Abstract
The understanding of magma ascent dynamics is essential in forecasting the scale, style and timing of volcanic eruptions. The monitoring of near-field deformation is widely used to gain insight into these dynamics, and has been linked to stress changes in the upper conduit. The ascent of magma through the conduit exerts shear stress on the conduit wall, pulling up the surrounding edifice, whilst overpressure in the upper conduit pushes the surrounding edifice outwards. H\sout{owever, h}ow much shear stress and pressure is produced during magma ascent, and the relative contribution of each to the deformation, \sout{is yet to be fully understood and quantified}\textcolor{blue}{has until now only been explored conceptually}. By combining flow and deformation modelling using COMSOL Multiphysics, we \sout{are} for the first time \sout{able to}\textcolor{blue}{present a quantitative model that links magma ascent to deformation. We} quantify how both shear stress and pressure vary spatially within a conduit, and show that shear stress generally dominates observed changes in tilt close to the conduit during activity at Tungurahua volcano, Ecuador, between 2013 and 2014. However, the relative contribution of pressure is not insignificant, and \sout{the full stress tensor comprising} both pressure and shear stress must be considered when interpreting deformation data. We demonstrate that significant changes in tilt \sout{can occur as magma refills an empty conduit, or} can be driven by changes in the driving pressure gradient or volatile content of the magma. The relative contribution of shear stress and pressure to the tilt varies considerably depending on these parameters. Our work provides insight into the range of elastic moduli that should be considered when modelling edifice-scale rock masses, and we show that even where the edifice is modelled as weak, shear stress \textcolor{blue}{generally} dominates the near field deformation over pressurisation of the conduit.
Highlights
Being able to understand what drives temporal variations in seismicity and deformation at volcanoes is essential in interpreting how volcanic systems evolve through time
The melt viscosity increases gradually as magma ascends and volatiles exsolve. Moving on from these studies, we investigate whether observed deformation can be explained using realistic depthdependent pressure and shear stress profiles, obtained through flow modeling
We adopt the concept of a Bingham rheology by scaling up the bulk viscosity derived in Equation (16) by a factor of 10.000, in order to yield suitable values for the ascent velocity that are low enough for magma to ascend from chamber depth (7.5–9.5 km, Samaniego et al, 2011) to the surface without fragmentation over the 3 months between each Vulcanian explosion
Summary
Being able to understand what drives temporal variations in seismicity and deformation at volcanoes is essential in interpreting how volcanic systems evolve through time. It is possible to simulate realistic magma ascent, where the governing parameters are based on results of several disciplines within volcanology This allows us to quantify how both pressure and shear stress vary within a volcanic conduit. Shear stress on the order of MPa is only modeled in the uppermost section of the conduit if at all This poses the question; can shear stress sufficient to explain observed deformation realistically be achieved during magma ascent?. The melt viscosity increases gradually as magma ascends and volatiles exsolve Moving on from these studies, we investigate whether observed deformation can be explained using realistic depthdependent pressure and shear stress profiles, obtained through flow modeling. Whilst we use observed data at Tungurahua to constrain the model, our findings provide important insight into the source of near-field deformation at silicic volcanoes in general
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