SEARCH · Geology Reports

Results for “Proceedings”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 541 records · Page 30Linked to original sources

USGS1985

Regression modeling of ground-water flow

Nonlinear multiple regression methods are developed to model and analyze groundwater flow systems. Complete descriptions of regression methodology as applied to groundwater flow models allow scientists and engineers engaged in flow modeling to apply the methods to a wide range of problems. Organization of the text proceeds from an introduction that discusses the general topic of groundwater flow modeling, to a review of basic statistics necessary to properly apply regression techniques, and then to the main topic: exposition and use of linear and nonlinear regression to model groundwater flow. Statistical procedures are given to analyze and use the regression models. A number of exercises and answers are included to exercise the student on nearly all the methods that are presented for modeling and statistical analysis. Three computer programs implement the more complex methods. These three are a general two-dimensional, steady-state regression model for flow in an anisotropic, heterogeneous porous medium, a program to calculate a measure of model nonlinearity with respect to the regression parameters, and a program to analyze model errors in computed dependent variables such as hydraulic head. (USGS)

Open-File Report↗

USGS1994

Crude-oil spill research project near Bemidji, Minnesota - Bibliography, 1984-1994

The U.S. Geological Survey began a research project at the site of a crude-oil spill near Bemidji, Minnesota in 1983. The project is part of the U.S. Geological Survey's Toxic Substances Hydrology Program. The objectives of research at the site are to obtain an understanding of the mobilization, transport, and fate of petroleum derivatives in the shallow subsurface and to use this understanding to develop predictive models of contaminant behavior. This report presents a bibliography of scientific papers that have been published from 1984 to early 1994. Research results have been published in scientific journals, conference proceedings, U.S. Geological Survey Reports, and academic theses.

Open-File Report↗

USGS1995

Collection of short papers on Beaver Creek watershed studies in West Tennessee, 1989-94

In 1989, the U.S. Geological Survey began a scientific investigation to evaluate the effect of agricultural activities on water quality and the effectiveness of agricultural best management practices in the Beaver Creek watershed, West Tennessee. The project is being conducted jointly with other Federal, State, county agencies, the farming community, and academic institutions, in support of the U.S. Department of Agriculture's Hydrologic Unit Area program. The Beaver Creek project has evolved into a long-term watershed assessment and monitoring program. In 1991, a grant was received to develop and evaluate sampling strategies for higher order streams. During the summer of 1992, a reconnaissance of water-quality conditions for the shallow aquifers in Shelby, Tipton, Fayette, and Haywood Counties was conducted and included 89 domestic wells in the Beaver Creek watershed. Results from this effort lead to the development of a 1-year program to evaluate cause- and-effect relations that can explain the observed water-quality conditions for the shallow aquifers in the watershed. In 1992 the USGS, in cooperation with the Soil Conservation Service and the Shelby County Soil Conservation District, began an evaluation of in-stream processes and in-stream resource-management systems. In 1993, a biomonitoring program was established in the watershed. This collection of eight articles and abstracts was originally published in the American Water Resources Association National Symposium on Water Quality Proceedings for the national conference held in Chicago in 1994 and describes what has been learned in the study to date.

Tennessee↗

USGS1995

EAST93: Geophysical traverse from the Transantarctic Mountains to the Wilkes Basin, East Antarctica

