Longevity of a small shield volcano revealed by crypto-tephra studies (Rangitoto volcano, New Zealand): Change in eruptive behavior of a basaltic field
Phil Shane, Maria Gehrels, Aleksandra Zawalna-Geer, Paul Augustinus,
Jan Lindsay, Isabelle Chaillou
PII:
DOI:
Reference:
S0377-0273(13)00110-8
doi: 10.1016/j.jvolgeores.2013.03.026
VOLGEO 5120
To appear in:
Journal of Volcanology and Geothermal Research
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Accepted date:
29 October 2012
27 March 2013
Please cite this article as: Shane, Phil, Gehrels, Maria, Zawalna-Geer, Aleksandra, Augustinus, Paul, Lindsay, Jan, Chaillou, Isabelle, Longevity of a small shield volcano
revealed by crypto-tephra studies (Rangitoto volcano, New Zealand): Change in eruptive behavior of a basaltic field, Journal of Volcanology and Geothermal Research (2013),
doi: 10.1016/j.jvolgeores.2013.03.026
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Longevity of a small shield volcano revealed by crypto-tephra studies (Rangitoto volcano,
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New Zealand): change in eruptive behavior of a basaltic field
Phil Shane a, Maria Gehrels b, Aleksandra Zawalna-Geer a, Paul Augustinus a, Jan Lindsay a,
School of Environment, University of Auckland, Private Bag 92019, Auckland 1142, New
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a
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Isabelle Chaillou a
b
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Zealand
School of Geography, Earth and Environmental Sciences, Plymouth University, Plymouth
Phil Shane
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Corresponding author:
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PL4 8AA, UK
Email: pa.shane@auckland.ac.nz
Phone: 64-9-9237083
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Abstract
The life-span of small volcanoes in terrestrial basaltic fields, commonly considered
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‘monogenetic’, can be difficult to assess due to a paucity of datable materials capable of
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providing a 102-103 -year age resolution. We have used microscopic tephra layers (cryptotephra) in lake sediments to determine the longevity of Rangitoto volcano, a small shield that
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represents the most recent volcanism in the Auckland Volcanic Field (AVF), New Zealand.
Previous studies suggested construction in a relatively short interval at ~550-500 cal yrs BP.
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In contrast, the tephra record shows evidence of intermittent activity from 1498 ± 140 to (at
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least) 504 ± 6 cal yrs BP, a longevity of ~1000 years. Rangitoto volcano is thought to
represent about half the magma erupted in the 250-ka-history of AVF. Thus, the AVF has
experienced a dramatic shift to prolonged and voluminous central-vent volcanism in its most
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recent history. This demonstrates the difficulty in determining time-erupted volume
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relationships in such fields. Previous AVF hazard-risk modeling based on isolated, short-lived
(< 1 year) phenomena at sites that have not experienced activity needs to be revisited in light
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of the new Rangitoto chronology.
Keywords: shield; monogenetic; basaltic volcano; crypto-tephra; Auckland Volcanic Field
1. Introduction
Terrestrial basaltic fields commonly comprise numerous small edifices including
cinder cones, tuff rings, maars, shield volcanoes and lava flows. Eruptions are infrequent (10-4
– 10-5 events/yr) (e.g., Kuntz et al., 1986; Condit and Connor, 1996; Conway et al., 1998) and
individual events generally emit small volumes of magma (102 - 109 m3) in a short-lived
interval lasting days to years (e.g., Wood, 1980). Although infrequent, such volcanism is
significant to society where infrastructure has been or will be developed on basaltic fields.
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Some small edifices have been considered to be multi-episodic, erupting over thousands of
years (e.g., Bradshaw and Smith, 1994), although such longevity has been questioned partly
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due to the paucity of datable materials (Valentine et al., 2006). Understanding the life of
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individual edifices in basaltic fields is critical to assessing future hazards.
Auckland City, New Zealand is built directly on the late Quaternary Auckland
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Volcanic Field (AVF) (Kermode, 1992) (Fig. 1). The AVF has been the intense focus of
petrological (e.g., Smith et al., 2008; McGee et al., 2011), geochronological (e.g., Cassidy,
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2006; Cassata et al., 2008), and hazard/risk studies (e.g., Houghton et al., 2006; Lindsay et al.,
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2010; Sandri et al., 2012), because of Auckland city’s large population (~1.4 million people),
and its economic significance to New Zealand. These studies are fundamentally based on the
concept of ‘monogenetic’ volcanism, where edifices or eruption centres are considered to
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represent short-lived episodes of activity (< 1 year) by comparison to small volcanic
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landforms and eruptions elsewhere (e.g., Wood 1980). Obliteration of stratigraphic sequences
by urban development in the Auckland region, coupled with deep weathering in a temperate
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climate but limited erosional dissection hinder the evaluation of volcano longevity.
Anomalous amongst the small ‘monogenetic’ landforms in the AVF is Rangitoto
Island (volcano), a shield volcano representing about half of the estimated erupted magma
volume of the field (Kermode, 1992; Needham et al., 2011). The lack of deep exposures on
the volcano provides little insight to its history. In addition to its size, the volcano is of
particular interest because it is the only site of volcanism in the field during the last 10 ka. The
Rangitoto eruptions are also of archeological significance because early cultural layers were
buried by the tephra indicating human habitation before and after the activity (summarized by
Lowe et al., 2000). The anomalous size of the edifice and type of structure (lava shield) raises
the question whether the AVF has undergone a change in volcanic regime to one of
‘polygenetic’ eruptions from a central vent region.
