Fracture, friction and fragmentation - brittle processes at lava dome volcanoes

The extent to which transitions from dominantly viscous to dominantly brittle magma deformation regulate eruptive activity has not been widely explored in volcanology. Within this thesis, investigations combining experiments, petrology and geophysical signals are presented to help decipher and understand the role of brittle deformation during lava dome eruptions. Lava domes are commonly associated with explosions and dome collapse events, both of which generate volcanic ash. In order to recognise and discriminate fragmentation mechanisms from ash samples, the physical properties and mineralogy of natural ash produced during a typical Vulcanian explosion and a dome collapse event were compared. Measurements of the componentry of several thousand ash particles were conducted using QEMSCAN® Particle Mineralogical Analysis, a rapid automated SEM-EDS mapping technique. Analysis of images obtained by QEMSCAN® reveals that the relative distribution of plagioclase and glass present along the ash particle boundaries varied for both generation mechanisms. Deconvolution of particle size distributions and particle shape analyses shows that ash ejected in Vulcanian explosions has a more complex fragmentation and transport history, while ash produced in pyroclastic flows shows the dominance of a single process. These results suggest mechanism-dependent controls on the surface composition and componentry of volcanic ash – future work is required to discriminate fragmentation mechanisms from ash characteristics through the use of QEMSCAN® data. Explosive fragmentation at lava dome volcanoes is likely to be triggered by tensile failure of magma following stress accumulation. In order to investigate pressure-driven fracturing of conduit magma, Brazilian tensile tests were conducted on lava samples from Santiaguito volcano at ambient and magmatic temperatures. These tests reveal that deformation style becomes sensitive to small changes in temperature and strain rate at temperatures of 750-800 °C. Higher temperatures generated increasingly viscous deformation, while faster strain rates promoted more brittle behaviour. Experimental constraints on the strain rate and strain leading to failure can be compared to natural deformation timescales recorded in cycles of inflation preceding explosions at Santiaguito, which shows that a viscous component accompanies deformation and suggests that fractures propagate away from a pressure source prior to explosive eruption. Following fracture propagation, any remaining energy is likely to be accommodated by fault slip along the fracture plane. These faulting events are investigated using a high-velocity rotary shear apparatus, showing that the response to faulting generates temperatures sufficient to produce frictional melt within ~10 cm of slip under the slip rates and normal stresses constrained through monitoring of the volcanic behaviour at Santiaguito. The range of mechanical response to slip events in the volcanic conduit and their relation to concurrent seismic signals are investigated in greater detail during the extrusion of a lava spine at Mt Unzen (Japan). Examination of textures at the spine margins and similarity of the seismic signals that accompanied its extrusion has determined that spine emplacement proceeds by incremental fault slip events in the shallow edifice. Waveform analysis together with spine growth observations allow calculation of the average slip distance (8.9 cm) and slip velocity (0.75 m.s-1). These calculations are combined with results from laboratory measurements in a high-velocity rotary shear apparatus to define the range of depths where average faulting events would induce viscous remobilisation (at >275m) and frictional melting along the fault plane (at >500 m). The frictional properties of the dome rocks and the viscosity of the frictional melt in the fault zone suggest that at shallow depths frictional melts act as a viscous brake to fault slip, potentially augmenting stick-slip spine growth. Taken together, the failure, faulting and fragmentation of dome-forming magma demonstrates that our interpretation of eruptive activity at lava dome volcanoes requires a fundamental understanding of brittle processes