Abstract
The explosive eruption from Tonga's Hunga Volcano on 15 January 2022 sent air-pressure and tsunami waves around the globe. Particles created by this powerful eruption were ejected into both, the atmosphere and ocean. We have analysed the morphology and fracture patterns on grain-bounding surfaces of 1 phi and 4 phi ash particles from bulk samples representing material deposited on Tonga over the course of the approximately 11-hour eruption, collected approximately 1 week after the event.
Samples were sieved into half-phi fractions, from which 1 phi particles and 4 phi particles were selected and mounted on carbon tape for image collection using an SEM Backscattered Electron Detector (BSD). High resolution, high magnification images were collected for analysing particle fracture patterns, while higher contrast images were processed through the PARTIcle Shape ANalyzer (PARTISAN) MatLab tool. Particle populations were statistically analysed using DendroScan, another MatLab tool, to assess morphological similarities and dissimilarities with particles created in other eruptions (Havre 2012, Ubehebe, Rotomahana 1886, and experimentally created Havre particles). The shapes of Hunga ash grains most closely match those of Havre submarine rhyolite ash.
A striking feature of the exteriors of Hunga particles is the great abundance of diagnostic fracture surfaces. 1 phi particles exhibited many such surfaces, with >95% of particles analysed presenting shock wave traces (3D stepped surfaces, conchoidal fractures and 2D hackle lines). Most (>80%) 4 phi particles also exhibited these diagnostic fracture patterns. While similarity of morphology between Hunga and Havre particles indicates that a thermohydraulic mechanism drove explosions in both of these eruptions, the energy released - producing diagnostic fracture patterns across nearly all grains - was much higher at Hunga. These particles record the fragmentation process at work during one of the most powerful explosive eruptions ever recorded.