Fragmentation in crystal-rich basaltic systems: Ash generation at Volcán de Fuego, Guatemala
Abstract
Volcanic ash, particularly fine ash (<63 µm), poses a considerable hazard to health, aviation and local and regional infrastructure. The final grain size distribution of a tephra deposit reflects the combined influence of primary volatile-driven fragmentation and secondary abrasive processes. The low viscosities of basaltic magmas generally inhibit fine fragmentation, in the absence of external water. However, paroxysmal fountaining events in hydrous, comparatively crystal-rich, arc systems are often accompanied by abundant fine-grained ash. It is critical to further understand the influence of crystals on the fragmentation mechanism.
Volcan de Fuego, Guatemala, exhibits frequent ash-producing explosive activity. Several Strombolian explosions occur every hour and larger eruptions with widespread ash fall (>10 km) and sustained fountains typically occur every 3-5 weeks. A major eruption on 3 June 2018 resulted in the evacuation of 12,800 people, as pyroclastic flows inundated the surrounding villages. Prevailing westerly winds transported ash as far as Guatemala City (40 km NE of the volcano), forcing closure of the international airport. Here, we analyse a timeseries of ash samples collected during large eruptions between 2011 and 2018, leading up to the climactic event of 3 June 2018. Many of the samples derive from a network of ash collectors installed in February 2016 as part of a citizen science initiative. We analyse separately the componentry of ash particles over a range of size fractions, using both external morphology and internal texture to classify components. Together these data explore the influence of bubbles and crystals on the GSD and, by inference, the fragmentation process. Ash produced during the 3 June 2018 eruption was dominated by juvenile material (51%) and free crystals/fragments (40%), with a minor lithic fraction (<10%). Subdivision of the juvenile population shows that the proportion of dense fragments increases with decreasing grain size, from 14% (1φ; 500-1000 µm) to 30% (3φ; 125-250 µm), such that fine ash is almost entirely dense. Free crystals with little adhering glass are abundant; the relative proportion of intact crystals to crystal fragments reduces abruptly from 85% (1φ) to 14% (2φ, 3φ), suggesting that modal particles sizes may be related to the phenocryst population.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V44A..05L
- Keywords:
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- 8404 Volcanoclastic deposits;
- VOLCANOLOGYDE: 8428 Explosive volcanism;
- VOLCANOLOGYDE: 8445 Experimental volcanism;
- VOLCANOLOGYDE: 8486 Field relationships;
- VOLCANOLOGY