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During the ongoing (2013–present) eruption of Sinabung volcano, north Sumatra, we have routinely used a variety of satellite remote sensing data to observe and forecast lava dome and lava flow collapse events, to map the resulting pyroclastic deposits, and to estimate effusion rates. In this paper, we focus on the first two years of the current eruption (September 2013–December 2015), and we summarize major events in 2016. We divide the eruption into 5 major phases: 1) phreatomagmatic (July 2013–18 December 2013), 2) first dome growth and collapse (18 December 2013–10 January 2014), 3) lava-flow (10 January 2014–mid-September 2014), 4) second lava dome and collapse (mid-September 2014–July 2015), 5) lava dome collapse and ash explosion phase (August 2015–present). Throughout the eruption, remotely sensed information has been instrumental in assessing the stability of the lava dome and flow and to forecast collapse events that produce pyroclastic density currents (PDCs: block-and-ash flows, co-ignimbrite surges, and blasts). Forecasts based on remote sensing data in combination with seismic, geodetic and gas-monitoring data have also helped inform decisions related to alert levels and evacuations. Relatively unusual aspects of the Sinabung eruption include the transition from dome to flow morphology (phase 2 to phase 3 transition) and the frequent occurrence during phase 3 of collapses from the lava flow-front and flow-margins—collapses that produced extensive pyroclastic density currents. By analogy to the well-known “Merapi type” collapses and pyroclastic deposits, we propose that lava flow-front and flow-margin collapses with associated PDCs be known as “Sinabung type.” Although detailed study of deposits has not been possible due to continuing hazards, our observations suggest that the transition from lava dome to lava flow and the occurrence of flow-front and flow-margin collapses reflect a particular combination of lava viscosity and steepness of slope. Our observations also show clear evidence of at least one slope-parallel high-velocity and dilute PDC (a “blast”) that emanated from a lava-margin collapse site 500 m downslope from the vent. This 1 February 2014 blast downed and singed a forest out to at least 3.9 km from the collapse site and killed 16 people. We also use a combination of field and remotely sensed data to map the distribution of Sinabung deposits. We estimate eruptive volumes and extrusion rates by combining sequential measurements of lava surface and pyroclastic flow areas with thickness estimates derived from simple geometric assumptions, oblique photographs and Digital Elevation Models (DEMs) derived from remotely sensed data. Our estimates of short-term effusion rates vary widely on a daily to weekly basis, from 20 m3 s−1. In a few cases, periods of increased extrusion precede lava flow-front collapses by a few days to a week, suggesting delays in transmittance of effusion pulses as lava moves from vent to flow front. We find that, as of 1 January 2016, the total area of deposits is 107 m2, and their approximate deposit volume is about 0.3 km3, equivalent to 0.2 km3 Dense Rock Equivalent (DRE). We anticipate that our deposit maps will be valuable in the future as a framework for the study of the magmatic and textural evolution of eruptive products through time.

Volcano disaster mitigation is a priority in Indonesia's national development plan due to the large number of active volcanoes, high frequency of eruptions and dense population within volcanic hazards zones. As a consequence of these factors and the many decades of experience in mitigation of volcanic risk, Indonesia demonstrates that effective community response is critical to avoid disasters. Understanding of hazards, risks and early-warning systems are the main factors in building an effective community response. The level of understanding of these factors is proportional to a community's independence in taking appropriate actions and is reflected in the community's preparedness. Further, the experience gained from responses to the eruptions of Sinabung and Kelud as well as from other Indonesian volcanoes shows that a local leader plays an important role in mobilizing a community. Such a leader utilizes knowledge of local culture (local wisdom) and an understanding of the community's character to encourage community members to participate and empower themselves. Sharing of information and the formulation of a disaster mitigation plan by government and community also results in a shared commitment to increase participation and community empowerment. Indonesia uses four volcano alert levels to activate community mitigation plans. In order of increasing criticality, these are: Normal, Advisory (Waspada), Watch (Siaga) and Warning (Awas). Implementation of policies, strategies and actions are tied to each level. In addition, mandatory actions by certain stakeholders are required at certain alert levels. These mandatory actions increase the effectiveness of disaster mitigation by both government and community. A comparison of the crisis responses of the Sinabung and Kelud communities is carried out here in order to better understand problems, learn lessons and improve the process of Indonesia's community mitigation policies. Local culture, the start time of community involvement (in normal versus crisis time), degree and level of government involvement, roles of local leaders, local perception of hazards, and political intervention are all factors that influence a community's response at the time of eruption. We stress that an understanding of the “community character,” which includes a number of cultural, social and knowledge parameters is essential for effective crisis management. The responses of Sinabung and Kelud illustrate both the key role of community in disaster mitigation and the synergy that can result from close coordination and collaboration between government and community.

Eruptions of Sinabung volcano, Indonesia have been ongoing since 2013. Since that time, the character of eruptions has changed, from phreatic to phreatomagmatic to magmatic explosive eruptions, and from production of a lava dome that collapsed to a subsequent thick lava flow that slowly ceased to be active, and later, to a new lava dome. As the eruption progressed, event trees were constructed to forecast eruptive behavior six times, with forecast windows that ranged from 2 weeks to 1 year: November 7–10, December 12–14, and December 27, 2013; and January 9–10, May 13, and October 7, 2014. These event trees were successful in helping to frame the forecast scenarios, to collate current monitoring information, and to document outstanding questions and unknowns. The highest probability forecasts closely matched outcomes of eruption size (including extrusion of the first dome), production of pyroclastic density currents, and pyroclastic density current runout distances. Events assigned low probabilities also occurred, including total collapse of the lava dome in January 2014 and production of a small blast pyroclastic density current in February 2014.

We draw on our experience in assisting with international crises through the volcano disaster assistance program (VDAP) and during the eruptions of Mount St. Helens in 1980–1986 and 2004–2008 to offer recommendations for successful observatory operations during times of crisis. The degree of success in responding to a crisis is profoundly affected by the degree of preparation before a crisis arises—including the building of monitoring systems and databases to improve forecasting and establish effective partnerships with civil protection authorities and communities at risk. Success further depends on teamwork and communication during the crisis and on the level, progression, and duration of unrest itself (the crisis timeline). Some factors lie within the purview of the observatory to control; others are external and difficult or impossible to control. We focus on the first. A myriad of specific tasks must be remembered and accomplished before, during, and after crises. Just as airline pilots use checklists to ensure that key items for aviation safety and aircraft performance are not overlooked, we recommend that volcanologists do the same. We offer here a checklist for volcanic crisis responses and encourage observatory scientists and managers to review and revise it to best suit their needs.

