The 26.5 ka Oruanui eruption, New Zealand: an introduction and overview
Introduction
Several deposits of young explosive volcanism at Taupo volcano in New Zealand have become archetypes for studies of large-scale ‘wet’ explosive interaction between silicic magma and external water (Self and Sparks, 1978, Walker, 1981, Self, 1983). Of these, the deposits generated in an eruption at (Wilson et al., 1988; equivalent to ∼26.5 calendar ka, using the second-order polynomial U/Th- to radiocarbon-age calibration of Bard, 1998) represent the largest and most widespread products of wet volcanism documented. Many names have been applied to this eruption and its deposits (Self and Healy, 1987, for summary). Here and in previous papers I use the term ‘Oruanui’ because (a) it was the first name applied (by Vucetich and Pullar, 1969) that recognised the essence of the deposits (viz., an extremely widespread ash-fall deposit accompanied by a voluminous ignimbrite), and (b) the alternative term ‘Wairakei’ used by Self, 1983, Self and Healy, 1987 had previously been and is applied to a wholly different, older welded ignimbrite (Grindley, 1965).
In this paper, I present an overview of the Oruanui eruption and its deposits to provide a framework within which subsequent papers can be placed in context. I discuss the stratigraphy and lateral correlations of the fall deposits in some detail here as these differ significantly from those previously proposed. In addition, the wide range of Oruanui pyroclastic density current (PDC) deposits are described in outline as a contribution to the recognition of a broader-than-usual spectrum of pyroclastic ‘flow’ deposits. Further papers will present the detailed data for individual units within the fall and PDC deposits and consider the dynamics of their parental eruptive phases.
The Oruanui eruption represents one of the youngest caldera-forming events in the rhyolite-dominated central Taupo Volcanic Zone (TVZ) in New Zealand (Houghton et al., 1995, Wilson et al., 1995a; Fig. 1). Taupo volcano and its neighbour Maroa to the north (Fig. 1) are built on the remnants of an older large caldera associated with the 340–320 ka Whakamaru-group welded ignimbrites (Wilson et al., 1986, Houghton et al., 1995). At Taupo the modern activity of overwhelmingly pyroclastic (>95%) volcanism, from vents mostly now concealed beneath Lake Taupo, began around 65 ka and has waxed coevally with the waning of lava-dominated activity at Maroa. Pyroclastic deposits exposed in the Maroa–Taupo area represent >9 eruptions from ∼64 to ∼27 ka (CJNW, work in progress; cf. Vucetich and Howorth, 1976a), the Oruanui eruption, then a sequence of 28 eruptions, all but three of them in the last ca. 12,000 years (Wilson, 1993).
Previous workers (Vucetich and Pullar, 1969, Vucetich and Howorth, 1976b, Self and Sparks, 1978, Self, 1983, Self and Healy, 1987) had recognised that the Oruanui eruption was large, involved large-scale magma:water interaction and was unusually complex. My work shows that the Oruanui deposits are significantly larger than previously thought, representing one of the largest documented eruptions in the last 250,000 years, only exceeded by the 74 ka youngest Toba eruption (Rose and Chesner, 1987). Previous stratigraphies of the Oruanui deposits had culminated in a 6-member division of the proximal deposits, comprising three fall members overlain by finer-grained ignimbrite (Member 4) a fall deposit (Member 5) then coarser-grained ignimbrite (Member 6: Self and Sparks, 1978, Self, 1983, Self and Healy, 1987). In distal areas, the six members were also recognised, with Members 4 and 6 being fall material inferred to be coeval with, and derived from, the corresponding proximal ignimbrite. This 6-member subdivision is not followed here because of the recognition by me of a more-detailed stratigraphic sequence (illustrated schematically in Fig. 2 and summarised in Table 1), significant miscorrelations in the fall deposits, and the presence of largely coeval generation of PDC and fall deposits throughout the eruption.
The chemistry of the Oruanui eruption products is detailed elsewhere (Sutton, 1995, Sutton et al., 1995; Blake et al., manuscript in prep.) and only specifically relevant in one respect to the eruptive stratigraphy. Of the extra-caldera Oruanui eruptives >99% are rhyolite (SiO2=71.8–76.7%, calculated volatile-free, but 70% of all analyses are 74.5–76.0% SiO2). However, the remaining <1% juvenile products are more-mafic material (52.3–63.3% SiO2) that occurs from ash-grade to 10–15 cm fragments. Hand-specimen mafic clasts are inferred to be juvenile from their often-cauliform shape, ubiquitous chilled margins and the presence of white pumiceous rhyolite material coating the surface and ingested into the clasts. Although juvenile mafic material is ubiquitous in most of the deposits (typically 0.1–0.5 wt%), it is notably abundant (1–4 wt%) in fall units 3+4, 7 and 9 and parts of the ignimbrite, forming ‘spikes’ used in stratigraphic correlation (Fig. 3; see below).