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To investigate the longevity of activity at Rangitoto volcano, we have examined tephra
layers preserved in sediments within 10 km of the edifice. Previous tephra studies have
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demonstrated the potential of constraining the timing of AVF eruptions where their tephra is
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bracketed by well-dated tephra layers sourced from other (more distal) volcanoes in New
Zealand (Molloy et al., 2009). Unlike previous work, we have also examined microscopic
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tephra concentrations in sediments (crypto-tephra, e.g., Lowe 2011; Pyne-O’Donnell, 2011)
that have the potential to record events less favorable for preservation due to highly
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directional wind dispersal or less explosive and/or less voluminous eruptions. We show that
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the crypto-tephra record provides a different perspective on the nature of past activity in the
AVF which has implications for the evolution of the field and the assessment of future
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volcanic hazards.
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2. Rangitoto volcano
The AVF is situated in a continental intra-plate setting with a crustal thickness of ~30
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km (Horspool et al., 2006). The field consists of ~50 basaltic cinder cones, maars, and
associated lava fields (Kermode, 1992), with an estimated total volume of magma erupted of
~3-4 km3. The known vents are scattered over a 360 km2 area (Fig. 1). Activity commenced at
~ 250 ka (Shane and Sandiford, 2003) and continued intermittently to ~0.5 ka.
Rangitoto Island (volcano) is a symmetrical, ~ 6 km wide, shield structure rising ~260
m above sea level, and has an estimated dense-rock volume of 1.78 km3 (Kermode 1992;
Needham et al., 2011). The summit region comprises a central scoria cone (Central Cone)
flanked to the north and south by remnant scoria mounds and ridges from older cones (North
and South Cones). Central and South Cones erupted sub-alkalic basalts of narrow
compositional range (~49-50 wt % SiO2; anhydrous whole rock). North Cone erupted lowerSiO2 alkaline basalt (~45-47 wt %). Most of the volcano edifice comprises gently dipping
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(~12° near summit to ~4° near the coastal flanks) pahoehoe and aa lava fields of sub-alkalic
basalt, compositionally similar to the Central and South Cones (Needham et al., 2011).
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Pyroclastic phases of the Rangitoto eruptions are poorly preserved, occurring as
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scattered exposures of <50 cm-thick tephra on the adjacent Motutapu Island (Fig. 1). One of
the better exposed and documented outcrops (because it is associated with archaeological
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horizons) is known as the Sunde site on the west coast of the island (Fig. 1). Previous
sediment coring in several swamps on Motutapu Island revealed two macroscopic basaltic
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tephra layers (Needham et al., 2011). The lower layer has an alkalic basalt composition based
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on bulk-ash analyses, while a single bulk-ash analysis of the upper tephra was reputed to be
sub-alkalic basalt, thus reflecting the bimodality of magma compositions erupted from the
scoria cones (Needham et al., 2011). However, pyroclastic fragmentation and hydraulic
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sorting of glass and crystal components in tephra makes the interpretation of bulk tephra
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compositions problematic. West of the volcano, two basaltic tephra layers are preserved in
lake sediments of Lake Pupuke (Fig. 1) (Horrocks et al., 2005; Molloy et al., 2009). These
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tephra layers have been correlated to Rangitoto volcano on the basis of their stratigraphic
position above the 1.7 ka rhyolite Taupo tephra from Taupo Volcano, and by the composition
of their glass shards. There have been numerous attempts to constrain the age(s) of Rangitoto
volcano (summarized by Nichol, 1992; Lowe et al., 2000; Lindsay et al., 2011). A cluster of
radiocarbon ages around 550-500 cal yr BP associated with tephra on Motutapu Island, are
preferred by Lindsay et al., (2011). The occurrence of two tephra layers variously separated
by peat or lake sediments demonstrate more than one eruption occurred, but the hiatus is
considered to be short (~50 years, Needham et al., 2011).
3. Sediment cores
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We have focused on tephra recorded in sediment cores to decipher the history of
Rangitoto volcano (Fig. 1, 2). Lake Pupuke was the target of our crypto-tephra study because
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of the finely-laminated lithology of the sediments deposited in relatively anoxic conditions.
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Hence, the sediments lack bioturbation that could potentially mask primary crypto-tephra
deposition. We also examined tephra in peat on Motutapu Island because thick macroscopic
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tephra occurrences had been reported.
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3.1. Lake Pupuke
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Core collection, sedimentological and geochemical work on the Lake Pupuke cores
have been previously described (Horrocks et al., 2005; Augustinus et al., 2006; 2008). We
examined two cores (06-06 and 08-06) containing laminated sediments (Fig. 2,3). The
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laminations are millimeter to sub-millimeter-thick alternating dark and light layers. The
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sediment is mud and comprises diatoms, sponge spicules, fine organic material and minor
clastic grains. The laminated appearance results from diatom-rich layers contrasting with
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background sedimentation (Augustinus et al., 2008). The Rangitoto eruption is represented by
two macroscopic basalt tephra layers of coarse ash and fine lapilli, typically < 1 cm thick,
separated by 1-2 cm of lake sediment. Sediments above this tephra are generally massive.
