DYNAMICS OF FRICTION, FRICTIONAL MELTING AND GENERATION OF PSEUDOTACHYLYTES IN VOLCANIC CONDUITS

Magmas in volcanic systems are heterogeneous geomaterials in highly dynamic environments that are commonly subjected simultaneously to high temperatures, pressures and differential stresses. As such, in the inevitable process of strain localisation at very high viscosity in dome-building events, they are candidate materials to fracturing, followed by frictional slip, and possibly melting – a phenomenon preserved in the rock record as a pseudotachylyte. This scenario is expressed at the surface by the exogenic extrusion of a lava spine.

Here, we experimentally test the ability of volcanic rocks containing different glass content to sustain friction using a high-velocity rotary apparatus. We find that obsidian-obsidian faults difficultly experience slip without ultimate failure. Alternatively, obsidian can be slipped against a crystalline material for some distances before succumbing to failure. In contrast, andesite-andesite and basalt-basalt faults can sustain extensive slip and friction, leading to melting (but not fracturing). We find that melting occurs in disequilibrium, resulting in chemically heterogeneous local melt batches, which rapidly homogenize into a mixed melt whose complex non-Newtonian shear viscosity controls the shear stress along the slip surface. The shear stress monitored in the presence of a melt in the slip zone is higher than during rock-rock friction, which suggest that frictional melting (of volcanic rocks) does not lubricate slip zones, but rather impede the ability to slip.

The ability of crystalline rock-rock to slip along restricted zones compared to obsidian-obsidian, which tend to shatter, carries important implication to the dynamics of magma ascent and formation of lava domes. Our findings suggest that the width of the slip zone decreases with the presence of crystals. We conclude that the comminution of crystals is a requirement to the development of a localised slip zone. In absence of crystals, magma and/or obsidian are forced to shatter catastrophically, promoting wide damage zones. Comparison of our observations with structures developed in lava domes indicate that crystal-poor magma forms wide fracture zones, which hinder the extrusion of spines, whereas crystal-rich magma can undergo comminution and easily evolve into exogenic spines.