William Hutchison

Structural controls on fluid pathways in an active rift system: A case study of the Aluto volcanic complex

In volcanically and seismically active rift systems, preexisting faults may control the rise and eruption of magma, and direct the flow of hydrothermal fluids and gas in the subsurface. Using high-resolution airborne imagery, field observations, and CO 2 degassing data on Aluto, a typical young silicic volcano in the Main Ethiopian Rift, we explore how preexisting tectonic and volcanic structures control fluid pathways and spatial patterns of volcanism, hydrothermal alteration and degassing. A new light detection and ranging (lidar) digital elevation model and evidence from deep geothermal wells show that the Aluto volcanic complex is dissected by rift-related extensional faults with throws of 50–100 m. Mapping of volcanic vent distributions reveals a structural control by either rift-aligned faults or an elliptical caldera ring fracture. Soil-gas CO 2 degassing surveys show elevated fluxes (>>100 g m –2 d –1 ) along major faults and volcanic structures, but significant variations in CO 2 flux along the fault zones reflect differences in near-surface permeability caused by changes in topography and surface lithology. The CO 2 emission from an active geothermal area adjacent to the major fault scarp of Aluto amounted to ∼60 t d –1 ; we estimate the total CO 2 emission from Aluto to be 250–500 t d –1 . Preexisting volcanic and tectonic structures have played a key role in the development of the Aluto volcanic complex and continue to facilitate the expulsion of gases and geothermal fluids. This case study emphasizes the importance of structural mapping on active rift volcanoes to understand the geothermal field as well as potential volcanic hazards.

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-δ13C of −4.2‰ to −4.5‰). 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.

Airborne thermal remote sensing of the Volcán de Colima (Mexico) lava dome from 2007 to 2010

Abstract We investigate high-resolution digital photographs and infrared images of the lava dome eruption at Volcán de Colima, from 2007 to 2010. Qualitative observations provide insight into active volcanic processes (e.g. rockfalls and fracturing) and show that, as the dome advances a substantial cooled talus apron develops, which stabilizes the structure. Progressive collapse of the talus apron as it reaches the crater rim corresponds with the development of a lava lobe, extruding hot lava from deeper within the dome. Quantitative dome surface temperature time-series show that the highest temperature hotspots migrate from the dome sides (250–380 °C) to the top (150–300 °C) and finally to the lava lobe (220–400 °C) as the structurally unstable areas expose fresh material. Net surface heat loss from the dome ranges from 5 to 30 MW, comparable to other dome forming systems. Heat budget calculations confirm that the lava dome retained a hot viscous core throughout the period 2007–2010. We propose that the mechanical stability of the Volcán de Colima dome arises from the shear strength of flanking talus which stabilizes the hot viscous core.

A pulse of mid-Pleistocene rift volcanism in Ethiopia at the dawn of modern humans

The Ethiopian Rift Valley hosts the longest record of human co-existence with volcanoes on Earth, however, current understanding of the magnitude and timing of large explosive eruptions in this region is poor. Detailed records of volcanism are essential for interpreting the palaeoenvironments occupied by our hominin ancestors; and also for evaluating the volcanic hazards posed to the 10 million people currently living within this active rift zone. Here we use new geochronological evidence to suggest that a 200 km-long segment of rift experienced a major pulse of explosive volcanic activity between 320 and 170 ka. During this period, at least four distinct volcanic centres underwent large-volume (>10 km3) caldera-forming eruptions, and eruptive fluxes were elevated five times above the average eruption rate for the past 700 ka. We propose that such pulses of episodic silicic volcanism would have drastically remodelled landscapes and ecosystems occupied by early hominin populations.