Despite dense cloud cover, satellite-borne commercial Synthetic Aperture Radar (SAR) enabled frequent monitoring of Merapi volcano's 2010 eruption. Near-real-time interpretation of images derived from the amplitude of the SAR signals and timely delivery of these interpretations to those responsible for warnings, allowed satellite remote sensing for the first time to play an equal role with in situ seismic, geodetic and gas monitoring in guiding life-saving decisions during a major volcanic crisis. Our remotely sensed data provide an observational chronology for the main phase of the 2010 eruption, which lasted 12 days (26 October–7 November, 2010). Unlike the prolonged low-rate and relatively low explosivity dome-forming and collapse eruptions of recent decades at Merapi, the eruption began with an explosive eruption that produced a new summit crater on 26 October and was accompanied by an ash column and pyroclastic flows that extended 8 km down the flanks. This initial explosive event was followed by smaller explosive eruptions on 29 October–1 November, then by a period of rapid dome growth on 1–4 November, which produced a summit lava dome with a volume of ~ 5 × 10 6 m 3 . A paroxysmal VEI 4 magmatic eruption (with ash column to 17 km altitude) destroyed this dome, greatly enlarged the new summit crater and produced extensive pyroclastic flows (to ~ 16 km radial distance in the Gendol drainage) and surges during the night of 4–5 November. The paroxysmal eruption was followed by a period of jetting of gas and tephra and by a second short period (12 h) of rapid dome growth on 6 November. The eruption ended with low-level ash and steam emissions that buried the 6 November dome with tephra and continued at low levels until seismicity decreased to background levels by about 23 November. Our near-real-time commercial SAR documented the explosive events on 26 October and 4–5 November and high rates of dome growth (> 25 m 3 s − 1 ). An event tree analysis for the previous 2006 Merapi eruption indicated that for lava dome extrusion rates > 1.2 m 3 s − 1 , the probability of a large (1872-scale) eruption was ~ 10%. Consequently, the order-of-magnitude greater rates in 2010, along with the explosive start of the eruption on 26 October, the large volume of lava accumulating at the summit by 4 November, and the rapid and large increases in seismic energy release, deformation and gas emissions were the basis for warnings of an unusually large eruption by the Indonesian Geological Agency's Center for Volcanology and Geologic Hazard Mitigation (CVGHM) and their Volcano Research and Technology Development Center (BPPTK) in Yogyakarta — warnings that saved thousands of lives.

To understand the processes that made the 2010 eruption of Merapi much larger and more explosive than most dome-forming eruptions of the past century, we investigated the geochemistry, petrology, and pre-eruptive conditions of magmas erupted in 2006 and 2010. The juvenile rocks of 2010 are plagioclase, two-pyroxene basaltic andesites with seriate textures and minor amounts of reaction-free amphibole, Fe–Ti oxides, and rare crystals of olivine and biotite. The bulk-rock composition, mineral paragenesis, and textures are similar to those of juvenile blocks from the much less explosive eruption of 2006. One of the key differences is that most amphiboles in 2010 don't have breakdown reaction rims, whereas those of 2006 are largely reacted. We acquired > 80 X-ray distribution maps of major and minor elements of large areas (> 1 cm2) and single crystals, backscattered electron images, electron microprobe analyse, and compositional traverses across crystals. The data reveal that both the 2006 and 2010 samples are heterogeneous at various spatial scales, with numerous reaction textures between pyroxenes and amphiboles, dissolution textures, and large variations of crystal sizes, morphologies, and compositions. These features record open-system magmatic processes involving the assimilation of carbonate rocks, and interactions between various parts of Merapi's plumbing system, including a degassed shallow magma system and deep hotter and more volatile rich magma intrusions. The petrological complexity of the samples makes unraveling the pre-eruptive conditions of Merapi magmas a petrological puzzle. We applied five different geothermobarometers and performed thermodynamic modeling with the MELTS algorithm, and we propose that there are at least three crystallization zones or environments below Merapi. A deep reservoir at about 30 (+/− 3) km depth is suggested by some amphiboles and high-Al clinopyroxenes. Here is where the high-Al basaltic andesites from Merapi are generated probably by water-rich fractionation of more primitive magmas. Such deep magmas are volatile-rich and at near-liquidus conditions (≥ 4–6 wt.% H2O, ≥ 0.15 wt.% SO2, and an undetermined amount of CO2, at about 1050 °C) when they start moving towards the surface. A second crystallization zone is recorded by another type of amphibole at about 13 (+/− 2) km. Here high-Al clinopyroxene may also grow together with Ca-rich plagioclase. Assimilation of limestone may also occur at this level as recorded by the very Ca-rich plagioclases found in the cores of some crystals. At this location the water content of the melt must remain high enough to stabilize amphibole (4–6 wt.% H2O) but CO2 and SO2 are probably already degassing and contribute to gas changes observed by the monitoring system at the surface. Finally, a shallower part of the system ( We propose that the 2006 and 2010 eruptions were driven by basically the same processes and magma types. The main difference is the much larger size of the deep and volatile-rich magma replenishment that took place in 2010, which had large effects on the kinetics and dynamics of the plumbing system and processes. In 2006, and perhaps also in most of the typical small dome-forming historical eruptions at Merapi, the direct ascent of deep and gas-rich magmas towards the surface is slowed down and partially arrested by the shallower crystal-rich zones of left-over magma from previous events. However, this was not possible in 2010, where the much larger (up to 10 times) size of the magma intrusions overwhelmed the crystal-rich eruption filter. In 2010 the deep magma probably resided for only a short time at intermediate to shallower depths which allowed it to proceed to the surface still carrying most of its deep gas cargo. The larger magma intrusion probably induced higher rates of crustal carbonate assimilation and production of additional CO2 gas at shallow depths. This contributed to the much faster than usual ascent rates and larger explosivities in 2010 than in 2006. These inferences are supported by the shorter interaction times calculated from the diffusion models of clinopyroxene compositions for the 2010 magmas, by the fact that most amphiboles are not broken down in 2010 as opposed to 2006, and also by the much shorter times of escalating monitoring signals (seismicity and deformation) in 2010 compared to 2006. A puzzling observation is that despite the multiple explosive phases of the 2010 eruption, pumiceous materials are rare, and were only found in the last part of the eruption. This contrasts with the abundant tephra layers and vesiculated deposits of older historical explosive events like 1872, and suggest that syn-eruptive processes in 2010 were also different from standard models. The rarity of expanded pumices in 2010 may be due to rapid degassing and re-welding of magma as it ascended from intermediate depths. Given the near constant bulk composition of Merapi magmas erupted in the last decades, and the similarity of textures and minerals in 2006 and 2010, our study suggests that most Merapi magmas are intrinsically capable of explosive eruptions. Here we propose that whether they do so or not mainly depends on the degree of interaction and magma mass proportions between the upper crystal-rich parts of the system (including carbonates) and the deeper and more gas-rich replenishing magmas. Older historical explosive eruptions at Merapi such as in 1872 were driven by more mafic magmas than those erupted in 2006 and 2010 and thus might be caused by different processes from those discussed here. The still unanswered and vexing questions remain as to why in 2010 a much larger amount of magma was segregated from depths and whether this will happen again in the near future.