Radiocarbon dating was not attempted because of the documented problems with
obtaining reliable data on these sediments (Horrocks et al., 2005; Augustinus et al., 2006;
2008). Sediment isotope and chemistry studies suggest erroneous old ages can be attributed to
in-washing of old carbon from the catchment, in particular soil-derived bicarbonate. Primary
age constraints for the cores include (1) a chemical marker consisting of two black anoxic
layers near the top of the sequence resulting from the addition of copper sulphate in AD1932
and 1939 (mid-point ~AD1934 = 16 yrs cal BP); (2) basaltic tephra from Rangitoto volcano
using the ages for the two tephra layers (552±7 and 504±5 cal yr BP) considered to best
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constrain the main tephra dispersal event (Needham et al., 2011; Lindsay et al., 2011); and (3)
rhyolite Taupo tephra (1718 ± 10 cal yr BP; Lowe et al., 2008), previously identified on the
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basis of stratigraphy, mineralogy and glass chemistry (Horrocks et al., 2005; Molloy et al.,
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2009).
We developed an age-depth model for the cores using Bayesian statistics (Bronk
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Ramsey, 2009) that systematically combines age data with other ‘prior’ information such as
depth in the core (Table 1). Ages for un-dated basalt crypto-tephra horizons were estimated by
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linear interpolation between the chronologically ordered posterior distributions of the dated
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tephra horizons, similar to the approach described by Wohlfarth et al. (2006). Poisson
distribution (P_Sequence) was applied in the age-depth modelling using OxCal 4.1.7
software. This assumes random deposition of a given number of events (parameter k)
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occurring in a fixed interval of time providing approximate proportionality to the depth
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(Bronk Ramsey, 2009). Since the Pupuke cores were sampled in intervals of 1 cm thick, the
parameter of k = 1 (100 m-1) is used. In addition, we were able to add the well-dated Kaharoa
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tephra (636 ± 12 cal yrs BP; Lowe et al., 2008) to the age model based on the crypto-tephra
investigation (see section 6.1).
Lake Pupuke is a maar that has been dated at 207 ± 6 ka (Cassata et al., 2008). The tuffring surrounding the lake is a potential source of basaltic detritus in the lake sediments. Thus, we
collected samples of coarse ash and fine lapilli through the tuff-ring sequence, exposed in a quarry
and the western margin of the lake (Fig. 1), for chemical fingerprinting. Magmatic deposits of
well-sorted vesicular, juvenile material are more common in the upper part of the sequence while
finer-grained, poorly sorted phreatomagmatic deposits dominated the lower sequence.
3.2. Motutapu Island peat cores
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Cores containing at least two macroscopic basaltic tephra collected from swamps on
Motutapu Island have been described by Needham et al. (2011). The tephra layers are <10 cm
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thick and variously separated by 20-100 cm of peat. In this study, we examined tephra from
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cores MVC5 and 6 of Needham et al. (2011) to chemically fingerprint the glass shards (Fig. 1,
2). We also collected two additional piston cores from the site of MVC6 (BGS 21 and 23).
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The piston coring involves pushing a 1.5 m long, 3.5 cm wide barrel into the substrate and
extracting sediments in 1.5 m long sections. The two cores revealed a stratigraphy similar to
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that found in previous coring (Fig. 2). Core BGS21 contained two macroscopic basaltic layers
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separated by peat. The lower tephra layer was deposited on clay, representing weathered
Miocene sandstones/mudstones. The peats in both BGS 21 and 23 cores contain zones of
coarse volcanic ash that is difficult to detect by eye. However, they are easily delineated by
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magnetic susceptibility peaks that contrast with the organic-rich background deposits (Fig. 2).
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Macroscopic tephra and high magnetic susceptibility tephra zones were sampled for
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geochemical fingerprinting.
4. Laboratory methods
4.1. Crypto-tephra
Two Lake Pupuke sediment cores (06-06 and 08-06) were sub-sampled in 1-cm thick
contiguous slices (Fig. 4). Methods for extracting dispersed glass shards in sediments have
been described elsewhere (e.g., Davies et al., 2005; Gehrels et al., 2006; 2008). Organic
matter in the samples was removed via digestion in heated H2O2 solution, and biogenic silica
(diatoms, spicules) were dissolved in heated NaOH solution. Fine clays and other silicate
materials were removed by wet sieving at 25 microns. For core 06-06 samples, an additional
step was applied using sodium polytungsate (>2 gcm3) to remove sponge spicules. This
density separated component did not contain glass shards. To test the possible effect of
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chemical treatment on the composition of glass shards, we analysed via electron microprobe
aliquots of shards that had been chemically treated and compared them to un-treated shards.
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No geochemical differences were revealed. The residue glass shards were spiked with
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Lycopodium spores to facilitate measurement of glass-shard concentrations, and mounted on
slides. The slides were examined at 400X magnification with a polarizing microscope to
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distinguish and count glass shards. The glass shards were recorded alongside Lycopodium
spores, a quantification method adapted from palynology. Glass shard concentrations are
4.2. Geochemical analysis of shards
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reported as shards per milligram dry weight as described by Gehrels et al. (2006).
Glass shards were embedded in epoxy resin, polished, and analysed by a Jeol JXA-
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840 probe fitted with a PGT Prism 2000 EDS detector at University of Auckland. An
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absorbed current of 1.5 nA at 15 kV was used. The beam was defocused to 20 µm to limit loss
of Na. Calibration was achieved by AstimexTM mineral standards. Glasses in Lake Pupuke
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core 08-06 were analysed by a CAMECA SX100 electron microprobe at the University of
Edinburgh using a10 kV accelerating voltage, 10 nA beam current, and a 4 μm spot size.
Analysis times for 'first cycle' elements (Si, K, Mn, Al, Na) were 30 s peak, 30 s background.
