Abstract
Volcanic ash particle properties depend upon their genetic fragmentation processes. Here, we introduce QEMSCAN Particle Mineralogical Analysis (PMA) to quantify the phase distribution in ash samples collected during activity at Santiaguito, Guatemala and assess the fragmentation mechanisms. Volcanic ash from a vulcanian explosion and from a pyroclastic density current resulting from a dome collapse were selected. The ash particles resulting from both fragmentation modes are dense and blocky, typical of open-vent dome volcanoes and have a componentry consistent with their andesitic composition. We use image analysis to compare the fraction of each phase at particle boundaries compared to the total particle fraction. Our results show that the explosion-derived ash has an even distribution of plagioclase and glass, but boundaries enriched in pyroxene and amphibole. In contrast, the ash generated during dome collapse has an increased fraction of glass and decreased fraction of plagioclase at particle boundaries, suggesting that fractures preferentially propagate through glass during abrasion and milling in pyroclastic flows. This study presents QEMSCAN PMA as a new resource to identify generation mechanisms of volcanic ash, which is pertinent to volcanology, aviation, respiratory health and environmental hazards, and highlights the need for further experimental constraints on the fragmentation mechanism fingerprint.
Highlights
Volcanic ash, as the product of fragmentation, is a near-ubiquitous phenomena at active volcanoes[1], irrespective of the failure modes and mechanisms at play across a broad spectrum of eruptive activity
Volcanic ash produced during a vulcanian explosion and a dome-collapse event is analysed using SEM and QEMSCAN to provide a quantitative map showing the distribution of phases within the ash particles
We use image analysis to measure the relative fraction of constituent phases at ash particle boundaries and within the bulk for all particles within size bins from 2–8 Ф
Summary
As the product of fragmentation, is a near-ubiquitous phenomena at active volcanoes[1], irrespective of the failure modes and mechanisms at play across a broad spectrum of eruptive activity. Primary fragmentation mechanisms produce ash directly from magma within volcanoes: through gas overpressure fragmenting foamed magma[2], interaction between magma and water[3], or through strain-induced shear failure at conduit boundaries[4]. Secondary fragmentation processes occur via particle interactions in pyroclast-laden flows[5] and during slip in fault gouge[6], and can modify existing particles and create new ash-sized particles[7,8]. Secondary fragmentation processes include particles produced by milling[9], abrasion[5,10,11] and attrition[6,11]. The size, composition, dispersal and sedimentation of ash contribute to the degree and nature of hazards to respiratory health, the natural environment, infrastructure and aviation[15,16], and of benefits to the biosphere[17,18,19,20]
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