Ian A. Nairn

Basalt triggering of the c. AD 1305 Kaharoa rhyolite eruption, Tarawera Volcanic Complex, New Zealand

Abstract The ∼AD 1305 Kaharoa eruption episode of Tarawera Volcanic Complex was the largest (∼4 km3 magma) eruption in New Zealand in the last 1000 years. High-silica (∼76% SiO2) rhyolite was erupted from seven vents along an 8-km linear zone to form pumiceous pyroclastic deposits and lava domes. Initial vent-clearing explosions were followed by a series of plinian pumice eruptions that spread tephra downwind, and pyroclastic density current deposits over the volcano slopes. The early pyroclastic eruptions were followed by extrusion of a dome in the summit vent and the migration of further plinian activity to adjacent vents. The episode finished with the extrusion of three summit lava domes, accompanied by block-and-ash flows. Two main types of rhyolite were erupted: (1) a low-Zr, low-Sr, high-Rb type comprising the early plinian pyroclastics and (2) a high-Zr, high-Sr, low-Rb type comprising the late eruptives including the summit domes. Basaltic material (basalt and basaltic andesite) is commonly present as free clasts within the Kaharoa pyroclastic deposits, along with rare clasts of granodiorite, diorite, gabbro and olivine clinopyroxenite. The basaltic clasts are often veneered by Kaharoa rhyolite. Basaltic material is also common as inclusions within rhyolite pumices, along with olivine and augite xenocrysts derived from the basalt. The basaltic clasts and inclusions contain complexly zoned plagioclase and corroded olivine (commonly with a reaction rim) consistent with disequilibrium caused by magma mixing. Some basaltic clasts and inclusions are hornblende-free; others contain groundmass hornblende. ‘Plumes’ of brown glass often extend from hornblende-bearing basaltic inclusions into the surrounding rhyolite. These plumes demonstrate magma mixing, as does the presence of rhyolitic glass and quartz xenocrysts in some basaltic inclusions. Most basaltic inclusions have crenulate boundaries, suggesting they were fluidal when coming into contact with the rhyolite magma. Diorite, gabbro and olivine clinopyroxenite clasts have mineralogy similar to the basalts, commonly contain patches of fine-grained basaltic material, and are considered co-magmatic with the basalts. Intrusion of basalt magma into the base of the rhyolite magma chamber appears to have occurred for some time before the Kaharoa eruptions began, allowing considerable mixing with the rhyolite magma, transfer of water to some basalt inclusions and formation of hornblende in their groundmass. Conversely, the hornblende-free basalt inclusions did not have time to react with the rhyolite magma and must have been intruded shortly before the start of the eruption. Intrusion of this hornblende-free basalt appears to have triggered the Kaharoa eruption, after the magma chamber had been primed by the earlier intrusions.

Rhyolite magma processes of the ∼AD 1315 Kaharoa eruption episode, Tarawera volcano, New Zealand

Abstract Products of the ∼5 km3 (DRE), ∼5-yr duration Kaharoa eruption episode display two main high-silica rhyolite compositions; T1 erupted early (as plinian pyroclastics), and T2 erupted late (mostly as lavas). The T1 and T2 eruptive types are defined by crystal contents and compositional variations in whole rock, glass, plagioclase and biotite. Stratigraphically intermediate pyroclastic deposits have an intermediate composition (T1+2). A small volume of rhyodacite pyroclastics, mingled with injected basalt, was also erupted. The Kaharoa rhyolites were erupted from multiple sources spread along an 8-km linear vent zone, but the changes in eruptive compositions were largely controlled by position in the eruption sequence and magma discharge rates, rather than vent locations. Data from the Kaharoa eruptive types, vent locations, eruption sequence and discharge rates can be combined with concepts of magma chamber evacuation processes to produce a preliminary dimensional model of the pre-eruption rhyolite magma body. Our model magma body is sill-like, ∼8 km long, 1 km wide, 1.4 km thick, and located at ∼6–7 km depth in the upper crust. T1 magma overlay T2 magma in the upper levels of the chamber, with each magma layer internally mixed to a homogeneous composition along an axial extent defined by the vent locations. An underlying third rhyolite magma (T3) is recognised as the silicic end-member that was modified by basalt to form the rhyodacite eruptives. The rhyolite magma stratification survived multiple injections of basalt magma, which primed and finally triggered the Kaharoa eruptions. The T1+2 eruptives resulted from syn-eruption mingling in the conduit of the two main rhyolite magma types. Thickness of the T1 layer in the model can be estimated at 0.25 km; the T2 layer was somewhat thicker. Thicknesses of the underlying T3 and basalt layers are uncertain. Post-eruption geothermal heat flow indicates a residual magma volume of ≥6 km3, suggesting that the pre-eruption magma volume was ≥11 km3.