For Na and Si the 30 s peak time was split into 6.5 sec intervals and a decay curve procedure
employed to correct for count rate decrease or increase in Na and Si, respectively. Second
cycle elements counting times were 20 s peak and 20 s background. The instrumental settings
in both laboratories produce high accuracy and precision, as demonstrated by replicate
analyses on glass standards (see Supplementary data). Error values associated with analyses
are dependent on elemental abundance, and were assessed from replicate analyses on
homogeneous glass (Supplementary data). Analytical totals on unknown glasses are in the
range 91-100% (mostly 94-98%); deficiencies being attributed to variable post-eruption
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meteoric hydration, a feature common in tephra of all ages and compositions (e.g., Shane,
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2000). Glass analyses on unknown glasses were recalculated to 100% to aid comparisons.
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5. Results
5.1. Glass shard concentrations
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Glass shards in the Lake Pupuke cores were classified based on color and petrographic
features. Colorless shards are typically pumiceous or cuspate in morphology and free of
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groundmass microlites, but may contain a few crystal inclusions. They are similar to shards
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found in rhyolite tephra layers in the Auckland region (Molloy et al., 2009). Brown shards
display a range of color variants from pale brown to nearly black. They are vesicular or
blocky in morphology and a few display fluidal shapes. Groundmass microlites of silicate
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minerals are common. The shards are similar to those found basaltic tephra (e.g., Molloy et
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al., 2009). In core 06-06, we further attempted to subdivide the colored shards into a pale
yellow group that are finer grained and highly pumiceous and microlite-rich. They are similar
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to shards found in andesite-dacite tephra layers (e.g., Molloy et al., 2009). The shard
concentrations are highly variable both within the cores and between the cores (Fig. 4).
Excluding macroscopic tephra layers and sediments immediately above them, colorless shards
are the most abundant and occur in concentrations up to ~200 shards/mg, but mostly <100
shards/mg in core 06-06 and <60 shards/mg in core 08-06. Brown shards occur in more
localized zones or peaks in both cores with concentrations <60 shards/mg. Yellow shards are
of distinctly lower abundance.
5.2. Rhyolite and andesite crypto-tephra in Lake Pupuke
The low concentration of shards in most samples and their small size (<60 m) made
it impractical to analyze the entire sample suite. Instead, we focused on either localized high
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concentrations of shards in the cores or simply shards large enough to analyze (>40 microns)
with an electron beam without interference from vesicles or microlites (Fig. 4; Supplementary
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data). Previous studies have described the identification and source of tephra layers in
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Auckland, based on stratigraphy, mineralogy, and glass chemistry (Shane, 2005; Molloy et
al., 2009). Although not designed for glass compositions, the total alkali–silica diagram of Le
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Maitre (1984) provides a simple method of classifying tephra layers and recognizing the
volcanic provenance (Fig. 5A). The stratigraphy and geochemical fingerprint of rhyolite
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tephra from Okataina and Taupo Volcanic Centres (OVC and TVC) in central North Island
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(Fig. 1) are well established (Nairn, 2002; Smith et al., 2005; 2006; Lowe et al., 2008). Hence,
they are the primary tephra horizons for erecting stratigraphic frameworks.
The colorless shards in the Lake Pupuke cores are rhyolitic and compositionally match
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tephra erupted from OVC and TVC. Most of the shard populations are heterogeneous,
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variously matching the composition of the 9.5 ka Rotoma, 8 ka Mamaku and 5.5 ka
Whakatane tephra (Fig. 5B), erupted from OVC (Nairn, 2002). A few also match Holocene-
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aged tephra erupted from TVC, including the 1.7 ka Taupo tephra, the youngest event from
TVC. We did not attempt to distinguish between the various TVC tephra layers because of
their high degree of similarity in composition (Smith et al., 2005; Lowe et al., 2008). A few
shards are peralkaline in composition and are similar to the 7 ka Tuhua tephra from Mayor
Island volcano (e.g., Shane et al., 2006). These zones are considered to be the product of
reworking (see section 6.1).
A peak in colorless (and pale yellow) shards at 65 cm in core 06-06 and 78 cm in core
08-06 (Fig. 4), comprise shards of rhyolite and andesite composition (Fig. 5C,D). The rhyolite
composition matches glass in the Kaharoa tephra erupted from OVC (Nairn, 2002) at 636 ±
12 cal yrs BP (~0.7 ka) (Lowe et al., 2008). We consider this horizon to represent an in-situ
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crypto-tephra (see section 6.1). The andesite shards are chemically similar to those erupted
from Ruapheu volcano of the Tongariro Volcanic Centre (e.g., Moebis et al., 2011) (Fig. 5D).
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Other samples containing andesite shards also display affinity to Tongariro volcanic
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centre-sourced tephra, including those from Tongariro, Ngauruhoe and Ruapehu volcanoes
(Fig. 5D). A few shards also show affinities to tephra from Taranaki volcano (e.g., Shane
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2005; Platz et al., 2007). We note that the geochemical fingerprint of more widely dispersed
tephra from the andesite volcanoes is not well established, and thus individual eruptions
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and/or number of eruptions cannot always be delineated.
