Eruptions of intermediate magma may be explosive or effusive. The development of open system degassing has been proposed as a pre-requisite for effusion of intermediate magma, however processes leading to open system degassing are poorly understood. To better understand degassing processes during lava dome extrusion we report high temporal-resolution SO2 emission rate measurements collected with an ultra violet imaging camera at Santiaguito, Guatemala. Santiaguito is an ideal case study as the dome lava is compositionally very similar to products of the 1902 Plinian eruption of the parental Santa María volcano. We find that degassing is weak (0.4–1 kg s− 1) but continuous, and explosions are associated with small increases in emission rates (up to 2–3 kg s− 1). Continuous repose degassing occurs through a shallow cap rock which likely represents a proto-crust on the block lava flow which is extruded from the same vent. The continual permeability of the upper conduit argues against a mechanism of explosion triggering in which gas pressure builds beneath a viscous cap rock or plug. Rather, we consider degassing data better consistent with a model of shear-fracturing at the conduit margins. Using field constraints, we model the viscosity of Santiaguito magma as a function of depth and show that conditions for shear-fracturing are met from 150–600 m to the surface. This is in line with independent estimates of explosion initiation depth. We show that repose timescales are orders of magnitude longer than the timescale for shear fracture, and suggest that explosions are triggered when a continuous network of smaller-scale fractures develops, at which point decompression occurs and an explosion is triggered. Fracture healing occurs by viscous relaxation however near to the surface where viscosity is highest, an unconsolidated gouge layer may develop. Our model implies that the observed explosions are a by-product of extrusion. Shear-fracturing can drive open system degassing of crystal rich intermediate magma at shallow levels in the conduit, as high magma viscosity is able to overcome the low strain rates associated with slow ascent of magma.
- UV camera
Holland, P., Watson, IM., Phillips, JC., Caricchi, L., & Dalton, M. P. (2011). Degassing processes during lava dome growth: insights from Santiaguito lava dome, Guatemala. Journal of Volcanology and Geothermal Research, 202(1-2), 153-166. https://doi.org/10.1016/j.jvolgeores.2011.02.004