Experimental simulations of the May 18, 1980 directed blast at Mount St. Helens, WA

The 1980 directed blast at Mount St. Helens erupted from a high-pressure magma chamber into atmospheric conditions at a pressure ratio of ~150:1, producing a high-velocity dusty gas flow. Decompression from even modestly high pressure ratios (>2:1) produces supersonic flow and thus, this event was modeled as a supersonic underexpanded jet by Kieffer (1981). Steady-state underexpanded jets have a complex geometrical structure in which there is an abrupt, stationary, normal shock wave, called the Mach disk shock. For steady flow, a log-linear relationship between pressure ratio and Mach disk standoff distance, known as the Ashkenas-Sherman relation, is valid for pressure ratios above 15:1 given by x/D=0.67(Rp)^(0.5) where Rp is the pressure ratio, and x/D is the standoff distance normalized to vent diameter. The effects of unsteady discharge from a finite reservoir and application to Mount St. Helens have not been previously investigated. In order to simulate the blast, we use laboratory and numerical experiments of unsteady flow from a finite reservoir to examine jet structure. The reservoir and test section correspond to the magma chamber and ambient atmospheric conditions at Mount St. Helens respectively. We completed a series of laboratory experiments in which we varied the initial pressure ratio, reservoir length and reservoir gas (nitrogen, helium). The numerical simulations show that the Mach disk initially forms close to the vent and then travels downstream to its equilibrium position. The experiments show that as the reservoir pressure continuously decreases during the venting, or “blowdown”, the Mach disk shock continuously moves back toward the reservoir after its formation at the equilibrium position. Results of these experiments indicate that above a pressure ratio of 15:1, the Mach disk standoff distance for unsteady flow falls on the empirical Ashkenas-Sherman curve for steady flow. We present a new relation for the location of the Mach disk shock for pressure ratios below 15:1 given by x/D=0.41(Rp)^(0.66). The results indicate no dependence of the normalized Mach disk location on the finiteness of the reservoir. These results may be of interest not only for high pressure eruptions such as Mount St. Helens, but to low pressure steam eruptions as well because helium is a good analog to steam

Experimental simulation of volcanic steam blasts and jets at high pressure ratios

End-member compositions of plumes from volcanic eruptions range from nearly pure steam to heavily particle-laden gas flows. In all cases, if the plumes erupt from a high-pressure reservoir, they are initially supersonic jets that may have complex internal flow structures not easily documented in the field. In the laboratory, some properties of volcanic jets can be investigated with particle-laden flows, but other properties can only be investigated in optically transparent flows. We examine the relation of unsteady jet structure to reservoir conditions for optically transparent flows. We have developed an experimental shock tube facility capable of achieving pressure ratios up to ~150 with reservoirs of different shapes. Time-resolved schlieren visualization is combined with pitot pressure measurements to interrogate the structure of the underexpanded jet flow. We have done preliminary experiments at a pressure ration of 40 with air, with two reservoirs that are 12.6 and 20 cm in length. These initially produce well-defined supersonic jets that have properties (shape of the underexpanded jet; barrel shocks, Mach disk shocks) which we have bench-marked against other experiments and simulations. Estimated durations of the supersonic portions of the flow from pressure decay calculations are ~45 and ~75 ms, respectively. On these time-scales, the experimental jets collapse; the plume boundary and internal barrel shocks tighten and the Mach disk shock moves toward the vent, until subsonic conditions occur

Flow of supersonic jets across flat plates: Implications for ground-level flow from volcanic blasts

We report on laboratory experiments examining the interaction of a jet from an overpressurized reservoir with a canonical ground surface to simulate lateral blasts at volcanoes such as the 1980 blast at Mount St. Helens. These benchmark experiments test the application of supersonic jet models to simulate the flow of volcanic jets over a lateral topography. The internal shock structure of the free jet is modified such that the Mach disk shock is elevated above the surface. In elevation view, the width of the shock is reduced in comparison with a free jet, while in map view the dimensions are comparable. The distance of the Mach disk shock from the vent is in good agreement with free jet data and can be predicted with existing theory. The internal shock structures can interact with and penetrate the boundary layer. In the shock-boundary layer interaction, an oblique shock foot is present in the schlieren images and a distinctive ground signature is evident in surface measurements. The location of the oblique shock foot and the surface demarcation are closely correlated with the Mach disk shock location during reservoir depletion, and therefore, estimates of a ground signature in a zone devastated by a blast can be based on the calculated shock location from free jet theory. These experiments, combined with scaling arguments, suggest that the imprint of the Mach disk shock on the ground should be within the range of 4–9 km at Mount St. Helens depending on assumed reservoir pressure and vent dimensions

Introduction: geoethics goes beyond the geoscience profession

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Experiments on the Gas Dynamics of the Mt. St. Helens 1980 Lateral Blast

Field evidence suggests that the lateral blast in the 1980 Mt. St. Helens eruption behaved like an underexpanded jet flow. We conduct two experiments to investigate this hypothesis. In our first experiment, we use the compressible flow--shallow-water analogy to measure the geometry of the shock structure around the underexpanded jet, which is comparable with the position of the interface between the direct and channelized blast zones described by Kieffer (1981). Also, Kieffer and Sturtevant (1988) identified furrows created by the blast which were possibly formed by scouring due to Goertler vortices induced by curvature in the terrain. In our second experiment, carried out in a compressible flow laboratory, we investigate an additional Goertler vortex generation mechanism due to the curvature of the shear layer adjacent to the intercepting shocks in the underexpanded jet. These experiments allow for a more-detailed scrutiny of the underexpanded jet--lateral blast analogy proposed by Kieffer (1981)

