COMITÉ DE CONSULTORES EXTERNOS El Comité Asesor de Expertos del PEEC de Química

The first numerical age assigned to a prehistoric tephra deposit from Mt. Taranaki has been defined in 1833 by A.W. Burrell, who documented pumice fragments lodged in the forks of matai (Prumnopitys taxifolia) trees near Stratford. Using tree-ring counts

Burrell estimated an age of c. AD 1430 for this deposit, formally known as Burrell Lapilli (Oliver, 1931). Dating of charcoals from a Maori oven buried beneath the Burrell Lapilli produced similar ages of 400±60 yr BP and 360±60 yr BP (Oliver, 1931; Fergusson and Rafter, 1957). The most frequently used age of the Burrell eruption is based on dendrochronological analysis of kaikawaka (Libocedrus bidwillii) trees and

yields an age of AD 1655 (Druce, 1966). The pioneering studies of Oliver (1931) may qualify as the first application of tephrochronology in New Zealand (Lowe, 1990).

First detailed tephrochronological work on Mt. Taranaki was mainly based on soil and peat surveys. Grange and Taylor (1933) recognised two widespread tephra ‘showers’ (airfalls) by mapping several soil-forming tephra deposits in the Taranaki region. The Stratford Shower (Ash), which is mainly dispersed to the east, and the overlying Egmont Shower (Ash), which is mainly dispersed from the north-east to the south-east. Further studies along the northern coastline of Taranaki by Wellman (1962) documented numerous andesitic ash layers, which were correlated with the Burrell Lapilli (Oliver, 1931), Newall Ash and Stratford Ash (Taylor, 1954). Beneath these ashes another three were grouped into the Stent Ash and another two into the Onaero Pumice (Wellman, 1962). The stratigraphy and chronology of the Stent Ash (silicic, Taupo-derived tephra) is described in detail by Alloway et al. (1994).

The first comprehensive synthesis of the volcanic eruption history of Mt. Taranaki was made by Druce (1966), who mapped and dated pyroclastic deposits on the upper flanks of the volcano during a general soil and vegetation survey. Nine andesitic ash layers, which were erupted during the last phase of volcanic activity at Mt. Taranaki, were grouped into three formations: (1) Tahurangi Formation (AD 1755), (2) Burrell Formation (AD 1655), and (3) Newall Formation (prior AD 1604). Shortly afterwards, Druce’s stratigraphy was refined by Tonkin (1970) who studied the influence of these tephras on soil formation in the Dawson Falls region, and Topping (1972) who presented new radiocarbon dates and a detailed description of the widespread Burrell Lapilli, including its deposition and dispersal pattern. Subsequently, Cronin et al. (2003) introduced the term ‘Maero Eruptive Period’ to represent the latest stage of volcanic activity at Mt. Taranaki.

The investigations of Neall (1972) demonstrated a more extensive andesitic tephrochronological record. He studied the north-western flanks of Mt. Taranaki and documented ten additional ash and lapilli formations deposited during the last 100 ka. Deposits include the formally named Inglewood and Korito Tephra (p2 and p1, respectively of Druce, 1966), the Egmont Shower of Grange and Taylor (1933), which was split into two separate tephra units: (1) Eg 1 – Oakura Tephra (~6970±76 yr BP) and (2) Eg 2 – Okato Tephra and Aa – Ahuahu Lapilli (c. 14,000 yr BP), as well as the underlying Gg – Saunders Ash, which is considered to be a nuées ardentes deposit

(pyroclastic flow - PDC) dated at 16,100±220 yr BP (Neall, 1972). The Koru Lapilli represents a widespread marker bed in older pyroclastic deposits recognised (34,400±1500 yr BP; Neall, 1972). The bottom of the record is characterised by an unconformity, which separates a major ash-group, termed the “New Plymouth Ashes and buried soils”, from the overlying deposits. Neall (1972) suggests an age range of 70-100 ka for the lowermost formation, but these were only based on relative stratigraphy, because radiocarbon dating had only been completed on post-12 ka deposits (Alloway et al., 1995).

Whitehead (1976) and Franks (1984) subsequently added new data to previous defined tephra deposits. Whitehead (1976) informally named the Manganui Tephra and Kaupokonui Tephra (p4 of Druce, 1966) as well created detailed isopach maps, which clearly show a multi-lobe distribution pattern (SE, E, and N) for the Manganui Tephra, which imply Fanthams Peak as its source. The documented age of the Manganui Tephra (3200 yr BP) and the age of the Kaupokanui Tephra (1200 yr BP) are estimated ages from their stratigraphic position between dated tephra and/or lahar deposits. Franks (1984) studied the tephra layers on the eastern flanks of Mt. Taranaki. She described and analysed several tephra units below the prominent Manganui Tephra, encompassing the informally named Mahoe, Konini (E1), Kaponga (E2), Waipuku (E3), Tariki (E4), and Mangatoki (E5) tephras, which were correlated to the previously defined Oakura and Okato Tephra Formations of Neall (1972). Although Alloway et al. (1995) outlined the most complete record of the eruptive history of Mt. Taranaki, the pioneering work of Druce (1966), Neall (1972) and Franks (1984) form the basis of all following tephrostratigraphic and tephrochronological investigations.

