During volcanic eruptions, domes of solidifying magma can form at the volcano summit. As magma ascends it often forms a plug bounded by discrete fault zones, a process accompanied by drumbeat seismicity. The repetitive nature of this seismicity has been attributed to stick-slip motion(1) at fixed loci between the rising plug of magma and the conduit wall(2,3). However, the mechanisms for such periodic motion remain controversial(4-7). Here we simulate stick-slip motion in the laboratory using high-velocity rotary-shear experiments on magma-dome samples collected from Soufriere Hills Volcano, Montserrat, and Mount St HelensVolcano, USA. We frictionally slide the solid magma samples to generate slip analogous to movement between a magma plug and the conduit wall. We find that frictional melting is a common consequence of such slip. The melt acts as a viscous brake, so that the slip velocity wanes as melt forms. The melt then solidifies, followed by pressure build up, which allows fracture and slip to resume. Frictional melt therefore provides a feedback mechanism during the stick-slip process that can accentuate the cyclicity of such motion. We find that the viscosity of the frictional melt can help define the recurrence interval of stick-slip events. We conclude that magnitude, frequency and duration of drumbeat seismicity depend in part on the composition of the magma

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

Abstract. The strength and macroscopic deformation mode (brittle vs. ductile) of rocks is generally related to the porosity and pressure conditions, with occasional considerations of strain rate. At high temperature, molten rocks abide by Maxwell's viscoelasticity and their deformation mode is generally defined by strain rate or reciprocally by comparing the relaxation timescale of the material (for a given condition) to the observation timescale – a dimensionless ratio known as the Deborah (De) number. Volcanic materials are extremely heterogeneous, with variable concentrations of crystals, glass–melt, and vesicles (of different sizes), and a complete description of the conditions leading to flow or rupture as a function of temperature, stress and strain rate (or timescale of observation) eludes us. Here, we examined the conditions which lead to the macroscopic failure of variably vesicular (0.09–0.35), crystal-rich (∼ 75 vol %), pristine and altered dome rocks (at ambient temperature) and lavas (at 900 °C) from Mt. Unzen volcano, Japan. We found that the strength of the dome rocks decreases with porosity and is commonly independent of strain rate; when comparing pristine and altered rocks, we found that the precipitation of secondary mineral phases in the original pore space caused minor strengthening. The strength of the lavas (at 900 °C) also decreases with porosity. Importantly, the results demonstrate that these dome rocks are weaker at ambient temperatures than when heated and deformed at 900 °C (for a given strain rate resulting in brittle behaviour). Thermal stressing (by heating and cooling a rock up to 900 °C at a rate of 4 °C min−1, before testing its strength at ambient temperature) was found not to affect the strength of rocks.In the magmatic state (900 °C), the rheology of the dome lavas is strongly strain rate dependent. Under conditions of low experimental strain rate (≤ 10−4 s−1), ductile deformation dominated (i.e. the material sustained substantial, pervasive deformation) and displayed a non-Newtonian shear thinning behaviour. In this regime, the apparent viscosities of the dome lavas were found to be essentially equivalent, independent of vesicularity, likely due to the lack of pore pressurisation and efficient pore collapse during shear. At high experimental strain rates ( ≥ 10−4 s−1) the lavas displayed an increasingly brittle response (i.e. deformation resulted in failure along localised faults); we observed an increase in strength and a decrease in strain to failure as a function of strain rate. To constrain the conditions leading to failure of the lavas, we analysed and compared the critical Deborah number at failure (Dec) of these lavas to that of pure melt (Demelt = 10−3–10−2; Webb and Dingwell, 1990). We found that the presence of crystals decreases Dec to between 6.6×10−4 and 1×10−4. The vesicularity (φ), which dictates the strength of lavas, further controls Dec following a linear trend. We discuss the implications of these findings for the case of magma ascent and lava dome structural stability.

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.