Passive degassing during quiescence as trigger of volcanic unrest

Abstract:

Unravelling the mechanisms that trigger the ascent of magma to the surface is crucial for making progress towards improved eruption forecasts. This is especially important for persistent degassing volcanoes, like Etna (Italy) or Mayon (Philippines), since they are the most active sub-aerial volcanoes around the world. Their activity consists of quiescent periods, dominated by abundant passive gas emissions, that alternate with unrest episodes and eruptions occurring every few months or years. In this study, we wonder how passive degassing during quiescence can affect the eruptive dynamics of persistent degassing volcanoes. We have developed a new lumped parameter model that correlates the pressure of shallow magma reservoirs with the mean degassing rates measured with monitoring systems. The model accounts for the conduit-reservoir size, the viscoelastic properties of the crust, the exsolution and expansion of bubbles at depth, the magma density changes, and the connectivity between the shallow reservoir and deeper magma sources. Our theoretical analysis demonstrates that there are many realistic scenarios under which depressurizations between 1-10 MPa occur in only a few months or years, that is, within the inter-eruptive timescale of persistent degassing volcanoes. Our results are consistent with geophysical and geodetical observations at volcanoes like Llaima (Chile), Asama (Japan), Satsuma-Iwojima (Japan), and Masaya (Nicaragua), and suggest that degassing-induced depressurization could induce magma replenishment, sudden bubble expansion at depth, collapse of the crater floor, and fractures in the reservoir wall-rock. All these processes can, in turn, lead to new unrest episodes and eruption.