Experimental constraints on phreatic eruption processes at Whakaari (White Island volcano)

Type of content
Journal Article
Thesis discipline
Degree name
Publisher
University of Canterbury. Geological Sciences
Journal Title
Journal ISSN
Volume Title
Language
Date
2015
Authors
Mayer, K.
Scheu, B.
Gilg, H.A.
Heap, M.J.
Kennedy, B.M.
Lavallée, Y.
Letham-Brake, M.
Dingwell, D.B.
Abstract

Vigorous hydrothermal activity interspersed by sequences of phreatic and phreatomagmatic eruptions occur at Whakaari (White Island volcano), New Zealand. Here, we investigate the influence of sample type (hydrothermally altered cemented ash tuffs and unconsolidated ash/lapilli) and fragmentation mechanism (steam flashing versus gas expansion) on fragmentation and ejection velocities as well as on particle-size and shape. Our rapid decompression experiments show that fragmentation and ejection speeds of two ash tuffs, cemented by alunite and amorphous opal, increase with increasing porosity and that both are significantly enhanced in the presence of steam flashing. Ejection speeds of unconsolidated samples are higher than ejection speeds of cemented tuffs, as less energy is consumed by fragmentation. Fragmentation dominated by steam flashing results in increased fragmentation energy and a higher proportion of fine particles. Particle shape analyses before and after fragmentation reveal that both steam flashing and pure gas expansion produce platy or bladed particles from fracturing parallel to the decompression front. Neither fragmentation mechanisms nor sample type show a significant influence on the shape. Our results emphasize that, under identical pressure and temperature conditions, eruptions accompanied by the process of liquid water flashing to steam are significantly more violent than those driven simply by gas expansion. Therefore, phase changes during decompression and cementation are both important considerations for hazard assessment and modeling of eruptions in hydrothermally active environments.

Description
Citation
Mayer, K., Scheu, B., Gilg, H.A., Heap, M.J., Kennedy, B.M., Lavallée, Y., Letham-Brake, M., Dingwell, D.B. (2015) Experimental constraints on phreatic eruption processes at Whakaari (White Island volcano). Journal of Volcanology and Geothermal Research, 302, pp. 150-162.
Keywords
Fragmentation, Hydrothermal alteration, Alunite, Rock strength, Steam flashing, Volcanic ash
Ngā upoko tukutuku/Māori subject headings
ANZSRC fields of research
Fields of Research::37 - Earth sciences::3705 - Geology::370512 - Volcanology
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