The East Antarctic Seismic Traverse (EAST93) was a geophysical traverse designed to image the bedrock under the East Antarctic ice cap. The traverse started 10 km west of the Taylor Dome drill site and 25 km west of the exposed bedrock of the Transantarctic Mountains at Lashly Mt. and ended 323 km west of the drill site over the Wilkes subglacial basin (Fig. 1). The traverse was located subparallel to latitude 78° S starting 30-50 km north of the Victoria Land Traverse (1958-1959). It was carried out jointly by the U.S. Geological Survey and Stanford University, U.S.A., together with the Institute of Geological and Nuclear Sciences, and Victoria University, New Zealand, during December 1993 and January 1994. The geophysical traverse included 236 km of multichannel seismic reflection data at 150 m shot intervals, 312.5 km of gravity data collected at intervals of 2.1 km, 312.5 km of magnetic data (total field intensity) collected at average intervals of 0.5 km, and 205 km of ground penetrating radar at intervals of 77 m. Relative locations and elevations of the entire traverse were measured at intervals of 150 m by traditional surveying methods, and tied to three absolute locations measured by the Global Positioning System (GPS). EAST93 is the first large-scale geophysical traverse on the polar plateau to our knowledge since the early 1960s. As such, the experiment presented several logistical challenges: (1) how to collect regional seismic profiles during the short Antarctic summer; (2) how to keep the scientific instruments running with minimal protection in harsh conditions; and (3) how to combine daily moves of camp with full days of work. The scientific and logistical aspects of the project proceeded, in general, according to plan despite the harsh conditions and our lack of previous experience on the polar plateau. Two unanticipated problems affected the progress of the work: the strong wind which slowed seismic acquisition, and the break-down of one of the large traverse vehicles. The major operational lessons of this project are. (1) Primacord laid close to the surface is not an adequate seismic source for imaging under the thick East Antarctic ice sheet, despite positive prior tests on the Ross Ice Shelf. (2) It is necessary to reduce the 6-7 hours spent daily on camp move and other chores by integrating the living quarters into the working teams, and by improving vehicle warming methods and generator housing. The following report details the operational and logistical aspects of the work, the weather and ground conditions, the technical aspects of acquisition of geophysical data, and lessons and recommendations for future geophysical traverses.

Open-File Report↗

USGS1996

Reconnaissance of hydrologic monitoring sites and preliminary monitoring plan for the Vale, Oregon, geothermal area

The Bonneville Power Administration is working with private industry to develop a geothermal demonstration project in the Known Geothermal Resources Area (KGRA) near Vale, Oregon. Hydrologic monitoring in the area is planned in order to evaluate any impacts from the proposed development. The hydrology in and around the Vale KGRA is not well known. Additionally, little is known about the targeted geothermal reservoir and the nature of its connection to the shallow ground-water system. Given this uncertainty, a variety of features were selected to ensure adequate monitoring coverage. Wells and springs in and around the geothermal area were evaluated, and 19 were selected as potential monitoring sites. In selecting wells and springs for monitoring, particular emphasis was placed on those with a known or probable connection with the geothermal system because they would most likely be the first to show any effects from development. The selected features include thermal wells in the hot-spring area near the town of Vale and a hot spring south of the KGRA. Several warm wells (70 to 90 degrees Fahrenheit) near the KGRA were also selected because it is likely that the water produced from these wells includes a component of geothermal water. In order to identify any effects of development, it is necessary to have an understanding of natural and man-caused variations and trends prior to development. A quarterly measurement schedule is proposed to help characterize these variations and trends. It is anticipated that the proposed monitoring plan will be modified as exploration and development proceed and more is learned about the geothermal system.

Oregon↗

USGS1998

Simulation of ground-water flow in the Albuquerque Basin, central New Mexico, 1901-95, with projections to 2020

The ground-water-flow model of the Albuquerque Basin (Kernodle, J.M., McAda, D.P., and Thorn, C.R., 1995, Simulation of ground-water flow in the Albuquerque Basin, central New Mexico, with projections to 2020: U.S. Geological Survey Water-Resources Investigations Report 94-4251, 114 p.) was updated to include new information on the hydrogeologic framework (Hawley, J.W., Haase, C.S., and Lozinsky, R.P., 1995, An underground view of the Albuquerque Basin: Proceedings of the 39th Annual New Mexico Water Conference, November 3-4, 1994, p. 37-55). An additional year of ground-water-withdrawal data was appended to the simulation of the historical period and incorporated into the base for future projections to the year 2020. The revised model projects the simulated ground-water levels associated with an aerally enlarged occurrence of the relatively high hydraulic conductivity in the upper part of the Santa Fe Group east and west of the Rio Grande in the Albuquerque area and north to Bernalillo. Although the differences between the two model versions are substantial, the revised model does not contradict any previous conclusions about the effect of City of Albuquerque ground-water withdrawals on flow in the Rio Grande or the net benefits of an effort to conserve ground water. Recent revisions to the hydrogeologic model (Hawley, J.W., Haneberg, W.C., and Whitworth, P.M., in press, Hydrogeologic investigations in the Albuquerque Basin, central New Mexico, 1992-1995: Socorro, New Mexico Bureau of Mines and Mineral Resources Open- File Report 402) of the Albuquerque Basin eventually will require that this model version also be revised and updated.

