Pyroclastic density currents at Ruapehu volcano; New Zealand
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Pyroclastic density currents (PDCs) are hazardous mixtures of volcanic particles and gas that travel along the flanks of a volcano due to a higher bulk density than the surrounding atmosphere. Understanding the frequencies, magnitudes, and different PDC generation and transport processes is essential for understanding the PDC hazard. At Mount Ruapehu, a much-visited active volcano in the North Island of New Zealand, future PDCs represent a significant threat to life and infrastructure. However no extensive historical PDCs and very few prehistoric deposits have been studied at this volcano. Here, we develop a new confidence-based system for identifying and distinguishing small-volume PDC deposits from other proximal volcaniclastic deposits in the field, and use this to identify 12 young (<13.6 ka) PDC units exposed on Ruapehu’s eastern flanks. Field investigations of deposit morphologies and textures show that Ruapehu’s PDCs were generated by a variety of eruption styles. These ranged from (1) collapsing plinian eruption columns that emplaced massive pumice-rich PDC deposits (Units 1-5, ~13.6 - 11.6 ka), through to (2) welded scoriaceous deposits that resulted from periodic collapses of spatter/cinders that first accumulated on steep proximal slopes (Units 6, ~11.6 ka, and 7, unknown age). Additionally, (3) several small-volume deposits containing denser pyroclasts (Units 8-10, <11.6 ka, and 11-12, ~13.6-11.6 ka) are interpreted to result from smaller eruptions not dissimilar to Ruapehu’s historical activity. Detailed studies of (a) bulk and glass pyroclast chemistries, (b) pyroclast density distributions, (c) vesicle textures, and (d) rhyolite- MELTS modelled storage conditions provide further insight into the underlying magmatic processes that led to generation of these PDCs. These show that magma storage depths and temperatures, magma mingling between new and relict magmas, and open vs. closed systems strongly influenced the amounts of pre-eruptive degassing and bulk pyroclast densities. This in turn affected the buoyancy of the erupting mixture, and hence the tendency to generate PDCs. In most cases, heterogeneous storage and ascent pathways at Ruapehu appear to have favoured PDC generation, and this may be an important consid-eration when assessing the future PDC hazard. Furthermore, the deposit ages, textures, and distributions indicate that many of Ruapehu’s PDCs encountered glacial ice during transport, and this is interpreted to have affected the PDC dynamics and preservation of the deposits. By combining results from microphysical pyroclast-ice contact experiments with high-resolution mutiphase numerical simulations, we here model the large-scale effects of PDC transport over ice for the first time. Simulations based on interpreted prehistoric ice extents at Ruapehu suggest that transport over ~2km of ice strongly affects the PDC dynamics, increasing the runout distance of the hazardous high-particle concentration bedload and generating meltwater quantities equivalent to ~25% of the PDC bedload volume. This may then generate secondary debris flows which, following flow bulking, have volumes equivalent to at least 50% of the bedload volume of the primary PDC. These results have implications for assessing the PDC and associated hazards at Ruapehu and other glaciated volcanoes worldwide.