Section snippets
Oruanui stratigraphic framework
The Oruanui stratigraphic framework is founded on recognition of 10 fall units (Table 1). Nine of these can be correlated for ≥150 km as distinct units; the tenth is a ‘catch-all’ representing the poorly-preserved (but volumetrically dominant) remainder of the fall deposit. The eruption is thus divided into 10 phases, each of which corresponds to the respective fall unit. Oruanui PDC deposits are referred to by the relevant phase of the eruption that they were generated in, and are correlated
Overview of the fall deposits
The Oruanui fall units are all of wide to extremely wide dispersal (e.g. Self, 1983, Wilson, 1994). The deposits also vary in grading, sorting, and evidence for degrees of ‘wetness’ on deposition that vary quasi-systematically both within individual units and also over specific geographic areas. Here I discuss these features insofar as they reflect the evolution of the eruption. Detailed field and grain size data for individual units will be presented elsewhere.
Eruption timing and episodicity
A characteristic feature of wet volcanism in general is its non-uniform nature, seen in observed eruptions (e.g. White and Houghton, 2000) and inferred from prehistoric deposits (e.g. Houghton et al., 1999) to occur on two time scales. The first, of seconds to minutes, reflects fluctuations in the degree of magma:water interaction, leading to individual observed explosions, and/or inferred formation of individual laminae in deposits. The second, of hours to tens of hours, is driven by changes
Conclusions and comparisons
The Oruanui eruption in total represents a quite extraordinary event, both in terms of size and complexity, and represents the largest extreme of ‘wet’ volcanism yet documented (Self, 1983). Total bulk volumes inferred are ∼750 km3 for extracaldera material, and ∼420 km3 for primary intracaldera deposits, equivalent to ∼530 km3 of magma (230 km3 from intracaldera material and ∼300 km3 from extracaldera material, the latter assuming a mean dry bulk density of 1100 kg m−3 and a mean lithic content of 15
Acknowledgements
I thank Steve Self, George Walker, Bruce Houghton and Kate Wilson for their encouragement and support in this work, Brent Alloway, Iain Campbell, Bruce Houghton, Zinzuni Jurado-Chichay, Ted Lloyd, Ian Nairn, Brad Pillans, Steve Self, and George Walker for locality information, and Mike Rosenberg, Brad Scott and Jason Britten for technical assistance. I also acknowledge many landowners for access, in particular the forestry companies of the central North Island and especially Colin Dunstan (New
References (89)
- et al.
Rhyolitic fallout and pyroclastic density current deposits from a phreatoplinian eruption in the eastern Aegean Sea, Greece
J. Volcanol. Geotherm. Res.
(1998) Geochemical and geophysical implications of the radiocarbon calibration
Geochim. Cosmochim. Acta
(1998)- et al.
Thickness variations and volume estimates of tephra fall deposits: the importance of particle Reynolds number
J. Volcanol. Geotherm. Res.
(1998) - et al.
Fall-out and deposition of volcanic ash during the 1979 explosive eruption of the Soufrière of St Vincent
J. Volcanol. Geotherm. Res.
(1982) - et al.
Abyssal circulation around New Zealand — a comparison between observations and a global circulation model
Mar. Geol.
(1999) - et al.
Lithic types in ignimbrites as a guide to the evolution of a caldera complex: Taupo volcanic centre, New Zealand
J. Volcanol. Geotherm. Res.
(1998) - et al.
Computer simulation of transport and deposition of the Campanian Y-5 ash
J. Volcanol. Geotherm. Res.
(1983) - et al.
Gravity, magnetic and seismic surveys of the Taupo caldera, North Island, New Zealand
J. Volcanol. Geotherm. Res.
(1998) - et al.
Mobility of a large-volume pyroclastic flow — emplacement of the Campanian ignimbrite, Italy
J. Volcanol. Geotherm. Res.
(1993) - et al.
