Dating the Taupo and Kaharoa eruptions
Taupo
The Taupo ignimbrite is full of charcoal (Fig. 12.6)
and so was an obvious target when radiocarbon (
14 C)
dating was first undertaken in New Zealand. NZ-1 (New
Zealand’s first carbon date published in 1953) gave a
date of 1820 ± 150
14 C yr BP (68% probability). Since
then, dozens of samples from Taupo eruptives have been
dated but various problems, including in-built age and
stratigraphic considerations, and imprecision in calibrating ages from
14 C years to calendar years, meant that
the date of the eruption could have been at any time
between about AD 130 and AD 320. A solution to
deriving a much more precise date arose from a quirk of
the eruption itself. During its emplacement, the Taupo
ignimbrite flattened a remnant of forest at Pureora
(Fig. 12.4) which was then perfectly preserved, as
a buried forest, by peat development because of a
fortuitous alteration in drainage. After developing a
high-precision kauri (Agathis australis)-based
14 C calibration curve with the assistance of dendrochronologists, Alan Hogg (Waikato Radiocarbon Dating
Laboratory) and colleagues used a large tanekaha (celery pine, Phyllocladus trichomanoides) log from Pureora, and a technique called
14 C wiggle-matching, to
derive a new calendar date of AD 232 ± 10 (95%
probability). That date put paid to various dates obtained
previously including a date based on ancient Roman and
Chinese records as well as erroneous ice-core-based
dates. The Taupo eruption took place in late summer to
early autumn (typically late March to early April) on the
basis of fruit and seeds preserved at Pureora and the lack
of an outer latewood ring on preserved logs. Preserved
insect assemblages suggested the ignimbrite was
deposited in the late afternoon.
Kaharoa
As was the case for Taupo,
14 C dates obtained for the
Kaharoa eruption were not able to be calibrated with
much precision. However, after a fruitless search for
suitable wood on Mt Tarawera, a partially carbonised
tanekaha log was discovered in Kaharoa pyroclastic
flow and surge deposits at the foot of the mountain. With
the help of dendrochronologist Jonathan Palmer, Alan
Hogg used the
14 C wiggle-matching method to obtain a
date of AD 1314 ± 12 (95% probability). The outermost ring of the log included both early and late wood,
meaning the tree was killed in the period between
growth cessation—because of the onset of winter—but
before the start of spring growth. The exact month of the
eruption is uncertain, but contemporary tree-ring studies
of the same species indicate that the wintery period from
May to September is likely.
12.3 Soil-Landscape Relationships
12.3.1 Forming and Modifying Landscapes
and Changes in Pumice Soils Over
Distance
The deposition of tephra-fall beds from the Taupo and
Kaharoa eruptions, and the emplacement of the Taupo ignimbrite, resulted in sudden and marked changes to the
landscape. The fall beds draped the pre-existing land, specially making it smoother (a landscape-modifying event).
The emplacement of the Taupo ignimbrite, in contrast, was
mainly a landscape-forming event in which pre-existing
valleys became plains upon infilling. The deposition of the
ignimbrite veneer deposits on high points in the landscape
mimicked the effect of the fall beds in generally rounding the
landscape contours (Fig. 12.9).
The soils on the valley-ponded ignimbrite deposits are
typically much more compact and dense (Impeded Pumice
Soils) in contrast with those on the ‘soft’ and loose ignimbrite veneer deposits (Orthic Pumice Soils).
The patterns of deposition of fall and flow deposits, and
the inherent properties of the deposits, especially layering and
thickness, have led to differences in the Pumice Soils
depending on their location. Despite an initial period of
landscape instability and reworking the young Pumice Soils
were generally well preserved by vegetation cover once forests became established. Hence at sites relatively close to
Lake Taupo, especially immediately east where the thickest
deposits occur, the soils are formed entirely in pyroclastic fall
and flow deposits >2 m deep (e.g. Fig. 12.5). Beyond about
50 km, the Taupo eruptives make up profiles of about 1 m or
less in thickness. Near Mt Tarawera, the proximal Kaharoa
fall beds about 8 km southeast of the mountain are nearly 2 m
thick (Fig. 12.7) but the thickness drops off markedly to only
about 0.5 m or less within a few kilometres (Fig. 12.4).
At distal sites where the Taupo or Kaharoa eruptives are
only about 0.5 m thick or less, the soils are multi-layered and
complex, with Taupo or Kaharoa deposits forming just the
upper part of the soil profile. Such soils occur on the margins
of the distribution of Taupo ignimbrite, towards Taumarunui,
Tokoroa, Waiouru, and Murupara, for example (Fig. 12.10).
12.3.2 Tephrostratigraphy and Paleopedology
In the many multi-layered profiles, such as that at Murupara
(Fig. 12.10) the various tephra deposits are able to be
identified using their positions in the sequence together with
their physical and (if necessary) mineralogical and glass
compositional properties (derived from laboratory analysis).