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 More than a million small and low-frequency "drumbeat" earthquakes with coda magnitudes (M d ) 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.

Soputan is a high-alumina basalt stratovolcano located in the active North Sulawesi-Sangihe Islands magmatic arc. Although immediately adjacent to the still geothermally active Quaternary Tondono Caldera, Soputan’s magmas are geochemically distinct from those of the caldera and from other magmas in the arc. Unusual for a basalt volcano, Soputan produces summit lava domes and explosive eruptions with high-altitude ash plumes and pyroclastic flows—eight explosive eruptions during the period 2003–2011. Our field observations, remote sensing, gas emission, seismic, and petrologic analyses indicate that Soputan is an open-vent-type volcano that taps basalt magma derived from the arc-mantle wedge, accumulated and fractionated in a deep-crustal reservoir and transported slowly or staged at shallow levels prior to eruption. A combination of high phenocryst content, extensive microlite crystallization and separation of a gas phase at shallow levels results in a highly viscous basalt magma and explosive eruptive style. The open-vent structure and frequent eruptions indicate that Soputan will likely erupt again in the next decade, perhaps repeatedly. Explosive eruptions in the Volcano Explosivity Index (VEI) 2–3 range and lava dome growth are most probable, with a small chance of larger VEI 4 eruptions. A rapid ramp up in seismicity preceding the recent eruptions suggests that future eruptions may have no more than a few days of seismic warning. Risk to population in the region is currently greatest for villages located on the southern and western flanks of the volcano where flow deposits are directed by topography. In addition, Soputan’s explosive eruptions produce high-altitude ash clouds that pose a risk to air traffic in the region.

In response to NASA's announced requirement for Earth hazard monitoring sensor-web technology, a multidisciplinary team involving sensor-network experts (Washington State University), space scientists (JPL), and Earth scientists (USGS Cascade Volcano Observatory (CVO)), have developed a prototype of dynamic and scalable hazard monitoring sensor-web and applied it to volcano monitoring. The combined Optimized Autonomous Space - In-situ Sensor-web (OASIS) has two-way communication capability between ground and space assets, uses both space and ground data for optimal allocation of limited bandwidth resources on the ground, and uses smart management of competing demands for limited space assets. It also enables scalability and seamless infusion of future space and in-situ assets into the sensor-web. The space and in-situ control components of the system are integrated such that each element is capable of autonomously tasking the other. The ground in-situ was deployed into the craters and around the flanks of Mount St. Helens in July 2009, and linked to the command and control of the Earth Observing One (EO-1) satellite.

Volcanic fields at the eastern margin of the Red Sea rift were regarded as seismically quiet until a swarm of 30,000 earthquakes struck in 2009. Geological analyses reveal the intrusion of a volcanic dyke and indicate that rift-related extension is spread over a broad region far from the rift axis.

We investigated plagioclase phenocrysts in dacite of the 2004-5 eruption of Mount St. Helens to gain insights into the magmatic processes of the current eruption, which is characterized by prolonged, nearly solid-state extrusion, low gas emission, and shallow seismicity. In addition, we investigated plagioclase of 1980-86 dacite. Light and Nomarski microscopy,were used to texturally characterize plagioclase crystals. Electron microprobe analyses measured their compositions. We systematically mapped and categorized all plagioclase phenocrysts in a preselected area according to the following criteria: (1) occurrence of zones of acicular orthopyroxene inclusions, (2) presence of dissolution surface(s), and (3) spatial association of 1 and 2. Phenocrysts fall into three main categories; one category contains four subcategories. The range of anorthite (An) content in 2004-5 plagioclase is about An 57-35 during the last 30-40 percent crystallization of plagioclase phenocrysts. Select microphenocrysts (10-50 μm) range from An 30 to An 42 . Anorthite content is lowest near outermost rims of phenocrysts, but zonation patterns between interior and rim indicate variable trends that correlate with textural features. Crystals without dissolution surfaces (about 14 percent of total) show steadily decreasing An content outward to the crystal rim (outer ~80 μm). All other crystals are banded as a consequence of dissolution; dissolution surfaces are band boundaries. Such crystals display normal outward An zoning within a single band that, following dissolution, is then overgrown abruptly by high-An material of the next band. Swarms of acicular orthopyroxene inclusions in plagioclase are characteristic of 2004-5 dacite. They occur mostly inward of dissolution surfaces, where band composition reaches lowest An content. The relative proportions of the three crystal types are distinctly different between 2004-5 dacite and 1980s dome dacite. We propose that crystals with no dissolution surfaces are those that were supplied last to the shallow reservoir, whereas plagioclase with increasingly more complex zoning patterns (that is, the number of zoned bands bounded by dissolution surfaces) result from prolonged residency and evolution in the reservoir. We propose that banding and An zoning across multiple bands are primarily a response to thermally induced fluctuations in crystallinity of the magma in combination with recharge; a lesser role is ascribed to cycling crystals through pressure gradients. Crystals without dissolution surfaces, in contrast, could have grown only in response to steady(?) decompression. Some heating-cooling cycles probably postdate the final eruption in 1986. They resulted from small recharge events that supplied new crystals that then experienced resorption-growth cycles. We suggest that magmatic events shortly prior to the current eruption, recorded in the outermost zones of plagioclase phenocrysts, began with the incorporation of acicular orthopyroxene, followed by last resorption, and concluded with crystallization of euhedral rims. Finally, we propose that 2004-5 dacite is composed mostly of dacite magma that remained after 1986 and underwent subsequent magmatic evolution but, more importantly, contains a component of new dacite from deeper in the magmatic system, which may have triggered the new eruption.

