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Understanding aspects of andesitic dome-forming eruptions through the last 1000 yrs of volcanism at Mt. Taranaki, New Zealand : a dissertation presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science, Massey University, Palmerston North, New Zealand
Andesitic volcanoes are notorious for their rapid and unpredictable changes in eruptive
style between and during volcanic events, a feature normally attributed to shallow crustal
and intra-edifice magmatic processes. Using the example of eruptions during the
last 1000 yrs at Mt. Taranaki (the Maero Eruptive Period), deposit sequences were
studied to (1) understand lava dome formation and destruction, (2) interpret the causes
of rapid shifts from extrusive to explosive eruption styles, and (3) to build a model of
crustal magmatic processes that impact on eruption style.
A new detailed reconstruction of this period identifies at least 10 eruptive episodes
characterised by extrusive, lava dome- and lava flow-producing events and one sub-
Plinian eruption. To achieve this, a new evaluation procedure was developed to purge
glass datasets of contaminated mineral-glass analyses by using compositional diagrams
of mineral incompatible-compatible elements. Along with careful examination of
particle textures, this procedure can be broadly applied to build a higher degree of
resolution in any tephrostratigraphic record. Geochemical contrasts show that the
products of the latest Mt. Taranaki eruption, the remnant summit dome (Pyramid Dome)
was not formed during the Tahurangi eruptive episode but extruded post-AD1755. Its
inferred original maximum volume of 4.9×106 m3 (DRE) was formed by simultaneous
endogenous and exogenous dome growth within days. Magma ascent and extrusion
rates are estimated at =0.012 ms-1 and =6 m3s-1, respectively, based on hornblende
textures. Some of the Maero-Period dome effusions were preceded by a vent-clearing
phase producing layers of scattered lithic lapilli around the edifice [Newall Ash (a),
Mangahume Lapilli, Pyramid Lapilli]. The type of dome failure controlled successive
eruptive phases in most instances. The destruction of a pressurised dome either caused
instantaneous but short-lived magmatic fragmentation (Newall and Puniho episodes), or
triggered a directed blast-explosion (Newall episode), or initiated sustained magmatic
fragmentation (Burrell Episode). The transition from dome effusion to a sustained, sub-
Plinian eruption during the Burrell Lapilli (AD1655) episode was caused by unroofing a
conduit of stalled magma, vertically segregated into three layers with different degrees
of vesiculation and crystallisation. The resultant ejecta range from brown, grey and
black coloured vesicular clasts to dense grey lithics. Bulk compositional variation of
erupted clasts can be modelled by fractionation of hornblende, plagioclase,
clinopyroxene, and Fe-Ti oxides. Pre-eruption magma ascent for the Maero Period
events is assumed to begin at depths of c.9.5 km.