5.3. Basaltic tephra and crypto-tephra in Lake Pupuke
In core 08-06, samples targeted for geochemical analyses were brown shard
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concentration peaks at 105, 75, 66 and 54 cm depths (Fig. 4B,6A), in addition to the
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macroscopic layer at 63 cm depth. In core 06-06, samples analyzed were from small
concentration peaks at 138, 125, 101, 42 and 22 cm depths (Fig. 4A, 6B), and the
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macroscopic tephra at 59-57 cm depth. Basaltic shards in all of these samples show broad
compositional bimodality. A low-SiO2 (~43-47 wt %, anhydrous) shard group with higher
K2O (and CaO) can be readily distinguished from a high-SiO2 group (~50-52 wt %) with
lower K2O (and CaO) content (Fig. 6A,B). The two shard groups are otherwise similar in
petrography and shard size. The various samples contain either one or both of the high- and
low-SiO2 shard groups, but there are no temporal trends. As previously documented, the
macroscopic basalt tephra layer is preserved as two layers in some cores variously separated
by up to 2 cm of sediment (Horrocks et al., 2005; Molloy et al., 2009). However, the
separation is not discernable in all cores. The lower layer comprises the low-SiO2 shard
group, while the upper layer comprises the high-SiO2 shard group.
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Glass shards and interstitial glass in fine lapilli from the Pupuke volcano tuff-ring
form a compositional array from ~45 to 50 wt % SiO2, somewhat filling the compositional
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gap between the bimodal shards found in the lake sediments (Fig. 6C). Most of the Pupuke
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volcano tuff-ring shards have SiO2 contents of ~45-46 wt %, but are distinguished from the
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low-SiO2 shard group of tephra in the lake sediments by higher TiO2 (and K2O) contents.
5.4. Tephra on Motutapu Island and Rangitoto scoria cones
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Glass in macroscopic tephra layers in Motutapu Island cores form a compositional
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array similar to the low-SiO2 shard group in Lake Pupuke sediments, but also extend to higher
SiO2 contents (~47 wt %) (Fig. 6D). Indeed, most of the shards have SiO2 contents in the
range ~46-47 wt %, somewhat higher than the main low-SiO2 shard population in Lake
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same chemical signature.
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Pupuke sediments. All macroscopic tephra layers and tephra-rich zones in the peat display the
The tephra outcrop at the Sunde site on Motutapu Island also comprises shards that are
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broadly chemically similar to the low-SiO2 shard group in Lake Pupuke sediments and tephra
layers in Motutapu swamp sediments. However, the Sunde site shards display slightly lower
SiO2 contents for the same K2O content, when compared to shards at other sites (Fig. 6D).
The bimodality in whole rock compositions with North Cone comprising alkali basalt,
and Central-South Cones (and lava fields) comprising subalkalic basalt (higher SiO2 and
lower K2O) (Needham et al., 2011) (Fig. 6D), is also reflected in the glass compositions of
these rocks. The glasses display higher SiO2 and K2O than their respective whole-rock host
because the glasses lack crystals (mostly plagioclase and olivine). In contrast, much of the
compositional array of the tephra glasses in the sediment cores extends to lower SiO2
contents. This is inconsistent with these tephra glasses being crystal-free counterparts of the
whole-rock samples, and demonstrates a wider range of magmas were erupted from the
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volcano than that represented by the cones. Interstitial glass in fine lapilli and glass shards
from the Central and South Scoria Cones are compositionally homogeneous and match the
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high-SiO2 shard group from tephra deposited in Lake Pupuke (Fig. 6D). Glasses from the
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North Scoria Cone are compositionally unique amongst the samples, plotting separately on
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geochemical diagrams (Fig. 6D).
6. Discussion
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6.1. Crypto-tephra record
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The sediment in Lake Pupuke records primary and secondary deposition of glass
shards from local and distal volcanoes. The tephra from distal rhyolite volcanoes (TVC and
OVC) are significant because their chronology and geochemical fingerprints are well
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established (e.g., Shane, 2000; Lowe et al., 2008). Hence, they provide a basis for
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constructing a stratigraphy and assessing reworking as demonstrated in previous studies of the
AVF (e.g., Molloy et al., 2009).
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In addition to macroscopic tephra layers (1.7 ka Taupo and ~0.6 ka Rangitoto), cryptotephra are pervasive through the sequences (Fig. 4). Core 6-06 (60 m depth) was collected 239
m south of core 08-06 (55 m depth) on the broadly flat floor of the basin. Although
synchronous in time, the cores record different patterns of shard abundance. In particular,
colorless (rhyolite) shards occur in high abundance in the post-Rangitoto tephra part of core
06-06, but not in core 08-06 (Fig. 4). We note that microscope shard counting was conducted
independently by different operators (core 08-06 = MG; core 06-06 = AZ). This could have
resulted in some un-detected bias. However, there is no systematic difference for the entire
sequence (Fig. 4), and both cores record similar stratigraphic features such as multiple
occurrences of basaltic shard zones and rhyolite Kaharoa tephra (see below), beneath
Rangitoto tephra. Thus, we consider most of the differences to reflect depositional history.
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Spatiotemporal variation in shard deposition is common within lake basins, and is controlled
by catchment inlets and basin geometry (e.g., Pyne-O’Donnell, 2011 and others). Localized
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lake floor relief can contribute to sediment focusing, and past disturbance events induced by
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storms, earthquakes and gravitational instability can remobilize sediment subaqueously,
resulting in discontinuous deposition.
