Benjamin J. Andrews

Effects of topography on pyroclastic density current runout and formation of coignimbrites

Laboratory experiments of dilute mixtures of warm talc powder in air simulate dilute pyroclastic density currents (PDCs) and show the effects of topography on current runout, buoyancy reversal, and liftoff into buoyant plumes. The densimetric and thermal Richardson, Froude, Stokes, and settling numbers for our experiments match those of natural PDCs. The laboratory currents are fully turbulent, although the experiments have lower Reynolds numbers than PDCs. In sum, our experiments are dynamically similar to natural currents. Comparisons of currents traversing flat topography or encountering barriers show that runout distance is not significantly reduced for currents that traverse barriers with height

Explosive dome eruptions modulated by periodic gas‐driven inflation

Volcan Santiaguito (Guatemala) “breathes” with extraordinary regularity as the edifices conduit system accumulates free gas, which periodically vents to the atmosphere. Periodic pressurization controls explosion timing, which nearly always occurs at peak inflation, as detected with tiltmeters. Tilt cycles in January 2012 reveal regular 26 ± 6 min inflation/deflation cycles corresponding to at least ~101 kg/s of gas fluxing the system. Very long period (VLP) earthquakes presage explosions and occur during cycles when inflation rates are most rapid. VLPs locate ~300 m below the vent and indicate mobilization of volatiles, which ascend at ~50 m/s. Rapid gas ascent feeds pyroclast-laden eruptions lasting several minutes and rising to ~1 km. VLPs are not observed during less rapid inflation episodes; instead, gas vents passively through the conduit producing no infrasound and no explosion. These observations intimate that steady gas exsolution and accumulation in shallow reservoirs may drive inflation cycles at open-vent silicic volcanoes.

Thermal vesiculation during volcanic eruptions

Terrestrial volcanic eruptions are the consequence of magmas ascending to the surface of the Earth. This ascent is driven by buoyancy forces, which are enhanced by bubble nucleation and growth (vesiculation) that reduce the density of magma. The development of vesicularity also greatly reduces the ‘strength’ of magma, a material parameter controlling fragmentation and thus the explosive potential of the liquid rock. The development of vesicularity in magmas has until now been viewed (both thermodynamically and kinetically) in terms of the pressure dependence of the solubility of water in the magma, and its role in driving gas saturation, exsolution and expansion during decompression. In contrast, the possible effects of the well documented negative temperature dependence of solubility of water in magma has largely been ignored. Recently, petrological constraints have demonstrated that considerable heating of magma may indeed be a common result of the latent heat of crystallization as well as viscous and frictional heating in areas of strain localization. Here we present field and experimental observations of magma vesiculation and fragmentation resulting from heating (rather than decompression). Textural analysis of volcanic ash from Santiaguito volcano in Guatemala reveals the presence of chemically heterogeneous filaments hosting micrometre-scale vesicles. The textures mirror those developed by disequilibrium melting induced via rapid heating during fault friction experiments, demonstrating that friction can generate sufficient heat to induce melting and vesiculation of hydrated silicic magma. Consideration of the experimentally determined temperature and pressure dependence of water solubility in magma reveals that, for many ascent paths, exsolution may be more efficiently achieved by heating than by decompression. We conclude that the thermal path experienced by magma during ascent strongly controls degassing, vesiculation, magma strength and the effusive–explosive transition in volcanic eruptions.

Turbulent dynamics of the 18 May 1980 Mount St. Helens eruption column

Volcanic eruption columns that fail to entrain and heat enough air to become buoyant collapse and generate devastating pyroclastic flows. Turbulent eddies along the column margins entrain and mix air into the column interior, allowing the column to become buoyant. Currently, the turbulent velocity fields of volcanic eruption columns are unknown, and therefore numerical models of eruption columns remain untested against geologic observations. Through extensive reevaluation of video and photographs of the 18 May 1980 eruption of Mount St. Helens (United States), we report the first measurements of the turbulent velocity field of a volcanic column. During the buoyant, B2, phase of eruption, eddies along the column margins were larger and the fluctuating (turbulent) component of velocity was lower than during partial column collapse in the B3 phase of eruption. We propose that the turbulent structure of the column margins reflects the thickness of and velocity gradients within the column boundary layer in communication with the atmosphere. Thus eddy size scales with the column radius when the entire column becomes buoyant, whereas eddy size is controlled by the thickness of a buoyant annulus surrounding a dense, collapsing core during periods of partial column collapse.

Earth's Volcanoes and Their Eruptions: An Overview

Volcanoes are not random phenomena, but owe their existence, location, morphology, and eruptive styles to tectonic plate motions. Volcanic landforms vary widely in size and morphology from the stereotypical towering glacier-clad stratovolcano. They range from small spatter cones to massive shield volcanoes several thousand cubic kilometers in volume or broad volcanic depressions, cinder cones being the most common volcanic construct. Their eruptions likewise span many orders of magnitude in volume. Evaluation of eruption magnitudes and frequencies contrasts the vast majority of mild-to-moderate eruptions (∼80% with volcanic explosivity index, or VEI ≤2) with the more infrequent larger, higher-impact events (5% with VEI ≥4). Increased risk from population growth in proximity to volcanoes has been mitigated by enhanced monitoring and hazard-reduction efforts.

