Modalidades de evaluación del ABP.

Mt. Taranaki (Egmont volcano) is a basaltic to andesitic-dacitic strato-volcano that rises in near perfect conical form to 2518 m above sea level (Fig. 3.1). It is the westernmost expression of current volcanic activity within New Zealand, lying some 180 km behind the active volcanic arc (Cole 1990). Although currently in a state of quiescence, recent studies (Alloway et al. 1995; Turner et al. 2008a; Appendix 1) have shown it to have been very active, with VEI 4+ pumice eruptions occurring interspersed with relatively smaller scale explosive and lava extrusion episodes. The frequency-volume curve estimated from the Umutekai lake core by Bebbington et al. (2008) shows a logarithmic relation with slope approximately 1. Approximately 15% of the estimated volumes are greater than 0.1km3, with the largest at 0.94 km3. Turner et al. (2008a) noted that there appears to be no significant relationship between eruption size and the following (or preceding) repose period. This type of activity, and also the geochemistry/petrology of the resulting deposits, is very similar to those of several recently highly active volcanoes, including Merapi (Indonesia), Unzen (Japan), Colima (Mexico) and Augustine (Alaska) and hence Mt. Taranaki must be seen as having a similar hazard potential as these volcanoes. The date of the most recent eruption from Mt. Taranaki is controversial. Druce (1966) used dendrochronology to date a distinct, locally distributed ash within the soil profile of the edifice to AD 1755. Paleovegetation studies and pollen evidence (Lees and Neall 1993) indicated that the last tephra producing event was around AD 1860, at or near the time of European settlement in the area. Recent stratigraphic and geochemical evidence confirms that two post AD 1755 events occurred at c. AD 1800 and 1854 (Platz et al. 2006).

Figure 3.1: Map of the Taranaki region showing the locations of the sample sites;

Lake Umutekai and Lake Rotokare in relation to Mt. Taranaki’s edifice. 300 m contours shown in grey with the >1500 m highlighted. (S) Summit vent, (F) Fanthams Peak vent, (P) Pouakai Volcano (extinct), (K) Kaitake Volcano (extinct). The grey squares T-1 and T-3 are the sample sites used by Alloway et al. (1994) to date the rhyolitic marker bed: the Stent Ash. The black square gives the location of Eltham Township and main river channels which direct flows towards the lakes are shown in dark grey (see text). Insert: North Island, New Zealand.

In order to compile as complete as practicable eruption dataset for Mt. Taranaki, three separate eruption records were acquired. Two of these were obtained from sediment cores from lakes that are located sufficiently near to the volcano to record minor eruptive events, but at a great enough distance to be mostly unaffected by debris and pyroclastic flow deposits. Lake Umutekai, lying 25 km NNE of the summit of Mt. Taranaki (Fig. 3.1), is a lake approximately 0.02 km2 _{and up to 2 m-deep formed within}

a swampy depression on a planar volcaniclastic surface. The surrounding landscape was densely forested throughout the Holocene (McGlone and Neall 1994). Lake Rotokare lies 30 km southeast of Taranaki’s summit (Fig. 3.1). It is a Y-shaped lake, formed by a landslide damming event. It is 0.25 km2 in area with a maximum width of 200 m and a

maximum water depth of 12 m at the apex of the Y-junction. Lake Rotokare and its head waters are surrounded by steep unstable hills of weakly consolidated Pleistocene and Pliocene marine blue-grey mudstones (cf., Fleming 1976).

Several cores were collected in 1 m sections from each lake using a 35 mm diameter hand-operated piston corer. The cores were imaged with an X-Ray camera to identify mineral layers within the organic host sediments, particularly sub-mm units. No evidence of erosion (non-deposition or sediment disturbance) was found in the cores. All of the mineral layers identified within the Umutekai core were confirmed to be primary tephras by optical microscopy (well sorted angular grains, including glass shards or glass-coated particles). However, the mudstone hills surrounding Lake Rotokare have periodically contributed layers of mineral-rich, mudstone material. Therefore each mineral layer identified within the X-ray images was carefully examined under microscope and primary tephras layers were identified and sampled for further analysis.

