Magma fracturing and degassing associated with obsidian formation: The explosive–effusive transition

Journal of Volcanology and Geothermal Research
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Abstract

This paper explores the role of melt fracturing in degassing rhyolitic volcanic systems. The Monte Pilato-Rocche Rosse eruptions in Italy evolved from explosive to effusive in style, and H2O content in quenched glasses changed over time from relatively H2O-rich (~ 0.90 wt.%) to H2O-poor dense obsidian (~ 0.10–0.20 wt.%). In addition, healed fractures have been recorded in all different eruptive materials, from the glass of early-erupted tube pumice and rinds of breadcrusted obsidian pyroclasts, to the glass of late-erupted dense obsidian pyroclasts, and throughout the final effusive Rocche Rosse lava flow. These rocks show multiple fault sets, some with crenulated fault planes indicating resumption of viscous flow after faulting, complex obsidian breccias with evidence for post-brecciation folding and stretching, and centimetre- to metre-thick tuffisite preserved in pyroclasts and lava, representing collapsed foam due to fracturing of vesicle walls. These microstructural observations indicate that multiple fracturing and healing events occurred during both explosive and effusive eruptions. H2O content in glass decreases by as much as 0.14 wt.% towards healed fractures/faults and decreases in stretched obsidian breccias towards regions of intense brecciation. A drop in pressure and/or increase in temperature along fractures caused diffusive H2O migration through melt towards fracture surfaces. Repetitive and pervasive fracturing and healing thereby create conditions for diffusive H2O loss into fractures and subsequent escape through permeable paths. This type of progressive magma degassing provides a potential mechanism to explain the formation of dense obsidian and the evolution from explosive to effusive eruption style.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Magma fracturing and degassing associated with obsidian formation: The explosive–effusive transition
Series title Journal of Volcanology and Geothermal Research
DOI 10.1016/j.jvolgeores.2014.12.014
Volume 298
Year Published 2015
Language English
Publisher Elsevier
Publisher location Amsterdam
Contributing office(s) Volcano Science Center
First page 71
Last page 84
Country Italy
Other Geospatial Monte Pilato-Rocche Rosse
Online Only (Y/N) N
Additional Online Files (Y/N) N