Search

The Tongariro Te Maari crater, New Zealand, erupted for the first time in over 80 years on August 6, 2012. Because it occurred at night, the short-lived eruption prevented direct visual observations, hence presenting some challenges for understanding the complex eruptive sequence and reach of the various explosions. Using 1 Hz Global Positioning System (GPS) data from the local volcano monitoring network, we computed the GPS LC phase residuals and SNR data at the time of the eruption and show that several processes successively affected the signal at the closest GPS station. Firstly, the signal from one of the satellites shows two successive drops of SNR caused by the presence of two distinct dense ash-laden plumes near the vent, produced successively by the second and last of the main eruption pulses. Secondly, we show that a gas-charged debris avalanche and a lateral blast successively entrained hot and/or humid air over 3 km away from the vent, atop the VGKR GPS station. While the limited amount of data prevents fully quantitative analyses and estimates of temperature or humidity in the lateral air flow, our results document the first recorded detection of a small-scale lateral plume by GPS technology. They highlight the potential of GPS data analysis for improving our understanding of eruptive sequences when visual observations are limited. [ABSTRACT FROM AUTHOR]

Abstract: Strikingly similar examples of edifice collapse and directed blast are the 18 May 1980 eruption of Mount St. Helens (MSH), Washington, USA, and the 30 March 1956 eruption of Bezymianny Volcano (BZ), Kamchatka, Russia. In these cases, flank failures led to near-instantaneous decompression and fragmentation of intra-edifice cryptodome magma, which produced catastrophic, laterally directed blasts. In both instances, the blast products consisted of juvenile material with bimodal density/vesicularity distributions: low- and high-density modes at 1900 and 2400kgm−3 for BZ, 1600 and 2300kgm−3 for MSH, although the proportion of high-density material is greater at BZ. Blast materials also exhibit striking variety in groundmass crystallinity (<40 to >90vol.%) despite having fairly uniform pheno-crystallinities, suggesting that degassing-driven groundmass crystallization occurred to varying extents within cryptodome magma at both volcanoes. New bulk-rock H2O and ∂D measurements confirm that progressive open-system outgassing occurred prior to both blasts. The correlations between crystallinity, clast density, and bulk H2O contents suggest that syn-blast magma expansion was modulated both by non-uniform volatile distribution within the cryptodome and rheological controls associated with non-uniform crystal content. Spatial heterogeneities in volatiles and crystallinity within a given cryptodome are attributed to distance from the wallrock margin, which probably correlates with timing of magma injection. The greater proportion of high-density material at BZ is speculatively related to lower blast energy compared with MSH. [ABSTRACT FROM AUTHOR]