Table 3.1: Radiocarbon dates from lake sites used in this study Date Code or age source Age (Yrs B.P.)a Material Sample Depth (mm) Source Umutekai core S39º 05.27’ E174 º08.22’

NZ23065 1559 ± 40 Peat 148-163 Turner et al. (2008a) NZ23066 1848 ± 35 Peat 356-379 Turner et al. (2008a) NZ23067 2098 ± 35 Peat 570-585 Turner et al. (2008a) NZ23117 2369 ± 30 Peat 678-695 Turner et al. (2008a) NZ23068 3142 ± 35 Peat 1029-1045 Turner et al. (2008a) NZ23069 3507 ± 35 Peat 1189-1205 Turner et al. (2008a) NZ23085 4345 ± 40 Peaty silt

loam 1699-1715 Turner et al. (2008a) NZ23089 7068 ± 55 Peaty silt

loam 2399-2415 Turner et al. (2008a) NZ23118 9219 ± 45 Peaty silt

Table 3.1: cont. Rotokare core S39º 27.10’ E174 º24.64’

NZ5511 547 ± 35 Peat 260-273 This Study NZ5512 2120 ± 35 Peat 2077-2090 This Study NZ5513 3065 ± 30 Peat 2630-2645 This Study NZ5463 3749 ± 30 Peat 3400-3415 This Study NZ5464 4662 ± 30 Peaty silt _loam 4093-5005 This Study NZ5465 6074 ± 40 Peaty silt _loam 4776-4790 This Study

Slices (1 cm) of the organic sediments were sampled for radiocarbon age determination using the AMS method at the Rafter Radiocarbon Laboratory, New Zealand. Nine samples (a tenth was contaminated, and discarded) were dated from the Umutekai core (Turner et al. 2008a) and six from Rotokare (Table 3.1). In addition to these dated horizons, the rhyolitic ‘Stent Ash’ sourced from the Taupo Volcanic Centre ~250 km NE of Mt. Taranaki produces a common horizon within each record (cf., Alloway et al. 1994). These dates (Table 3.1) have normally distributed errors (Stuvier and Pearson 1992) and along with the true sedimentation depth of each core (i.e., with instantaneous depositional events, such as tephra fall being subtracted), have been used to construct depositional/sedimentation rates for each lake (Fig. 3.2). The sedimentation rate of Lake Umutekai is considerably slower than that of Lake Rotokare, with the 3.5 m core obtained from Lake Umutekai containing 104 tephras between ~1550 and ~10,150 years B.P.; compared to 42 tephras between 500 and ~6250 years B.P. of Lake Rotokare’s 5 m core.

Ash layers preserved within the lacustrine environment may represent anywhere between a few hours to approximately a year of near continuous volcanic activity. The relatively slow sedimentation rates within the lakes means that time intervals of a few months between individual eruption plumes will not be separated by organic sediment. Therefore each of the identified ash layers within these records are termed events

because the Simkin and Siebert (1994) event definition is of a series of linked eruptions separated by intermittent quiescence of no more than three months.

Table 3.2: Radiocarbon dates of ‘near source events’ used in this study

Date Code or age source

Age (yrs B.P.)a

Location Material Material Source

Lat (ºS)

Long (ºE)

Estimated historical

ageb 96 N.A. N.A N.A. Turner et al. (2008a)

Estimated historical

ageb 150 N.A. N.A. N.A. Turner et al. (2008a)

NZ5593 <250 39º 16.30’ 174º 00.00’ Charcoal Turner et al. (2008a) Dendrochronology

agec 195 N.A. N.A. Tree rings Druce (1966)

NZ721 249 ± 55 39º 20.30’ 174º 13.70’ Wood Grant-Taylor and _{Rafter (1971)} Wk 2376 300 ± 60 39º 15.41’ 173º 56.10’ Charcoal This study Wk11590 305 ± 39 39o _16.26’ 174o

00.80’ Charcoal Turner et al. (2008a) NZ63 300 ± 60 39º 18.46’ 174º 07.50’ _{(anthropogenic)} Charcoal Fergusson and Rafter ₍₁₉₅₇₎

NZ64 400 ± 60 39º 18.46’ 174º 07.50’ Charcoal Fergusson and Rafter ₍₁₉₅₇₎

R-Combine of NZ63

and NZ64d 356 ± 42 N.A. N.A N.A. N.A.

