The crystalline silica respiratory hazard from rhyolitic lava dome eruptions in New Zealand's Taupo Volcanic Zone: A case study from the 1315 CE Kaharoa eruption

Abstract

The rhyolitic Kaharoa 1315 CE eruption was a complex, long-lived event from Tarawera volcano, New Zealand. Explosive phases were followed by around 5 years of lava dome extrusion and collapse which produced block-and-ash flows (BAF). Lava domes generate crystalline silica in the form of cristobalite, and rhyolitic magmas often contain quartz phenocrysts. Fine-grained ash containing crystalline silica can be formed through dome collapses or explosions, generating a respiratory health hazard for communities affected by ashfall. The aims of this study are to: i) determine whether the Kaharoa eruption dome-forming phase generated substantial quantities of crystalline silica and, therefore, to determine the potential for future dome-forming eruptions of Tarawera to do the same; ii) consider the potential hazard of the crystalline silica by studying the crystal habit and chemistry compared to other lava domes, globally; and iii) assess the particle size and crystalline silica content of the Kaharoa ash, to inform a respiratory hazard assessment.

Five co-BAF ash samples and one co-ignimbrite (explosive) ash sample from the Kaharoa pyroclastic deposits were analysed for health-pertinent factors: particle size distribution and crystalline silica content. Eight dome-rock samples were collected from the dome complex and associated BAF deposits and groundmass texture (especially forms of crystalline silica) and quantity of crystalline silica were assessed.

Cristobalite was present in the 4 ash samples analysed by X-ray diffraction (XRD; 1.3–3.7 wt%) as was quartz (5.7–12.5 wt%). For the 4 dome samples analysed by XRD, all samples contained quartz (4.1–10.4 wt%) and two contained significant quantities of cristobalite (24.7 and 27.3 wt%). Of the two dome samples with minimal cristobalite (visible as individual vapour-phase crystals by SEM but not quantifiable by XRD), one was from the non-devitrified dome carapace and the other was from the compacted interior but had not undergone devitrification. The two dome samples with substantial cristobalite were from dome interiors and were highly devitrified, with well-developed spherulitic textures. Using energy-dispersive X-ray spectroscopy, cristobalite in all samples contained minor aluminium, as has been seen for volcanic cristobalite from other lava domes, which may ameliorate its toxicity. By laser diffraction, the quantities of ash in the health pertinent size fractions varied, with a range of 1.3–8.1 vol% for particles of <4 μm diameter and 1.7–15.6 vol% for particles of <10 μm diameter, which is lower than measured in ash from large-scale dome collapse events at other volcanoes.

The findings suggest a potential for substantial crystalline silica to be formed in future Kaharoa-style eruptions, but that cristobalite generation is site-specific, depending on location within the dome and whether the dome remains sufficiently hot for spherulite formation and glass devitrification. Respiratory hazard will therefore vary depending on the collapse of (or explosions through) individual lobes – although all lava is expected to contain quartz phenocrysts – as well as the size and energy of those collapses, which will influence particle size and quantity of ash generated and dispersed.