Structural controls on volcanism and degassing in the Main Ethiopian Rift

Thesis

The interactions between magmatism and crustal structure in continental rifts are manifold, and affect both rift dynamics and the populations that live within them. This thesis explores these interactions, focussing on structural controls on volcanism and degassing in the Main Ethiopian Rift, an archetypal continental rift. The rift hosts up to 10 large silicic volcanoes that have been active in the Quaternary, along with extensive basaltic volcanic fields and widespread geothermal activity. Using a range of geological, geochemical and remote sensing techniques, I examine past volcanism and present degassing with conclusions relevant for the local, regional and global scale.

I use new diffuse CO₂ surveys to show the variability of fault-controlled degassing in the rift, and combine the results with a new compilation of geothermal sites to extrapolate for a rift-scale estimate of emissions. Whilst rift-related diffuse CO2 flux is clearly important for the global carbon budget, the new estimate for degassing in East Africa is lower than previously suggested.

High-resolution digital elevation models produced for 3 silicic volcanoes in the Main Ethiopian Rift and 1 volcano in Afar show vent and crater alignments along both rift-parallel and cross-rift structures. I have mapped previously unidentified deposits, and compared the relative frequency, type, and magnitude of eruptions at each volcano. Morphological constraints suggest emplacement viscosities for peralkaline lavas of ~10⁸–10¹⁰ Pa s, similar to or lower than calc-alkaline rhyolites.

I compare the morphology of basaltic volcanic fields using digital elevation models, and calculate average volumes. Isolated cones adjacent to faults in East Ziway suggests dike capture, and morphometric analysis (e.g., height to width ratios and rim curvature) suggests an older age than cones in the Butajira field on the rift flank. Superimposed cones and maars at Butajira were likely emplaced by fissure eruptions and phreatomagmatic activity fed by deflected dikes.

Authors: Hunt, J

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Volcanology; Remote sensing