The concluding episode of activity during the recent eruption of Mt. Unzen (October 1994to February 1995 was characterized by incremental spine extrusion, accompanied by seismicity. Analysis of the seismic record reveals the occurrence of two dominant long-period event families associated with a repeating, nondestructive source mechanism, which we attribute to magma failure and fault-controlled ascent. We obtain constraints on the slip rate and distance of faulting events within these families. That analysis is complemented by an experimental thermomechanical investigation of fault friction in Mt. Unzen dacitic dome rock using a rotary-shear apparatus at variable slip rates and normal stresses. A power density threshold is found at 0.3 MW m À2, above which frictional melt forms and controls the shear resistance to slip, inducing a deviation from Byerlee's frictional law. Homogenized experimentally generated pseudotachylytes have a similar final chemistry, thickness, and crystal content, facilitating the construction of a rheological model for particle suspensions. This is compared to the viscosity constrained from the experimental data, to assess the viscous control on fault dynamics. The onset of frictional melt formation during spine growth is constrained to depths below 300 m for an average slip event. This combination of experimental data, viscosity modeling, and seismic analysis offers a new description of material response during conduit plug flow and spine growth, showing that volcanic pseudotachylyte may commonly form and modify fault friction during faulting of dome rock. This model furthers our understanding of faulting and seismicity during lava dome formation and is applicable to other eruption modes.

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Editor: T.A. MatherKeywords: amphibole reaction rim heating andesite Magmatic minerals record the pre-eruptive timescales of magma ascent and mixing in crustal reservoirs and conduits. Investigations of the mineral records of magmatic processes are fundamental to our understanding of what controls eruption style, as ascent rates and magma mixing processes are well known to control and/or trigger potentially hazardous explosive eruptions. Thus, amphibole reaction rims are often used to infer pre-eruptive magma dynamics, and in particular to estimate magma ascent rates. However, while several experimental studies have investigated amphibole destabilization during decompression, only two investigated thermal destabilization relevant to magma mixing processes. This study examines amphibole decomposition experimentally through isobaric heating of magnesiohornblende phenocrysts within a natural high-silica andesite glass. The experiments first equilibrated for 24 h at 870 • C and 140 MPa at H 2 O-saturated conditions and ƒO 2 ∼ Re-ReO prior to rapid heating to 880, 900, or 920 • C and hold times of 3-48 h. At 920 • C, rim thicknesses increased from 17 μm after 3 h, to 55 μm after 12 h, and became pseudomorphs after longer durations. At 900 • C, rim thicknesses increased from 7 μm after 3 h, to 80 μm after 24 h, to pseudomorphs after longer durations. At 880 • C, rim thicknesses increased from 7 μm after 3 h, to 18 μm after 36 h, to pseudomorphs after 48 h. Reaction rim microlites vary from 5-16 μm in size, with no systematic relationship between crystal size and the duration or magnitude of heating. Time-averaged rim microlite growth rates decrease steadily with increasing experimental duration (from 3.97 × 10 −7 mm s −1 to 3.1 to 3.5 × 10 −8 mm s −1 ). Time-averaged microlite nucleation rates also decrease with increasing experimental duration (from 1.2 × 10 3 mm −3 s −1 to 5.3 mm −3 s −1 ). There is no systematic relationship between time-averaged growth or nucleation rates and the magnitude of the heating step. Ortho-and clinopyroxene together constitute 57-90 modal % mineralogy in each reaction rim. At constant temperature, clinopyroxene abundances decrease with increasing experimental duration, from 72 modal % (3 h at 900 • C) to 0% (48 h at 880 • C, and 36 h at 900 and 920 • C). Fe-Ti oxides increase from 6-12 modal % (after 3-6 h) to 26-34 modal % (after 36-48 h). Plagioclase occurs in relatively minor amounts (<1-11 modal %), with anorthite contents that increase from An56 to An88 from 3 to 36 h of heating. Distal glass compositions (>500 μm from reacted amphibole) are consistent with inter-microlite rim glasses (71.3-77.7 wt.% SiO 2 ) within a given experiment and there is a weakly positive correlation between increasing run duration and inter-microlite melt SiO 2 (68.9-78.5 wt.%). Our results indicate that experimental heating-induced amphibole reaction rims have thicknesses, textures, and mineralogies consistent with many of the natural reaction rims seen at arc-andesite volcanoes. They are also texturally c...

Volcanic environments often represent structurally active settings where strain localisation can promote faulting, frictional deformation, and subsequent melting along fault planes. Such frictional melting is thermodynamically a disequilibrium process initiated by selective melting of individual mineral phases and softening of volcanic glass at its glass transition as a response to rapid frictional heating. The formation of a thin melt layer on a fault plane surface can drastically accelerate or terminate slip during fault motion. A comprehensive understanding of the physical and chemical properties of the frictional melt is required for a full assessment of slip mechanism, as frictional rheology depends on the contributions from selectively melted mineral and glass phases as well as the physical effects of restite fragments suspended in the frictional melt. Here, we experimentally investigate the impact of host-rock mineralogy on the compositional and textural evolution of a frictional melt during slip. High-velocity rotary shear (HVR) experiments were performed under controlled, volcanically relevant, coseismic conditions (1 m s -1 slip rate and 1 MPa normal stress) using three intermediate dome lavas with contrasting mineral assemblages, sampled from volcanic systems where fault friction is evident: (1) an amphibole-bearing andesite (Soufrière Hills Volcano, Montserrat); (2) an amphibole-poor dacite (Santiaguito dome complex, Guatemala); and (3) an amphibole-free andesite (Volcán de Colima, Mexico). For each sample, five HVR experiments were terminated at different stages of frictional melt evolution, namely: (1) at the onset of melting and (2) formation of a steady-state melt layer; and (3) after 5 m, (4) 10 m, and (5) 15 m of slip at steady-state conditions. Progressive mixing and homogenisation of selective, single-phase melts within the frictional melt layer through double-diffusion convection demonstrates the dependence of melt composition on slip behaviour. Amphiboles melted preferentially, leading to lower shear stress (~1 MPa) and pronounced shear weakening during the frictional melting of amphibole-bearing lavas. The results highlight the implications of mineral assemblage on volcanic conduit flow processes, which may influence the explosivity of eruptions, and run-out distances of rapid granular flows.

Deep below the coast of North Yorkshire, UK, scientists are conducting world‐class research into elusive dark matter and life on other planets, and are using the products of exploding stars to combat climate change and revolutionize how we monitor the storage of CO2 emitted from industry. Boulby Underground Laboratory is hosted within Boulby Mine, the deepest active mine in the UK. Funded by the UK government through the Science and Technology Facilities Council (STFC), the laboratory sits within a tunnel hewn from halite at a depth of 1100 m. Originally built for research into particle and astrophysics, recent years have seen a move unprecedented among similar facilities around the world—a significant expansion of research into a wide variety of other scientific fields. Today, Earth science research at Boulby includes Muon tomography for monitoring deep carbon sequestration, ultra‐low‐background gamma spectroscopy, studies of life in extreme environments, and developing Mars rover equipment to analyse Martian geology.