Textural, compositional, and mineralogical data are reported and interpreted for a large population of clinoamphibole phenocrysts in 22 samples from the seven successive dacite spines erupted at Mount St. Helens between October 2004 and January 2006. Despite the uniformity in bulk composition of magma erupted since 2004, there is striking textural and compositional diversity among amphibole phenocrysts and crystal fragments that have grown from, partly dissolved in, or been accidentally incorporated in the new dacite. This study demonstrates that magma erupted throughout the current dome-building episode is the end product of small-scale, thorough mixing of multiple generations of crystal-laden magma. The mixed amphibole population provides important clues to magma conditions within the dacite magma reservoir prior to ascent and, to some extent, the dynamics of mixing and ascent. The predominant amphibole in new dome rock ranges from moderate- to high-alumina tschermakite and magnesiohastingsite compositions. As substantiated by major- and trace-element geochemistry and barometry calculations, this compositional range of crystals, along with plagioclase, orthopyroxene, and iron-titanium oxide, is likely to have precipitated from dacite magma over a range of pressures and temperatures consistent with experimentally determined phase relations (~900°C to ~800°C between 100 MPa and ~350-400 MPa or ~4-km and 13.5-15-km depth). Along with trace-element characteristics, textural and compositional data help to distinguish some low-alumina magnesiohornblende crystals as xenocrysts. The diverse range in composition of amphibole in all samples of 2004-6 dacite, and the complex zonation observed in many phenocrysts, suggests a well-mixed source magma with components that are subjected to repeated heating and (or) pressurization within this pressure-temperature window. Amphibole textural and compositional diversity suggest dynamic conditions in the upper-reservoir zone, which has been tapped steadily during ~2 years of continuous and monotonous eruption. This well-mixed crystal mush is likely to have been subjected to repeated injection of hotter magma into cooler crystal-laden magma while simultaneously assimilating earlier generations of dacitic roof material and surrounding gabbroic rock. Decompression-related reaction rims around subhedral, rounded, resorbed, and fragmented amphibole phenocrysts, regardless of composition, indicate that this mixed-crystal assemblage was being broken, abraded, and dissolved in the magma as a result of mechanical mixing before and during early stages of ascent from conduit roots extending into a mushy cupola of the shallow reservoir. In the earliest lava samples (October 2004), amphiboles with

Eighteen years after dome-forming eruptions ended in 1986, and with little warning, Mount St. Helens began to erupt again in October 2004. During the ensuing two years, the volcano extruded more than 80×10 6 m 3 of gas-poor, crystal-rich dacite lava. The 2004-6 dacite is remarkably uniform in bulk-rock composition and, at 65 percent SiO 2 , among the richest in silica and most depleted in incompatible elements of the magmas erupted at Mount St. Helens during the past 500 years. Since shortly after the first spine of lava appeared, samples have been collected using a steel box dredge ("Jaws") suspended 20-35 m below a helicopter and, occasionally, by hand sampling. As of the spring of 2006, 25 age-controlled samples have been collected from the seven spines of the new lava dome. Samples were obtained from both the interiors of spines and from their carapaces, which are composed of fault gouge and cataclasite 1-2 m thick. The dacite lava is crystal rich, with 40-50 percent phenocrysts. The groundmass is extensively crystallized to a cotectic assemblage of quartz, tridymite, and Na- and K-rich feldspar microlites, raising the total crystal content to more than 80 percent on a vesicle-free basis in all but the earliest erupted.

We report the results of in-situ laser ablation ICP-MS analyses of anorthite content, trace-element (Li, Ti, Sr, Ba, La, Pr, Ce, Nd, Eu, Pb) concentrations, and Pb-isotope compositions in plagioclase from eight dome-dacite samples collected from the 2004-5 eruption of Mount St. Helens and, for comparison, from three dome samples from 1981-85. For 2004-5 samples, plagioclase phenocrysts range in composition from An 30 to An 80 , with the majority An 42 -An 65 . With the exception of Li, the range of trace-element abundances in plagioclase phenocrysts is largely constant in material erupted between October 2004 and April 2005 and is broadly consistent with the 1983-85 dome samples. Anomalously high Li contents in the early stage of the eruption are thought to reflect addition of Li to the upper part of the magma chamber immediately before eruption (within ~1 year) by transfer of an alkali-enriched, exsolved vapor from deep within the magma chamber. Other trace elements show significant correlations (at >99 percent confidence limits) with anorthite content in plagioclase phenocrysts-Ba, light rare-earth elements (LREE), and Pb show positive correlations, whereas Ti and Sr correlate negatively. Variations in plagioclase-melt partitioning as a function of anorthite content cannot explain trace-element variations-in particular predicting trends for Ti and Sr opposite to those observed. A simple model involving closed-system fractional crystallization of plagioclase + hypersthene + amphibole + oxides largely reproduces the observed trends. The model requires no gain or loss of plagioclase and is consistent with the lack of europium anomalies in bulk dacite samples. Analytical traverses within individual plagioclase phenocrysts support this model but also point to a diversity of melt compositions present within the magma storage zone in which plagioclase crystallized. Plagioclase crystals from gabbronorite inclusions in three dacite samples have markedly different trace-element and Pb-isotope compositions from those of plagioclase phenocrysts, despite having a similar range of anorthite contents. Inclusions show some systematic differences from each other but typically have higher Ti, Ba, LREE, and Pb and lower Sr and have lower 208 Pb/ 206 Pb and 207 Pb/ 206 Pb ratios than coexisting plagioclase phenocrysts. The compositions of plagioclase from inclusions cannot be related to phenocryst compositions by any reasonable petrologic model. From this we suggest that they are unlikely to represent magmatic cumulates or restite inclusions but instead are samples of mafic Tertiary basement from beneath the volcano.

The 2004-5 eruption of Mount St. Helens, still ongoing as of this writing (September 2006), has comprised chiefly lava dome extrusion that produced a series of solid, fault-gouge-mantled dacite spines. Vertical aerial photographs taken every 2 to 4 weeks, visual observations, and oblique photographs taken from aircraft and nearby observation points provide the basis for two types of photogeologic maps of the dome—photo—based maps and rectified maps. Eight map pairs, covering the period from October 1, 2004, through December 15, 2005, document the development of seven spines: an initial small, fin-shaped vertical spine; a north-south elongate wall of dacite; two large and elongate recumbent spines ("whalebacks"); a tall and elongate inclined spine; a smaller bulbous spine; and an initially endogenous spine extruded between remnants of preceding spines. All spines rose from the same general vent area near the southern margin of the 1980s lava dome. Maps also depict translation and rotation of active and abandoned spines, progressive deformation affecting Crater Glacier, and distribution of ash on the crater floor from phreatic and phreatomagmatic explosions. The maps help track key geologic and geographic features in the rapidly changing crater and help date dome, gouge, and ash samples that are no longer readily correlated to their original context because of deformation in a dynamic environment where spines extrude, deform, slough, and are overrun by newly erupted material.

