Eruption cycles and magmatic processes at a reawakening volcano, Mt. Taranaki, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand
Realistic probabilistic hazard forecasts for re-awakening volcanoes rely on making an
accurate estimation of their past eruption frequency and magnitude for a period long
enough to view systematic changes or evolution. Adding an in-depth knowledge of the
local underlying magmatic or tectonic driving processes allows development of even
more robust eruption forecasting models. Holocene tephra records preserved within
lacustrine sediments and soils on and surrounding the andesitic stratovolcano of Mt.
Taranaki (Egmont Volcano), New Zealand, were used to 1) compile an eruption
catalogue that minimises bias to carry out frequency analysis, and 2) identify magmatic
processes responsible for variations in activity of this intermittently awakening volcano.
A new, highly detailed eruption history for Mt. Taranaki was compiled from sediment
sequences containing Holocene tephra layers preserved beneath Lakes Umutekai and
Rotokare, NE and SE of the volcano’s summit, respectively, with age control provided
by radiocarbon dating. To combine the two partly concurrent tephra records both
geochemistry (on titanomagnetite) and statistical measures of event concurrence were
applied. Similarly, correlation was made to proximal pyroclastic sequences in all sectors
around the 2518 m-high edifice. This record was used to examine geochemical
variations (through titanomagnetite and bulk chemistry) at Mt. Taranaki in
unprecedented sampling detail.
To develop an unbiased sampling of eruption event frequency, a technique was
developed to distinguish explosive, pumice-forming eruptions from dome-forming
events recorded in medial ash as fine-grade ash layers. Recognising that exsolution
lamellae in titanomagnetite result from oxidation processes within lava domes or plugs,
their presence within ash deposits was used to distinguish falls elutriated from blockand-
ash flows. These deposits are focused in particular catchments and are hence
difficult to sample comprehensively. Excluding these events from temporal eruption
records, the remaining, widespread pumice layers of sub-plinian eruptions at a single
site of Lake Umutekai presented the lowest-bias sampling of the overall event
frequency. The annual eruption frequency of Mt. Taranaki was found to be strongly
cyclic with a 1500-2000 year periodicity.
Titanomagnetite, glass and whole-rock chemistry of eruptives from Mt. Taranaki’s
Holocene history all display distinctive compositional cycles that correspond precisely
with the event frequency curve for this volcano. Furthermore, the largest known
eruptions from the volcano involve the most strongly evolved magmas of their cycle
and occur during the eruptive-frequency minimum, preceding the longest repose
Petrological evidence reveals a two-stage system of magma differentiation and
assembly operating at Mt. Taranaki. Each of the identified 1500-2000 year cycles
represent isolated magma batches that evolved at depth at the base of the crust before
periodically feeding a mid-upper crustal magma storage system.