Mechanical behaviour of dacite from Mount St. Helens (USA): A link between porosity and lava dome extrusion mechanism (dome or spine)?
Résumé
There is a rich diversity in lava dome morphology, from blocky domes and lobes to imposing spine and whaleback structures. The latter extrude via seismically active, gouge-rich conduit-margin faults, a manifestation of a brittle failure mode. Brittle versus ductile behaviour in volcanic rocks is known to be porosity dependent, and therefore offers a tantalising link between the properties of the material near the conduit margin and the extrusion mechanism (dome or spine). We test this hypothesis by complementing published data on the mechanical behaviour of dacites from the 2004-2008 spine-forming eruption at Mount St. Helens (MSH) with new data on dacite lavas collected from the 1980 dome. The 1980 dacite samples were deformed at room temperature under a range of pressures (i.e., depths) to investigate their mechanical behaviour and failure mode (brittle or ductile). Low-porosity dacite (porosity 0.19) is brittle up to an effective pressure of 30 MPa (depth 1 km) and is ductile at 40 MPa (depth 1.5 km). High-porosity dacite (porosity 0.32) is ductile above an effective pressure of 5 MPa (depth 200 m). Samples deformed in the brittle regime show well-developed ( 1 mm) shear fracture zones comprising broken glass and crystal fragments. Samples deformed in the ductile regime feature anastomosing bands of collapsed pores. The combined dataset is used to explore the influence of strain rate, temperature, and porosity on the mechanical behaviour and failure mode of dacite. A decrease in strain rate does not influence the strength of dacite at low temperature, but reduces strength at high temperature (850 °C). Due to the extremely low glass content of these materials, such weakening is attributed to the increased efficiency of subcritical crack growth at high temperature. However, when strain rate is kept constant, temperature does not significant impact strength reflecting the highly crystallised nature of dacite from MSH. Dacite from the 2004-2008 eruption is stronger than 1980 dome material and remains brittle even at high effective pressures, a consequence of their low preserved porosities. Only the porous (porosity 0.32) 1980 dome material deformed in a ductile manner (i.e., no macroscopic shear fracture) at effective pressures relevant for edifice deformation. Spine formation typically involves the extrusion of low-porosity material along faults that envelop the magma-filled conduit (i.e., brittle deformation), suggesting that the extrusion mechanism (dome or spine) may be a consequence of slow ascent rates and efficient pre-eruptive outgassing. Well-outgassed, low-porosity (slow ascent rate) materials favour a brittle mode of failure promoting spine extrusion, while poorly-outgassed, high-porosity (fast ascent rate) materials result in blocky domes or lobes. A crystal content-porosity map for brittle (spine) versus ductile (blocky dome) behaviour demonstrates that the window for brittle deformation is small and offers an explanation as to why spine and whaleback structures are relatively rare in nature.