Syn-eruptive crystallisation in silicic magmas

: The case of Volcán de Colima, Mexico

Abstract

Crystallisation occurs as water-saturated magmas ascend during eruptions as a result of a degree of undercooling induced by decompression. Crystal batch textures (crystallinity, morphology, size and number density), zoning patterns and compositions record the kinetics associated with the dynamics of ascent. I evaluate syn-eruptive crystal textures of lavas and pyroclasts that encompass a broad range of eruptive intensities and styles produced at Volcán de Colima, Mexico. Volcán de Colima is an excellent case study given the monotonous bulk compositions of its recent lavas and nearly continuous records of eruptive activity in the last couple of centuries. A sub-Plinian eruption at Colima today, similar to those that occurred in 1818 and 1913, would pose a direct threat to nearly 300,000 people; therefore, developing robust evaluation tools is essential to improve our perceptions of risks associated with its activity. Of the textural parameters measured, crystal aspect ratio and crystal area fraction (crystallinity) are the best correlated with the average discharge rates of the studied suite of steady eruptions. There is a linear relationship between crystallinity and logarithmic discharge rate for products of steady eruptions of Colima that is very similar to trends for other intermediate volcanoes. I applied textural correlations established with well-documented eruptions to estimate eruptive intensities for poorly-observed eruptions of Volcán de Colima such as the 1818 sub-Plinian and the 2005 Vulcanian crises. Syn-eruptive textures in products from the 2005 and 2013 Vulcanian sequences allowed me to study the dynamics of both magma ascent and downwards-fragmentation processes. Finally, I extended the textural analyses to phenocryst-size crystals and found that larger crystals experience significant rim overgrowth during syn-eruptive magma ascent, particularly during low-intensity eruptions. This thesis demonstrates the applicability and efficacy of integrated analysis of late-stage crystal textures as petrological tools to infer and reconstruct syn-eruptive conditions for hazard assessment of volcanic processes.

Date of Award	23 Jan 2019
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Alison C Rust (Supervisor) & Katharine Cashman (Supervisor)