Some Aspects of Quaternary History of Tongariro Volcanic Centre

Abstract

The Tongariro Volcanic Centre, at the southern end of the Taupo Volcanic Zone, North Island, New Zealand, consists of five large andesite volcanoes – Ruapehu, Tongariro, Ngauruhoe, Pihanga and Kaharamea-Tihia – with several associated minor volcanoes and vents all of Quaternary age. In the northern part of the Centre the north-westerly trending Kakaramea-Tihia Massif and Pihanga form a chain of volcanoes which last erupted more than 20,000 years BP, but from which large mudflows have recurred up to as recently as 1910 A.D. By contrast the other two multiple vent volcanoes – Ruapehu and the Tongariro Massif – include several vents (four and possibly five) that have been active during the last 100 years and, in particular, Ngauruhoe and Ruapehu Crater Lake. In the first part of the thesis the tephrostratigraphy of the Tongariro area is described. Thirteen rhyolitic tephras erupted from Okataina, Maroa and Taupo Volcanic Centres over the last 20,000 years are interbedded with local andestitic tephras. The order of succession of the 13 tephras is established by stratigraphy, and is confirmed by their mineralogical composition and by chemical analysis of their titanomagnetites. Seven of the rhyolitic tephras have been previously dated by 14C, four more (Rotorua Ash, Karapiti Lapilli, Papanetu Tephra and Poronui Tephra) are dated hare, and two others are not yet dated. Six further rhyolitic tephras are of less use as marker beds. Four of these are tentatively correlated with rhyolite tephras from the more northerly volcanic centres and all but one are older than, 20,000 years BP. These rhyolitic tephra marker beds have provided valuable time control which, together with 12 further radiocarbon dated events has allowed a chronology of the andesitic tephras erupted from vents within the Tongariro Volcanic Centre. Two previously named tephras, Ngauruhoe Tephra and Magatawai Tephra, are given formation status, and a further four formations (Papakai Tephra, Mangamate Tephra, Okupata Tephra and Tetoaira Lapilli) are defined. Ngauruhoe Tephra (0-1819 years BP), Mangatawai Tephra (c. 1819-2,500 years BP), and Papakai Tephra (c. 3,400-9,700 years BP) have all accumulated as a result of intermittent activity from several vents. Mangamate Tephra, with six named members, was erupted mainly from the Tama Lakes-Ngauruhoe area between c. 9,700-9,780 years BP. Four of the members were deposited eastwards from the Tongariro Massif. The oldest member, Te Rato Lapilli, was erupted c. 9,780 years BP probably from North Crater. It was distributed in a single lobe to the NNE and is the best andesitic tephra marker bed in rhyolite tephra sequences further north. The youngest member, Poutu Lepilli, was erupted as three lobes (NW, NE, SE) from a source probably at Blue about 9,700 years BP. Poutu Lapilli has a volume of 0.9 km3 and the tephra of the total formation is likely to have destroyed more than 1,000 km2 of the pre-Mangamate forest. Ruapehu was very active between c. 10-14,000 years BP. Most of the lapilli and ash beds were deposited east of the volcano and most of the remainder were deposited to the north. The youngest of the units erupted northwards, Okupata Tephra, underlies Te Rato Lapilli and probably has an age between 10-12,000 years BP. Rotoaira Lapilli is prominent north-east of the Tongariro Massif. It was probably erupted from lower Te Meri Crater c. 13,800 years BP and has been used to define the base of the Tongariro Sub-group. This subgroup is of Aranuian (post-glacial) age and includes all tephras accordant with the present day topography and which have been erupted from the Tongariro Volcanio Centre. Much of the tephra record between 20,000 and 14,000 years BP was lost through contemporaneous erosion and andesitic tephras have been reworked in with Oruanui tephra to from loess beds of late Otiran age. A cool-climate origin for these loesses is consistent with the pollen record of the Tongariro area which shows that between 20,000 and 14,000 years BP there was a change from Nethofagus forest, grassland and shrubland to podocarp-broadleaf forest. This implies a change to a warmer climate during that period. The major erosional unconformities in the Tongariro area are also of this age. An unconformity at the top of the Mangamate Formation (9,700-9,780 years BP) resulted from erosion during the time taken fro vegetation to recolonise the surface of the thick tephra sequence following total devastation of the forest. Periods of widespread erosion followed deposition of the thick Oruanui Breccia and Taupo Pumice tephra flow units. Widespread present day erosion is probably due primarily to extensive pre-Eurpoean and early Erurpean fires. Late Otiran glacial moraines recognised in five valleys about the Tongariro Massif are older than 14,700 years BP and moraine on eastern Ruapehu is probably older than 20,000 years BP. The amount of soil development undergone by andesitic tephra is related to the net rate tephra accumulation. Mangatawai Tephra shows least soil development and had the fastest accumulation rate; Papakai Tephra shows most soil development and had the slowest accumulation rate. The degree of paleosol development also varies according to the distribution of the tephras from the source vents and their related particle sizes; soils around Tongariro were least well development ESE of the massif where the rate of tephra accumulation was highest, and were best developed in the west to south quadrant where the rate of tephra accumulation was lowest. Eruptions of Mangamate Tephra were violent and the tephras were generally very well sorted. These eruptions were from magma of a “new” chemical composition and their depositions was associated with the first tephras erupted for about 10,000 years from the Taupo Volcanic Centre. It is likely that the upward