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Jürgen Kienle - One of the best experts on this subject based on the ideXlab platform.
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Depth of the Ash Flow deposit in the Valley of Ten Thousand Smokes, Katmai National Park, Alaska
Geophysical Research Letters, 1991Co-Authors: Jürgen KienleAbstract:In anticipation of a drill hole, seismic refraction and gravity profiles acquired more than 20 years ago have been reinterpreted in light of new density data to determine the thickness of the Ash Flow deposit in the Valley of the Thousand Smokes (VTTS). The data show the Ash Flow is at least 170m thick and most likely welded in its lower portion. Its average density is ∼1800 kg m−3. Seismic velocities range from 0.6 to 1 km/sec for the deposit's unwelded 30-to-50m-thick top section and 1.8 to 2.5 km/sec for the lower welded section. The data confirm the first realistic estimate of the volume of the Ash Flow of about 11 km³ by Curtis (1968), based on a geomorphic analysis of stream profiles.
Steven E. Ingebritsen - One of the best experts on this subject based on the ideXlab platform.
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Ongoing hydrothermal heat loss from the 1912 Ash-Flow sheet, Valley of Ten Thousand Smokes, Alaska
Journal of Volcanology and Geothermal Research, 2005Co-Authors: Noor Hogeweg, Terry E.c. Keith, Elizabeth M. Colvard, Steven E. IngebritsenAbstract:Abstract The June 1912 eruption of Novarupta filled nearby glacial valleys on the Alaska Peninsula with Ash-Flow tuff (ignimbrite), and post-eruption observations of thousands of steaming fumaroles led to the name ‘Valley of Ten Thousand Smokes’ (VTTS). By the late 1980s most fumarolic activity had ceased, but the discovery of thermal springs in mid-valley in 1987 suggested continued cooling of the Ash-Flow sheet. Data collected at the mid-valley springs between 1987 and 2001 show a statistically significant correlation between maximum observed chloride (Cl) concentration and temperature. These data also show a statistically significant decline in the maximum Cl concentration. The observed variation in stream chemistry across the sheet strongly implies that most solutes, including Cl, originate within the area of the VTTS occupied by the 1912 deposits. Numerous measurements of Cl flux in the Ukak River just below the Ash-Flow sheet suggest an ongoing heat loss of ∼250 MW. This represents one of the largest hydrothermal heat discharges in North America. Other hydrothermal discharges of comparable magnitude are related to heat obtained from silicic magma bodies at depth, and are quasi-steady on a multidecadal time scale. However, the VTTS hydrothermal flux is not obviously related to a magma body and is clearly declining. Available data provide reasonable boundary and initial conditions for simple transient modeling. Both an analytical, conduction-only model and a numerical model predict large rates of heat loss from the sheet 90 years after deposition.
A Ryan - One of the best experts on this subject based on the ideXlab platform.
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Conditions and timescales for welding block-and-Ash Flow deposits
Journal of Volcanology and Geothermal Research, 2014Co-Authors: Michael J. Heap, S. Kolzenburg, J. Welles, Jamie I Farquharson, J K Russell, M. E. Campbell, A RyanAbstract:Welding of pyroclastic deposits to reform a coherent rock mass is a common phenomenon, especially for pumiceous pyroclastic density current deposits (i.e., ignimbrites). However, and despite the pervasive abundance of block-and-Ash Flow (BAF) deposits in the geological and modern record, instances of strongly welded BAF deposits are few. Here, we present a series of high-temperature (800-900. °C) compaction experiments designed to map the conditions (deposit thickness/stress and temperature/viscosity) and timescales that permit or inhibit the welding of BAF deposits. Our experiments were performed on unconsolidated aggregates (containing an Ash and lapilli component) derived from crushed and sieved lava blocks (containing 25% crystals) taken from the well-documented welded BAF deposit at Mount Meager volcano (British Columbia, Canada). The experiments demonstrate that welding efficiency increases with increasing time and temperature. Progressive welding is expressed by increasing axial strain, porosity loss, and bulk density. The rate of change of each of these physical properties reduces as welding progresses. Microstructural analysis of the experimental products shows that the loss of interclast porosity during welding results from the progressive sintering and amalgamation of vitric fragments, and that the pore shape changes from sub-equant pores to stretched lenses sandwiched between vitric and crystal fragments. The coincidence between the microstructure and rock physical properties of the natural and experimental samples highlight that we have successfully reproduced welded BAF in the laboratory. Furthermore, our permeability measurements highlight a hysteresis in the return journey of the ". there-and-back-again" volcanic permeability cycle (expressed by an increase in permeability due to vesiculation and fragmentation followed by a decrease due to welding). This hysteresis cannot be described by a single porosity-permeability power law relationship and reflects the change in pore shape and connectivity during welding. Finally, we show that a simple model for welding can accurately forecast the welding timescales of the BAF deposit at Mount Meager (as reconstructed from the collapse of the Lillooet River valley dam) using our experimental data. We use this validation as a platform to provide a universal window for the welding of BAF deposits, also applicable for comparable welded deposits (e.g., welded autobreccias in block-lavas and lava domes), for a broad range of deposit thickness (or stress) and effective viscosity.
