Search

Data collected during well-observed eruptions can lead to dramatic increases in our understanding of volcanic processes. However, the necessary prioritization of public safety and hazard mitigation during a crisis means that scientific opportunities may be sacrificed. Thus, maximizing the scientific gains from eruptions requires improved planning and coordinating science activities among governmental organizations and academia before and during volcanic eruptions. One tool to facilitate this coordination is a Scientific Advisory Committee (SAC). In the USA, the Community Network for Volcanic Eruption Response (CONVERSE) has been developing and testing this concept during workshops and scenario-based activities. The December 2020 eruption of Kīlauea volcano, Hawaii, provided an opportunity to test and refine this model in real-time and in a real-world setting. We present here the working model of a SAC developed during this eruption. Successes of the Kīlauea SAC (K-SAC) included broadening the pool of scientists involved in eruption response and developing and codifying procedures that may form the basis of operation for future SACs. Challenges encountered by the K-SAC included a process of review and facilitation of research proposals that was too slow to include outside participation in the early parts of the eruption and a decision process that fell on a small number of individuals at the responding volcano observatory. Possible ways to address these challenges include (1) supporting community-building activities between eruptions that make connections among scientists within and outside formal observatories, (2) identifying key science questions and pre-planning science activities, which would facilitate more rapid implementation across a broader scientific group, and (3) continued dialog among observatory scientists, emergency responders, and non-observatory scientists about the role of SACs. The SAC model holds promise to become an integral part of future efforts, leading in the short and longer term to more effective hazard response and greater scientific discovery and understanding.

One of the biggest challenges in volcanic hazard assessment is to understand how and why eruptive style changes within the same eruptive period or even from one eruption to the next at a given volcano. This review evaluates the competing processes that lead to explosive and effusive eruptions of silicic magmas. Eruptive style depends on a set of feedback involving interrelated magmatic properties and processes. Foremost of these are magma viscosity, gas loss and external properties such as conduit geometry. Ultimately, these parameters control the speed at which magmas ascend, decompress and outgas en route to the surface, and thus determine eruptive style and evolution.

The textures of minerals in volcanic and plutonic rocks testify to a complexity of processes in their formation that is at odds with simple geochemical models of igneous differentiation. Zoning in plagioclase feldspar is a case in point. Very slow diffusion of the major components in plagioclase means that textural evidence for complex magmatic evolution is preserved, almost without modification. Consequently, plagioclase affords considerable insight into the processes by which magmas accumulate in the crust prior to their eventual eruption or solidification. Here, we use the example of the 1980-1986 eruptions of Mount St. Helens to explore the causes of textural complexity in plagioclase and associated trapped melt inclusions. Textures of individual crystals are consistent with multiple heating and cooling events; changes in total pressure (P) or volatile pressure ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) are less easy to assess from textures alone. We show that by allying textural and chemical analyses of plagioclase and melt inclusions, including volatiles ([H.sub.2]O, C[O.sub.2]) and slow-diffusing trace elements (Sr, Ba), to published experimental studies of Mount St. Helens magmas, it is possible to disambiguate the roles of pressure and temperature to reconstruct magmatic evolutionary pathways through temperature-pressure-melt fraction (T-[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]-F) space. Our modeled crystals indicate that (1) crystallization starts at [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] > 300 MPa, consistent with prior estimates from melt inclusion volatile contents, (2) crystal cores grow at [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] = 200-280 MPa at F = 0.65-0.7, (3) crystals are transferred to [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] = 100-130 MPa (often accompanied by 10-20°C of heating), where they grow albitic rims of varying thicknesses, and (4) the last stage of crystallization occurs after minor heating at [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] * 100 MPa to produce characteristic rim compositions of [An.sub.50]. We hypothesize that modeled [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] decreases in excess of ~ 50 MPa most likely represent upward transport through the magmatic system. Small variations in modeled [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], in contrast, can be effected by fluxing the reservoir with C[O.sub.2]-rich vapors that are either released from deeper in the system or transported with the recharge magma. Temperature fluctuations of 20-40°C, on the other hand, are an inevitable consequence of incremental, or pulsed, assembly of crustal magma bodies wherein each pulse interacts with ancestral, stored magmas. We venture that this 'petrological cannibalism' accounts for much of the plagioclase zoning and textural complexity seen not only at Mount St. Helens but also at arc magmas generally. More broadly we suggest that the magma reservoir below Mount St. Helens is dominated by crystal mush and fed by frequent inputs of hotter, but compositionally similar, magma, coupled with episodes of magma ascent from one storage region to another. This view both accords with other independent constraints on the subvolcanic system at Mount St. Helens and supports an emerging view of many active magmatic systems as dominantly super-solidus, rather than subliquidus, bodies. Keywords Plagioclase * Mount St. Helens * Volatiles * Magma chamber, Introduction The enduring paradigm of chemical differentiation via crystallization of magmas provides a valuable framework within which to view the evolution of igneous rock suites. However, even a cursory look [...]

Historical eruptions from Mt. Ruapehu (New Zealand) have been small ( Keywords Andesite * Volatile * Melt inclusions * Ruapehu * Crystal mush * Antecryst * [H.sub.2]O * C[O.sub.2] * Magma mixing, Introduction Magma erupted from andesitic volcanoes often records a complex history, and interactions between recharge and antecedent magmas and/or crystal mush zones are common (e.g., Nakamura 1995; Murphy et al. [...]

