Thermal Damage and Pore Pressure Effects of the Brittle-Ductile Transition in Comiso Limestone

Volcanic edifices are commonly unstable, with magmatic and non‐magmatic fluid circulation, and elevated temperature gradients having influence on the mechanical strength of edifice and basement rocks. We present new mechanical characterization of the Comiso limestone of the Mount Etna Volcano (Italy) basement to constrain the effects of regional ambient conditions associated with the volcanic system: the effects of pore fluid on rock strength and the effects of distal magmatic heating (~20 °C to 600 °C) at a range of simulated depths (0.2 to 2.0 km). The presence of water promotes ductile behaviour at shallow depths and causes a significant reduction in brittle rock strength compared to dry conditions. Thermal stressing, in which specimens were heated and cooled before mechanical testing at room temperature, has a variable effect for dry and saturated cases. In dry conditions, thermal stressing up to 450 °C homogenizes the strength of the specimen such that the majority of the specimens exhibit the same peak stress; at 600 °C, the brittle failure is promoted at lower differential stress. The presence of water in thermally‐stressed specimens promotes ductile behaviour and reduces peak strength. Acoustic emission monitoring suggests that accumulated damage is associated with the heating–cooling sequence, particularly in the 300–450‐600°C. Based on conduction modeling, we estimate this temperature range could affect basement rocks up to 300 m away from minor sheet intrusions and much further with larger bodies. Considering the dyke spacing beneath Etna, these conditions may apply to a significant percentage of the basement, promoting ductile behaviour at relatively shallow depths. Plain Language Summary: Volcanoes can collapse as a result of underground magma, gas and/or water flows, and temperature effects on its basement rock's strength. We did laboratory experiments to further test the strength of surface samples of a limestone present beneath Mount Etna Volcano (Italy). We kept some samples ‘as‐collected’ and heated and cooled the others at different temperatures (up to 600oC) prior to their deformation as if some magma bodies had flown in the vicinity of these rocks. We then water saturated a sample of each temperature condition applied and compared their strengths at different simulated depths ranging about 0.2 to 2.0 km (i.e. confinement ranging between 7 and 50 MPa) to the ones of the corresponding samples kept dry. Our results show that water presence lowers the limestone strength compared to dry conditions but also the conditions of pressure at which the failure behaviour transitions between brittle and ductile regimes. For temperature up to 450 °C, the strength of this limestone seems to be independent of the treatment's temperature with the maximum strength values being higher in dry conditions than in water saturated conditions. The rock fails only in the brittle regime when thermally treated prior deformation at all temperatures, saturation and confinement applied. Considering the dyke spacing beneath Mount Etna, these combinations of water, temperature and pressure conditions may apply to a significant percentage of the basement, promoting weaker and ductile behaviour at relatively shallow depths.