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The influence of thermal and cyclic stressing on the strength of rocks from Mount St. Helens, Washington |
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| Authors: | Jackie Evan Kendrick Rosanna Smith Peter Sammonds Philip G Meredith Matthew Dainty John S Pallister |
| Institution: | 1. Rock and Ice Physics Laboratory, Department of Earth Sciences, UCL, London, UK 2. Department of Earth and Environmental Sciences, LMU, Munich, Germany 5. Department for Earth, Ocean and Ecological Sciences, University of Liverpool, Liverpool, UK 3. Institute for Risk and Disaster Reduction, UCL, London, UK 4. USGS Cascades Volcano Observatory, Vancouver, WA, USA |
| Abstract: | 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. |
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