Fracturing of volcanic systems: Experimental insights into pre-eruptive conditions

Conditions for fracturing are a primary control on the behaviour of volcanic systems, especially during the approach to eruption. We here present the results of deformation experiments under simulated volcanic conditions on a porhyritic andesite from ancestral Mount Shasta. Andesite was chosen as a representative material because it is common at subduction-zone volcanoes, among both erupted products and country rock. We deformed the lava in tension and triaxial compression tests at strain rates of 10− 5 s− 1, confining pressures from 0 to 50 MPa and temperatures up to 900 °C. We also concurrently recorded acoustic emissions (AE), in order to monitor cracking activity. The results show that deformation behaviour changes significantly in the temperature range 600-750 °C. Thus, as temperatures increased across this interval, the tensile fracture toughness increased from 2.5 ± 0.5 MPa m1/2 to 3.5 ± 1 MPa m1/2, the compressive strength decreased from 110 ± 30 MPa to 55 ± 35 MPa (at 900 °C) and the corresponding Young's Modulus decreased from 20 ± 4 GPa to 6 ± 4 GPa. The changes occur when the deformation of the sample changes from elastic-brittle to brittle-ductile behaviour, which we attribute to the blunting of crack tips due to melting of the glass phase and enhanced crystal plasticity at high temperature. AE activity was observed in all experiments, indicating that earthquakes can be generated not only in country rock, but also in hot magma, such as may be found in lava domes and at the margins of magma conduits. In addition, the trends in accelerating AE event rates before sample failure were comparable to those seen in earthquakes before some volcanic eruptions and a minimum in the seismic b-value coincided with sample failure. Applied to volcanic systems, the results suggest that (1) andesite strength and elasticity will not be affected by temperature or pressure beyond ~ 10-100 m from active magma, (2) before eruptions, fractures propagate preferentially through weaker horizons in a mechanically heterogeneous volcano, and (3) volcanic rocks have characteristic seismic b-values that are perturbed during the approach to bulk failure. Each of these conditions provides quantitative constraints on models for seismic precursors to eruption or intrusion.