Disequilibrium between 210 Po, 210 Pb, and 226 Ra was measured on rocks and plagioclase mineral separates erupted during the first year of the ongoing eruption of Mount St. Helens. The purpose of this study was to monitor the volatile fluxing and crystal growth that occurred in the weeks, years, and decades leading up to eruption. Whole-rock samples were leached in dilute HCl to remove 210 Po precipitated in open spaces. Before leaching, samples had variable initial ( 210 Po) values, whereas after leaching, the groundmasses of nearly all juvenile samples were found to have had ( 210 Po) ≈ 0 when they erupted. Thus, most samples degassed 210 Po both before and after the magmas switched from open- to closed-system degassing. All juvenile samples have ( 210 Pb)/( 126 Ra) ratios within 2σ of equilibrium, suggesting that the magmas involved in the ongoing eruption did not have strong, persistent fluxes of 222 Rn in or out of magmas during the decades and years leading to eruption. These equilibrium values also require a period of at least a century after magma generation and the last significant differentiation of the Mount St. Helens dacites. Despite this, the elevated ( 210 Pb)/( 226 Ra) value measured in a plagioclase mineral separate from lava erupted in 2004 suggests that a significant proportion of this plagioclase grew within a few decades of eruption. The combined dataset suggests that for most 2004-5 lavas, the last stage of open-system degassing of the dacite magmas at Mount St. Helens is confined to the period between 1-2 years and 1-2 weeks before eruption, whereas plagioclase large enough to be included in the mineral separate grew around the time of the 1980s eruption or earlier.

This chapter synthesizes the monitoring of active volcanoes using a wide range of techniques and instrumentation. We cover definitions, concepts of instrument design and use, patterns and processes that occur before, during, and after eruptions. Ideas, trends, and the interplay between monitoring and research are discussed for both under-monitored and well-monitored “laboratory volcanoes,” including a recent case history of the 2010 eruption of Merapi Volcano, Indonesia. We conclude with a discussion of: the importance of using multiple techniques and databases in forecasting, the reliability of forecasts, and future directions in volcano monitoring.

Bayesian event trees are now constructed before or during many volcanic crises, to reflect multiple possible scenarios and the relative and absolute likelihoods of each scenario. Group discussion followed by formal expert elicitation is an excellent way to estimate probabilities at each branch of a tree, but commonly neither sufficient time exists nor a large group of experts is available for a new formal elicitation. This chapter describes a simpler, easily understandable method that can be used in advance of more formal elicitations. This “method of multiple data sets” intentionally uses a number of different data sets at each node. At some nodes, quite different types of data are used, e.g., seismic, gas, and dates of historical eruptions. At other nodes, data sets differ only in the time periods or subsets of volcanoes that are considered. The goal is to define a robust range of plausible probabilities at each branch. The best estimate is probably within this range, and the spread between high and the low values is a rough indication of the uncertainty. We offer a generic Excel-based template that can be modified by any user as needed, case histories of several applications in past crises, and ideas for future improvements and uses of event trees.

The 2004-05 eruption of Mount St Helens exhibited sustained, near-equilibrium behaviour characterized by relatively steady extrusion of a solid dacite plug and nearly periodic shallow earthquakes. Here we present a diverse data set to support our hypothesis that these earthquakes resulted from stick-slip motion along the margins of the plug as it was forced incrementally upwards by ascending, solidifying, gas-poor magma. We formalize this hypothesis with a dynamical model that reveals a strong analogy between behaviour of the magma-plug system and that of a variably damped oscillator. Modelled stick-slip oscillations have properties that help constrain the balance of forces governing the earthquakes and eruption, and they imply that magma pressure never deviated much from the steady equilibrium pressure. We infer that the volcano was probably poised in a near-eruptive equilibrium state long before the onset of the 2004-05 eruption.

Stratigraphic and field data are used in conjunction with textural and chemical evidence (including data from scanning electron microscope, electron microprobe, X-ray fluorescence, X-ray diffraction, and instrumental neutron activation analysis) to establish that the 2003 eruption of Anatahan volcano was mainly phreatomagmatic, dominated by explosive interaction of homogeneous composition low-viscosity crystal-poor andesite magma with water. The hydromagmatic mode of eruption contributed to the significant height of initial eruptive columns and to the excavation and eruption of altered rock debris from the sub-volcanic hydrothermal system. Volatile contents of glass inclusions in equilibrium phenocrysts less abundances of these constituents in matrix glass times the estimated mass of juvenile magma indicate minimum emissions of 19 kt SO2 and 13 kt Cl. This petrologic estimate of SO2 emission is an order-of-magnitude less than an estimate from TOMS. Similarly, inferred magma volumes from the petrologic data are an order of magnitude greater than those modeled from deformation data. Both discrepancies indicate additional sources of volatiles, likely derived from a separate fluid phase in the magma. The paucity of near-source volcanic–tectonic earthquakes preceding the eruption, and the dominance of sustained long-period tremor are attributed to the ease of ascent of the hot low-viscosity andesite, followed by a shallow phreatomagmatic mode of eruption. Phreatomagmatic eruptions are probably more common at emergent tropical island volcanoes, where shallow fresh-water lenses occur at near-sea-level vents. These relations suggest that phreatomagmatic explosions contributed to the formation of many of the near-sea-level craters and possibly even to the small calderas at the other Mariana islands.

1. IntroductionAnatahan Volcano, situated in the Commonwealthof the Northern Mariana Islands (CNMI), erupted forthefirst timeinrecordedhistoryonMay10,2003.Theeruption prompted a period of intensive study andcollaboration between researchers from a number ofgovernmental agencies, institutes and universities, andresultedinaspecialsessiondedicatedtotheeruptionatthe Fall Meeting of the American Geophysical Union(AGU)in December of thesame year (seeEOS. Trans.AGU 84(46), p. 1561–1563). This volume of thirteenpapersgrewoutoftheAGUspecialsessionandcoversfour principal themes: (a) geological studies of Anata-han carried out prior to the May 2003 event includingmapping of both the submarine and subaerial portionsof the volcano and monitoring of the local seismicityfollowing activity in 1990; (b) geophysical signals ofthe 2003 eruption; (c) petrology and geochemistry ofthe eruptive products of the 2003 eruption; and (d)impacts associated with the 2003 eruption. In this briefintroduction, we summarize the major findings asso-ciatedwitheachofthesethemes.Wealsooffersugges-tionstowhereupdatedmaterialandresultsofon-goingstudies can be found. At the time of writing (January2005), Anatahan Volcano continues to be active—albeit at a lower level compared to the spring of2003, so remains the focus of considerable researchand monitoring efforts.2. Anatahan Volcano—background geologyThe island of Anatahan is situated about 120 kmnorth of Saipan—the capital of the CNMI. It lies atthe southern end of the 1500-km-long Izu-Bonin-Mariana (IBM) arc system where the oldest oceaniccrust of the Pacific Plate is subducting beneath youn-ger crust of the Philippine Plate. The island representsthe summit of a large strato-volcano, which rises fromdepths of ~3000 m below sea level to an elevation of788 m.