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In the Lake Pupuke cores, zones of compositionally heterogeneous shards whose
compositions match pre-Taupo tephra-age eruptions from OVC and TVC (Fig. 5B) are the
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product of reworking because calc-alkaline rhyolite eruptions of their age (post-1.7 ka) did
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not occur in New Zealand. Most of the shard compositions (including those of peralkaline
affinity) match those in tephra layers that occur as macroscopic horizons stratigraphically
below the sequence examined here (Taupo, Tuhua, and Rotoma) (Horrocks et al., 2005;
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Augustinus et al., 2008; Molloy et al., 2009). Lake Pupuke is a topographically enclosed
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basin that lacks fluvial or marine sediment inlets due to a surrounding basaltic tuff-ring. The
absence of tuff-ring-sourced shards in the cores (see below) indicates that reworking from
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external sources was insignificant. Instead, subaqueous remobilization of lake sediments
around the margin of the basin and transport into the basin centre is the most likely cause of
remobilized shards.
Despite the ubiquitous background deposition of shards in the sequences, discrete
volcanic events are also preserved. One crypto-tephra zone, beneath Rangitoto tephra in both
cores (65 cm in core 06-06; 78 cm in core 08-06), comprises rhyolite shards that match those
of the ~0.7 ka Kaharoa tephra erupted from OVC, and do not match other OVC-sourced
tephra (Fig. 5C). Their stratigraphic position above the 1.7 ka Taupo tephra precludes
correlation to any other event because Kaharoa is the only rhyolite eruption in the time
interval (e.g., Nairn, 2002; Lowe et al., 2008).
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Discrete concentrations of basaltic shards are found in both cores up to ~80 cm
beneath the macroscopic Rangitoto tephra layer (Fig. 4), in the interval between 1.7 ka Taupo
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and 0.7 ka Kaharoa horizons (Table 1). Downward density-settling of the shards from the
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macroscopic layer is seemingly unlikely because of the non-disturbed, sub-mm-laminated
lithology of the sediments (Fig. 3) which is inconsistent with the passage of water-saturated
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sediment, or bioturbation. Density settling of tephra is associated with sediment load
structures and intrusion (e.g., Beierle and Bond, 2002). It is also unlikely that the basaltic
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crypto-tephra zones represent reworked detritus from the Pupuke tuff-ring because of their
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compositional differences (Fig. 6C). Instead the composition of the basalt crypto-tephra zones
in core 08-06 has a distinct bimodality that variously matches the composition of the
macroscopic Rangitoto tephra and Central-South cones of Rangitoto volcano (Fig. 6D). There
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is no evidence for post-10 ka volcanism at other sites in the AVF (e.g., Lindsay et al., 2011).
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Thus, we consider the crypto-tephra zones to represent Rangitoto eruptions.
Shards in the basaltic crypto-tephra zones in core 06-06 display the high-SiO2
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geochemical affinity (Fig. 6B), while those in core 08-06 display both high- and low-SiO2
affinities (Fig. 6A). This is likely to be another artifact of preservation variability, typical of
lake sediments (as described above). Thus, the two cores partly record different phases of the
Rangitoto eruption sequence.
The cores also contain significant peaks in basaltic shard concentrations above the
Rangitoto tephra that display the same compositional bimodality of shards in the macroscopic
tephra (Fig. 4, 6A,B). However, their depositional mechanism is less certain. They could
represent fallout from eruptions or remobilization of sediments. There is a significant increase
in the abundance of remobilized rhyolite shards in the upper part of core 06-06, and the
sediments are finer grained and more massive than the rest of the core (Horrocks et al., 2005).
This interval also shows a decline in tree pollen and appearance of charcoal and Pteridium
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spores interpreted to reflect Polynesian deforestation in the region (Horrocks et al., 2005).
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Hence, there is a strong likelihood for sediment reworking in this part of the core.
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6.2. Chronology of Rangitoto volcano
The timing and duration of activity at Rangitoto volcano has been extensively
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reviewed (Nichol, 1992; Lowe et al., 2000; Lindsay et al., 2012). Erroneously old ages have
been obtained from K-Ar dating of lava due to the presence of excess (non-degassed) Ar
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(McDougall et al., 1969). A limited paleomagnetic investigation of lava flows (Robertson,
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1996) revealed some scatter in the demagnetized remnant directions, but much of the data is
within wide error limits. A cluster of radiocarbon ages on organic material around 550-500 cal
yr BP is associated with tephra layers on Motutapu Island. These date a major episode of
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tephra dispersal that previous workers equate to the construction of the volcano in one
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relatively short episode. We note that some of the reported radiocarbon ages are not consistent
with stratigraphic order (e.g., MVC6 core; Fig. 2). Thus, some ages are erroneous making
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data selection problematic. Interestingly, the only radiocarbon determinations associated with
lava flows on Rangitoto Island produced widely disparate ages of 214 ± 129 cal yr BP for
wood beneath a lava flow and 1161 ± 72 cal yr BP for marine shells in mud baked by a
(different) lava flow. These were dismissed by Nichol (1992) as representing young tree roots
penetrating the lavas, and relict shells pre-dating the eruption, respectively.
We can make several observations that relate to the chronology of the volcano:
(1) Identification of the well-dated Kaharoa tephra (636 ± 12 cal yrs BP) beneath macroscopic
Rangitoto tephra in Lake Pupuke is consistent with most of the reported radiocarbon ages for
the latter.
(2) The composition of the lower macroscopic layer in Lake Pupuke (low-SiO2 group shards)
partly overlaps that of multiple tephra layers deposited on Motutapu Island (Fig. 6D), where
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radiocarbon ages have been determined. However, the latter is dominated by geochemically
distinct shards. The upper tephra layer in Lake Pupuke is also distinct (high-SiO2 shard
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group). This points to multiple tephra eruptions with different dispersal patterns.
