Fluidal pyroclasts reveal the intensity of peralkaline rhyolite pumice cone eruptions

Ben Clarke (), Eliza S. Calder, Firawalin Dessalegn, Karen Fontijn, Joaquín A. Cortés, Mark Naylor, Ian Butler, William Hutchison and Gezahegn Yirgu
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Ben Clarke: University of Edinburgh, Grant Institute
Eliza S. Calder: University of Edinburgh, Grant Institute
Firawalin Dessalegn: CNCS, Wollega University
Karen Fontijn: Université Libre de Bruxelles
Joaquín A. Cortés: University of Edinburgh, Grant Institute
Mark Naylor: University of Edinburgh, Grant Institute
Ian Butler: University of Edinburgh, Grant Institute
William Hutchison: University of St. Andrews, Irvine Building
Gezahegn Yirgu: Addis Ababa University, King George VI Street

Nature Communications, 2019, vol. 10, issue 1, 1-10

Abstract: Abstract Peralkaline rhyolites are medium to low viscosity, volatile-rich magmas typically associated with rift zones and extensional settings. The dynamics of peralkaline rhyolite eruptions remain elusive with no direct observations recorded, significantly hindering the assessment of hazard and risk. Here we describe uniquely-preserved, fluidal-shaped pyroclasts found within pumice cone deposits at Aluto, a peralkaline rhyolite caldera in the Main Ethiopian Rift. We use a combination of field-observations, geochemistry, X-ray computed microtomography (XCT) and thermal-modelling to investigate how these pyroclasts are formed. We find that they deform during flight and, depending on size, quench prior to deposition or continue to inflate then quench in-situ. These findings reveal important characteristics of the eruptions that gave rise to them: that despite the relatively low viscosity of these magmas, and similarities to basaltic scoria-cone deposits, moderate to intense, unstable, eruption columns are developed; meaning that such eruptions can generate extensive tephra-fall and pyroclastic density currents.

Date: 2019
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DOI: 10.1038/s41467-019-09947-8

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