Unusual geologic and geochemical relations are preserved along the contact between intracaldera tuff and a resurgent intrusion within the 26.9 Ma Turkey Creek caldera of southeast Arizona. Thick intracaldera tuff is weakly argillically altered throughout, except in zones within several hundred meters of its contact with the resurgent intrusion, where the groundmass of the tuff has been variably converted to granophyre and unaltered sanidine phenocrysts are present. Dikes of similarly granophyric material originate at the tuff-resurgent intrusion contact and intrude overlying intracaldera megabreccia and tuff. Field relations indicate that the resurgent intrusion is a laccolith and that it caused local partial melting of adjacent intracaldera tuff. Geochemical and petrographic relations indicate that small volumes of partially melted intracaldera tuff assimilated and mixed with dacite of the resurgent intrusion along their contact, resulting in rocks that have petrographic and compositional characteristics transitional between those of tuff and dacite. Some of this variably contaminated, second-generation magma coalesced, was mobilized, and was intruded into overlying intracaldera rocks. Interpretation of the resurgent intrusion in the Turkey Creek and other calderas as intracaldera laccoliths suggests that intrusions of this type may be a common, but often unrecognized, feature of calderas. Development of granophyric and anatectic features such as those described here may be equally common in other calderas. The observations and previously undocumented processes described here can be applied to identification and interpretation of similarly enigmatic relations and rocks in other caldera systems. Integration of large-scale field mapping with detailed petrographic and chemical data has resulted in an understanding of otherwise intractable but petrologically important caldera-related features.

“Wet volcanoes” with crater lakes and extensive hydrothermal systems pose challenges for monitoring and forecasting eruptions. That's because their lakes and hydrothermal systems serve as reservoirs for magmatic heat and fluid emissions, filtering and delaying the surface expressions of magmatic unrest.

Stratovolcanoes and lava domes are particularly susceptible to sector collapse resulting from wholesale rock failure as a consequence of decreasing rock strength. Here, we provide insights into the influence of thermal and cyclic stressing on the strength and mechanical properties of volcanic rocks. Specifically, this laboratory study examines the properties of samples from Mount St. Helens; chosen because its strength and stability have played a key role in its history, influencing the character of the infamous 1980 eruption. We find that thermal stressing exerts different effects on the strengths of different volcanic units; increasing the heterogeneity of rocks in situ. Increasing the uniaxial compressive stress generates cracking, the timing and magnitude of which was monitored via acoustic emission (AE) output during our experiments. AEs accelerated in the approach to failure, sometimes following the pattern predicted by the failure forecast method (Kilburn 2003). Crack damage during the experiments was tracked using the evolving static Young’s modulus and Poisson’s ratio, which represent the quasi-static deformation in volcanic edifices more accurately than dynamic elastic moduli which are usually implemented in volcanic models. Cyclic loading of these rocks resulted in a lower failure strength, confirming that volcanic rocks may be weakened by repeated inflation and deflation of the volcanic edifice. Additionally, volcanic rocks in this study undergo significant elastic hysteresis; in some instances, a material may fail at a stress lower than the peak stress which has previously been endured. Thus, a volcanic dome repeatedly inflated and deflated may progressively weaken, possibly inducing failure without necessarily exceeding earlier conditions.

We use geologic field mapping and sampling, photogrammetrlc analysis of oblique aerial photographs, and digital elevation models to document the 2008-2009 eruptive sequence at Chaitén Volcano and to estimate volumes and effusion rates for the lava dome. We also present geochemical and petrologic data that contribute to understanding the source of the rhyolite and its unusually rapid effusion rates. The eruption consisted of five major phases: 1. An explosive phase (1-11 May 2008); 2. A transitional phase (11-31 May 2008) in which low-altitude tephra columns and simultaneous lava extrusion took place; 3. An exogenous lava flow phase (June-September 2008); 4. A spine extrusion and endogenous growth phase (October 2008-February 2009); and 5. A mainly endogenous growth phase that began after the collapse of a prominent Peléean spine on 19 February 2009 and continued until the end of the eruption (late 2009 or possibly earliest 2010). The 2008-2009 rhyolite lava dome has a total volume of approximately 0.8 km³. The effusion rate averaged 66 m³s-1 during the first two weeks and averaged 45 m³s-1 for the first four months of the eruption, during which 0.5 km³ of rhyolite lava was erupted. These are among the highest rates measured world-wide for historical eruptions of silicic lava. Chaitén's 2008-2009 lava is phenocryst-poor obsidian and microcrystalline rhyolite with 75.3±0.3% SiO2. The lava was erupted at relatively high temperature and is remarkably similar in composition and petrography to Chaitén's pre-historic rhyolite. The rhyolite's normative composition plots close to that of low pressure (100-200 MPa) minimum melts in the granite system, consistent with estimates of approximately 5 to 10 km source depths based on phase equilibria and geodetic studies. Calcic plagioclase, magnesian orthopyroxene and aluminous amphibole among the sparse phenocrysts suggest derivation of the rhyolite by melt extraction from a more mafic magmatic mush. High temperature and relatively low viscosity enabled rapid magma ascent and high effusion rates during the dome-forming phases of the 2008-2009 eruption.

Available geophysical and geologic data provide a simplified model of the current magmatic plumbing system of Mount St. Helens (MSH). This model and new geochemical data are the basis for the revised hazards assessment presented here. The assessment is weighted by the style of eruptions and the chemistry of magmas erupted during the past 500 years, the interval for which the most detailed stratigraphic and geochemical data are available. This interval includes the Kalama (A. D. 1480–1770s?), Goat Rocks (A.D. 1800–1857), and current eruptive periods. In each of these periods, silica content decreased, then increased. The Kalama is a large amplitude chemical cycle (SiO2: 57%–67%), produced by mixing of arc dacite, which is depleted in high field-strength and incompatible elements, with enriched (OIB-like) basalt. The Goat Rocks and current cycles are of small amplitude (SiO2: 61%–64% and 62%–65%) and are related to the fluid dynamics of magma withdrawal from a zoned reservoir. The cyclic behavior is used to forecast future activity. The 1980–1986 chemical cycle, and consequently the current eruptive period, appears to be virtually complete. This inference is supported by the progressively decreasing volumes and volatile contents of magma erupted since 1980, both changes that suggest a decreasing potential for a major explosive eruption in the near future. However, recent changes in seismicity and a series of small gas-release explosions (beginning in late 1989 and accompanied by eruption of a minor fraction of relatively low-silica tephra on 6 January and 5 November 1990) suggest that the current eruptive period may continue to produce small explosions and that a small amount of magma may still be present within the conduit. The gas-release explosions occur without warning and pose a continuing hazard, especially in the crater area. An eruption as large or larger than that of 18 May 1980 (≈0.5 km3 dense-rock equivalent) probably will occur only if magma rises from an inferred deep (≥7 km), relative large (5–7 km3) reservoir. A conservative approach to hazard assessment is to assume that this deep magma is rich in volatiles and capable of erupting explosively to produce voluminous fall deposits and pyroclastic flows. Warning of such an eruption is expectable, however, because magma ascent would probably be accompanied by shallow seismicity that could be detected by the existing seismic-monitoring system. A future large-volume eruption (≥0.1 km3) is virtually certain; the eruptive history of the past 500 years indicates the probability of a large explosive eruption is at least 1% annually. Intervals between large eruptions at Mount St. Helens have varied widely; consequently, we cannot confidently forecast whether the next large eruption will be years decades, or farther in the future. However, we can forecast the types of hazards, and the areas that will be most affected by future large-volume eruptions, as well as hazards associated with the approaching end of the current eruptive period.