I. Shock metamorphism of the Coconino sandstone at Meteor Crater, Arizona. II. The specific heat of solids of geophysical interest

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. PART I: A study of the shocked Coconino sandstone from Meteor Crater, Arizona, was undertaken to examine the role of porosity in the compression of rocks and in the formation of high-pressure phases. A suite of shocked Coconino specimens collected at the crater is divided into five classes, arranged in order of decreasing quartz content. The amounts of coesite, stishovite (measured by quantitative x-ray diffraction) and glass vary systematically with decreasing quartz content. Coesite may comprise one-third by weight of some rocks, whereas the stishovite content does not exceed 1%. The five classes of rocks have distinct petrographic properties, correlated with the presence of regions containing coesite, stishovite or fused silica. Very few occurrences of diaplectic glass are observed, in striking contrast to its abundant occurrence in the non-porous rocks from the Ries Crater. In the lowest stages of shock metamorphism (Class I), the quartz grains are fractured and the voids in the rock are filled with myriads of small chips derived from neighboring grains. The fracture patterns in the individual quartz grains are controlled by the details of the initial morphology of the colliding grains. In one weakly shocked rock, it was possible to map the general direction of shock passage by recording the apparent direction of collision of individual grains. The principal mechanism of energy deposition by a shock wave in a porous material is the reverberation of shock and rarefaction waves through grains due to collisions with other grains. A one-dimensional model of the impact process can predict the average pressure, volume and temperature of the rock if no phase changes occur, but cannot predict the observed nonuniformity of energy deposition. In all rocks shocked to higher pressure than was necessary to close the voids, high-pressure and/or high-temperature phases are present. Locally high pressures enduring for microseconds and high temperatures enduring for milliseconds controlled the phases of SiO2 which formed in the rock. Collapsing pore walls became local hot spots into which initial deposition of energy was focused. Microcrystalline coesite in Class II rocks occurs in symplektic regions on quartz grain boundaries which were regions of initial stress and energy concentration, or in sheared zones within the grains. The occurrence and morphology of the coesite-rich regions can be explained only if the transformation from quartz to coesite proceeds slowly in the shock wave. In Class III rocks, microcrystalline coesite occurs in opaque regions which surround nearly isotropic cores of cryptocrystalline coesite. The cores are interpreted to be the products of the inversion of stishovite (or a glass with Si in six-fold coordination) which initially formed in the shock front in regions of grains shocked to pressures near 300 kb. Stishovite is preserved only in the opaque regions, which are believed to have been cooler than the cores. In Class IV rocks, vesicular glass occurs in core regions surrounded by opaque regions containing coesite. The relation of the glass to the coesite and quartz suggests that the glass was formed by inversion of stishovite formed above 350 kb upon release to lower pressure. Class V rocks are composed almost entirely of glass with vesicles uniformly distributed in the glass. These vesicles were probably formed by exsolution of water that had been dissolved in melted SiO2 during passage of the shock. PART II: The use of Debye temperatures as parameters for material properties of silicate minerals is becoming common in geophysical studies. The elastic Debye temperature, [...] alone is, in general, insufficient to specify properties which depend on lattice vibrations. Two effects ignored by the Debye model are shown to be important: high frequency lattice vibrations and the dispersion relation. As an alternative to the Debye model, a somewhat more complicated model is proposed that is still reasonably convenient and is able to account much better than the Debye model does for the variation of specific heat of complex substances over a wide range of temperature. This model is designated the acoustic-optic model. The parameters required for this model are the maximum lattice vibrational frequency, the elastic Debye temperature, and the specific heat at a single (say, room) temperature. Adequate approximations to these parameters are generally available. To consider heat capacity data and compare the data with either the Debye or acoustic-optic model, the calorimetric Debye temperature, [...] is considered. [...] is the value of Debye temperature that will reproduce the specific heat at constant volume at the temperature T. For silicates, a large increase at high temperatures in [...] above the elastic Debye temperature is due to the presence of oscillators between the elastic Debye frequency and a maximum vibrational frequency which exceeds the Debye frequency. Vibrations of these oscillators cause the spectral lines observed at infrared frequencies. The proposed model takes these oscillators into account by adding a constant valued continuum to an assumed low-frequency Debye spectrum. In all substances considered, [...] at low temperatures initially decreases to values below [...]. This decrease is believed due to the dispersion relation, i.e., the nonlinear relation between the wave vector and the frequency. In two substances of geophysical interest, NaCl and MgO, the maximum observed vibrational frequency is depressed below the observed elastic Debye frequency as a consequence of the dispersion relation. The acoustic-optic model is capable of predicting the specific heat of these two substances and the acoustic-optic spectra which result from application of the model to these substances describe qualitatively the spectra that result from the dispersion relation. The nonlinearity of the dispersion relation dominates the specific heat behavior of NaCl and MgO and influences the low temperature behavior of all silicates with varying degrees of severity. The effect of the dispersion relation can be ignored for some silicates if specific heats only at high temperatures (T > 100[degrees]K) are considered. The effect cannot be ignored in the case of rutile or stishovite