Natalia Pardo

New insights into Holocene eruption episodes from proximal deposit sequences at Mt. Taranaki (Egmont), New Zealand

Upper stratovolcano flanks contain the most nuanced depositional record of long eruption episodes, but steep, irregular terrain makes these sequences difficult to correlate and interpret. This necessitates development of a detailed and systematic approach to describing localized depositional facies and relating these to eruptive processes. In this work, the late-Holocene eruption history of Mt. Taranaki/Egmont, New Zealand, was re-assessed based on a study of proximal deposits spanning the 14C-dated age range of ~5.0–0.3xa0calxa0ka B.P. Mt. Taranaki is a textbook-example stratovolcano, with geological evidence pointing to sudden switches in scale, type and frequency of eruptions over its ~130xa0ka history. The proximal stratigraphy presented here almost doubles the number of eruptions recognized from previous soil-stratigraphy studies. A total of 53 lithostratigraphic bed-sets record eruptions of the summit crater and parasitic vents like Fanthams Peak (the latter between ~3.0 and 1.5xa0calxa0ka B.P.). At least 12 of the eruptions represented by these bed-sets comprise deposits comparable with or thicker than those of the latest sub-Plinian eruption of AD 1655. The largest eruption episode represented is the 4.6–4.7-calxa0ka B.P. Kokowai. Contrasting eruption styles were identified, from stable basaltic-andesite eruption columns at Fanthams Peak, to andesitic lava-dome extrusion, blasts and partial collapse of unstable eruption columns at Mt. Taranaki’s summit. The centemetre-scale proximal deposit descriptions were used to identify several previously unknown, smaller eruption events. These details are indispensable for building a comprehensive probabilistic event record and in the development of realistic eruptive scenarios for complex eruption episodes prior to re-awakening of a volcano.

Diverse dynamics of Holocene mafic-intermediate Plinian eruptions at Mt. Taranaki (Egmont), New Zealand

Over the last 5000xa0years, at least 53 eruptive episodes have occurred at Mt. Taranaki (western North Island, New Zealand), from either its summit crater (~xa02500xa0m) or a satellite vent on Fanthams Peak (~xa01900xa0m). The magmas erupted have a wide range of compositions from basaltic to trachy-andesitic (~xa048–60xa0wt% SiO2). Five large-magnitude episodes from this sequence were studied so as to characterize a typical range of explosive eruption styles at andesitic stratovolcanoes, including three eruptions from the summit crater and two from Fanthams Peak. Sustained eruption columns characterized the climactic phase of all five eruptions, but these were interspersed with pulsating, collapsing, or oscillating conditions. Eruption columns reached between 14 to 29xa0km in height and ejected minimum volumes of 0.1–1.1xa0km3 at mass discharge rates of 1xa0×xa0107–2xa0×xa0108xa0kg/s, indicating magnitudes of 4.1 to 5.1. The simplest eruptions occurred from Fanthams Peak with basaltic magmas producing high-climactic eruption columns rapidly after vent opening, followed by gentle waning phases or a passage into a lava-fountaining phase. Eruptions of higher-silica magmas at the summit vent, by contrast, showed longer pre-climactic eruptive phases with either dome growth or complex phases of vent clearance and blockage producing unsteady eruption columns. The latter eruption types produced block-and-ash flows, lateral-blast surges, and column-collapse pumice-and-ash flows, with run-out distances of 3–19xa0km, covering 5–70xa0km2 with volumes of up to 0.022xa0km3. Our results demonstrate that very different eruption scenarios may occur at different vent locations, or with subtly different compositions erupted, on the same stratovolcano so that emergency management planning must take such a range of possibilities into account.