Distribution, stratigraphy, and history of proximal deposits from the c. AD 1305 Kaharoa eruptive episode at Tarawera Volcano, New Zealand

Abstract The c. AD 1305 Kaharoa eruptive episode consisted of a complex sequence of basalt‐triggered high‐silica rhyolite eruptions from at least seven vents along an 8 km linear zone across Tarawera Volcano. Initial plinian eruptions from a summit vent spread Kaharoa Tephra southeast across the North Island, accompanied by phreatomagmatic explosions and pyroclastic density currents from vents opened on the northern flanks of the volcano. The early plinian phase was ended by extrusion of Crater Dome in the summit vent, with explosive activity migrating to two adjacent vents (Tarawera and Ruawahia) to the southwest and northeast. Renewed plinian eruptions, apparently from the northern vents, produced tephra falls dispersed northeast‐northwest from the volcano. Extrusion of the three summit lava domes was accompanied by voluminous block‐and‐ash flows generated by collapse of the growing Ruawahia and Wahanga Domes, forming large fans to north, southeast, and northwest of the volcano. Calculation of Kaharoa lava volumes and comparisons with the extrusion rates of observed dome‐building eruptions suggest a duration of c. 4 yr for the c. 4 km3 Kaharoa eruptive episode. This estimate, of years rather than days or weeks, is significant for planning an effective response to a future similar rhyolite episode in New Zealand.

The ∼10 ka multiple vent pyroclastic eruption sequence at Tongariro Volcanic Centre, Taupo Volcanic Zone, New Zealand:: Part 1. Eruptive processes during regional extension

Abstract A sequence of six major (each ∼1 km3) and several minor pyroclastic eruption episodes occurred in the Tongariro Volcanic Centre (TgVC) during a period of uniquely intense activity at ∼10 ka ( 14 C years B.P.). This andesitic/dacitic sequence, named Pahoka–Mangamate (PM), had an apparent duration of ∼200 to 400 years. Rhyolite tephras erupted from Taupo are interbedded within the PM deposits. Our mapping of PM ejecta distribution has shown that multiple vents between Tongariro and Ruapehu volcanoes were active during each of the ∼10 ka episodes. The PM vents are located on a 20-km long NNE-trending linear vent zone, lying within a graben structure defined by regional fault zones. Some of these faults appear to have ruptured during the ∼10 ka eruptive sequence. The pattern of fault and vent zones suggests that an episode of accelerated extension (rifting) occurred across the TgVC at 10 ka, marked by normal displacements of the graben margin faults, and by dike intrusion beneath the vent zone on the graben axis. Juvenile ejecta from each of the PM eruptions is chemically and petrographically diverse, indicating that a number of separate magma bodies were tapped during the ∼10 ka eruption sequence (see companion paper). Despite this compositional diversity, each of the PM pyroclastic deposits is characterised by poorly vesicular juvenile lapilli with fracture-bounded surfaces, indicating interaction at depth between rapidly rising magmas and abundant groundwater. Widespread (plinian) dispersal of the PM tephras demonstrates that relatively high eruption columns were produced, despite this water-cooling of the erupting magmas. The PM eruptions and associated faulting were triggered by rifting processes. The coincident eruption of voluminous rhyolite tephras from a linear multiple vent zone in the Taupo Volcanic Centre, 50 km to NE of Tongariro, demonstrates that the 10 ka extensional episode was of regional extent.

Reactivation of a rhyolitic magma body by new rhyolitic intrusion before the 15.8 ka Rotorua eruptive episode: implications for magma storage in the Okataina Volcanic Centre, New Zealand