Wk11593 382 ± 39 39o _15.91’ 173o

58.73’ Charcoal Turner et al. (2008a) Wk11585 387 ± 43 39o _16.65’ 174o

00.32’ Charcoal Turner et al. (2008a) NZA941 404 ± 44 39º 15.10’ 173º 56.10’ Charcoal Neall (1979) NZ720 439 ± 55 39º 16.5’ 174º 00.1’ Leaves Grant-Taylor and _{Rafter (1971)} NZ1141 447 ± 40 39º 15.10’ 173º 56.10’ Charcoal Turner et al. (2008a) Wk11589 453 ± 43 39o _16.26’ 174o

00.80’ Charcoal Turner et al. (2008a)

R-Combine of NZ720, NZ1141 and

Wk11589d 447 ± 26 N.A. N.A N.A. N.A.

Wk11584 468 ± 42 39o _{16.65’ 174}o _{00.32’ Charcoal Turner et al. (2008a)}

Wk11591 475 ± 42 39o _{16.26’ 174}o _{00.80’ Charcoal Turner et al. (2008a)} R-Combine of

Wk11584 and

NZ567 537 ± 55 39º 18.10’ 173º 59.00’ Charcoal Grant-Taylor and _{Rafter (1971)} NZ1256 620 ± 74 39º 16.31’ 174º 00.01’ Charcoal Neall (1979) Wk11594 605 ± 42 39o _{15.91’ 173}o _{58.73’ Charcoal Turner et al. (2008a)} R-Combine of

NZ1256 and

Wk11594d 609 ± 37 N.A. N.A N.A. N.A.

Wk11592 649 ± 49 39o _{16.26’ 174}o _{00.80’ Charcoal Turner et al. (2008a)}

Wk11586 878 ± 39 39o _{16.66’ 174}o _{00.32’ Charcoal Turner et al. (2008a)}

Wk2378 970 ± 40 39º 05.10’ 173º 56.90’ Wood Neall (1999) NZ5596 1000 ± 60 39º 16.30’ 174º 00.00’ Charcoal Turner et al. (2008a)

R-Combine of Wk2378 and

NZ5596d 979 ± 33 N.A. N.A N.A. N.A.

Wk-16391 1130 ± 34 39º 17.11’ 174º 06.00 Charcoal Turner et al. (2008a) NZ6508 1390 ± 150 39º19.01’ 174º 08.60’ Peat McGlone et al (1988) ‘Stent ash’ Combined average of: Wk1259, Wk1032d

3920± 59 N/A N/A N/A Alloway et al. (1994)

a _{Radiocarbon age in years before present (B.P.) using Libby half-life} b _{Eruption age estimated from historical evidence (T. Platz, unpublished data)} c _{Dendrochronology estimate from damage to living trees by Druce (1966)} d _{Average radiocarbon dates, combined using OxCal v.3.9, Brook-Ramsay (2003).}

The soft, saturated and unconsolidated lake sediment within the upper 300-500 mm of the lake was difficult to sample because it could not be easily extracted using the piston- core method. Hence the last c. 1500 years B.P. from the Umutekai record and c. 500 years B.P. of the Rotokare record are missing (Fig. 3.2). Turner et al. (2008a; Appendix 1) completed the Umutekai record by compiling a dataset of 23 dated eruptions deposited on the edifice (Table 3.2). These eruptions occurred between the most recent known event at 96 years B.P. to one that occurred approximately 1400 years B.P. This dataset has also been included in the eruption hazard model section of this study.

Figure 3.2: Depth-age curves. Dated events indicated by circles, imputed events by

their estimated age 2 standard deviations.