ALBERT

Description: The 2004–2008 eruption of Mount St. Helens produced seven dacite spines mantled by cataclastic fault rocks, comprising an outer fault core and an inner damage zone. These fault rocks provide remarkable insights into the mechanical processes that accompany extrusion of degassed magma, insights that are useful in forecasting dome-forming eruptions. The outermost part of the fault core consists of finely comminuted fault gouge that is host to 1- to 3-mm-thick layers of extremely fine-grained slickenside-bearing ultracataclasite. Interior to the fault core, there is an ~2-m-thick damage zone composed of cataclastic breccia and sheared dacite, and interior to the damage zone, there is massive to flow-banded dacite lava of the spine interior. Structures and microtextures indicate entirely brittle deformation, including rock breakage, tensional dilation, shearing, grain flow, and microfaulting, as well as gas and fluid migration through intergranular pores and fractures in the damage zone. Slickenside lineations and consistent orientations of Riedel shears indicate upward shear of the extruding spines against adjacent conduit wall rocks. Paleomagnetic directions, demagnetization paths, oxide mineralogy, and petrology indicate that cataclasis took place within dacite in a solidified steeply dipping volcanic conduit at temperatures above 500 °C. Low water content of matrix glass is consistent with brittle behavior at these relatively high temperatures, and the presence of tridymite indicates solidification depths of 〈1 km. Cataclasis was coincident with the eruption’s seismogenic zone at 〈1.5 km. More than a million small and low-frequency "drumbeat" earthquakes with coda magnitudes (M d ) 〈2.0 and frequencies 〈5 Hz occurred during the 2004–2008 eruption. Our field data provide a means with which to estimate slip-patch dimensions for shear planes and to compare these with estimates of slip patches based on seismic moments and shear moduli for dacite rock and granular fault gouge. Based on these comparisons, we find that aseismic creep is achieved by micron-scale displacements on Riedel shears and by granular flow, whereas the drumbeat earthquakes require millimeter to centimeter displacements on relatively large (e.g., ~1000 m 2 ) slip patches, possibly along observed extensive principal shear zones within the fault core but probably not along the smaller Riedel shears. Although our field and structural data are compatible with stick-slip models, they do not rule out seismic and infrasound models that call on resonance of steam-filled fractures to generate the drumbeat earthquakes. We suggest that stick-slip and gas release processes may be coupled, and that regardless of the source mechanism, the distinctive drumbeat earthquakes are proving to be an effective precursor for dome-forming eruptions. Our data document a continuous cycle of deformation along the conduit margins beginning with episodes of fracture in the damage zone and followed by transfer of motion to the fault core. We illustrate the cycle of deformation using a hypothetical cross section of the Mount St. Helens conduit, extending from the surface to the depth of magmatic solidification.

Description: Over the past 25 years, our understanding of the physical processes that drive volcanic eruptions has increased enormously thanks to major advances in computational and analytical facilities, instrumentation, and collection of comprehensive observational, geophysical, geochemical, and petrological data sets associated with recent volcanic activity. Much of this work has been motivated by the recognition that human exposure to volcanic hazard is increasing with both expanding populations and increasing reliance on infrastructure (as illustrated by the disruption to air traffic caused by the 2010 eruption of Eyjafjallajökull volcano in Iceland). Reducing vulnerability to volcanic eruptions requires a thorough understanding of the processes that govern eruptive activity. Here, we provide an overview of our current understanding of how volcanoes work. We focus particularly on the physical processes that modulate magma accumulation in the upper crust, transport magma to the surface, and control eruptive activity.

Description: To assess the complexity of eruptive activity within mafic volcanic fields, we present a detailed geologic investigation of Holocene volcanism in the upper McKenzie River catchment in the central Oregon Cascades, United States. We focus on the Sand Mountain volcanic field, which covers 76 km 2 and consists of 23 vents, associated tephra deposits, and lava fields. We find that the Sand Mountain volcanic field was active for a few decades around 3 ka and involved at least 13 eruptive units. Despite the small total volume erupted (~1 km 3 dense rock equivalent [DRE]), Sand Mountain volcanic field lava geochemistry indicates that erupted magmas were derived from at least two, and likely three, different magma sources. Single units erupted from one or more vents, and field data provide evidence of both vent migration and reoccupation. Overall, our study shows that mafic volcanism was clustered in space and time, involved both explosive and effusive behavior, and tapped several magma sources. These observations provide important insights on possible future hazards from mafic volcanism in the central Oregon Cascades.

Description: Magma degassing can trigger crystal growth by increasing the magma liquidus temperature. As crystallization greatly increases magma viscosity, this process can strongly influence eruptive dynamics. We use a microscope and heated stage to obtain the first direct observations of degassing-driven crystal growth in natural basaltic melts at magmatic temperatures. Samples from Mount Etna, Italy (0.39 wt% H 2 O), and Kilauea volcano, Hawaii (0.18 wt% H 2 O) were heated in air at 1 bar, and held isothermally for 0.5–17 h between 1190 °C and 1270 °C, before cooling to solidus temperatures. On heating, bubble growth at 〉900 °C indicated volatile exsolution. In the hydrous Etna sample, isothermal conditions produced numerous new plagioclase crystals that grew to ≤160 μm at maximum rates of 5.2–18 x 10 –6 cm s –1 . Growth rates and crystal morphologies (tabular to spherulitic) depended on dwell temperature. Growth slowed dramatically after 20 min as equilibrium was approached. In the H 2 O-poor Kilauea sample, few new crystals appeared; they grew at maximum rates of 1.7–6.5 x 10 –6 cm s –1 . On cooling, crystal nucleation and growth were strongly influenced by preexisting crystal textures, highlighting the importance of studying natural samples. Our results document rapid crystal growth triggered by melt devolatilization when the H 2 O content of the glass is sufficiently high. Resultant swift, substantial changes in magma texture are a key control on lava rheology at Etna and elsewhere.