Open-File Report↗

USGS1997

Annotated bibliography of selected publications, through 1996, Cheyenne municipal well field areas, Cheyenne, Wyoming

Annotated bibliographies for 55 hydrology and geology manuscripts pertaining to the Cheyenne municipal wells fields are listed in this report. For each manuscript, a citation is provided, a summaryparagraph is presented, key words are listed, and a location of the report is given. The report lists manuscripts, conference proceedings, and guidebooks published by the U. S. Geological Survey, State of Wyoming, Geological Society of America, Wyoming State Geological Survey, private consultants, and University of Wyoming. Information on geological formations, structural geology, aquifer characteristics, water levels, well- field production, water-demand projections, and water quality is included in the manuscripts. The Cheyenne Board of Public Utilities, the University of Wyoming, and the U. S. Geological Surveycooperatively produced this annotated bibliography to allow easy access and efficient utilization of existing data. The manuscripts were authored between 1910 and 1996, reflecting work completed over a long period of development in the Cheyenne, Wyoming area. Some manuscripts did not receive broad distribution and indexing, thus they have been difficult to locate in the past. By having the references and summaries within one report, time and effort to gather previous study results will be minimized.

Wyoming↗

USGS1997

The U. S. Geological Survey in the early 1900's - Reminiscences of D. Foster Hewett, Adolph Knopf, and Frank C. Calkins

In December, 1962, 81-year-old Donnel Foster Hewett hosted a dinner party for what was then the “younger crop”of U.S. Geological Survey geologists in Menlo Park, California. His aim was to give them the benefit of some collective wisdom about what life in the Survey was like in the “good old days,” with particular attention paid to the personalities of some of the men who had played prominent roles in the Survey's development. To this end he recruited two other Survey old-timers: Adolph Knopf (80), a Visiting Professor at nearby Stanford University, and Frank Calkins (84), a USGS Annuitant in the Menlo Park headquarters. Following a convivial dinner, these three distinguished gentlemen delivered their remarks to an appreciative and responsive audience. In an act of great foresight, someone, perhaps Hewett himself, had arranged to have the proceedings tape-recorded. Walden P. Pratt, of the USGS in Denver, Colo., obtained a copy of the tape and transcribed it.

Open-File Report↗

USGS1997

Coal availability in the Hilight Quadrangle, Powder River Basin, Wyoming; a prototype study in a western coal field

The U.S. Geological Survey (USGS), in cooperation with the Bureau of Land Management (BLM), Geological Survey of Wyoming, and U.S. Bureau of Mines (USBM), has produced an estimate of the amount of available coal in an area about 35 miles south of Gillette, Wyo., where the Wyodak coal bed is, in places, more than 100 ft thick. Available coal is the quantity of the total coal resource that is accessible for mine development under current regulatory, land-use, and technologic constraints. This first western coal availability study, of the Hilight 7 1/2-minute quadrangle, indicates that approximately 60 percent (2.7 billion short tons) of the total 4.4 billion tons of coal in-place in the quadrangle is available for development. (There has been no commercial mining in the Hilight quadrangle.) Approximately 67 percent (1.9 billion tons) of the Main Wyodak coal bed is considered available. All tonnage measurements in this report are given in short tons. Coal-development considerations in the quadrangle include dwellings, railroads, pipelines, power lines, wildlife habitat (eagles), alluvial valley floors, cemeteries, and the Hilight oil and gas field and gas plant. Some of these considerations could be mitigated so that surface mining of the coal may proceed; others could not be mitigated and would preclude mining in their vicinity. Other technological constraints that influence the availability of the coal include overburden thickness, coal beds too thin, and areas of clinker.