movement of this andesite of “new” composition and its initial eruption about 10,000 years ago provided the trigger for eruption of the oldest Taupo Sub-group tephras. Timing of subsequent eruptions of rhyolitic tephra of a “new” magma composition was probably controlled by increases of vapour pressure over confining pressure with subsequent healing of fractures and a new build up of pressure. The concurrent intermittent andesitic volcanism probably had no significant effect on the timing of the youngest rhyolitic eruptions. Ngauruhoe, a composite cone about 900 m high at the southern end of the Tongariro Massif is considered to have first erupted about 2,500 years ago and to have completed most of its growth during deposition of Magatawai Tephra. Red Crater which lies within a small scoria cone on the central part of the Tongariro Massif is mainly younger than 3,400 years BP although the oldest flew can only be dated as younger than 9,700 years BP. The six lavas extruded before 1819 years BP are olivine andesites; the five lavas extruded since 1819 years BP are basalts. The Te Mari Craters on northern Tongariro have been active from at least 13,800 years ago to 1896 A.D. Lava was extruded following the eruption of Rotoaira Lapilli (13,800 years BP) and a large flank extrusion occurred between 9,700 and 6,000 years BP. Lava flows from the upper Te Mari Crater were probably extruded in the early 16th century. North Crater, near the north-western end of the Tongariro Massif probably dated from abut 70,000 years ago (Stipp 1968) and most of the cone is older than 10,000 years. A nuée ardente was erupted about 10,000 years BP and Te Rato Lapilli was erupted about 9,780 years BP. The upper part of the cone probably dated from 4,800 years BP and all major eruptions had ceased by 1819 years BP. Blue Lake was the site of the Poutu Lapilli eruption c. 9,700 years BP and has probably been active more recently. The Upper and Lower Tama Lakes new occupy the sites of many of the Mangamate Tephra eruptions of 9,700-9,780 years BP. Some eruptions of Papakai Tephra were probably also from this area. Pukeenake Scoria Cone, west of the Tongariro Massif, and the two plugs to its north define a fissure from which extensive olivine andesites were extruded more than 20,000 years ago. Pukekaikore dome west of Ngauruhoe is possibly an old cumuledome of labradorite andesite flows. Small olivine andesite flows and associated tephra were erupted from a vent near the summit of the dome about 14,700 years BP. While most of the lahars from the Tongariro Massif are older than 20,000 years BP at least five occurred on northern Tongariro during deposition of Papkai Tephra, and tree-ring dating provides an estimated age of 1895 A.D. for a historic mudflow from Ketetahi Springs, also on northern Tongariro. Eruptions from the Kakaramea-Tihia Massif are all older than 20,000 years BP and the youngest may be represented by a nuée ardente deposit from a vent WSW of Tihia. This deposit cams to rest above its Curie point and was radiocarbon dated at >40,300 years BP. Eruptions from Rotopounamu Graben and Pihanga are also older than 20,000 years BP. Lahars from these centres may date back to 0.6 m.y. but there have been several Aranuian lahars and the most recent were in 1846 and 1910 A.D. Ruapehu erupted numerous tephra beds from 10-14,000 years BP, probably from a northern crater. An extensive as flow from the north-western summit area has an age between 5-10,000 years BP, similar to the age of lava flows from a parasitic vent lower on the northern side. Murimotu Lahars on the lower north-western slopes of Ruapehu span the final 3,000 years of the tephra deposition. One of the largest lahars, that formed conical mounds, came to rest above its Curie point and is likely to have been initiated by displacement of a crater lake by lave or a growing lava dome. It is probably that this 'hot' lahar, aged between 9,540 and 12,450 years BP, occurred during the Gothenburg reverse polarity event. The vent presently occupied by the Crater Lake was initiated probably about 2,500 years BP and most lahars since that time have been confined to the Whangaehu River. Since 1859 eruptions through Crater Lake have caused two large lahars in Whangaehu River and there have been four cold lahars. The most recent of the cold lahars, on 24 December, 1953 destroyed the railway bridge at Tangiwai about five minutes before an express reached the bridge and 151 lives were lost. Lavas of Hauhungatahi cone, 12 km north-west of Ruapehu, are older than 20,000 years BP. The explesion craters at Ohakune, about 19 km south-west of the summit of Ruapehu, are also older than 20,000 years BP. Tephrochronology provides minimum ages for several rhyolite and dacite domes within or near the northern and of the Tongariro Volcanic Centre: Papanetu Tephra was erupted from the site of Kuharua Dome c. 9,785 years BP towards the end of dome formation; Puketarata Ash was erupted from the site of Puketarata Dome in the Maroa Volcanic Centre towards the end of dome formation approximately 13,500 years BP; Manganamu Dome is younger than 1819 years BP and Koroi Dome is likely to be close to 1819 years BP; Pukekaikiore Dome is older than 9,790 years BP; Horohoro Dome is older than 14,000 years BP; and Motuepuhi Island, Lake Rotoaira, is older than 20,000 years BP. Ages for several fault movements near the Tongariro Massif are given. At least one fault movement was associated with eruption of the Mangamate Formation. Fault movements at both ends of Lake Rotoaira occurred after eruption of Taupo Pumice and it is suggested that they were in response to eruption of some of the Taupo Pumice from within that lake. The reverse polarity of andesite lava in Tongariro River suggests its extrusion during the Matuyama reverse polarity epoch. The age of >0.69 m.y. is the oldest age for lava within Tongariro Volcanic Centre.