Shallow-seated controls on styles of explosive basaltic volcanism: a case study from New Zealand
J. Volcanol. Geotherm. Res.
(1999)
Quaternary silicic pyroclastic deposits of Atitlan caldera, Guatemala
J. Volcanol. Geotherm. Res.
Plinian pumice fall deposit of the Campanian Ignimbrite eruption (Phlgraean Fields, Italy)
J. Volcanol. Geotherm. Res.
Large-scale phreatomagmatic silicic volcanism: a case study from New Zealand
J. Volcanol. Geotherm. Res.
Volcanic ash clusters: tephra rafts and scavengers
J. Volcanol. Geotherm. Res.
An outline geochemistry of rhyolite eruptives from Taupo volcanic centre, New Zealand
J. Volcanol. Geotherm. Res.
Characteristics of two phreatoplinian ashes and their water-flushed origin
J. Volcanol. Geotherm. Res.
Ignimbrite types and ignimbrite problems
J. Volcanol. Geotherm. Res.
The ground layer of the Taupo ignimbrite: a striking example of sedimentation from a pyroclastic flow
J. Volcanol. Geotherm. Res.
An ignimbrite veneer deposit: the trail marker of a pyroclastic flow
J. Volcanol. Geotherm. Res.
Volcanic and structural evolution of Taupo Volcanic Zone, New Zealand: a review
J. Volcanol. Geotherm. Res.
Stratigraphy of the Kos Plateau Tuff: product of a major Quaternary explosive rhyolitic eruption in the eastern Aegean, Greece
Int. J. Earth Sci.
Formation of the Aira caldera, southern Kyushu, ∼22,000 years ago
J. Geophys. Res.
Turbulent transport and deposition of the Ito pyroclastic flow: determinations using anisotropy of magnetic susceptibility
J. Geophys. Res.
The volcanic deposits of Scinde Island. With special reference to the pumice bodies called chalazoidites
Trans. Proc. NZ Inst.
A reappraisal of ignimbrite emplacement: progressive aggradation and changes from particulate to non-particulate flow during emplacement of high-grade ignimbrite
Bull. Volcanol.
Bimodal grain size distribution and secondary thickening in air-fall ash layers
Nature
Influence of convective sedimentation on the formation of widespread tephra fall layers in the deep sea
Geology
Influence of particle aggregation on deposition of distal tephra from the May 18, 1980, eruption of Mount St Helens volcano
J. Geophys. Res.
Quantitative models of the fallout and dispersal of tephra from volcanic eruption columns
Bull. Volcanol.
Correlation, dispersal and preservation of the Kawakawa Tephra and other late Quaternary tephra layers in the southwest Pacific Ocean
NZ J. Geol. Geophys.
The plinian eruptions of 1912 at Novarupta, Katmai National Park, Alaska
Bull. Volcanol.
Mechanism of deposition from pyroclastic flows
Am. J. Sci.
Brazos River bar: a study in the significance of grain size parameters
J. Sedim. Petrol.
The origin of accretionary lapilli
Bull. Volcanol.
Charge measurements on particle fallout from a volcanic plume
Nature
The geology, structure and exploitation of the Wairakei geothermal field, Taupo, New Zealand
NZ Geol. Surv. Bull.
Geochemical correlation of genetically related rhyolitic ash-flow and air-fall ashes, central and western Guatemala and the equatorial Pacific
Geol. Soc. Am. Spec. Pap.
The Hachinohe Ash: an example of accretionary lapilli-fall deposit from Towada Volcano, Japan
Bull. Volcanol. Soc. Jpn.
Physical oceanography of the waters over the Chatham Rise
NZ Oceanogr. Inst. Oceanogr. Summ.
Volcán Quizapu, Chilean Andes
Bull. Volcanol.
The 1976–1982 strombolian and phreatomagmatic eruptions of White Island, New Zealand: eruptive and depositional mechanisms at a wet volcano
Bull. Volcanol.
Chronology and dynamics of a large silicic magmatic system: central Taupo Volcanic Zone
New Zealand. Geology
Phreatoplinian volcanism
Estudio sobre los fenómenos volcánicos y material caı́do durante la erupción del grupo el ‘Descabezado’ en el mes de abril de 1932
Anal. Mus. Nac. Hist. Nat. ‘Bernadino Rivadavia’ (Buenos Aires)
Cited by (245)
A review of approaches for submarine landslide-tsunami hazard identification and assessment
2024, Marine and Petroleum GeologyLithostratigraphy of the ignimbrite-dominated Miocene Bükk Foreland Volcanic Area (Central Europe)
2024, Journal of Volcanology and Geothermal ResearchMobility and emplacement of an ancient, large-volume pyroclastic flow, Ongatiti Ignimbrite, North Island, New Zealand
2023, Journal of Volcanology and Geothermal ResearchAsh aggregate-rich pyroclastic density currents of the 431 CE Tierra Blanca Joven eruption, Ilopango caldera, El Salvador
2023, Journal of Volcanology and Geothermal Research