Reconstructing the largest explosive eruptions of Mt. Ruapehu, New Zealand: lithostratigraphic tools to understand subplinian–plinian eruptions at andesitic volcanoes

Abstract

We analysed the tephra record of Mt. Ruapehu for the period 27,097±957 to~10,000 cal. years BP to determine the largest-scale explosive eruptions expected from the most active New Zealand andesitic volcano. From the lithostratigraphic analysis, a systematic change in the explosive behaviour is identified from older deposits suggesting dry magmatic eruptions and steady eruptive columns, characterised by frothy to expanded pumice fabrics, to younger deposits that are products of unsteady conditions and collapsing columns, characterised by microvesicular, fibrous, and colour-banded pumice fabrics. The end-members were separated by eruptions with steady columns linked to water-magma interaction and highly unstable conduit walls. Dry magmatic eruptions producing steady plinian columns were most common between 27,097±957 and shortly after 13,635+ 165 cal. years BP. Following this time, activity continued with eruptions that produced dominantly oscillating unsteady columns, which engendered pyroclastic density currents, until~10 ka when there was an abrupt transition at Mt. Ruapehu since which eruptions have been an order of magnitude lower in intensity and volume. These data demonstrate long-period transitions in eruption behaviour at an andesitic stratovolcano, which is critical to understand if realistic time-variable hazard forecasts are to be developed.

Andesitic Plinian eruptions at Mt. Ruapehu: quantifying the uppermost limits of eruptive parameters

“…Ruapehu took place during the Late Pleistocene (Topping 1973), and the tephras produced during one of the most active periods are grouped into the~27 to~10 ka Bullot Formation (Donoghue 1991;Donoghue et al 1995;Neall et al 1995;Cronin et al 1996a, b;. New stratigraphic data presented by Pardo et al (2011) reveal at least 33 distinct eruptive units within this formation, which they grouped into products of six eruptive periods that reflect important variations in eruptive behaviour, particularly in column stability.…”

“…Ruapehu's eruptive behaviour has systematically changed since~27 ka from older explosive eruptions characterised by steady eruptive columns, to younger events characterised by unsteady, partially collapsing columns (Pardo et al 2011). These stages were separated by an interval between 17,625+425 cal years BP and shortly after 13,635+165 cal years BP, when eruptive columns were steady, but powerful and involved large proportions of accessory and accidental lithic fragments.…”

“…SH: state highways connect the urban centres (red squares). Study sites indicated by blue circles; main reference type sections labelled as B deposits with contrasting lithofacies associations, which Pardo et al (2011) classified into three main types: Type 1 association comprises a few (<3) overlapping fall deposits characterised by massive to normally graded, framework-supported, pumice lapilli beds and scarce porphyritic lithics. Usually, the initial eruptive products include a basal concentration of fresh andesitic lithics or angular to rounded dense pumice clasts.…”

“…vesiculation state, composition and volatile content of the erupting magma, fragmentation mechanisms) and environmental conditions (i.e. conduit/vent geometry, inflow of external water into the conduit), ultimately affecting the steadiness of the eruptive column and tephra dispersion (Pardo et al 2011). In this paper, we present new isopach and isopleth maps and quantify the main corresponding physical eruptive parameters of the five eruptive units representing the largest explosive eruptions known in the Late Pleistocene record of Mt.…”

Petrological record from young Ruapehu eruptions in the 4.5 ka Kiwikiwi Formation, Whangaehu Gorge, New Zealand

“…These are generally at least an order of magnitude smaller than those reported for earlier eruptions at 10 ka (Moebis et al 2011;Pardo et al 2011). Kiwikiwi pyroclasts represent magma that developed in a crustal mush column below Ruapehu, as has also been inferred for other Ruapehu andesites (Nakagawa et al 2002;Stewart 2010;Price et al 2012).…”