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Fracture of Basalts under Simulated Volcanic Conditions

Valentina Rocchi

High temperature fracturing of basaltic rock is an integral process in a number of volcanic phenomena, in particular in lava dynamics and vent opening of volcano flanks. Experimental data is essential to investigate the effects of external physical parameters in the deformation processes of basaltic rocks under high-pressure and low-temperature conditions. Data under these specific conditions from this study provide support in the understanding of flow dynamics, particularly to investigate the limiting conditions of flow advance dictated by the combination of extrinsic and intrinsic factors acting on lava flows.

Fracture toughness and uniaxial compression experiments were performed to quantify the mechanical properties of basalts under these specific conditions. A new triaxial rock deformation apparatus has been designed to perform experiments at confining pressures between 0 and 50MPa and temperatures between 25°C and 1000°C on cylindrical samples of 25mm diameter and 75mm length. Results of experiments performed on Etnean and Vesuvian rocks are presented and discussed in terms of the effects of temperature, confining pressure and strain rate on the mechanical properties of the basalts and in terms of the implications in the study of the volcanic phenomena.

The data has been applied to lava flows to investigate the fracturing of the crust and the deformation mechanism causing it. It was found that the new data could be reconciled with existing lava flow models by allowing most of the strain energy before failure to be stored in the crust as a result of shear deformation and not tensile deformation as previously thought. Furthermore the data was used to investigate fracturing of the flow through its entire thickness. This phenomenon represents a limiting condition for the flow advance and therefore a key aspect in hazard reduction.

The experimental data was also used to develop fracture a criterion and a creep power law for basaltic rocks, which were used to construct principal stress maps where all fracture and flow data were plotted as a function of pressure, strain and temperature.