High-precision tephrostratigraphy : tracking the time-varying eruption pulse of Mt. Taranaki, North Island, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science, Massey University, Palmerston North, New Zealand
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Date
2017
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Massey University
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Abstract
In this research it was proposed that a more robust record of volcanic activity for Mt.
Taranaki (New Zealand) could be derived from tephras (pyroclastic fall deposits) within
cores from several lakes and peatlands across a 120o arc, NE-SE of the volcano,
covering a range of prevailing down-wind directions. These data were integrated with
previous tephrochronology studies to construct one of the longest and most complete
volcanic eruption history records ever developed for an andesitic stratovolcano. Using
44 new radiocarbon dates, electron microprobe analysis of glass shard and
titanomagnetite chemical composition, along with whole-rock chemistry, a chrono- and
chemostratigraphy was established. The new record identifies at least 272 tephraproducing
eruptions over the last 30 cal ka BP. Six chemo-stratigraphic groups were
identified: A (0.5 – 3 cal ka BP), B (3 – 4 cal ka BP), C (4 – 9.5 cal ka BP), D (9.5 – 14
cal ka BP), E (14 – 17.5 cal ka BP), and F (23.5 – 30 cal ka BP). These were used to
resolve previous stratigraphic uncertainties at upper-flank (proximal) and ring-plain
(medial) sites. Several well-known “marker tephras” are now recognized as being
~2000 years older than previously determined (e.g., Waipuku, Tariki, and Mangatoki
Tephra units) with the prominent Korito Tephra stratigraphically positioned above the
Taupo-derived Stent Tephra. Further, new markers were identified, including the
Kokowai Tephra unit (~4.7 cal ka BP), at a beach-cliff exposure, 40-km north-east of
the volcano. Once age-models were established for each tephra, units were matched
between sites using statistical methods. Initial statistical integration showed that the
immediate past high-resolution tephrochronological record suffered from a distinctive
“old-carbon” effect on its ages (Lake Rotokare). This had biased the most recent
probabilistic forecasting and generated artificially high probability estimates (52-59%
eruption chance over the next 50 years). Once the Rotokare record was excluded and
chemostratigraphy constraints were applied, a reliable multi-site tephra record could be
built only for the last ~14 ka BP. The new data confirms a highly skewed distribution of
mainly (98% of cases) short intervals between eruptions (mode of ~9 years and average
interval ~65 years). Long intervals (up to 580 years) as seen in earlier records were
reduced to 2% of the record, but can now be considered real, rather than missing data.
The new data confirm a cyclic pattern of varying eruption frequency (with a five-fold
range in annual frequency) on a period of ~1000-1500 years. The new time-varying
frequency estimates suggest a lower probability for a new eruption at Mt. Taranaki over
the next 50 years of 33-42%. The newly established chemostratigraphy was further used
to investigate time-related compositional changes. Whole-lapilli analyses highlighted
that a specific very evolved Ca-rich and Fe-poor composition was only found within the
easterly and south-easterly depositional sites. This was explained by eruption of a
stratified magma reservoir, which holds greater modal proportions of plagioclase and
lower proportions of pyroxene within low-density, gas-rich upper conduit regions.
During the most explosive phases of eruptions, when plumes reach the stratospheric jetstream,
the lowest-density pumice is thus dispersed by high-level stable westerly winds.
Further, two distinct evolutional trends were seen in the long and new
tephrochronological record; from 17.5 to 3 cal ka BP and <3 cal ka BP; with wholelapilli,
glass, and titanomagnetite compositions overall evolving over time. The former
compositional trend indicates a crystallising and cooling magma source in the deep
crust, with multiple, spatially separated magma source regions forming, each generating
magmas (i.e., magma batches) with unique titanomagnetite compositions. This trend is
interrupted by a distinct shift towards less-evolved compositions and the initiation of a
second parasitic vent (Fanthams Peak at the southern flank of Mt. Taranaki).
Description
Keywords
Tephrochronology, Geology, Stratigraphic, Volcanic eruptions, Mount Egmont, Mount Taranaki, New Zealand