Description: Mafic lava flows are common; for this reason, they have long been a focus of volcanological studies. However, field studies of both older and active flows have been hampered by difficulties in field access; active flows are hot, whereas older flows have rough and jagged surfaces that are difficult to traverse. As a result, morphometric studies of lava flows have generally lagged behind theoretical studies of flow behavior. The advent of laser scanning (LS) (i.e., lidar, light detection and ranging) technologies, both airborne mapping (ALSM) and terrestrial (TLS), is promoting detailed studies of lava flows by generating data suitable for production of high-resolution digital elevation models (DEMs). These data are revolutionizing both the visual and quantitative analysis of lava flows. First and foremost, this technology allows accurate mapping of flow boundaries, particularly in vegetated areas where bare earth imaging dramatically improves mapping capabilities. Detailed imaging of flow surfaces permits mapping and measurement of flow components, such as channels, surface folds, cracks, blocks, and surface roughness. Differencing of preeruptive and posteruptive DEMs allows analysis of flow thickness variations, which can be related to the dynamics of lava emplacement. Multitemporal imaging of active flows provides information not only on the rates and locations of individual flow lobes, but also measurement of pulsed lava transport. Together these new measurement capabilities can be used to test proposed models of channel formation, lava tube formation, rates of flow advance, and flow conditions within lava channels; they also provide new ways to assess the hazard and risk posed by lava flow inundation. Early published studies illustrate the potential of applying lidar to volcanic terrain; it is clear, however, that the availability of high-resolution digital topography is poised to revolutionize the study of mafic lava flows.

Description: Pyroclastic flows produced by large volcanic eruptions commonly densify after emplacement. Processes of gas escape, compaction, and welding in pyroclastic-flow deposits are controlled by the physical and thermal properties of constituent material. Through measurements of matrix porosity, permeability, and electrical conductivity, we provide a framework for understanding the evolution of pore structure during these processes. Using data from the Shevlin Park Tuff in central Oregon, United States, and from the literature, we find that over a porosity range of 0%–70%, matrix permeability varies by almost 10 orders of magnitude (from 10 –20 to 10 –11 m 2 ), with over three orders of magnitude variation at any given porosity. Part of the variation at a given porosity is due to permeability anisotropy, where oriented core samples indicate higher permeabilities parallel to foliation (horizontally) than perpendicular to foliation (vertically). This suggests that pore space is flattened during compaction, creating anisotropic crack-like networks, a geometry that is supported by electrical conductivity measurements. We find that the power law equation: k 1 = 1.3 x 10 –21 x 5.2 provides the best approximation of dominant horizontal gas loss, where k 1 = permeability, and = porosity. Application of Kozeny-Carman fluid-flow approximations suggests that permeability in the Shevlin Park Tuff is controlled by crack- or disk-like pore apertures with minimum widths of 0.3 and 7.5 μm. We find that matrix permeability limits compaction over short times, but deformation is then controlled by competition among cooling, compaction, water resorption, and permeable gas escape. These competing processes control the potential for development of overpressure (and secondary explosions) and the degree of welding in the deposit, processes that are applicable to viscous densification of volcanic deposits in general. Further, the general relationships among porosity, permeability, and pore geometry are relevant for flow of any fluid through an ignimbritic host.

Description: The abundant fine ash produced in the 2011 subglacial eruption of Grímsvötn, Iceland, highlights the fragmentation efficiency of mafic hydromagmatic eruptions, which is considerably higher than for comparable "dry" eruptions. Ash from the 2011 eruption can be divided into three morphological components—vesicular particles, shards, and dense fragments—distinguished by the size and abundance of constituent vesicles. We use the vesicle characteristics to define a new shape factor, the concavity index, which provides an unbiased way to classify individual ash particles as either bubbly (vesicular particles and shards) or dense. The relative proportion of bubbly and dense particles varies systematically with grain size, with the proportion of bubbly grains decreasing as the particle size approaches the modal bubble diameter. Measured bubble volume distributions are similar to those of rapidly quenched pyroclasts from Hawaiian fountains and suggest a comparable degassing history during magma ascent. Yet concordance between the size distributions of ash and of bubbles in the Grímsvötn samples stands in contrast to the size distributions in Hawaiian fountains, where pyroclasts are orders of magnitude larger than individual bubbles. We propose that the Grímsvötn ash formed by brittle disintegration of vesicular pyroclasts and that fragmentation efficiency was amplified by residual thermal stresses in glass quenched by glacial water. The strong control of resulting particle sizes and morphologies by the size and spatial distribution of bubbles demonstrates that the bubble population cannot be ignored when modeling hydromagmatic fragmentation.