Alan Pullar and Colin Vucetich identified and mapped tephra
layers, mainly in the 1950s to 1970s, in the central North
12.2 Soil Profile Genesis
189
Taupo
The Taupo ignimbrite is full of charcoal (Fig. 12.6)
and so was an obvious target when radiocarbon (
14 C)
dating was first undertaken in New Zealand. NZ-1 (New
Zealand’s first carbon date published in 1953) gave a
date of 1820 ± 150
14 C yr BP (68% probability). Since
then, dozens of samples from Taupo eruptives have been
dated but various problems, including in-built age and
stratigraphic considerations, and imprecision in calibrating ages from
14 C years to calendar years, meant that
the date of the eruption could have been at any time
between about AD 130 and AD 320. A solution to
deriving a much more precise date arose from a quirk of
the eruption itself. During its emplacement, the Taupo
ignimbrite flattened a remnant of forest at Pureora
(Fig. 12.4) which was then perfectly preserved, as
a buried forest, by peat development because of a
fortuitous alteration in drainage. After developing a
high-precision kauri (Agathis australis)-based
14 C calibration curve with the assistance of dendrochronologists, Alan Hogg (Waikato Radiocarbon Dating
Laboratory) and colleagues used a large tanekaha (celery pine, Phyllocladus trichomanoides) log from Pureora, and a technique called
14 C wiggle-matching, to
derive a new calendar date of AD 232 ± 10 (95%
probability). That date put paid to various dates obtained
previously including a date based on ancient Roman and
Chinese records as well as erroneous ice-core-based
dates. The Taupo eruption took place in late summer to
early autumn (typically late March to early April) on the
basis of fruit and seeds preserved at Pureora and the lack
of an outer latewood ring on preserved logs. Preserved
insect assemblages suggested the ignimbrite was
deposited in the late afternoon.
Kaharoa
As was the case for Taupo,
14 C dates obtained for the
Kaharoa eruption were not able to be calibrated with
much precision. However, after a fruitless search for
suitable wood on Mt Tarawera, a partially carbonised
tanekaha log was discovered in Kaharoa pyroclastic
flow and surge deposits at the foot of the mountain. With
the help of dendrochronologist Jonathan Palmer, Alan
Hogg used the
14 C wiggle-matching method to obtain a
date of AD 1314 ± 12 (95% probability). The outermost ring of the log included both early and late wood,
meaning the tree was killed in the period between
growth cessation—because of the onset of winter—but
before the start of spring growth. The exact month of the
eruption is uncertain, but contemporary tree-ring studies
of the same species indicate that the wintery period from
May to September is likely.
12.3 Soil-Landscape Relationships
12.3.1 Forming and Modifying Landscapes
and Changes in Pumice Soils Over
Distance
The deposition of tephra-fall beds from the Taupo and
Kaharoa eruptions, and the emplacement of the Taupo ignimbrite, resulted in sudden and marked changes to the
landscape. The fall beds draped the pre-existing land, specially making it smoother (a landscape-modifying event).
The emplacement of the Taupo ignimbrite, in contrast, was
mainly a landscape-forming event in which pre-existing
valleys became plains upon infilling. The deposition of the
ignimbrite veneer deposits on high points in the landscape
mimicked the effect of the fall beds in generally rounding the
landscape contours (Fig. 12.9).
The soils on the valley-ponded ignimbrite deposits are
typically much more compact and dense (Impeded Pumice
Soils) in contrast with those on the ‘soft’ and loose ignimbrite veneer deposits (Orthic Pumice Soils).
The patterns of deposition of fall and flow deposits, and
the inherent properties of the deposits, especially layering and
thickness, have led to differences in the Pumice Soils
depending on their location. Despite an initial period of
landscape instability and reworking the young Pumice Soils
were generally well preserved by vegetation cover once forests became established. Hence at sites relatively close to
Lake Taupo, especially immediately east where the thickest
deposits occur, the soils are formed entirely in pyroclastic fall
and flow deposits >2 m deep (e.g. Fig. 12.5). Beyond about
50 km, the Taupo eruptives make up profiles of about 1 m or
less in thickness. Near Mt Tarawera, the proximal Kaharoa
fall beds about 8 km southeast of the mountain are nearly 2 m
thick (Fig. 12.7) but the thickness drops off markedly to only
about 0.5 m or less within a few kilometres (Fig. 12.4).
At distal sites where the Taupo or Kaharoa eruptives are
only about 0.5 m thick or less, the soils are multi-layered and
complex, with Taupo or Kaharoa deposits forming just the
upper part of the soil profile. Such soils occur on the margins
of the distribution of Taupo ignimbrite, towards Taumarunui,
Tokoroa, Waiouru, and Murupara, for example (Fig. 12.10).
12.3.2 Tephrostratigraphy and Paleopedology
In the many multi-layered profiles, such as that at Murupara
(Fig. 12.10) the various tephra deposits are able to be
identified using their positions in the sequence together with
their physical and (if necessary) mineralogical and glass
compositional properties (derived from laboratory analysis).
Alan Pullar and Colin Vucetich identified and mapped tephra
layers, mainly in the 1950s to 1970s, in the central North
12.2 Soil Profile Genesis
189