Fluidal pyroclasts reveal the intensity of peralkaline rhyolite pumice cone eruptions

Ben Clarke,Karen Fontijn,Joaquín A Cortés,William Hutchison,Eliza S Calder,Gezahegn Yirgu,Ian Butler,Firawalin Dessalegn,Mark Naylor

doi:10.1038/s41467-019-09947-8

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.

Highlights

Peralkaline rhyolites are medium to low viscosity, volatile-rich magmas typically associated with rift zones and extensional settings
We present a study of unusual, yet uniquely well preserved, fluidal-shaped pyroclasts found in peralkaline rhyolite pumice-cone deposits at Aluto volcano in the Main Ethiopian Rift (MER)
For the first time, these pyroclasts are investigated in detail using scanning electron microscopy (SEM) and X-ray computed microtomography (XCT) providing unique insights into their 3D structure

Summary

Introduction

Peralkaline rhyolites are medium to low viscosity, volatile-rich magmas typically associated with rift zones and extensional settings. 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 quench in-situ. We present field and clast observations, undertake thermal modelling and develop the first comprehensive conceptual model for the formation of these peralkaline fluidal-shaped clasts, referred to as pumiceous achneliths This new understanding of pumiceous achneliths forms the basis of a conceptual model of pumice-cone forming peralkaline rhyolite eruptions and provides some important constraints on this hitherto ambiguous style of activity. This allows for a better understanding of the likely hazards associated with future eruptions of peralkaline rhyolite volcanoes, potentially worldwide

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Conclusion