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(3) The composition of the upper Lake Pupuke macroscopic layer matches that of Central and
South scoria cones (Fig. 6D). None of the other macroscopic tephra match any of the scoria
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cones. This indicates that the volcanic landforms do not record the entire pyroclastic history
of the volcano. We acknowledge our sampling of the scoria cones was not comprehensive due
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to lack of exposure, and more compositional affinities may have been erupted.
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(4) The two tephra layers and zones of tephra-rich peat deposited on Motutapu Island are
compositionally identical (Fig. 6D). They are not considered to result from local reworking
because the same stratigraphy is found at several sites (Needham et al., 2011; this study).
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Instead, these tephra occurrences reflect episodic eruptions. The sequences lack visible
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rhyolite tephra such as 1.7 ka Taupo tephra, and thus may represent shorter time periods than
that recorded in Lake Pupuke. Hence, much of the radiocarbon data for Rangitoto volcano
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pertains to just one part of the eruption history.
(5) The Lake Pupuke cores contain basaltic crypto-tephra layers that pre-date Kaharoa tephra
(636 ± 12 cal yrs BP), extending back to 1498 ± 140 cal yr BP (Table 1). We consider these to
represent early Rangitoto eruptions and hence, an extended life-span for the volcano of ~1000
yrs.
(6) Most of the crypto-tephra zones contain shards that compositionally match the high-SiO2
group, similar to those of Central and South Scoria cones (Fig. 6). This does not imply that
the two cones were long-lived. Instead, the crypto-tephra zones could represent periods of
earlier volcanism where proximal deposits are now buried beneath the cones. One cryptotephra horizon (at 75 cm) in core 08-06 matches the low-SiO2 group. Thus, the broad
compositional bimodality is a persistent signal through much of the volcano history. Such
18
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petrologic variability has been recognized at other small volcanoes (e.g., Wolff and Strong,
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2003; Brenna et al., 2011).
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The lack of deep erosional incision of Rangitoto volcano prevents any proximaloutcrop evaluation of its longevity. Thus, the compositional uniformity of the lava flows
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(Needham et al., 2011) and their broadly similar paleomagnetic directions (Robertson, 1996)
could simply reflect a veneer of late-stage effusive volcanism that covers older deposits. The
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volcano is thought to represent about half the volume of the magma erupted in the 250-ka-
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history of the AVF. Prolonged activity suggested by the crypto-tephra record (~1000 yrs)
would explain the anomalous volume of the volcano. Tephra dispersal is highly directional,
thus the lack of available sediment repositories at other azimuths around the volcano prevents
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the investigation of whether the prolonged activity was near-continuous or occurred as
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discrete episodes.
The frequency and locus of activity within a terrestrial basaltic field can change on a
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105-yr scale (e.g., Condit and Connor, 1996). Also, some fields display relatively constant
time-erupted volume relations on the 104-105 yr scale (e.g., Kuntz et al., 1986; Conway et al.,
1998). However, it is difficult to develop a unified tectono-volcanic hypothesis for the
modulation of tempo because of the different crustal settings of the fields. The new
perspectives on Rangitoto volcanism support the concept of changing volcanic style and
tempo in the AVF over short durations (103-104 yr). Geochronological studies suggest that
multiple, small volcanic centres were contemporaneously active across the field at around 30
ka (Cassidy, 2006; Cassata et al., 2008; Molloy et al., 2009). In contrast, there have been few
eruptions in the more recent history of the field. Well-constrained evidence for activity in the
interval 10-20 ka is scarce (Molloy et al., 2009; Lindsay et al., 2011), and a scoria cone and
tuff-ring complex was produced at ~10 ka (Mt Wellington centre, Shane and Zawalna-Geer,
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2011). Thus, the relatively voluminous Rangitoto volcano, perhaps representing over 1000 yrs
of activity in the last few millennia, reflects a dramatic regime change to central vent
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volcanism. This has significant implications for AVF hazard/risk models that have focused on
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a short-lived eruption (<1 yr) from a site that has not experienced recent volcanism. Future
modeling should consider the possibility of repeat eruptions from volcanic centers and
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prolonged activity. The socio-economic impact would differ from that resulting from short-
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lived phenomena.
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7. Concluding remarks
The value of crypto-tephra in reconstructing the longevity of a volcano is
demonstrated where the early history of the volcano is not exposed; datable deposits are
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scarce or stratigraphically ambiguous; and macroscopic tephra deposition was limited by
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highly directional fallout or preservation potential of the landscape. The tephra record of
activity at Rangitoto volcano shows it was not a simple ‘monogenetic’ volcano constructed on
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a 101-102 yr timescale. Instead, eruptions occurred between 1498 ± 140 and at least 504 ± 6
cal yrs BP, a time span of ~1000 yrs. This prolonged activity could explain why the edifice
represents about half the known volume of magma erupted in the AVF over its 250-ka life.
Thus, the tempo and style of volcanism in the volcanic field has undergone an dramatic
change in the last ~2 ka to prolonged central vent volcanism. This demonstrates that terrestrial
basaltic fields can significantly change character during their evolution in a short period of
time. Future hazard and risk modeling will have to consider this style of volcanism rather than
the traditional view of infrequently erupting, short-lived phenomena producing small volcanic
edifices.
Acknowledgements
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Pupuke coring and radiocarbon dating was funded by Marsden Fund grant UOA0517
to Augustinus (PI). Ian Snowball and Per Sandgrun provided valuable coring assistance.