Volcanic and shallow plutonic (hypabyssal) levels of the Turkey Creek caldera, located in southeast Arizona, are exposed as a consequence of uplift and erosion. The 40Ar/39Ar geochronology and paleomagnetic data indicate that the caldera cycle was relatively short lived and occurred at about 26.9 Ma, coincident with early phases of ductile extension in the southern Basin and Range. The caldera is a transitional calc-alkali to alkalic magmatic system and is similar to other relatively small volcanic-plutonic centers that formed after the main pulse of compressional calc-alkalic magmatism in the Cordillera. Trace element ratios and elemental distribution patterns for Turkey Creek rocks are consistent with origin in a transitional subduction to within-plate extensional setting. Field relations also suggest synextensional magmatism; regional northwest trending high-angle faults offset early caldera rocks but are buried by late moat rhyolite. Caldera collapse accompanied eruption of more than 500 km3 of high-silica rhyolite tuff (the Rhyolite Canyon Tuff). Eruption of the tuff was followed immediately by emplacement of a dacite porphyry intrusion, probably a thick laccolith, into the caldera fill and by extrusion of the dacite porphyry from ring-fracture-hosted feeders. Intracaldera tuff at the roof of the intrusion was metamorphosed and brecciated to produce low-pressure shock-metamorphic effects and was locally melted. Interpretation of the intrusion as an intracaldera laccolith readily explains a lack of floor rocks within the caldera and the absence of intracaldera equivalents of most of the outflow tuff. Intrusion of intracaldera laccoliths may represent a relatively common, though rarely recognized process within calderas. Stratigraphic relations, rare mingled rocks, and overlapping 40Ar/39Ar ages indicate that both high-silica rhyolite magma (Rhyolite Canyon Tuff) and dacite porphyry magma were present in the source reservoir (rhyolite above dacite) and were separated by a sharp interface. Dacite porphyry magma from beneath the interface was drawn up into and erupted from vents that previously fed ash flow eruptions; some dacite porphyry was trapped in and beneath the vents and solidified to form a ring dike at depth. Following an erosional hiatus of ≤0.3 m.y., rhyolite was again erupted, filling the caldera moat with ∼135 km3 of mainly aphyric high-silica rhyolite. Gradational contacts with underlying densely welded tuff, relatively large volumes, planiform aspect ratios, and superliquidus temperatures suggest that some of the laminated rhyolites are rheomorphic tuff. Eruption of moat rhyolites records generation of a voluminous new batch of mainly high-silica rhyolite with a distinct geochemical signature.

In response to NASA's announced requirement for Earth hazard monitoring sensor-web technology, a multidisciplinary team involving sensor-network experts (Washington State University), space scientists (JPL), and earth scientists (USGS cascade volcano observatory (CVO)), is developing a prototype dynamic and scaleable hazard monitoring sensor-web and applying it to volcano monitoring. The combined optimized autonomous space - in-situ sensor-web (OASIS) will have two-way communication capability between ground and space assets, use both space and ground data for optimal allocation of limited power and bandwidth resources on the ground, and use smart management of competing demands for limited space assets. It will also enable scalability and seamless infusion of future space and in-situ assets into the sensor-web. The prototype will be focused on volcano hazard monitoring at Mount St. Helens, which has been active since October 2004. The system is designed to be flexible and easily configurable for many other applications as well. The primary goals of the project are: 1) integrating complementary space (i.e., Earth observing one (EO-1) satellite) and in-situ (ground-based) elements into an interactive, autonomous sensor-web; 2) advancing sensor-web power and communication resource management technology; and 3) enabling scalability for seamless infusion of future space and in-situ assets into the sensor-web. To meet these goals, we are developing: 1) a test-bed in-situ array with smart sensor nodes capable of making autonomous data acquisition decisions; 2) efficient self-organization algorithm of sensor-web topology to support efficient data communication and command control; 3) smart bandwidth allocation algorithms in which sensor nodes autonomously determine packet priorities based on mission needs and local bandwidth information in real-time; and 4) remote network management and reprogramming tools. The space and in-situ control components of the system will be integrated such that each element is capable of autonomously tasking the other. Sensor-Web data acquisition and dissemination will be accomplished through the use of the open geospatial consortium sensor-web enablement protocols. The three-year project will demonstrate end-to-end system performance with the in-situ test-bed at Mount St. Helens and NASA's EO-1 platform.

This report presents a catalog of sample information and bulk-rock geochemical data for newly erupted rocks collected near the vent in Mount St. Helens crater between October 2004 and October 2007. An overview of sample collection and lithology along with specifics of analytical techniques are presented as a PDF file, which also includes a detailed explanation of sample information categories that are tabulated in spreadsheets. The Mount St. Helens 2004―2007 Dome Sample Catalog and geochemical data are downloadable in a separate Microsoft Excel file.

This report presents a catalog of sample information and bulk-ash, glass, and phenocryst geochemical data for tephra erupted at Mount St. Helens, Washington between October 2004 and August 2005. A summary of sampling techniques, descriptions of the categories of sample information (column headers) for the database fields, and analytical methods, are presented as a PDF file. The 2004-5 Mount St. Helens tephra sample catalog and geochemical database is reported in six tables and is downloadable in a separate Microsoft Excel file. Glass standard analyses are reported in Appendix 1.

