No description available.

The Springerville Volcanic Field (SVF) is a monogenetic volcanic field located in east-central Arizona and is the southernmost of several late Pliocene to Holocene volcanic fields along the margin of the Colorado Plateau. It encompasses an area of over 3000 km2 and consists of over 450 vents, most of which are cinder cones, which produced mainly basaltic flows, between 2.1 and 0.3 Ma. About 85% of the SVF was previously mapped in detail by Condit, Crumpler and Aubele (1999). In the summers of 2010 and 2011, mapping was completed in the remaining portion of the field known as the Yellow Jacket Cienega Subdivision (YJC). The YJC area is of great interest because it arguably contains the youngest and most evolved flows and represents the convergence of several different geographic subdivisions. The completed dataset, including the chemical analysis of 575 samples, allows for further study of the petrogenetic evolution of the SVF, with possibilities for thermobarometry and distinguishing isotopic reservoirs. The source rock for the SVF lavas was determined to be a garnet lherzolite with a higher clinopyroxene to garnet ratio than typical garnet lherzolite. Based on the methods derived by Lee et al (2009), depth of melting ranged from 75km to 130km, though the majority lie between 107 and 115 km depth. This could be inferred as a range of depths of melting, beginning in the garnet range and extending shallower, or scatter due to the assumptions made for these calculations. Despite the fairly limited isotopic data, SVF lavas seem to be derived from a Prevalent Mantle (PREMA) reservoir, with input of an enriched component, which is likely due to crustal contamination. The completed dataset for the SVF represents a unique resource, useful not only in studying the petrogenetic evolution of this volcanic field, but as a yardstick for comparing similar volcanic occurrences.

The bulk of the eruption of Askja in north central Iceland on March 28-29 1875 consisted of a plinian eruption that lasted 6-7 hours, produced 0.2 km3 of ash and rhyolitic pumice, and created a surge and partially welded ash/pumice fall deposit that crops out on the shore of the modern caldera lake (Sparks et al. 1981). We evaluate the volatile budget of the magma during the eruption and focus on water concentration in glass fragments and shards, glass adjacent to crystals, and melt inclusions (MIs). Sparks et al. (1981) estimated the gas exit velocity at the vent was 380 m/s during the plinian phase, and the water concentration at 2.8 wt%. Measurements of water concentration in basaltic and rhyolitic glass shards from layers C through E range from 0.15 to 0.5 wt%, with variations within layers, a drop in layer D, and increase in layer E. Plagioclase and pyroxene crystals from layers C through E contain rhyolitic MIs with water concentrations ranging from 0.1 to 1.8 wt%, some higher than the matrix glass. Magma underwent degassing on its way to the surface. Rhyolitic glass adjacent to crystals hosting MIs has the highest water concentration, from 0.4 to 2.18 wt%. This, and the initial phreatoplinian eruptive style, both suggest interaction of magma with meteoric water during the eruption. Intimate mixtures of basaltic glass compositions within samples and basaltic glass surrounded by rhyolitic glass support the conclusion of Sigurdsson and Sparks (1981) that magmas mingled prior to and during the eruption.

No description available.

No description available.

I describe an experimental apparatus used to perform deformation experiments relevant to volcanology. The apparatus supports low-load, high-temperature deformation experiments under dry and wet conditions on natural and synthetic samples. The experiments recover the transient rheology of complex (melt ± porosity ± solids) volcanic materials during uniaxial deformation. The key component to this apparatus is a steel cell designed for high-temperature deformation experiments under controlled water pressure. Experiments are run under constant displacement rates or constant loads; the range of accessible experimental conditions include: 25 - 1100 °C, load stresses 0 to 150 MPa, strain rates 10⁻⁶ to 10⁻² s⁻¹, and fluid pressures 0-150 MPa. I present a suite of high-temperature, uniaxial deformation experiments performed on 25 by 50 mm unjacketed cores of porous Φ∼0.8) sintered rhyolitic ash. The experiments were performed at, both, atmospheric (dry) and elevated water pressure conditions (wet). Dry experiments were conducted mainly at 900 °C, but also included a suite of lower temperature experiments at 850, 800 and 750 °C. Wet experiments were performed at ∼650 °C under water pressures of 1, 2.5, 3, and 5 MPa, and at a fixed PH2O of ∼2.5 MPa for temperatures of ∼385, 450, and 550 °C. During deformation, strain is manifest by shortening of the cores, reduction of porosity, flattening of ash particles, and radial bulging of the cores. The continuous reduction of porosity leads to a dynamic transient strain-dependent rheology and requires strain to be partitioned between a volume (porosity loss) and a shear (radial bulging) component. The effect of increasing porosity is to expand the window for viscous deformation for dry melts by delaying the onset of brittle deformation by ∼50 °C (875 °C to 825 °C). The effect is more pronounced in hydrous melts (∼0.67 — 0.78 wt. % H₂0) where the viscous to brittle transition is depressed by ∼140 to 150 °C. Increasing water pressure also delays the onset of strain hardening due to compaction-driven porosity reduction. These rheological data are pertinent to volcanic processes where high-temperature porous magmas I liquids are encountered (e.g., magma flow in conduits, welding of pyroclastic materials).

