliffs between Storm Point and Mary Bay gives insight into the geologic history of the Mary Bay hydrothermal explosion event. The Mary Bay explosion breccia deposits overlie sand above varved lake sediments and are separated locally into an upper and lower unit. The sand unit contains numerous small normal faults and is coextensive with the Mary Bay breccia in its northern extent. This sand may represent deposits of an earthquake-generated wave. Seismicity associated with the earthquake may have triggered the hydrothermal explosion responsible for development of the Mary Bay crater complex. Large hydrothermal explosions are rare events on a human time scale; however, the potential for additional future events of the sort in Yellowstone National Park is not insignificant. Based on the occurrence of large hydrothermal explosion events over the past 16,000 yr, an explosion large enough to create a 100-m-wide crater might be expected every 200 yr.

Wyoming↗
USGS2011

A geologic and anthropogenic journey from the Precambrian to the new energy economy through the San Juan volcanic field

The San Juan volcanic field comprises 25,000 km 2 of intermediate composition mid-Tertiary volcanic rocks and dacitic to rhyolitic calderas including the San Juan–Uncompahgre and La Garita caldera-forming super-volcanoes. The region is famous for the geological, ecological, hydrological, archeological, and climatological diversity. These characteristics supported ancestral Puebloan populations. The area is also important for its mineral wealth that once fueled local economic vitality. Today, mitigating and/or investigating the impacts of mining and establishing the region as a climate base station are the focuses of ongoing research. Studies include advanced water treatment, the acid neutralizing capacity (ANC) of propylitic bedrock for use in mine-lands cleanup, and the use of soil amendments including biochar from beetle-kill pines. Biochar aids soil productivity and revegetation by incorporation into soils to improve moisture retention, reduce erosion, and support the natural terrestrial carbon sequestration (NTS) potential of volcanic soils to help offset atmospheric CO 2 emissions. This field trip will examine the volcano-tectonic and cultural history of the San Juan volcanic field as well as its geologic structures, economic mineral deposits and impacts, recent mitigation measures, and associated climate research. Field trip stops will include a visit to (1) the Summitville Superfund site to explore quartz alunite-Au mineralization, and associated alteration and new water-quality mitigation strategies; (2) the historic Creede epithermal-polymetallic–vein district with remarkably preserved resurgent calderas, keystone-graben, and moat sediments; (3) the historic mining town of Silverton located in the nested San Juan–Silverton caldera complex that exhibits base-metal Au-Ag mineralization; and (4) the site of ANC and NTS studies. En route back to Denver, we will traverse Grand Mesa, a high NTS area with Neogene basalt-derived soils and will enjoy a soak in the geothermal waters of the Aspen anomaly at Glenwood Springs.

Colorado↗