Causes of unrest at silicic calderas in the East African Rift: New constraints from InSAR and soil-gas chemistry at Aluto volcano, Ethiopia

Restless silicic calderas present major geological hazards, and yet many also host significant
untapped geothermal resources. In East Africa, this poses a major challenge, although the calderas are
largely unmonitored their geothermal resources could provide substantial economic benefits to the region.
Understanding what causes unrest at these volcanoes is vital for weighing up the opportunities against the
potential risks. Here we bring together new field and remote sensing observations to evaluate causes of
ground deformation at Aluto, a restless silicic volcano located in the Main Ethiopian Rift (MER). Interferometric
Synthetic Aperture Radar (InSAR) data reveal the temporal and spatial characteristics of a ground deformation
episode that took place between 2008 and 2010. Deformation time series reveal pulses of
accelerating uplift that transition to gradual long-term subsidence, and analytical models support inflation
source depths of 5 km. Gases escaping along the major fault zone of Aluto show high CO2 flux, and a clear
magmatic carbon signature (CO2-d13C of 24.2&to 24.5&). This provides compelling evidence that
the magmatic and hydrothermal reservoirs of the complex are physically connected. We suggest that a
coupled magmatic-hydrothermal system can explain the uplift-subsidence signals. We hypothesize that
magmatic fluid injection and/or intrusion in the cap of the magmatic reservoir drives edifice-wide inflation
while subsequent deflation is related to magmatic degassing and depressurization of the hydrothermal system.
These new constraints on the plumbing of Aluto yield important insights into the behavior of rift volcanic
systems and will be crucial for interpreting future patterns of unrest