Understanding the volcanic response to edifice collapse : a case study of the Poto and Paetahi formations at Mt. Taranaki : a thesis submitted in partial fulfilment of a Philosophiae Doctor degree in Earth Science, Volcanic Risk Solutions, School of Agriculture and Environment, Massey University, New Zealand
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2024
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Massey University
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Abstract
Stratovolcanoes are unstable and prone to collapse. Depressurisation from collapse events can possibly impact the subvolcanic plumbing system. This may cause a change in the eruptive size, style and frequency of eruptive activity following a collapse.
Mt. Taranaki in New Zealand provides a unique opportunity to investigate the influence of edifice collapse on eruptive behaviour. The extensive ring plain around Mt. Taranaki is dominated by debris-avalanche deposits (DAD) recording the last > 200 kyr of eruptive history including at least fourteen events. The medial ring plain provides a stratigraphic record of the last 30 ka of eruptive history comprising four DADs including two of the largest to have occurred in Mt. Taranaki’s history the 27.3 ka (5.85 km³) Ngaere and 24.8 ka (> 7.5 Understanding the volcanic response to edifice collapse. A case study of the Poto and Paetahi Formations at Mt. Taranaki Understanding the volcanic response to edifice collapse. A case study of the Poto and Paetahi Formations at Mt. Taranaki km³ Pungarehu DAD. The (27.3-23.1 ka) Poto and Paetahi Formations deposited across the eastern and southeastern sector of the ring plain are used to investigate the effects of depressurisation on the magmatic system.
A detailed stratigraphic analysis of medial to distal exposures of the Poto and Paetahi Formations was undertaken across the eastern and southeastern sectors of the Taranaki ring plain. Changes in lithosedimentological characteristics were used to identify single and multiphase eruptive events. Isopach and Isopleth mapping of the deposits show a period of increased explosive activity within Mt. Taranaki’s history. The deposits were analysed for grain size distributions, componentry, juvenile and density, as well as X-ray tomography to define vesicle and crystal number densities and volumes. Geochemical analysis on whole rock, glass, feldspar, and pyroxene crystals was conducted to create a detailed account of the changes within the Poto and Paetahi Formations and infer the response to edifice collapse. This study found that the relative abundance of lithics informed processes of conduit stability throughout the eruptive period, with increase abundance reflecting conduit excavation. Components were divided into juveniles, lithics and free crystals with subcategories established for each lithology class. Micro-Computed X-ray tomography indicated the high percentage of small bubbles present within the juvenile deposits.
Twenty-eight subplinian eruptions produced at least ~3 km³ of tephra across the eastern and southeastern Taranaki ring plain within a ~4 kyr period, producing single and multiphase eruptive events with eruption column heights between 10-20 km and individual deposit volumes of 0.01-0.26 km³. Variations in the relative abundance of lithic clasts and density analysis of juvenile deposits reflect changing conduit conditions throughout the Poto and Paetahi Formations. Connected porosities and the abundance of juvenile clasts increased during stable conduit conditions due to the formation of gas flow pathways. A decrease in connected porosities and increase in the abundance of lithics indicated conduit excavation through unstable/ widening events, disrupting the formation of gas flow networks.
Large populations of small bubbles (2.75 x 10-7 mm-3) are indicated through high vesicle number densities (VND) (9.03 x 1015 – 1.74 x 1016 cm-3), reflecting the domination of late-stage bubble nucleation within the upper conduit by fast ascending magmas occurring throughout the Poto and Paetahi Formations. Vesicle size populations reflect the onset location of bubble nucleation within the system. Changes in vesicle size distributions and oscillatory crystal rims throughout the sequence reflect cycles of magma recharge and storage occurring below Mt. Taranaki. Single stage nucleation events reflect the rapid ascent of magma through the system, while bubble coalescence indicates some magma stalled within the mid-to-upper crustal system. Whole rock compositions from these tephra vary between 3.03 – 5.19 wt.% MgO and reflect an evolution in magmatic composition overtime. Depressurisation from the eastern Ngaere collapse resulted in an increase in MgO wt.% (from 4.06 wt.% to 4.55 wt.%), decrease in VND (from 1.53 x 1016 to 9.76 x 1015 cm-3), but uniform vesicle volumes (VV) (from 4.9 to 4.8 %). This indicates a change in magmatic overpressure and deactivation of the mid-to-upper crustal system. The younger (23.1 – 24.1 Ka) Paetahi Formation is more evolved than the Poto Formation (27.3-25 Ka), reflecting the re-activation of the mid-to-upper crustal system throughout the regrowth period. Continued evolution in magmatic composition (from 3.62 wt.% to 3.14 wt.% MgO), increase in VND (from 1.15 x1016 to 1.29x1016 cm-3) and decrease in VV (from 7.1 to 3.3%) following the western Pungarehu collapse (~2,500 years after the Ngaere) reflects no depressurisation on the shallow volcanic system. The observed differences in response to collapse events is due to the relative height of the edifice and location of the conduit/ vent.
The sedimentological, textural, and geochemical analysis of the Poto and Paetahi tephra formations demonstrate the changes to eruptive activity following collapse events. However, these results highlight the relationship between edifice height, lithostatic pressure and the magmatic system. The Ngaere collapse depressurized a fully grown edifice (~ 2500 m), shifting the vent location within the scar and destabilized the remanent cone. The relatively short time between collapse events (~2,500 years) saw the western remanent cone collapse before Mt. Taranaki had fully regrown. This did not cause a significant change on the magmatic system below and allowed for the continued regrowth. This study highlights a need to understand the relationship between hazardous volcanic phenomena to generate more accurate hazard scenarios for stratovolcanoes which are prone to edifice collapse.
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Keywords
Mt. Taranaki, andesite stratovolcano, edifice collapse, subplinian eruption, synchrotron microscopy