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

Abstract

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 CO 2 flux, and a clear magmatic carbon signature (CO 2 -d 13 C 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.

The Electrical Structure of the Central Main Ethiopian Rift as Imaged by Magnetotellurics: Implications for Magma Storage and Pathways

“…Nevertheless, the derived impedance transfer functions were very similar. No systematic differences were observed that could be associated with subsurface changes in the electrical structure in connection with the deformation events detected by both GPS and InSAR (Hutchison, Biggs, et al, ).…”

“…Pulse‐like surface deformation events on a number of volcanoes located on the central rift axis have been reported through the analysis of satellite imagery (interferometric synthetic aperture radar, InSAR; Biggs et al, ) and give rise to concern about potential geohazards associated with volcanic activity. The silicic peralkaline volcano Aluto has been the target of a number of geological, geochemical, and geophysical studies undertaken in recent years (Fontijn et al, ; Gleeson et al, ; Hutchison et al, ; Hutchison, Biggs, et al, ; Hutchison, Mather, et al, ; Saibi et al, ; Teklemariam et al, ; Wilks, Kendall, et al, ). It is the site for a hydrothermal power plant with three test drillings awaiting integration into the local power grid.…”

“…Periods of deformation of Aluto have been reported from InSAR and GPS signal analysis over the past 13 years with rapid uplift events with displacement of about 10–15 cm in 2004 and subsequent slower subsidence followed by another 10 cm uplift at the end of 2008 (Biggs et al, ). This deformation was modeled and attributed partly to a shallow source, representing processes in the hydrothermal system at 1‐ to 2 km depth and partly to a deeper source (5 km), which has been interpreted as the location of stored magma (Hutchison, Biggs, et al, ). The analysis of increased CO 2 degassing along the caldera faults (Hutchison et al, ) also indicates a deeper source of the magmatic processes.…”

The Geomorphology, Structure, and Lava Flow Dynamics of Peralkaline Rift Volcanoes From High‐Resolution Digital Elevation Models

“…Volcanic activity in the MER has formed both silicic peralkaline “central” volcanoes and distributed mafic cones and lavas (e.g., B. Abebe et al, ; Gibson, ). The poorly known volcanoes of the MER have become the focus of much recent work (Aspinall et al, ; Fontijn et al, ; Hutchison et al, , Hutchison, Fusillo, et al, , Hutchison, Biggs, et al, , Hutchison, Pyle, et al, , Hutchison et al, ; Lloyd, Biggs, Wilks, et al, , Lloyd, Biggs, Birhanu, et al, ; Martin‐Jones et al, ; McNamara et al, ; Rapprich et al, ; Siegburg et al, ; Tadesse et al, ; Vye‐Brown et al, ).…”

“…The silicic MER volcanoes are in a post‐caldera stage of evolution (Hutchison, Fusillo, et al, ). Four volcanoes have shown recent geodetic unrest (Aluto, Corbetti, Bora and Haledebi), with some deformation associated with ongoing geothermal activity (Biggs et al, ; Hutchison et al, , Hutchison, Biggs, et al, ; Lloyd, Biggs, Wilks, et al, , Lloyd, Biggs, Birhanu, et al, ). Corbetti and Aluto are considered the most active volcanic centers in the rift, with deposits from explosive eruptions implying an eruptive flux of 0.01–0.1 km 3 (magma)/kyr over the past 12 kyr (Fontijn et al, ).…”