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.

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Much effort has been undertaken for the estimation of propulsive force of swimmers in the front crawl. Estimation is typically based on steady flow theory: so-called the quasi-steady analysis. Flow fields around a swimmer, however, are extremely unsteady because the change direction of hand produces unsteady vortex motions. To evaluate the force correctly, it is necessary to know the unsteady properties determined from the vortex dynamics because that unsteadiness is known to make the force greater. Unsteady flow measurements were made for this study using a sophisticated technique called particle image velocimetry (PIV) in several horizontal planes for subjects swimming in a flume. Using that method, a hundred time-sequential flow fields are obtainable simultaneously. Each flow field was calculated from two particle images using the cross-correlation method. The intensity of vortices and their locations were identified. A strong vortex was generated near the hand and then shed by directional change of the hand in the transition phase from in-sweep to out-sweep. When the vortex was shed, a new vortex rotating in the opposite direction around the hand was created. The pair of vortices induced the velocity component in the direction opposite to the swimming. Results of this study show that the momentum change attributable to the increase of this velocity component is the origin of thrust force by the hand.

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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