Past, present and future volcanic activity at restless calderas in the Main Ethiopian Rift

Thesis

There is growing recognition that magma plays a key role in the extension and eventual break-up of the continental lithosphere. One of the most intriguing aspects of magmatism during the transition from continental rifting to sea-floor spreading is that large silicic magmatic systems develop within the rift zone. However, the processes by which these silicic magmatic centres develop as well as their larger role in rift development remains poorly understood.

This thesis focuses on silicic volcanism in the Main Ethiopian Rift; the most extensive active continental volcanic field in the world, and home to over 10 million people. I use a variety of geological, geochemical, geophysical and geochronological techniques to explore spatial and temporal variability in volcanism in this active rift segment. In particular I focus on Aluto volcano, a restless caldera volcano just ∼ 200 km south of Addis Ababa, but by extension I make inferences about what this means for past, present and future volcanism at other caldera systems in East Africa.

Aluto has been active for over 300,000 years, and has overwhelming erupted chemically-evolved peralkaline rhyolites that are derived through protracted (>80%) fractional crystallisation. Tectonic faults dissect the volcanic complex, and along with an underlying caldera ring fault provide key pathways for fluid release. Episodic ground deformation events occur at Aluto, and are characteristic of a coupled magmatic-hydrothermal system, where faults networks connecting the surface and deep reservoirs play an important role in the deflation. Finally, new ⁴⁰Ar/³⁹Ar geochronology suggest that silicic volcanism in this segment of the rift is pulsed, not steady, on human evolutionary timescales.

Authors: Hutchison, W

• DOI: 10.5287/ora-veaj07ayy
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Volcanology