Influence of ice load variations on shallow magma storage zones: Application to Katla volcano, Iceland

As a consequence of climate warming, many volcanoes are currently located under retreating ice caps. The resulting unloading can modify the eruptive activity as proven by observed correlation between deglaciation periods and eruptive activity in the past. Unloading can modify melt generation in the mantle, or modify magma storage conditions at shallow depth. Here we investigate how ice load variations at the Earth's surface act on shallow magma chambers. Numerical calculations are carried out in axisymmetric geometry for an elliptical chamber embedded in an elastic medium, taking magma compressibility into account. For variable chamber shape, size and depth, we quantify how unloading events induce magmatic pressure change as well as variation of the threshold pressure required for dyke initiation at the chamber wall. Influence on eruption likelihood is determined by the interplay between these two parameters. We evaluate the triggering effect of these surface events on onset of eruptions and find it depends strongly on the surface load location and magnitude, and the shape, depth and size of the magma chamber. We apply this model to Katla volcano, Iceland, which is covered by the Mýrdalsjökull ice cap. Ice load variations include long term thinning, as well as an annual load cycle, with up to 6 meters change in snow thickness from winter to summer. As the seasonal snow load is reduced, a pressure decrease of the same order of magnitude as the load is induced within the magma storage zone. The threshold pressure for failure is modified at the same time. Our model predicts that, in case of a spherical or horizontally elongated magma chamber, eruptions are more likely when the seasonal snow cover is smallest. This triggering effect is small, around few kPa, but appears consistent with the fact that all the nine last major historical eruptions at Katla occurred during the summer period. A long-term ice thinning due to global warming is also occurring, mainly at the periphery of the Myrdalsjökull ice cap. Our elastic model predicts that the short term effect of the Myrdalsjökull ice cap retreat, is to reduce the likelihood of eruptions at Katla. However, full consideration of long-term ice thinning influence on magmatic sytems requires a consideration of the viscosity of the upper mantle and lower crust.