The study of Alloway et al. (1995) comprises a comprehensive post-28 ka tephra synthesis, which they grouped into sixteen andesitic tephra formations. They recorded 76 eruptive events with volumes of >107 m3 and outlined three main tephra successions based on tephra characteristics and stratigraphic position. Those are (1) the upper tephra succession from Manganui to Mahoe Tephra(s) (3 to 11.5 ka period), (2) the upper sequence of the lower tephra succession from Kaihouri to Poto Tephra(s) (12 to 22.7 ka period) and (3) the lower sequence of the lower tephra succession from Tuikonga to Waitepuku Tephra(s) (23.4 to 28 ka period). The distributions of these tephras are

mainly to the NNE and SSE from the present vent, consistent with the dominant prevailing wind directions. Alloway et al. (1995) determined an average eruptive periodicity of one in every c. 330 years, which should be considered as a minimum, since tephras of lower magnitude eruptions are prone to rapid erosion and weathering in ring-plain environments (Lowe, 1986; Alloway et al., 1995). In contrast to previous studies of Turner et al. (2008a, 2009), who suggested a highly cyclic eruption frequency of one per 80 years with a 1500-2000 year periodicity using high-precision lake tephra records.

Alloway et al. (1995) also described two TVZ-sourced silicic tephra layers inter-bedded in the andesitic tephra successions from Mt. Taranaki. Both, the Stent Tephra (first recognised by Wellman, 1962) and the Aokautere Ash (first recognised by Rich, 1959 and named by Cowie, 1964), also known as Kawakawa Tephra (Vucetich and Howorth, 1976), are important stratigraphic marker beds used for age determinations and correlation purposes. The Stent Tephra has an error-weighted mean age of 3970±31 yr BP (Alloway et al., 1994) and the Kawakawa Tephra has a radiocarbon age of 25,360±160 cal yr BP (Vandergoes et al., 2013). Both tephra layers are easy to recognise on the basis of their distinct pale colour and glassy fine ash texture.

During a vegetation study by McGlone and Neall (1994) several andesitic tephra layers were identified in a peat core from the Eltham Swamp, c. 25 km to the SE of Mt. Taranaki’s summit (Fig. 1.3). Five tephra layers were correlated with the Burrell Lapilli (295 yr BP), Kaupokonui Tephra (1390±150 yr BP), Manganui Tephra (between 2890±100 to 3320±60 yr BP), Inglewood Tephra (3950±50 yr BP) and Korito Tephra (5000±90 yr BP). A prominent unnamed grey lapilli bed was dated at 10,150±100 yr BP, at the bottom of the peat-core. Dates were determined by radiocarbon dating of peat and wood samples above and/or below tephra layers. Another vegetation study by Lees and Neall (1993), which focused on the impact of volcanic eruption on the vegetation, suggested that the Tahurangi eruption is probably younger than previously estimated by Druce (1966). On the basis of pollen analysis they recognised a >70 year-long soil- forming interval between the Burrell Ash and Burrell Lapilli. Lees and Neall (1993) cored several peat bogs around the volcano including the Midhirst Swamp (Fig. 1.3). A

60 cm long peat core from the Midhirst Swamp contained the Burrell Tephra and the Kaupokonui Tephra.

