Extreme frictional processes in the volcanic conduit of Mount St. Helens (USA) during the 2004-2008 eruption

The 2004-2008 eruption of Mount St. Helens saw the extrusion of seven high-viscosity spines and formation of discrete shear zones along the conduit margin. At spine 7 this shear zone consists of four structurally distinct layers: the outer surface gouge (L1) crosscuts; a dark, banded layer (L2) which grades into; a moderately sheared layer (L3) and; undeformed rock (L4) inside the spine. Field observations, porosity measurements, geochemistry, mineralogy, microstructure, crystal size- and shape-distribution, kinetic properties and magnetic analyses chart the evolution of deformation processes and products throughout the eruption.Gouge formation was concomitant with characteristic microseismic "drumbeats" at depths 0.5-1 km. In addition, the seismic record shows two larger earthquakes with similar seismic signatures in August 2006, which we conclude represent larger slip amounts along the conduit margin of spine 7. Extensive slip resulted in frictional heating on the order of several hundreds of degrees, melting the highly-viscous, crystalline, ascending magma plug and forming a pseudotachylyte. High ambient temperatures in the conduit resulted in near-equilibrium melting and slow recrystallisation, thus impeding the development of signature pseudotachylyte characteristics and hindering identification. Thus, frictional melting and recrystallisation in ascending magma plugs may be a common, but unidentified, phenomena at composite volcanoes worldwide.