Montana, Wyoming↗

USGS1998

Maps showing geology, oil and gas fields and geologic provinces of the Arabian Peninsula

Introduction This digital map compilation, which includes geology, geologic provinces, and oil and gas fields of the Arabian Peninsula, is part of a map series of the world produced by the U.S. Geological Survey World Energy Project. The goal of the project is to produce a worldwide assessment of the undiscovered, technically recoverable oil and gas resources and report these results by the year 2000. To assess the world's petroleum, a sequence of steps is being undertaken proceeding from defining geologic provinces of the world at a comparable scale, allocating oil and gas fields to these provinces, defining petroleum systems within these provinces, and ultimately assessing the undiscovered petroleum potential of selected provinces of the world. A more in-depth discussion of the geologic provinces and their relative ranking in terms of total known petroleum volume is given in USGS Open File Report 97-463 (see Klett and others, 1997).

Open-File Report↗

USGS1998

Preliminary estimates of benthic fluxes of dissolved metals in Coeur d'Alene Lake, Idaho

This report presents porewater and selected water column data collected from Coeur d'Alene Lake in September of 1992. Despite probable oxidation of the porewater samples during collection and handling, these data are used to calculate molecular diffusive fluxes of dissolved metals (that is, Zn, Pb, Cu, and Mn) across the sediment-water interface. While these data and calculations provide preliminary information on benthic metal fluxes in Coeur d'Alene Lake, further work is needed to verify their direction and magnitude. The benthic flux calculations indicate that the sediment is generally a source of dissolved Zn, Cu, Mn, and, possibly, Pb to the overlying water column. These benthic fluxes are compared with two other major sources of metals to Coeur d'Alene Lake-the Coeur d'Alene and St. Joe Rivers. Comparisons indicate that benthic fluxes of Zn, Pb, and Cu are generally less than half of the fluxes of these metals into the lake from the Coeur d'Alene River. However, in a few cases, the calculated benthic metal fluxes exceed the Coeur d'Alene River fluxes. Benthic fluxes of Zn and, possibly, Pb may be greater than the corresponding metal fluxes from the St. Joe River. These results have implications for changes in the relative importance of metal sources to the lake as remediation activities in the Coeur d'Alene River basin proceed.

Idaho↗

USGS1999

A plan for assessing the occurrence and distribution of methyl tert-butyl ether and other volatile organic compounds in drinking water and ambient ground water in the Northeast and Mid-Atlantic regions of the United States

A plan to assess the occurrence and distribution of methyl tert-butyl ether (MTBE) and other volatile organic compounds (VOCs) in drinking water and ambient ground water in the Northeast and Mid-Atlantic regions of the United States was designed to meet two primary objectives. This study will provide the U.S. Environmental Protection Agency with information on potential human exposure to MTBE and other VOCs from drinking water. In addition, the study will further the goals of the U.S. Geological Survey's (USGS) National Water Quality Assessment Program (NAWQA) by providing additional information on the occurrence and distribution of VOCs in ambient ground water beneath a large, highly urbanized part of the Nation. The study will proceed in two phases-a drinking-water assessment (phase 1) and an ambient ground-water assessment (phase 2). The drinking-water assessment will involve compilation, review, and analysis of available water- quality and ancillary data for approximately 20 percent of the community water systems in 12 States in the Northeast and Mid-Atlantic regions. This effort will summarize the occurrence and distribution of MTBE and other VOCs in drinking water supplied by 2,110 community water systems. The ambient ground-water assessment will involve compilation, review, and analysis of data on MTBE and other VOCs from previous USGS studies in the 12-State area, including regional water-quality assessments conducted for the USGS's NAWQA, plus other available State or local datasets. These data will be related, to the extent allowed by the completeness and quality of the data, to land-use patterns, population density, and other anthropogenic and natural factors using statistical tests. The occurrence and distribution of MTBE and other VOCs in ambient ground water and, to the extent possible, drinking water in relation to such factors, will be evaluated.

Open-File Report↗

USGS1925

Correlation of the Eocene formations in Mississippi and Alabama

During Eocene time the site of the boundary between the States of Mississippi and Alabama fell within the transition zone between the Mississippi embayment and the open Gulf of Mexico. That different types of deposition proceeded simultaneously within these two regions may be inferred from the different facies which deposits of the same age exhibit on opposite sides of the State line. In general, much clay and carbonaceous material were laid down in Mississippi, while shell marl, laminated sand, and limestone were being formed in Alabama. The purpose of this paper is to point out the equivalences of formations of different facies in the two States. The formations of different facies in the two States. The correlation adopted in shown in the following table, all the formation names in which have been previously used, except Kosciusko sandstone, a new name proposed for a member of the Lisbon formation.

