Quantification of Volcano Deformation caused by Volatile Accumulation and Release

Magma stored in the crust may exsolve a significant amount of volatiles, primarily CO₂, but also H₂O and SO₂ if cooling promotes crystallisation and volatile exsolution. These volatiles may, over time, segregate and accumulate into a gas-rich foam at the roof of the magma body. This is the underpinning process to explain the frequently observed ‘excess gas’ produced in explosive eruptions, where the amount of erupted SO₂ is much larger than can be explained by the mass of erupted products and the initial dissolved S content.

Here, we examine and quantify the buoyancy force exerted on the crust due to the presence of accumulated volatiles in the roof of a magma reservoir of exsolved volatiles. This foam has a significantly lower density than magma or the crust, and will therefore produce a buoyancy force which will manifest as deformation of the volcanic edifice above. A key concept in this work is that the accumulation of the foam layer may occur slowly over long time periods and therefore be challenging to detect. However, upon eruption, the gas phase will be suddenly lost, and the removal of the buoyant volatiles will result in syn-eruptive subsidence, in addition to that expected from the eruption of lavas.

We present three-dimensional, visco-elasto-plastic, thermomechanical modeling results which quantify the ground deformation arising from the growth and sudden release of a volatile reservoir. We find that the deformation is independent from the thermal structure of the crust and the shapes of the volatile and magma reservoirs. Instead, it is a function of the volume, density and depth of the volatile reservoir and crustal rigidity. This allows us to derive a scaling law for the volatiles’ contribution to syn-eruptive subsidence.

Applying our scaling law to the April 2015 eruption of the Chilean stratovolcano Calbuco, together with estimates of the pre-accumulated volatile mass, suggests that up to 25% of the observed syn-eruptive subsidence can be explained by the release of a buoyant reservoir of exsolved volatiles. Our results highlight the key role that volatile-driven buoyancy can have in volcano deformation and show a new link between syn-eruptive degassing and deflation. They also highlight that shallow gas accumulation and release may have a major impact on ground deformation of volcanoes and can serve as an explanation for inflation/deflation of up to a few cm.