The 15.8 ka Rotorua rhyolite eruptive episode from the Okataina Volcanic Centre comprises a plinian pumice fall deposit followed by the extrusion of two rhyolitic lava domes or flows and their associated block-and-ash flows, with a total volume >1 km3 (dense rock equivalent). Variations in mineralogy, whole-rock, glass and mineral chemistry, and calculated magmatic properties suggest that two distinct rhyolitic magmas were sequentially tapped during the eruption. The first magma erupted (T1) is characterized by low SiO2 (c. 76.5 wt% in glass), calcic feldspars (An44), magnesian hornblendes (MgO c. 14.45 wt%), clinopyroxene, and high temperatures (c. 835 °C) and fO2 (1.8–2.1 ΔFMQ (where FMQ is the fayalite–magnetite–quartz buffer)). The second magma (T2) was more evolved and is characterized by higher SiO2 (c. 77.4 wt% in glass), Na-rich feldspars (An24), less magnesian hornblendes (MgO c. 11.8 wt%), biotite, and low temperatures (c. 750 °C) and fO2 (0.65–1.1 ΔFMQ). Both magmas are homogeneous, but evidence for some magma mingling indicates that they were in contact during eruption. However, there was only a minor degree of hybridization, perhaps reflecting the contrasting temperatures and viscosities of the two magmas. The crystal-rich, poorly vesicular nature of the T2 ejecta indicates that it originated from a cooling, high-level magma chamber that was reactivated by intrusion of hotter, volatile-rich T1 magma. The ponding of rhyolite magmas at shallow depth and their subsequent reactivation by later rhyolitic intrusion may be an important process in the compositional evolution and eruption dynamics of many Okataina Volcanic Centre rhyolite magma bodies.

Growth of the Tongariro volcanic complex: New evidence from K-Ar age determinations

Abstract New K‐Ar age determinations indicate that the exposed portion of the Tongariro volcanic complex has grown steadily since at least 275 ka, with intervals of vigorous cone growth at 210–200, 130–70, and 25 ka to the present day.

Late Pleistocene surface rupture history of the Paeroa Fault, Taupo Rift, New Zealand

Abstract The 30 km long Paeroa Fault is one of the largest and fastest slipping (c. 1.5 mm/yr vertical displacement rate) normal faults of the currently active Taupo Rift of North Island, New Zealand. Along its northern section, seven trenches excavated across 5 of 11 subparallel fault strands show that successive ruptures of individual strands probably occurred at the same time, but were individually and collectively highly variable in size and recurrence, and most fault strands have ruptured three or four times in the past 16 kyr. In the c. 16 kyrtimeframe, four surface‐rupturing earthquakes took place when Okataina volcano was erupting, and six occurred between eruptions. Large earthquakes on the Paeroa Fault comprise a significant component of the seismic hazard in the region between the Okataina and Taupo Volcanic Centres, and there are partial associations between these large earthquakes and volcanism.

A review of break‐out floods from volcanogenic lakes in New Zealand

Abstract New Zealand hosts numerous lakes in its active volcanic areas. These water bodies are developed in calderas, volcano‐tectonic collapse structures, explosion craters, and valleys dammed by lava, pyroclastic, or laharic flows. They range in scale and elevation from small shallow ponds a few metres above sea level in Auckland, through volcano‐tectonic collapse structures such as the Rotorua and Taupo calderas that hold many cubic kilometres of water at a height of a few hundred metres, to the summit Crater Lake of Mt Ruapehu, which contains c. 9 × 106 m3 of hot acidic water at an altitude of 2530 m. The combination of active volcanism and New Zealands temperate climate means that new lakes can form or old ones overfill rapidly following activity at any volcanic centre. Lakes in volcanic environments are often relatively shortlived features compared with those in tectonic settings, being prone to rapid formation and/or modification/destruction by both primary volcanic processes and the secondary effects of post‐eruptive landscape re‐adjustment. In addition to primary eruption‐related volcano‐hydrologic hazards, significant delayed hazards can also result from partial to total failure of impounding barriers of volcanic material. For example, New Zealands worst volcanic disaster, the 1953 Tangiwai lahar, which resulted in 151 deaths, was caused by partial failure of the rim of Mt Ruapehus Crater Lake 8 yr after the 1945 eruption had ended. Most recently, post‐1996 refilling of the lake behind a barrier of unconsolidated tephra laid down during the 1995/96 eruption sequence culminated in breaching and failure of the dam on 18 March 2007, resulting in the release of 1.3 × 106 m3 of water and generation of the largest historic lahar at the volcano. Historical reports and geomorphic/sedimentologic studies demonstrate that many of New Zealands volcanogenic lakes have been the source of large‐magnitude floods, some of which have caused major re‐organisation of regional drainage networks. Post‐eruption failures of the topographic rim of the Taupo caldera have produced catastrophic releases of up to 60 km3 of water, whereas intracaldera lakes hosted within the Okataina and Rotorua Volcanic Centres have also been the source of repeated floods. At least four events have been identified from Lake Tarawera (Okataina Volcanic Centre) in response to volcanogenic modification of the lake basin and catchment, most recently following the AD 1886 Tarawera eruption. Paleohydraulic reconstructions of dam‐breach hydrographs and downstream flood discharge rank some New Zealand caldera lake break‐outs as amongst the largest known Holocene floods on Earth.