Petrogenesis of antecryst-bearing arc basalts from the Trans-Mexican Volcanic Belt: Insights into along-arc variations in magma-mush ponding depths, H2O contents, and surface heat flux

Abstract The Trans-Mexican Volcanic Belt (TMVB) is known for the chemical diversity in its erupted products. We have analyzed the olivine, pyroxene, and plagioclase mineral chemistry of 30 geochemically well-characterized mafic eruptives from Isla Maria at the western end of the arc to Palma Sola in the east. The mineral major oxide data indicate the dominance of open system processes such as antecryst uptake, and the scarcity of mineral-mineral and mineral-melt equilibria suggests that apart from forming microlites, erupted melts do not significantly crystallize during ascent. A combination of plagioclase antecryst chemistry and MELTS thermodynamic modeling of H2O-saturated isobaric fractional crystallization was employed to develop a pressure sensor aimed at determining the ponding depths of the co-genetic magmas from which the erupted plagioclase crystal assemblage originates. We show that the depth of magma-mush reservoirs increase eastward along the TMVB. We suggest that magma ponding is triggered by degassing-induced crystallization during magma ascent, and that the pressure sensor can also be regarded as a degassing sensor, with more hydrous melts beginning to degas at greater depths.Modeled initial magma H2O contents at the Moho range from ~4 to ~9 wt%. Magma-mush ponding depth variations fully explain the observed westward increase of average surface heat flux along the TMVB, supporting a new model of mafic arc magma ascent, where rapidly rising, initially aphyric melts pick up their antecrystic crystal cargo from a restricted crustal depth range, in which small unerupted batches of previously risen co-genetic magmas typically stall and solidify. This implies that, globally, mafic arc magmas may be used to constrain the depths of degassing and mush zone formation, as well as the amount of H2O in the primary melts.

Ash production and dispersal from sustained low-intensity Mono-Inyo eruptions

Recent rhyolitic volcanism has demonstrated that prolonged low-intensity ash venting may accompany effusive dome formation. We examine the possibility and some consequences of episodes of extended, weak ash venting at the rhyolitic Mono-Inyo chain in Eastern California. We describe ash-filled cracks within one of the youngest domes, Panum Crater, which provide a textural record of ash venting during dome effusion. We use synchrotron-based X-ray computed tomography to characterize the particles in these tuffisites. Particle sizes in well-sorted tuffisite layers agree well with grain size distributions observed during weak ash venting at Soufrière Hills Volcano, Montserrat, and yield approximate upper and lower bounds on gas velocity and mass flux during the formation of those layers. We simulate ash dispersal with Ash3d to assess the consequences of long-lived Mono-Inyo ash venting for ash deposition and the accompanying volcanic hazards. Our results highlight the sensitivity of large-scale outcomes of volcanic eruptions to small-scale processes.

Ten years of satellite observations reveal highly variable sulphur dioxide emissions at Anatahan Volcano, Mariana Islands

Satellite remote sensing enables continuous multiyear observations of volcanic activity in remote settings. Anatahan (Mariana Islands) is a remote volcano in the western North Pacific. Available ground-based measurements of sulphur dioxide (SO2) gas emissions at Anatahan place it among thelargest volcanic SO2 sources worldwide. These ground-based measurements, however, are restricted to eruptive intervals. Anatahans activity since 2003 has been dominated temporally by prolonged periods of quiescence. Using 10 years of satellite observations from OMI, AIRS, SCIAMACHY, and GOME-2, we report highly variable SO2 emissions within and between eruptive and quiescent intervals at Anatahan. We find close correspondence between levels of activity reported at the volcano and levels of SO2 emissions detected from space. Eruptive SO2 emission rates have a mean value of ∼6400 t d−1, but frequently are in excess of 20,000 t d−1. Conversely, SO2 emissions during quiescent intervals are below the detection limit of space-based sensors and therefore are not likely to exceed ∼300 t d−1. We show that while Anatahan occupies a quiescent state for 85% of the past 10 years, only ∼15% of total SO2 emissions over this interval occur during quiescence, with the remaining ∼85% released in short duration but intense syn-eruptive degassing. We propose that the integration of multiyear satellite data sets and activity histories are a powerful complement to targeted ground-based campaign measurements in better describing the long-term degassing behavior of remote volcanoes.

Eruptive and Depositional Mechanisms of an Eocene Shallow Submarine Volcano, Moeraki Peninsula, New Zealand

Eocene Surtseyan lapilli tuff deposits with maximum stratigraphic thicknesses of at least 175 m record a submarine volcano that underwent two cycles of edifice construction and erosion at Moeraki Peninsula, South Island, New Zealand. Basalt dike fragments, basalt clasts exhibiting fluidal deformation, and angular schist derived xenoliths are present throughout the lapilli tuff units; together with petrographic examination of the lapilli tuff, these rocks indicate that a broad range of eruptive and fragmentary mechanisms took place in the Moeraki volcano. Processes involved probably included phreatomagmatic interaction of magma with pyroclast-mud-seawater slurries in the vent. Data from two measured stratigraphic sections show that the volcano was formed during two distinct phases of eruption, separated by quiescence during which 1.5 m of laminated volcaniclastic sandstones were deposited. Lapilli tuff units below the sandstones are generally massive, while those above are well bedded and often alternate between finer- and coarser-grained lenticular bedsets. The stratigraphy and lithology of rocks at Moeraki indicate that the volcano was constructed through the explosive eruption of lapilli and ash in water no deeper than 400 m. The cone built to a height of at least 100 m above the seafloor by a combination of fall and eruption fed density currents, before eruption ceased and erosion of the volcano occurred. Renewed volcanic activity resulted in a volcano that rose above storm wave base and may even have emerged. As with modern analogues Surtla and Kavachi, erosion of the volcano to storm wave base or below quickly followed cessation of eruption.