Stephen B. Castor - One of the best experts on this subject based on the ideXlab platform.
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eocene early miocene paleotopography of the sierra nevada great basin nevadaplano based on widespread Ash Flow tuffs and paleovalleys
Geosphere, 2012Co-Authors: Christopher D Henry, David A. John, Nicholas H Hinz, James E Faulds, Joseph P Colgan, Elwood R Brooks, Elizabeth J Cassel, Larry J Garside, David A Davis, Stephen B. CastorAbstract:The distribution of Cenozoic Ash-Flow tuffs in the Great Basin and the Sierra Nevada of eastern California (United States) demonstrates that the region, commonly referred to as the Nevadaplano, was an erosional highland that was drained by major west- and east-trending rivers, with a north-south paleodivide through eastern Nevada. The 28.9 Ma tuff of Campbell Creek is a voluminous (possibly as much as 3000 km 3 ), petrographically and compositionally distinctive Ash-Flow tuff that erupted from a caldera in north-central Nevada and spread widely through paleovalleys across northern Nevada and the Sierra Nevada. The tuff can be correlated over a modern area of at least 55,000 km 2 , from the western foothills of the Sierra Nevada to the Ruby Mountains in northeastern Nevada, present-day distances of ∼280 km west and 300 km northeast of its source caldera. Corrected for later extension, the tuff Flowed ∼200 km to the west, downvalley and across what is now the Basin and Range–Sierra Nevada structural and topographic boundary, and ∼215 km to the northeast, partly upvalley, across the inferred paleodivide, and downvalley to the east. The tuff also Flowed as much as 100 km to the north and 60 km to the south, crossing several east-west divides between major paleovalleys. The tuff of Campbell Creek Flowed through, and was deposited in, at least five major paleovalleys in western Nevada and the eastern Sierra Nevada. These characteristics are unusual compared to most other Ash-Flow tuffs in Nevada that also Flowed great distances downvalley, but far less east and north-south; most tuffs were restricted to one or two major paleovalleys. Important factors in this greater distribution may be the great volume of erupted tuff and its eruption after ∼3 Ma of nearly continuous, major pyroclastic eruptions near its caldera that probably filled in nearby topography. Distribution of the tuff of Campbell Creek and other Ash-Flow tuffs and continuity of paleovalleys demonstrates that (1) the Basin and Range–Sierra Nevada structural and topographic boundary did not exist before 23 Ma; (2) the Sierra Nevada was a lower, western ramp to the Nevadaplano; and (3) any faulting before 23 Ma in western Nevada, including in what is now the Walker Lane, and before 29 Ma in northern Nevada as far east as what is now the Ruby Mountains metamorphic core complex, was insufficient to disrupt the paleodrainages. These data are further evidence that major extension in Nevada occurred predominantly in the late Cenozoic. Characteristics of paleovalleys and tuff distributions suggest that the valleys resulted from prolonged erosion, probably aided by the warm, wet Eocene climate, but do not resolve the question of the absolute elevation of the Nevadaplano. Paleovalleys existed at least by ca. 50 Ma in the Sierra Nevada and by 46 Ma in northeastern Nevada, based on the age of the oldest paleovalley-filling sedimentary or tuff deposits. Paleovalleys were much wider (5–10 km) than they were deep (to 1.2 km; greatest in western Nevada and decreasing toward the paleo–Pacific Ocean) and typically had broad, flat bottoms and low-relief interfluves. Interfluves in Nevada had elevations of at least 1.2 km because paleovalleys were that deep. The gradient from the caldera eastward to the inferred paleodivide had to be sufficiently low so that the tuff could Flow upstream more than 100 km. Two Quaternary Ash-Flow tuffs where topography is nearly unchanged since eruption Flowed similar distances as the mid-Cenozoic tuffs at average gradients of ∼2.5–8 m/km. Extrapolated 200–300 km (pre-extension) from the Pacific Ocean to the central Nevada caldera belt, the lower gradient would require elevations of only 0.5 km for valley floors and 1.5 km for interfluves. The great eastward, upvalley Flow is consistent with recent stable isotope data that indicate low Oligocene topographic gradients in the Nevadaplano east of the Sierra Nevada, but the minimum elevations required for central Nevada are significantly less than indicated by the same stable isotope data. Although best recognized in the northern and central Sierra Nevada, early to middle Cenozoic paleodrainages may have crossed the southern Sierra Nevada. Similar early to middle Cenozoic paleodrainages existed from central Idaho to northern Sonora, Mexico, and persisted over most of that region until disrupted by major Middle Miocene extension. Therefore, the Nevadaplano was the middle part of an erosional highland that extended along at least this length. The timing of origin and location of this more all-encompassing highland indicates that uplift was predominantly a result of Late Cretaceous (Sevier) contraction in the north and a combination of Late Cretaceous–early Cenozoic (Sevier and Laramide) contraction in the south.