Alabama;Mississippi↗

USGS2001

Catastrophic debris flows transformed from landslides in volcanic terrains : mobility, hazard assessment and mitigation strategies

Communities in lowlands near volcanoes are vulnerable to significant volcanic flow hazards in addition to those associated directly with eruptions. The largest such risk is from debris flows beginning as volcanic landslides, with the potential to travel over 100 kilometers. Stratovolcanic edifices commonly are hydrothermal aquifers composed of unstable, altered rock forming steep slopes at high altitudes, and the terrain surrounding them is commonly mantled by readily mobilized, weathered airfall and ashflow deposits. We propose that volcano hazard assessments integrate the potential for unanticipated debris flows with, at active volcanoes, the greater but more predictable potential of magmatically triggered flows. This proposal reinforces the already powerful arguments for minimizing populations in potential flow pathways below both active and selected inactive volcanoes. It also addresses the potential for volcano flank collapse to occur with instability early in a magmatic episode, as well as the 'false-alarm problem'-the difficulty in evacuating the potential paths of these large mobile flows. Debris flows that transform from volcanic landslides, characterized by cohesive (muddy) deposits, create risk comparable to that of their syneruptive counterparts of snow and ice-melt origin, which yield noncohesive (granular) deposits, because: (1) Volcano collapses and the failures of airfall- and ashflow-mantled slopes commonly yield highly mobile debris flows as well as debris avalanches with limited runout potential. Runout potential of debris flows may increase several fold as their volumes enlarge beyond volcanoes through bulking (entrainment) of sediment. Through this mechanism, the runouts of even relatively small collapses at Cascade Range volcanoes, in the range of 0.1 to 0.2 cubic kilometers, can extend to populated lowlands. (2) Collapse is caused by a variety of triggers: tectonic and volcanic earthquakes, gravitational failure, hydrovolcanism, and precipitation, as well as magmatic activity and eruptions. (3) Risk of collapse begins with initial magmatic activity and increases as intrusion proceeds. An archetypal debris flow from volcanic terrain occurred in Colombia with a tectonic earthquake (M 6.4) in 1994. The Rio Piez conveyed a catastrophic wave of debris flow over 100 kilometers, coalesced from multiple slides of surflcial material weakened both by weathering and by hydrothermal alteration in a large strato- volcano. Similar seismogenic flows occurred in Mexico in 1920 (M -6.5), Chile in 1960 (M 9.2), and Ecuador in 1987 (M 6.1 and 6.9). Velocities of wave fronts in two examples were 60 to 90 km/hr (17-25 meters per second) over the initial 30 kilometers. Volcano flank and sector collapses may produce untransformed debris avalanches, as occurred initially at Mount St. Helens in 1980. However, at least as common is direct transformation of the failed mass to a debris flow. At two other volcanoes in the Cascade Range-- Mount Rainier and Mount Baker--rapid transformation and high mobility were typical of most of at least 15 Holocene flows. This danger exists downstream from many stratovolcanoes worldwide; the population at risk is near 150,000 and increasing at Mount Rainier. The first step in preventing future catastrophes is documenting past flows. Deposits of some debris flows, however, can be mistaken for those of less-mobile debris avalanches on the basis of mounds formed by buoyed megaclasts. Megaclasts may record only the proximal phase of a debris flow that began as a debris avalanche. Runout may have extended much farther, and thus furore flow mobility may be underestimated. Processes and behaviors of megaclast-bearing paleoflows are best inferred from the intermegaclast matrix. Mitigation strategy can respond to volcanic flows regardless of type and trigger by: (1) Avoidance: Limit settlement in flow pathways to numbers that can be evacuated after event warnings (flow is occurring). (2) Instrumental even

Professional Paper↗

USGS2008

Overview of the 2004 to 2006, and continuing, eruption of Mount St. Helens, Washington