The medial and distal tephra record of Mt. Taranaki is best preserved in lake and peat environments surrounding the volcano. Andesitic tephra deposits from Mt. Taranaki were identified in lake sediments of the Waikato lakes (Hamilton), c. 200 km to the NNE (Lowe, 1988a), in sediment successions of Lake Umutekai, c. 25 km to the NNE as well as Lake Rotokare, c. 30 km to the SE of the summit (Turner et al. 2008a, 2009; Fig. 1.3) and in sediments of Lake Tutira, c. 250 km to the east in Hawkes Bay (Eden et al., 1993). Further, numerous Mt. Taranaki-sourced tephra deposits occur in lake and maar sediments around Auckland City (i.e., Lake Pupuke, Onepoto Basin, Pukaki Lagoon, Orakei Basin, Hopua Crater), some 270 km N-NW from the source (Sandiford et al., 2001; Shane and Hoverd, 2002; Shane, 2005; Molloy et al., 2009). Distal tephra deposits preserved in lacustrine environments have been shown to be valuable records for high-resolution investigations on large- and small-scale eruption events, due to their continuous sedimentation and good preservation conditions (Lowe, 1986, 1988b). These records are also important for frequency and distribution analysis (Shane, 2005; Turner et al., 2008a, 2009; Bebbington et al., 2011; Green et al., 2013). The records recovered from Auckland sites show many andesitic tephra layers from Mt. Taranaki in the time interval 20-40 ka and minor tephras in the intervals 60-50 ka and 10 ka-present. This variable preservation is possibly explained by different atmospheric circulation conditions in the glacial periods (Horrocks et al., 2005; Molloy et al., 2009). However, near to the volcano, sediments in Lake Umutekai and Lake Rotokare (Turner et al., 2008a, 2009) reveal a high-precision post-10 ka eruption record for Mt. Taranaki. Within a 3.5 m-long core extracted from bottom sediments of Lake Umutekai, 104 individual tephra layers were identified and dated between c. 1550 – 10,000 yr BP, and 42 tephra layers dated between c. 500 – 6250 yr BP were recognised in a 5 m-long core from Lake Rotokare. These temporally-constrained tephra records from the lakes provided statistical data for subsequent volcanic event frequency analysis and probabilistic eruption forecast modelling (Turner et al., 2008a, 2009, 2011b). The data highlighted a pattern of highly cyclic variations in the eruption frequency at Mt. Taranaki, having a 1500-2000 year periodicity.

Further, Turner et al. (2008b) used titanomagnetites as geochemical fingerprints, because andesitic glass shards were geochemically heterogeneous (Shane, 2005) and commonly contain an abundance of microlites, which are difficult to avoid with large- scale electron microprobe beam areas required for glass analysis (Platz et al., 2007b). Turner et al. (2008b) distinguished two eruption styles by examining titanomagnetite textures: (1) slow-ascent eruptions (dome-growth) characterised by exsolved titanomagnetites and (2) fast-ascent eruptions (sub-Plinian) characterised by homogenous titanomagnetites. Further, Turner et al. (2009) predicted a probability of 0.52-0.59 for an eruption occurring in the next 50 years, using the combined eruption record and the mixture-of-Weibulls model (Bebbington and Lai, 1996). Thus, modern tephrochronology has not only focused on the overall chronostratigraphic background of Mt. Taranaki but has also used combined tephra records to understand its long-term eruption behaviour.

Latest work at proximal-medial sites (e.g., Platz et al.,, 2007a, 2012; Turner et al., 2008c, 2011a; Zernack et al., 2011a) focused on detailed re-mapping of ring-plain successions and costal sections so as to build up a more comprehensive record of eruptive behaviour. Interbedded tephra layers were used to build a detailed chronostratigraphic record for the whole volcanic-volcaniclastic succession of Mt. Taranaki. Zernack et al. (2011a) identified at least 14 debris avalanches, which occurred during the last ~170 ka suggesting one major edifice collapse on average every 10-14 ka, with an increase in frequency during the last 40 ka. The estimated age of c. 130 ka (Alloway et al., 2005) for the earliest volcanic activity at Mt. Taranaki has been revised to ~170 ka based on the emplacement age of the newly termed Mangati Formation (Zernack et al., 2011a). Further studies of Zernack et al. (2012) forecasted a potential edifice collapse of 7.9 km3 in the next 16.2 ka based on volume-frequency models of debris avalanche deposits on Mt. Taranaki. Platz (2007) provides a detailed reconstruction of the latest eruption period, known as the Maero Eruptive Period (<1000 yr BP), which encompasses at least 10 eruptive episodes beginning from the Kaupokonui eruptive event to the latest eruption, newly termed as the Sisters eruption (1785-1820 AD; Platz et al., 2012). Platz et al. (2007a) focused mainly on non- explosive dome-forming eruptions, but identified the causes of rapid changes from effusive to explosive behaviour. Turner et al. (2011a) additionally studied twelve key

stratigraphic locations on the ring-plain of Mt. Taranaki and correlated some of these proximal deposits to the lake record (i.e., Lake Rotokare) on the basis of titanomagnetite chemistry and canonical discriminant function analysis (DFA). This was realised to be the first attempt to connect on-cone deposits with fallout deposits in lake sites. However, correlation of proximal-medial-distal deposits still remains a difficult task at Mt. Taranaki, since no geochemical data is available from on-cone sites, and units in soil sequences are often plagued by poor time-control and incomplete preservation.

Tinkler (2013) studied pollen in a more recent core from the Eltham Swamp. They recovered an 18 m-long core dating back to c. 40,000 cal yr BP This Eltham Swamp core contained 130 tephra layers, which have been correlated to medial deposits of Alloway et al. (1994) on the basis of stratigraphic position, colour, grain-size and sorting, isopach thicknesses and radiocarbon ages. New radiocarbon ages have been determined for several tephra layers. Tinkler (2013) mainly concentrated on palaeo- environmental and -climate reconstructions and created a detailed palaeo-wind record for the Eltham area.