No description available.

I describe an experimental apparatus used to perform deformation experiments relevant to volcanology. The apparatus supports low-load, high-temperature deformation experiments under dry and wet conditions on natural and synthetic samples. The experiments recover the transient rheology of complex (melt ± porosity ± solids) volcanic materials during uniaxial deformation. The key component to this apparatus is a steel cell designed for high-temperature deformation experiments under controlled water pressure. Experiments are run under constant displacement rates or constant loads; the range of accessible experimental conditions include: 25 - 1100 °C, load stresses 0 to 150 MPa, strain rates 10⁻⁶ to 10⁻² s⁻¹, and fluid pressures 0-150 MPa. I present a suite of high-temperature, uniaxial deformation experiments performed on 25 by 50 mm unjacketed cores of porous Φ∼0.8) sintered rhyolitic ash. The experiments were performed at, both, atmospheric (dry) and elevated water pressure conditions (wet). Dry experiments were conducted mainly at 900 °C, but also included a suite of lower temperature experiments at 850, 800 and 750 °C. Wet experiments were performed at ∼650 °C under water pressures of 1, 2.5, 3, and 5 MPa, and at a fixed PH2O of ∼2.5 MPa for temperatures of ∼385, 450, and 550 °C. During deformation, strain is manifest by shortening of the cores, reduction of porosity, flattening of ash particles, and radial bulging of the cores. The continuous reduction of porosity leads to a dynamic transient strain-dependent rheology and requires strain to be partitioned between a volume (porosity loss) and a shear (radial bulging) component. The effect of increasing porosity is to expand the window for viscous deformation for dry melts by delaying the onset of brittle deformation by ∼50 °C (875 °C to 825 °C). The effect is more pronounced in hydrous melts (∼0.67 — 0.78 wt. % H₂0) where the viscous to brittle transition is depressed by ∼140 to 150 °C. Increasing water pressure also delays the onset of strain hardening due to compaction-driven porosity reduction. These rheological data are pertinent to volcanic processes where high-temperature porous magmas I liquids are encountered (e.g., magma flow in conduits, welding of pyroclastic materials).