Rapid onset of unrest at Mount St. Helens on September 23, 2004, initiated an uninterrupted lava-dome-building eruption that continues to the time of writing this overview (spring 2006) for a volume of papers focused on this eruption. About three weeks of intense seismic unrest and localized surface uplift, punctuated by four brief explosions, constituted a ventclearing phase, during which there was a frenzy of media attention and considerable uncertainty regarding the likely course of the eruption. The third week exhibited lessened seismicity and only minor venting of steam and ash, but rapid growth of the uplift, or welt, south of the 1980-86 lava dome proceeded as magma continued to push upward. Crystalrich dacite (~65 weight percent SiO 2 ) lava first appeared at the surface on October 11, 2004, beginning the growth of a complex lava dome of uniform chemical composition accompanied by persistent but low levels of seismicity, rare explosions, low gas emissions, and frequent rockfalls. Petrologic studies suggest that the dome lava is chiefly of 1980s vintage, but with an admixed portion of new dacite. Alternatively, it may derive from a part of the magma chamber not tapped by 1980s eruptions. Regardless, detailed investigations of crystal chemistry, melt inclusions, and isotopes reveal a complex magmatic history. Largely episodic extrusion between 1980 and 1986 produced a relatively symmetrical lava dome composed of stubby lobes. In contrast, continuous extrusion at mean rates of about 5 m 3 /s in autumn 2004 to <1 m 3 /s in early 2006 has produced an east-west ridge of three mounds with total volume about equal to that of the old dome. During much of late 2004 to summer 2005, a succession of spines, two recumbent and one steeply sloping and each mantled by striated gouge, grew to nearly 500 m in length in the southeastern sector of the 1980 crater and later disintegrated into two mounds. Since then, growth has been concentrated in the southwestern sector, producing a relatively symmetrical mound with steep gougecovered slabs on its east flank. Throughout the eruption, the position of the extrusive vent has remained more or less fixed. Lack of geodetic evidence for either volume increase or pressure increase in the deep magmatic system since about 1990 and geodetic modeling that can account for only 20 to 30 percent of the 2004-to-present dome volume puzzles geodesists. Better constraints on parameters such as magma-chamber volume, crustal properties, and magma compressibility are needed to improve the models. Development of the welt and the new dome bisected horseshoe-shaped Crater Glacier, which formerly wrapped around three sides of the 1980s dome, and fractured, compressed, and thickened the glacier’s surviving east and west arms. Doubling of ice thickness resulted in increased flow rate and advance of termini, although rapid infiltration of water into the highly porous glacier bed prevented substantial basal sliding. Overall, dome growth and disintegration has removed surprisingly little ice. The outcome of the ongoing eruption remains uncertain, but Mount St. Helens’ varied eruptive history suggests multiple possibilities. One dynamical model and several petrologic investigations regard the current eruption as an extension of 1980s dome building that may persist continuously or episodically for years to come.

Washington↗

USGS2008

Effects of lava-dome growth on the crater glacier of Mount St. Helens, Washington

The process of lava-dome emplacement through a glacier was observed for the first time as the 2004-6 eruption of Mount St. Helens proceeded. The glacier that had grown in the crater since the cataclysmic 1980 eruption was split in two by the new lava dome. The two parts of the glacier were successively squeezed against the crater wall. Photography, photogrammetry, and geodetic measurements document glacier deformation of an extreme variety, with strain rates of extraordinary magnitude as compared to normal temperate alpine glaciers. Unlike such glaciers, the Mount St. Helens crater glacier shows no evidence of either speed-up at the beginning of the ablation season or diurnal speed fluctuations during the ablation season. Thus there is evidently no slip of the glacier over its bed. The most reasonable explanation for this anomaly is that meltwater penetrating the glacier is captured by a thick layer of coarse rubble at the bed and then enters the volcano’s groundwater system rather than flowing through a drainage network along the bed. Mechanical consideration of the glacier-squeeze process also leads to an estimate for the driving pressure applied by the growing lava dome.

