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The impact of recurrence time on magmatic systems response to repeated edifice collapse.

Shannen Mills1, Jonathan Procter1, Anke Zernack1, Stuart Mead1, Georg Zellmer1, Mark Bebbington1, Shane Cronin2, Nessa D'Mello3

  • Affiliations:  1Volcanic Risk Solutions, School of Agriculture and Environment, Massey University, Palmerston North, New Zealand; 2School of Environment, University of Auckland, 23 Symonds Street, Auckland Central, New Zealand; 3Department of Earth Sciences, Southern Methodist University, Dallas, Texas 75205, United States of America. 

  • Presentation type: Poster

  • Presentation time: Tuesday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 198

  • Programme No: 3.2.21

  • Theme 3 > Session 2

Abstract

Understanding how a volcano responds to edifice collapse is important for forecasting post-failure changes in eruptive behaviour and associated hazards. At Mt. Taranaki, two collapse events occurred in close succession emplacing the 27.3ka Ngaere (5.85km3) and 24.8ka Pungarehu (7.5km3) debris-avalanche deposits to the east and west, respectively. This period in Mt. Taranaki's history was characterised by an increase in large explosive eruptions with at least 28 subplinian events comprising ~3 km3 of material recorded in the Poto and Paetahi tephra formations. Using 3D Micro-Tomography and geochemical analyses of pumice clasts from these formations, we investigated changes in the magmatic system throughout two collapse and regrowth cycles. Whole-rock compositions revealed an overall trend towards more evolved compositions over time. Bubble texture analysis showed that pyroclasts were dominated by small bubbles (2.7x10-7 mm3) and high vesicle number densities (VND). A clear magmatic response to the sudden decompression associated with the Ngaere collapse is reflected in a decrease in VND (from 1.53x1016 to 9.76x1015 cm-3) and an increase in MgO (from 4.06 to 4.55 wt.%), suggesting that the change in lithostatic pressure allowed primitive melts to ascend without impediment. This increased the eruptive frequency, partially rebuilding Mt. Taranaki before the Pungarehu collapse removed the opposite portion of the pre-Ngaere edifice 2,500 years later. However, no systematic response to decompression was observed with a gradual compositional evolution (from 3.62 to 3.14wt.% MgO) and increase in VND (from 1.15x1016 to 1.29x1016 cm-3), highlighting the influence of pre-failure edifice height/loading on the magmatic response to collapse.