Date

5-1-2019

Document Type

Thesis

Degree

Master of Science

Department

Earth and Environmental Sciences

First Adviser

Steve Peters

Abstract

Explosive, ash-producing volcanic eruptions represent a major natural hazard that can threaten life, economy, and infrastructure. Turrialba is an active stratovolcano located in the Central Cordillera of Costa Rica. The edifice is located only 35 km east-northeast of Costa Rica’s capital city San Jose and poses a threat to its central valley, the social and economic hub where more than half of the population resides. This study focuses on the eruptive event taking place between May 2016- May 2017; consisting of four eruptive phases. The multi-phase sequence of eruptions is characteristic of this particular volcano. Observations, through the use of SEM, reveal significant morphological differences between the phases, reflected by the presence of large quantities of dust and gypsum crystals in the early phases. The morphology of volcanic ash is fundamental to our understanding of magma fragmentation, and in transport modeling of volcanic plumes and ash clouds. Epoxy mounts were utilized to better observe the chemistry of the interior of the ash particles, by removing the dusty exterior. SEM-EDS was used to determine the chemical composition of the bulk ash samples, the crystals, and the dust. ICP-MS resulted in a bulk chemical and trace element analysis for the four eruptive phases. The eruptive phases displayed chemical heterogeneity, correlating with the differences observed morphologically. The correlation of ash morphology and chemistry suggests that the vesicularity and rheology of fragmenting magmatic foam controls both eruption energetics and ash morphology in a moderately explosive ash-producing eruption. The differences in the explosive nature and ash chemistry between the beginning phases and final phases indicate that ash morphological differences were driven by heterogeneity of bubble nucleation site density in the ascending magma, which was chemically evolving over the duration of the eruption. These initial results, linking ash morphology to ash chemistry, are a step toward better understanding eruption mechanisms and how and why these mechanisms are exhibited morphologically on the ash particles.

Available for download on Friday, February 26, 2021

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