Washington↗

USGS2008

Evolving magma storage conditions beneath Mount St. Helens inferred from chemical variations in melt inclusions from the 1980-1986 and current (2004-2006) eruptions

Major element, trace element, and volatile concentrations in 187 glassy melt inclusions and 25 groundmass glasses from the 1980-86 eruption of Mount St. Helens are presented, together with 103 analyses of touching FE-Ti oxide pairs from the same samples. These data are used to evaluate the temporal evolution of the magmatic plumbing system beneath the volcano during 1980-86 and so provide a framework in which to interpret analyses of melt inclusions from the current (2004-2006) eruption. Major and trace element concentrations of all melt inclusions lie at the high SiO 2 end of the data array defined by eruptive products of the late Quaternary age from Mount St. Helens. For several major and trace elements, the glasses define a trend that is oblique to the whole-rock trend, indicating that different mineral assemblages were responsible for the two trends. The whole-rock trend can be ascribed to differentiation of hydrous basaltic parents in a deep-seated magma reservoir, probably at depths great enough to stabilize garnet. In contrast, the glass trends were generated by closed-system crystallization of the phenocryst and microlite mineral assemblages at low pressures. The dissolved H 2 O content of the melt inclusions from 1980-86, as measured by the ion microprobe, ranges from 0 to 6.7 wt. percent, with the highest values obtained from the plinian phase of May 18, 1980. Water contents decrease with increasing SiO 2 , consistent with decompression-driven crystallization. Preliminary data for dissolved CO 2 in melt inclusions from the May 18 plinian phase from August 7, 1980, indicate that X H 2 O in a vapor phase was approximately constant at 0.80, irrespective of H 2 O content, suggestive of closed-system degassing with a high bubble fraction or gas streaming through the subvolcanic system. Temperature and f O 2 estimates for touching Fe-Ti oxides show evidence for heating during crystallization owing to release of latent heat. Consequently, magmas with the highest microlite crystallinities record the highest temperatures. Magmas also become progressively reduced during ascent and degassing, probably as a result of redox equilibria between exsolving S-bearing gases and magmas. The lowest temperature oxides have f O 2 ≈ NNO, similar to high-temperature fumarole gases from the volcano. The temperature and f O 2 of the magma tapped by the plinian phase of May 18, 1980, are 870-875°C and NNO+0.8, respectively. The dissolved volatile contents of the melt inclusions have been used to calculate sealing pressures; that is, the pressure at which chemical exchange between inclusion and matrix melt ceased. These are greatest for the May 18 plinian magma (120 to 320 MPa); lower pressures are recorded by samples of the preplinian cryptodome and by all post-May 18 magmas. Magma crystallinity, calculated from melt-inclusion Rb contents, is negatively correlated with sealing pressure, consistent with decompression crystallization. Elevated contents of Li in melt inclusions from the cryptodome and post-May 18 samples are consistent with transfer of Li in a magmatic vapor phase from deeper parts of the magma system to magma stored at shallower levels. The Li enrichment attains its maximum extent at ~150 MPa, which is ascribed to separation of a single vapor phase into H 2 O-rich gas and dense Li-rich brine at the top of the magma column. There are striking correlations between melt-inclusion chemistry and monitoring data for the 1980-86 eruption. Dissolved SO 2 contents of melt inclusions from any given event, multiplied by the mass of magma erupted during that event, correlate with the measured flux of SO 2 at the surface, suggesting that magma degassing and melt-inclusion sealing are closely related in time and space. Textural and chemical evidence indicates that melt inclusions became effectively sealed (physically or kinetically) shortly before eruption. Thus by converting pressure to depth using a density model and edifice-loading algorithm for the volcano, changing depths of magma extraction with time can be tracked and compared to the seismic record. The plinian eruption of May 18, 1980, involved magma stored 5-11 km below sea level; this is inferred to be the subvolcanic magma chamber. The preceding eruptions, including the May 18, 1980, blast, involved magma withdrawal from the cryptodome and conduit down to 5 km below sea level. Subsequent 1980 eruptions tapped magma down to depths of ≤10 km below sea level. Tapping of magma stored deeper than 2 km below sea level stopped abruptly at the end of 1980, coincident with the onset of extensive shallow seismicity and a change from explosive to effusive eruption style from 1981 to 1986. Overall, the 1980-86 eruption is consistent with the evisceration of a thin, vertically extensive body of magma extending from 5 to at least 11 km below sea level and connected to the surface by a thin conduit. In the absence of sustained high magma-supply rates from depth, decompression crystallization of magma ascending through the system leads eventually to plugging of the conduit. The current eruption of Mount St. Helens shares some similarities with the 1981-86 dome-building phase of the previous eruption, in that there is extensive shallow seismicity and extrusion of highly crystalline material in the form of a sequence of flows and spines. Melt inclusions from the current eruption have low H 2 O contents, consistent with magma extraction from shallow depths. Highly enriched Li in melt inclusions suggests that vapor transport of Li is a characteristic feature of Mount St. Helens. Melt inclusions from the current eruption have subtly different trace-element chemistry from all but one of the 1980-86 melt inclusions, with steeper rareearth-element (REE) patterns and low U, Th, and high-fieldstrength elements (HFSE), indicating addition of a new melt component to the magma system. It is anticipated that increasing involvement of the new melt component will be evident as the current eruption proceeds.