The Proterozoic Kruidfontein Volcanic Complex (KVC) is a collapsed carbonatitic caldera structure, preserved as a high-level feature within Transvaal Sequence sediments. An outer ring of hills contains silicate pyroclastic rocks composed of lithic and pumice fragments, crystals and recrystallized matrix. These rocks are the products of co-ignimbrite lithic breccias and partially welded ignimbrite flows. An inner caldera was filled with recrystallized carbonatitic bedded volcaniclastic rocks. Relic pyroclastic carbonate fragments, such as droplet and armoured lapilli, containing juvenile calcite laths, are present. Well preserved primary structure sequences indicate emplacement by pyroclastic flow, surge and air-fall. Together with some reworking and debris flow deposits. The volcanism spans from early eruption of phonolitic material, from ring vents associated with caldera collapse, to smaller volume carbonatitic eruptions, producing intracaldera deposits. The processes operating during emplacement of carbonatitic pyroclastic material are essentially the same as those of silicate tuffs. As well as numerous fragments of phonolitic pumice in the silicate tuffs, there are unusual banded fragments composed of alternating silicate and carbonate compositions which appear to have been originally magmas separated by liquid immiscibility. The fragments show replacement of Al by Fe, and have also been K-feldspathized. Sovite and alvikite carbonatite dykes show that variation between CaO, MgO and FeO is consistant with fractionation from sovite to Fe-rich alvikites. All the carbonatites are strongly enriched in REE. The alvikites are enriched in the incompatible elements La, Ce, Nd, Y, Th, compared with the sovites, but are depleted in Sr, P, Ti, because of early fractionation of Sr-rich calcite, apatite and Ti-Fe oxides. The alvikites also have more positive δ18O and less negative δl3C compositions compared with the sovites, with values trending away from "mantle" compositions. This interpretation is consistant with a carbonatite magma chamber beneath the KVC which fractionated to produce the carbonatites seen at the present day surface. The few, highly altered, KVC nephelinitic rocks have trace-element distributions suggesting that they are parental to the phonolites. Fractionation from nephelinites, to phonolites, to trachytes satisfactorilly accounts for the incompatible trace element distributions. Some of the rocks have suffered secondary alteration, but have retained their trace element signatures. Zr and Nd are residual, whilst crystal fractionation involving feldspar, magnetite, and apatite have depleted some rocks in P, REE, and Sr. The fractionation from phonolite to trachyte, which is the reverse of normally observed trends, is ascribed to increasingly high F contents in the fractionating KVC magma. Three types of fluorite mineralization are recognised at KVC: 1) Replacement and disseminated deposits, 2) Fluorite veins and fracture fillings, 3) Fluorite-rich carbonatite and related dykes. Only Type 1) deposits are of economic importance at Kruidfontein. Fluorite selectively replaces calcite rather than ankerite in the KVC rocks, with ankeritization preceeding and inhibiting fluorite mineralization. Shallow dipping ankeritic tuffs form the host rock for a large (Tilde with hyphen below 5xl06 tonnes) sub-economic horizontal stratiform fluorite orebody, emplaced after inward sag of the bedding.

Although numerous hazard models exist to assess possible ash fallout from explosive volcanic eruptions around the world, these models frequently neglect to consider ash aggregation or use a simple percent proxy to represent aggregation, without considering the varying processes at work throughout the volcanic flow. Eruption dynamics are sensitive to ash aggregation, and ash aggregates are commonly found in eruptive deposits, yet few experiments have been conducted on aggregation phenomena using natural materials. In this work, experiments were developed to produce both probabilistic and process-based relationships for the efficiency of ash aggregation with respect particle size, collision kinetic energy, atmospheric water vapor and residence time. A synthetic ash proxy, ballotini, and ash from the 2006 eruption of Tungurahua, Ecuador, and the 1980 eruption of Mount St. Helens, WA, were examined for their aggregation potential. Two aggregation regimes, wet and dry, were identified based on their potential for aggregation. The wet flow regime occurs when particles are circulated in high relative humidity environments long enough to develop a water layer with a thickness that exceeds the particle roughness scale. Hydrodynamic forces control aggregation in the wet flow regime. The dry flow regime includes particles in low relative humidity environments as well as those that circulate too briefly in high humidity environments to fully develop a water layer. Electrostatic forces control aggregation in the dry flow regime. Aggregation efficiency in both regimes was dominantly controlled by collision kinetic energy; however, this effect is significantly dampened in the wet flow regime. Equations governing the relationships between aggregation efficiency, collision kinetic energy and the related forcings in the wet or dry flow regimes have been developed for implementation into large-scale numerical volcanic models. The results of this experimental work have been developed into a probability distribution that has been integrated and incorporated into a multifluid numerical model. The numerical simulation was tested on a range of explosive depths and overpressure estimates from the 1790 eruption of Kilauea volcano, HI. The model output was compared to field data collected on the deposit thickness moving away from the source and the distribution, including both size and density, of aggregates. The mass fraction of ash removed from the eruption column in the form of aggregates was also calculated to examine how efficiently aggregation processes remove ash throughout the eruption. Cumulatively, the work presented here furthers our understanding of aggregation processes and the role they play in volcanic eruptions.