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Lindsay acknowledges funding from New Zealand Earthquake Commission and Auckland
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City Council. We thank editor Joan Marti and an anonymous reviewer for their comments.
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Figure captions
Fig. 1 (A) Map of Rangitoto volcano and environs showing core and sample sites. See
Supplementary data for sample site longitude and latitude. (B) Map of the Auckland Volcanic
Field (modified from Kermode, 1992) showing the distribution of volcanic landforms and
location of Rangitoto volcano. (C) Map of North Island, New Zealand showing the location of
Quaternary volcanoes that have contributed tephra to the Auckland region.
Fig. 2. Summary logs of cores containing Rangitoto tephra examined in this study, with
magnetic susceptibility data (MS in dimensionless SI units) for BGS 23 and 21. For site
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locations see Fig. 1A and Supplementary data. Macroscopic tephra layer samples used in
geochemical studies are marked as depths (in cm, right of log). Ages for MVC5 and 6 are
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calibrated radiocarbon data from Needham et al., (2011). The Sunde site is an outcrop on
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Motutapu Island.
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Fig. 3. X-ray density image of part of Lake Pupuke core 06-06 showing laminated sediments
enclosing the interval of a basaltic crypto-tephra at 101 cm. Bright shading reflects high
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density, and dark shading reflects low density.
Fig. 4. Concentration of shards in sediment samples from Lake Pupuke cores 06-06 (A) and
08-06 (B). Numbers represent depths (cm) of samples used in geochemical studies.
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Macroscopic tephra layers of Rangitoto and Taupo tephra are shown as grey bars. Note
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differences in scaling of shard concentrations (x-axis).
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Fig. 5 (A) Composition of individual glass shards from cores and outcrops examined in this
study. Compositional fields are from Le Maitre (1984). A - andesite; B - basalt; BA - basaltic
andesite; BS - basanite; D - dacite; F - foidite; T - trachyte; TA- trachyandesite; TD trachydacite and R – rhyolite. The tephra grouping represents all macroscopic and cryptotephra layers; cones represent North/Central/South Scoria Cones; and tuff-ring represents
Pupuke Volcano. (B) Composition of glass shards in heterogeneous crypto-tephra zones from
Lake Pupuke cores 06-06 and 08-06, compared to named Holocene tephra layers from distal
volcanoes (compositional fields from Smith et al., 2006; Shane et al., 2006; 2008; Shane
unpublished data). (C) Composition of glass shards in a crypto-tephra found in both core 0606 and -08 that matches Kaharoa tephra. Compositional fields as in (B). (D) Composition of
glass shards in andesite-dacite crypto-tephra in Lake Pupuke. Compositional field for
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Tongariro Volcanic Centre (Ruapehu, Nguaruhoe and Red Crater) from Moebis et al., (2011);
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and Taranaki volcano (Platz et al., 2007).
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Fig. 6 (A) Composition of basaltic shards in crypto-tephra zones of Lake Pupuke core 08-06.
See Figure 4 for sample positions. Lower and upper tephra is the compositional fields of the
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Rangitoto macroscopic tephra layers (data from Molloy et al., 2009 and this study). (B)
Composition of basaltic shards in crypto-tephra zones of Lake Pupuke core 06-06. Lower and
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upper tephra is Rangitoto tephra as in (A). (C) Composition of glass shards in the Pupuke
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volcano tuff-ring compared to all basaltic macroscopic and crypto-tephra in Lake Pupuke
sediment cores. (D) Composition of shards in macroscopic tephra from Motutapu Island cores
and the Sunde site; glass from the North/Central/South Scoria Cones; and all tephra and
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crypto-tephra from Lake Pupuke cores. Whole rock (WR) analyses from the scoria cones are
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shown for comparison (data from Needham et al., 2011).
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16
552
4
7
MA
7
63
66
75
78
105
212
638
12
1718
10
16
553
558
626
634
659
1718
4
7
34
34
12
76
10
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Core 08-06
Copper
Rangitoto
Basalt crypto-tephra
Basalt crypto-tephra
Kaharoa
Basalt crypto-tephra
Taupo
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Table 1
Ages for basaltic crypto-tephra layers in Lake Pupuke based on Bayesian modelling
Tephra
Depth Input age
Model age
(cm) (yrs BP) ± yr
(yrs BP) ± yr
Core 06-06
Copper a
5
16
4
16
4
Rangitoto upper
57
504
6
505
6
Rangitoto lower
60
552
7
551
8
Kaharoa
65
636 12
634 12
Basalt crypto-tephra
101
922 35
Basalt crypto-tephra
125
1040 164
Basalt crypto-tephra
138
1498 140
Taupo
159
1718 10
1718 10
AC
CE
PT
Assumes constant sedimentation rate between dated horizons. Poisson distribution using
OxCal 4.1 (ShCal04 dataset). Prior parameter k = 1 (see Bronk Ramsey, 2009). Errors = 1
sigma.
a
copper sulphate geochemical marker horizon represents AD1934 (16 yrs cal BP), the
average of two reported anthropogenic additions in AD1932 and 1939 (Augustinus et al.,
2006).
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PT
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MA
NU
SC
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P
T
ACCEPTED MANUSCRIPT
Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Fig. 6
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Highlights
Crypto-tephra reveal a previously unrecognized record of volcanism .
T
Rangitoto volcano considered ‘monogenetic’ but erupted for ~1000 years.
RI
P
Dramatic change in eruption style, volume and frequency in Auckland region.
AC
CE
PT
ED
MA
NU
SC
Hazard modeling needs to be revisited.
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