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Pyritic Ash-Flow tuff, Yucca Mountain, Nevada
Economic Geology, 1994Co-Authors: Stephen B. Castor, J. V. Tingley, H. F. BonhamAbstract:The Yucca Mountain site is underlain by a 1,500-m-thick Miocene volcanic sequence that comprises part of the southwestern Nevada volcanic field. Rocks of this sequence, which consists mainly of Ash-Flow tuff sheets with minor Flows and bedded tuff, host precious metal mineralization in several areas as near as 10 km from the site. In two such areas, the Bullfrog and Bare Mountain mining districts, production and reserves total over 60 t gold and 150 t silver. Evidence of similar precious metal mineralization at the Yucca Mountain site may lead to mining or exploratory drilling in the future, compromising the security of the repository. The authors believe that most of the pyrite encountered by drilling at Yucca Mountain was introduced as pyroclastic ejecta, rather than by in situ hydrothermal activity. Pyritic ejecta in Ash-Flow tuff are not reported in the literature, but there is no reason to believe that the Yucca Mountain occurrence is unique. The pyritic ejecta are considered by us to be part of a preexisting hydrothermal system that was partially or wholly destroyed during eruption of the tuff units. Because it was introduced as ejecta in tuff units that occur at depths of about 1,000 m, such pyritemore » does not constitute evidence of shallow mineralization at the proposed repository site; however, the pyrite may be evidence for mineralization deep beneath Yucca Mountain or as much as tens of kilometers from it.« less
Geoff Clayton - One of the best experts on this subject based on the ideXlab platform.
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Organic block coatings in block-and-Ash Flow deposits at Merapi Volcano, central Java
Geological Magazine, 2008Co-Authors: Eleanor Donoghue, Valentin R Troll, Lothar M. Schwarzkopf, Geoff Clayton, Robbie GoodhueAbstract:The 1954, 1994, 1998 and 2006 block-and-Ash Flow deposits from Merapi volcano, Indonesia, contain andesite blocks that display localized, dark surface coatings. These coated blocks are typically found in clusters, but may also occur isolated within the block-and-Ash Flow deposits. The coatings form irregular patches (tens of centimetres across) on block surfaces, and are characterized by a silver, metallic appearance. The origin and composition of the coatings have not previously been determined, and no comparable deposits have been described in detail from any other volcanically active region of the world. Carbon isotope analyses of coated and uncoated bulk rock material, and of a range of charred and uncharred wood samples, suggest that the dark block coatings at Merapi volcano are organic in origin. Scanning electron microscopy and reflected light microscopy indicate that such coatings may form due to the migration and subsequent deposition of organic carbon in cavities within the andesite blocks. The most recent field evidence from the 2006 block-and-Ash Flow deposits at Merapi indicates that in situ charcoalification of plant material after block deposition and/or during the final stages of Flow, is the major mechanism that forms localized concentrations of block coatings. Isolated coated blocks may result either from recycling of older deposits, or, more likely, from charcoalification and redistribution of plant material caught between colliding lava blocks during Flow.
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Organic coatings on blocks in block and Ash Flow deposits form Merapi volcano, Java, Indonesia.
Geological Magazine, 2008Co-Authors: Eleanor Donoghue, Valentin R Troll, Lothar M. Schwarzkopf, Robbie Goodhue, Geoff ClaytonAbstract:Organic coatings on blocks in block and Ash Flow deposits form Merapi volcano, Java, Indonesia.
