Thursday 25 November, 4 PM CET
DR. PAUL WALLACE
ABSTRACT:
Dome-building eruptions are commonly known for their ability to switch from effusive to explosive eruption style with little precursory warning. Such behaviour can be attributed to a complex interplay between deep (e.g., magma injection, deep convection, mush instabilities and volatile fluctuations) and shallow (e.g., volatile exsolution, degassing, crystallisation) magmatic processes, altering the magmas physico-chemical state, rheology, ascent style and ultimately its propensity to erupt. Yet, the importance of shear during magma ascent remains elusive. Here, I will present the evidence, extent and role of magma shearing and strain localisation at active lava domes to demonstrate its impact on magma ascent dynamics. Strain localisation was investigated through a petrological survey of a marginal shear zone of the 1994–1995 lava spine at Unzen volcano, Japan [1]. Evidence suggests crystals can behave as strain gauges during magma ascent through the viscous–brittle transition, via rearrangement, crystal plasticity and comminution. Owing to a thermo-mechanical response, shear can trigger disequilibrium conditions leading to mineral reactions, alteration of rock magnetic properties and compaction of the porous network, thus influencing outgassing efficiency of the system. Strain localisation can also trigger seismogenic magma failure followed by faulting and slip along fracture planes. In particular, frictional sliding near the conduit margins can cause localised melting, imposing important rheological controls on slip dynamics. I will experimentally demonstrate the importance of host-rock mineralogy on slip progression [2], notably that hydrous phases dramatically impact melt homogenisation and slip dynamics. Such shearing processes have been linked to regular small-to-moderate, gas-and-ash explosions at Santiaguito dome complex, Guatemala. However, in 2015–2016, activity shifted to larger, less frequent ash-rich explosions [3]. I will finish by reporting petrological and geochemical signatures of tephra deposits, together with geophysical monitoring, that indicate a switch from shallow shear-driven fragmentation to a deeper fragmentation mechanism.
References:
[1] Wallace et al. (2019) J. Petrol, 60(4):791-826, doi: https://doi.org/10.1093/petrology/egz016
[2] Wallace et al. (2019) Geochim. Cosmochim. Acta, 255:265-288, doi: https://doi.org/10.1016/j.gca.2019.04.010
[3] Wallace et al. (2020) Earth Planet. Sci. Lett., 536:116139, doi: https://doi.org/10.1016/j.epsl.2020.116139