Recent growth in the business of unmanned air vehicles (UAVs) both in the US and abroad has improved their overall capability, resulting in a reduction in cost, greater reliability and adoption into areas where they had previously not been considered. Uses in coastal and border patrol, forestry and agriculture have recently been evaluated in an effort to expand the observed area and reduce surveillance and reconnaissance costs for information gathering. The scientific community has both contributed and benefited greatly in this development. A larger suite of light-weight miniaturized sensors now exists for a range of applications which in turn has led to an increase in the gathering of information from these autonomous vehicles. In October 2004 the first eruption of Mount St Helens since 1986 caused tremendous interest among people worldwide. Volcanologists at the U.S. Geological Survey rapidly ramped up the level of monitoring using a variety of ground-based sensors deployed in the crater and on the flanks of the volcano using manned helicopters. In order to develop additional unmanned sensing methods that can be used in potentially hazardous and low visibility conditions, a UAV experiment was conducted during the ongoing eruption early in November. The

Middle Tertiary volcanic rocks of the central Chiricahua Mountains in southeast Arizona are the westernmost constituents of the Eocene-Oligocene Boot Heel volcanic field of southwestern New Mexico and southeastern Arizona. About two dozen volumetrically and stratigraphically significant volcanic units are present in this area. These include large-volume, regionally distributed ash-flow tuffs and smaller volume, locally distributed lava flows. The most voluminous of these units is the Rhyolite Canyon Tuff, which erupted 26.9 million years ago from the Turkey Creek caldera in the central Chiricahua Mountains. The Rhyolite Canyon Tuff consists of 500-1,000 cubic kilometers of rhyolite that was erupted from a normally zoned reservoir. The tuff represents sequential eruptions, which became systematically less geochemically evolved with time, from progressively deeper levels of the source reservoir. Like the Rhyolite Canyon Tuff, other ash-flow tuffs preserved in the central Chiricahua Mountains have equivalents in nearby, though isolated mountain ranges. However, correlation of these other tuffs, from range to range, has been hindered by stratigraphic discontinuity, structural complexity, and various lithologic similarities and ambiguities. New geochemical and geochronologic data presented here enable correlation of these units between their occurrences in the central Chiricahua Mountains and the remainder of the Boot Heel volcanic field. Volcanic rocks in the central Chiricahua Mountains are composed dominantly of weakly peraluminous, high-silica rhyolite welded tuff and rhyolite lavas of the high-potassium and shoshonitic series. Trace-element, and to a lesser extent, major-oxide abundances are distinct for most of the units studied. Geochemical and geochronologic data depict a time and spatial transgression from subduction to within-plate and extensional tectonic settings. Compositions of the lavas tend to be relatively homogeneous within particular units. In contrast, compositions of the ash-flow tuffs, including the Rhyolite Canyon Tuff, vary significantly owing to eruption from compositionally zoned reservoirs. Reservoir zonation is consistent with fractional crystallization of observed phenocryst phases and resulting residual liquid compositional evolution. Rhyolite lavas preserved in the moat of the Turkey Creek caldera depict compositional zonation that is the reverse of that expected of magma extraction from progressively deeper parts of a normally zoned reservoir. Presuming that the source reservoir was sequentially tapped from its top downward, development of reverse zonation in the rhyolite lava sequence may indicate that later erupted, more evolved magma contains systematically less wallrock contamination derived from the geochemically primitive margins of its incompletely mixed reservoir. New 4 0 Ar/ 3 9 Ar geochronology data indicate that the principal middle Tertiary volcanic rocks in the central Chiricahua Mountains were erupted between about 34.2 and 26.2 Ma, and that the 5.2 m.y. period between 33.3 and 28.1 Ma was amagmatic. The initial phase of eruptive activity in the central Chiricahua Mountains, between 34.2 and 33.3 Ma, was associated with a regional tectonic regime dominated by subduction along the west edge of North America.

High-silica rhyolite magma fuels Earth's largest and most explosive eruptions. Recurrence intervals for such highly explosive eruptions are in the 100- to 100,000-year time range, and there have been few direct observations of such eruptions and their immediate impacts. Consequently, there was keen interest within the volcanology community when the first large eruption of high-silica rhyolite since that of Alaska's Novarupta volcano in 1912 began on 1 May 2008 at Chaiten volcano, southern Chile, a 3-kilometer-diameter caldera volcano with a prehistoric record of rhyolite eruptions [Naranjo and Stern, 2004semi; Servicio Nacional de Geologia y Mineria (SERNAGEOMIN), 2008semi; Carn et al., 2009; Castro and Dingwell, 2009; Lara, 2009; Munoz et al., 2009]. Vigorous explosions occurred through 8 May 2008, after which explosive activity waned and a new lava dome was extruded.

THE eruptive products of calc-alkaline volcanos often show evidence for the mixing of basaltic and acid magmas before eruption (see, for example, refs 1, 2). These observations have led to the suggestion3 that the injection of basaltic magma into the base of a magma chamber (or the catastrophic overturn of a stably stratified chamber containing basaltic magma at its base) might trigger an eruption. Here we report evidence for the mixing of basaltic and dacitic magmas shortly before the paroxysmal eruptions of Pinatubo volcano on 15 June 1991. Andesitic scoriae erupted on 12 June contain minerals and glass with disequilibrium compositions, and are considerably more mafic than the dacitic pumices erupted on 15 June. Differences in crystal abundance and glass composition among the pumices may arise from pre-heating of the dacite magma by the underlying basaltic liquid before mixing. Degassing of this basaltic magma may also have contributed to the climatologically important sulphur dioxide emissions that accompanied the Pinatubo eruptions.

On 2 May 2008, a large eruption began unexpectedly at the inconspicuous Chaiten volcano in Chile’s southern volcanic zone. Ash columns abruptly jetted from the volcano into the stratosphere, followed by lava dome effusion and continuous low- altitude ash plumes [Lara, 2009]. Apocalyptic photographs of eruption plumes suffused with lightning were circulated globally. Effects of the eruption were extensive. Floods and lahars inundated the town of Chaiten, and its 4625 residents were evacuated. Widespread ashfall and drifting ash clouds closed regional airports and cancelled hundreds of domestic flights in Argentina and Chile and numerous international flights [Guffanti et al., 2008]. Ash heavily affected the aquaculture industry in the nearby Gulf of Corcovado, curtailed ecotourism, and closed regional nature preserves. To better prepare for future eruptions, the Chilean government has boosted support for monitoring and hazard mitigation at Chaiten and at 42 other highly hazardous, active volcanoes in Chile. The Chaiten eruption discharged rhyolite magma, a high-silica composition associated with extremes of eruptive behavior ranging from gentle lava effusion to violent, gas-driven explosions. As the first major rhyolitic eruption since that of Alaska’s Katmai-Novarupta in 1912, it permits observations that are benchmarks for future such events. It also reignites the debate on what processes rekindle long-dormant volcanoes, justifies efforts to mitigate rare but significant hazards through ground-based monitoring, and confi rms the value of timely satellite observations.

From introduction: This report supplements similar reports prepared for the four 7.5-minute quadrangles that cover the Turkey Creek caldera and geologic map of the caldera.