Deformation and eruption forecasting at volcanoes under retreating ice caps: Discriminating signs of magma inflow and ice unloading at Grimsvotn and Katla volcanoes, Iceland

Warmer climate is causing retreat of many of the world's ice caps that cover volcanoes. Such ice unloading may influence magma systems and lead to elastic/inelastic earth response such as glacio-isostatic uplift. In Iceland, a number of the most active volcanoes are under retreating ice caps, including the Grimsvotn and Katla volcanoes. Both volcanoes have calderas and shallow magma chambers, and are currently undergoing periods of unrest. At both volcanoes, repeated GPS measurements on nunataks (mountains sticking out of the ice) and around the ice caps, show uplift of about 1-3 cm/year, as well as horizontal displacements. At Katla, optical leveling tilt measurements, InSAR data, and continuous GPS measurements constrain the deformation field outside the ice-covered part of the volcano as well. For both volcanoes, the current uplift rates may eventually conform either to magma inflow or glacio-isostatic rebound. The origin of the deformation can be resolved by considering horizontal displacements and the ratio between horizontal and vertical displacements. Under ice caps and near their edges, the Earth response to ice unloading is mostly vertical, with horizontal displacements an order of magnitude smaller than vertical (ratio < 0.3). This holds true both for immediate elastic response to ice unloading, and even more for the final relaxed state approximated as the response of an elastic plate (brittle part of the crust) underlain by fluid (relaxed ductile part of the crust or a magmatic system). For a magmatic source approximated as a point source of pressure, the ratio between horizontal and vertical displacements is >0.5 at distances >0.5D, where D is the depth to the source. At Grimsvotn and Katla, the observed ratio is close to 1, suggesting the deformation is mainly caused by magma movements. Horizontal displacements at rates of 1-2 cm/year occur at the caldera boundaries, away from the magmatic sources. In addition to inflow of magma to the volcanoes, both of them have elevated seismicity and geothermal activity. Pressure in a shallow magma reservoir at Grimsvotn is likely to have exceeded pre-eruptive limit for its last eruption in 1998. Katla volcano is also considered to be close to failure, and an eruption of at least one of these volcanoes is anticipated within a few years, as long as magma continues to flow into their shallow magma chambers.