Drivers, Dynamics and Hazards of the largest pyroclastic eruptions of Tongariro Volcano, Central North Island, New Zealand

Abstract

Long lived composite volcanoes show large variations in eruption frequency and magnitude over time. It is important for hazard assessment to understand why these extremes occur. The ~11 ka BP Mangamate eruptive sequence produced the largest pyroclastic eruptions known from Mt. Tongariro. Within a <200 year period, seven sub-Plinian to Plinian eruption episodes produced at least 4.5 km³ of tephra. This "flare-up" far outstrips the averaged eruption rate of Mt. Tongariro of 0.1-1.0 km³/ka. Mapping of the individual tephra deposits of the Mangamate sequence revealed an 'unzipping' of eruption vents from north to south along the axis of the Tongariro Graben. The northern vents produced sustained Plinian eruption columns, while southern vents sourced unstable and collapsing sub-Plinian eruptions, the latter generating pyroclastic density currents. Peak eruption column heights for these episodes were between 16 and 32 km with volumes of individual tephra lobes reaching 0.76 km³. Via 2D and 3D textural studies, it is shown that these mostly micro-porous pumices erupted via crystallisation-driven second boiling and magmas erupted with a range of compositional, gas and crystal contents. Foam-like vesicular pumices were only erupted by more open northern vents in the more rapidly extending northern graben. Narrow, fault-bounded vents to the south were fed by more crystalline magmas in variable dykes and sills. In order to produce an eruptible magma volume of 4.5 km³ in such a short time span, the skeletal transcrustal magmatic system developed beneath Mt. Tongariro was expanded and connected by greater rates of intrusion - forming a N-S elongated heterogenous mush zone aligned with the graben axis. Mixing of more evolved residual and intruding primitive melt created mainly basaltic-andesitic whole-rock compositions. More evolved andesites, with dacitic to rhyolitic interstitial glass represent segregation of volatile-rich melt fractions driving Plinian eruptions in northern vents. Since the Mangamate sequence, the residual mush system was only locally activated with small-volume magma batches, making new eruptions of this scale unlikely until significant new intrusion begins. This case-study demonstrates how strongly the local tectonic environment affects eruption vent locations and dynamics.