Pre-eruption thermal rejuvenation and stirring of a partly crystalline rhyolite pluton revealed by the Earthquake Flat Pyroclastics deposits, New Zealand

The Earthquake Flat Pyroclastics form a c. 10 km3 rhyolite deposit erupted at c. 50 ka from the margin of Okataina Volcanic Centre, immediately following the caldera-forming eruption of the Rotoiti Pyroclastics (c. 100 km3) from vents c. 20 km to the NE. Earthquake Flat Pyroclastics deposits display textural and compositional complexity on a crystal-scale consistent with rejuvenation of a near-crystalline pluton in the upper crust. Quartz and plagioclase crystals are resorbed, whereas hornblende and biotite are euhedral. Fe–Ti oxides indicate large variations in pre-eruption temperatures (702–805 °C). Differences of up to 70 °C within pumice lapilli show that crystals were chaotically juxtaposed during magma stirring and evacuation. Chemical zoning within hornblende crystals is consistent with rimward increases of c. 50 °C. These features are consistent with a convective self-stirring process. Previous isotope studies demonstrate a long (>100 ka) crystallization history for the magma. Resorption of crystals deep in the magma may have produced a Ca-, Fe- and Mg-enriched rhyolite melt that allowed the growth of reverse-zoned hornblende. Microdiorite lithic fragments in the Earthquake Flat Pyroclastics and Rotoiti deposits and a basaltic eruption that immediately preceded the Rotoiti eruption suggest that mafic underplating beneath Okataina Volcanic Centre provided a major thermal and volatile pulse to drive the caldera eruptions.

The ∼10 ka multiple vent pyroclastic eruption sequence at Tongariro Volcanic Centre, Taupo Volcanic Zone, New Zealand: Part 2. Petrological insights into magma storage and transport during regional extension

Abstract The six eruption episodes of the ∼10 ka Pahoka–Mangamate (PM) sequence (see companion paper) occurred over a ?200–400-year period from a 15-km-long zone of multiple vents within the Tongariro Volcanic Centre (TgVC), located at the southern end of the Taupo Volcanic Zone (TVZ). Most TgVC eruptives are plagioclase-dominant pyroxene andesites and dacites, with strongly porphyritic textures indicating their derivation from magmas that ascended slowly and stagnated at shallow depths. In contrast, the PM pyroclastic eruptives show petrographic features (presence of phenocrystic and groundmass hornblende, and the coexistence of olivine and augite without plagioclase during crystallisation of phenocrysts and microphenocrysts) which suggest that their crystallisation occurred at depth. Depths exceeding ∼8 km are indicated for the dacitic magmas, and >∼20 km for the andesitic and basaltic andesitic magmas. Other petrographic features (aphyric nature, lack of reaction rims around hornblende, and the common occurrence of skeletal microphenocrystic to groundmass olivine in the andesites and basaltic andesites) suggest the PM magmas ascended rapidly immediately prior to their eruption, without any significant stagnation at shallow depths in the crust. The PM eruptives show three distinct linear trends in many oxide–oxide diagrams, suggesting geochemical division of the six episodes into three chronologically-sequential groups, early, middle and late. Disequilibrium features on a variety of scales (banded pumice, heterogeneous glassy matrix and presence of reversely zoned phenocrysts) suggest that each group contains the mixing products of two end-member magmas. Both of these end-member magmas are clearly different in each of the three groups, showing that the PM magma system was completely renewed at least three times during the eruption sequence. Minor compositional diversity within the eruptives of each group also allows the PM magmas to be distinguished in terms of their source vents. Because petrography suggests that the PM magmas did not stagnate at shallow levels during their ascent, the minor diversity in magmas from different vents indicates that magmas ascended from depth through separate conduits/dikes to erupt at different vents either simultaneously or sequentially. These unique modes of magma transport and eruption support the inferred simultaneous or sequential tapping of small separate magma bodies by regional rifting in the southern Taupo Volcanic Zone during the PM eruption sequence (see companion paper).