Washington↗

USGS2008

Growth of the 2004-2006 lava-dome complex at Mount St. Helens, Washington

The eruption of Mount St. Helens from 2004 to 2006 has comprised extrusion of solid lava spines whose growth patterns were shaped by a large space south of the 1980-86 dome that was occupied by the unique combination of glacial ice, concealed subglacial slopes, the crater walls, and relics of previous spines. The eruption beginning September 2004 can be divided (as of April 2006) into five phases: (1) predome deformation and phreatic activity, (2) initial extrusion of spines, (3) recumbent spine growth and repeated breakup, (4) southward extrusion across previous dome debris, and (5) normal faulting of the phase 4 dome to form a depression, a shift to westward extrusion and overthrusting of earlier phase 5 products. Overall, steady spine extrusion gradually slowed from 6 m 3 /s in November 2004 to 0.6 m 3 /s in February 2006. Thermal camera data show that phase 1 activity included low-temperature thermal features, such as fumaroles, fractures, and ground warming related to rapid uplift, as well as deformation in the south moat of the crater. The relatively cold (<160°C) phreatic eruptions of early October heralded activity at a subglacial vent situated along the south-sloping margin of the 1980–86 dome. Thermal infrared imagery, documenting increased heat flow, presaged phase 2 extrusion of the October 11–15, 2004, lava spine. The thermal images of the extruding spine revealed a hot basal margin and highest temperatures of 600–730°C. During phase 3, a recumbent whaleback-shaped spine with a low-temperature shroud of fault gouge and a hot, U-shaped basal margin extruded. This spine pushed southward along the bed of the glacier until it encountered the south wall of the 1980 crater, whereupon it broke up, decoupled, and regrew. Continued southward growth of the recumbent spine pushed cold deformed rock, hot dome rubble, and glacier ice eastward at a rate of 2 m/d. In April 2005, breakup of the whaleback and growth of a lava spine across previous dome rubble heralded phase 4 spine thrusting over previous spine remnants. During phase 4, the active spine pushed southward with an increasingly vertical component and increasing incidence of large rockfalls. In late July, the spine decoupled from its source, the vent reorganized, and a new spine began to grow westward at right angles to the previous growth direction, defining phase 5. Dome migration again plowed glacier ice out of the way at a rate of about 2 m/d, this time westward. In early October, the spine buckled near the vent and thrust over the previous one. A massive spine monolith had been constructed by December 2005, and growth of spines with increasingly steep slopes characterized activity through April 2006. The chief near-surface controls on spine extrusion during 2004-6 have been vent location, relict topographic surfaces from the 1980s, and spine remnants emplaced previously during the present eruption. In contrast, glacier ice has had minimal influence on spine growth. Ice as thick as 150 m has prevented formation of marginal angle-of-repose talus fans but has not provided sufficient resistance to stop spine growth or slow it appreciably. Spines initially emerged along a relict south-facing slope as steep as 40° on the 1980s dome. The open space of the moat between that dome and the crater walls permitted initial southward migration of recumbent spines. An initial spine impinged on the opposing slopes of the crater and stopped; in contrast, recumbent whaleback spines of phase 3 impinged on opposing walls of the crater at oblique angles and rotated eastward before breaking up. Once spine remnants occupied all available open space to the south, spines thrust over previous remnants. Finally, with south and east portions of the moat filled, spine growth proceeded westward. Although Crater Glacier had only a small influence on the growing spines, spine growth affected the glacier dramatically, initially dividing it into two arms and then bulldozing it hundreds of meters, first east and then west, and heaping it more than 100 m higher than its original altitude.

Washington↗