Author(s): Jon Blundy (corresponding author) [1]; Kathy Cashman [2]; Madeleine Humphreys [1] Explosive volcanic eruptions are driven by exsolution of H[sub.2 ] O-rich vapour from silicic magma [1]. Eruption dynamics [...]

Crystals in hydrous magmas can form in response to falling temperature (magma cooling) or degassing (magma decompression). It remains unclear which process dominates beneath explosive silicic volcanoes. Because decompression and cooling operate on very different time scales, resolving the driving force behind crystallization is of fundamental importance for determining magma dynamics and eruption hazard. Here we use ion-microprobe measurements of dissolved [H.sub.2]O in phenocryst-hosted melt inclusions from pumices erupted between May and October 1980 at Mount St. Helens volcano to show that all microlites and a significant proportion of phenocrysts were formed by near isothermal decompression. Magmas erupted after 18 May show evidence for subsequent crystallization of both phenocrysts and microlites, indicating that the time scales of crystal nucleation and growth are on the order of months or less. Keywords: Mount St. Helens, melt inclusions, volcano, ion probe, decompression crystallization.

We introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system Qz-Ab-Or-[H.sub.2]O in order to relate variations in volcanic glass chemistry to changing pressure (P) and temperature (T) conditions in the sub-volcanic magma system. By this means an important distinction can be made between ascent-driven and cooling-driven crystallisation under water-saturated or undersaturated conditions. In samples containing feldspar and a silica phase (quartz or tridymite), quantitative P-T estimates of the conditions of last equilibrium between crystals and melt can be made. Formation of highly silicic melts (i.e. >77 wt% Si[O.sub.2]) is a simple consequence of the contraction of the silica phase volume with decreasing pressure, such that high silica glasses can only form by crystallisation at low pressure. Resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation. Groundmass and inclusion glasses in dacites from the 1980-1986 eruption of Mount St Helens volcano (WA) span a wide range in Si[O.sub.2] (68-80 wt%, anhydrous). The compositions of the least evolved (Si[O.sub.2]-poor) inclusions in amphibole phenocrysts record entrapment of silicic liquids with [less than or equal to] 5.4 wt% water, corresponding to a water saturation pressure of ~200 MPa at 900 [degrees] C. The compositions of more evolved (higher Si[O.sub.2]) plagioclase-hosted inclusions and groundmass glasses are consistent with extensive ascent-driven fractional crystallisation of plagioclase, oxide and orthopyroxene phenocrysts and microlites to low pressures. During this polybaric crystallisation, plagioclase phenocrysts trapped melts with a wide range of dissolved water contents (3.5-5.7 wt%). Magmas erupted during the Plinian phase of the 18 May 1980 eruption were derived from a large reservoir at depths of [greater than or equal to] 6 km. Subsequent magmas ascended to varying depths within the sub-volcanic system prior to extraction. From glass chemistry and groundmass texture two arrest levels have been identified, at depths of 0.5-1 and 2-4 km. A single dome sample from February 1983 contains groundmass plagioclase, tridymite and quartz, testifying to temperatures of at least 885 [degrees] C at 11 MPa. These shallow storage conditions are comparable to those in the cryptodome formed during spring 1980. The corresponding thermal gradient, [less than or equal to] 0.2 [degrees] C M[Pa.sup.-1], is consistent with near-adiabatic magma ascent from ~8 km. We argue that the crystallisation history of Mount St Helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation. This challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980-1986 eruption. Because the crystallisation process is both polybaric and fractional, it cannot be simulated directly using isobaric equilibrium crystallisation experiments. However, calculation of the phase proportions in water-saturated 910 [+ or -] 15 [degrees] C experiments by Rutherford et al. (1985) over the pressure range 220-125 MPa reproduces the crystallisation sequence and phenocryst modes of Mount St Helens dacites from 18 May 1980. By allowing for the effects of fractional versus equilibrium crystallisation, entrained residual source material, and small temperature differences between nature and experiment, phase compositions can also be matched to the natural samples. We conclude that decompression of water-saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres.

Zones of deformation bands occur in unconsolidated late Pleistocene marine terrace sand in the footwall of the active McKinleyville thrust fault in Humboldt County, California. Individual deformation-band shear zones are as much as 8 cm wide and accommodate ~50 cm of reverse-dip separation. Like deformation bands described in Mesozoic sandstone of the Colorado Plateau, these structures formed in fine-grained, well-sorted, porous sand. We assessed the relative importance of compaction, grain breakage, and grain rotation during shear-zone development by measuring grain size, grain shape, grain orientation, porosity, and bulk strain both within and outside deformation-band shear zones. Sand grains within shear zones are smaller, more compacted, and have stronger preferred orientations and more elongate shapes than grains outside of deformation-band shear zones. Bulk strain analyses of sand within shear zones give strain ellipses that are compatible with dextral shear strains of ~0.1-0.5 and volume loss of 5%-15%. On the basis of these observations, we conclude that compaction, grain rotation, and extensive cataclasis all contribute to deformation-band shear-zone formation in these unconsolidated sands, despite very low confining pressures. In addition, the position of these deformation-band shear zones adjacent to an active fault with a history of episodic slip during large earthquakes suggests that they may form in conjunction with slip events. Keywords: cataclasis, deformation band, active fault.