Oruanui eruption

The Oruanui eruption of New Zealand's Taupō Volcano (also known as the Kawakawa eruption or Kawakawa/Oruanui event) was the world's most recent supereruption, and largest phreatomagmatic eruption characterised to date.

Geography
At the time of the eruption sea level was much lower than at present and the Taupō Volcano had been for over 100,000 years mainly located under a larger lake than the present Lake Taupō called Lake Huka. Lake Huka was destroyed in the eruption and other features of the local geography were changed significantly as outlined below.

Eruption
With a Volcanic Explosivity Index of 8, it is one of the largest eruptions ever to occur in New Zealand and the most recent supereruption. It occurred $25,675$ years BP in the Late Pleistocene and generated approximately 430 km3 of pyroclastic fall deposits, 320 km3 of pyroclastic density current (PDC) deposits (mostly ignimbrite) and 420 km3 of primary intracaldera material, equivalent to 530 km3 of rhyolitic magma, totalling 1170 km3 of total deposits. As such it is the largest phreatomagmatic eruption characterised to date. The eruption is divided into 10 different phases on the basis of nine mappable fall units and a tenth, poorly preserved but volumetrically dominant fall unit.

Modern-day Lake Taupō, 616 km2 in area and 186 m deep, partly fills the caldera generated during this eruption. A 140 km2 structural collapse is concealed beneath Lake Taupō, while the lake outline at least partly reflects volcano-tectonic collapse. Early eruption phases saw shifting vent positions; development of the caldera to its maximum extent (indicated by lithic lag breccias) occurred during phase 10.

Unusual features
The Oruanui eruption shows many unusual features: its episodic nature, a wide range of magma-water interaction, and complex interplay of pyroclastic fall and flow deposits. The erupted magma was very uniform in composition and this composition has not been seen since but had been seen before the eruption. Detailed compositional analysis has revealed the early phases of the eruption had a small amount of magma from outside the Taupō Volcano and are most consistent with a tectonic trigger. The eruption occurred through a lake system which was either the southern section of Lake Huka, recently separated by pre-eruption upwarping shortly before the eruption itself or some have suggested Lake Taupō had separated with a higher level than the remaining Lake Huka about a thousand years earlier, due solely to eruptive activity of the Poihipi volcano adjoining Mount Tauhara whose magma chamber is under Wairakei and that had erupted at Trig 9471 and the Rubbish Tip Domes about 27,000 years ago, filling that portion of Lake Huka. Accordingly, many of the deposits contain volcanic ash aggregates.

Eruption process
The timescale for the growth of the assumed Oruanui mush zone, which has a distinctive chemical and isotopic composition and zircon model-age spectra is now known to be from about 40,000 years ago from earlier Taupō Volcano eruptions. During crystal-liquid separation in this mush, large volumes of melt and crystals were carried upwards into a melt dominant magma body that formed at between 3.5 - 6 km depth. There is emerging evidence that much of the silicic magma produced was formed deeper than this in the middle or lower crust (some have suggested as deep as the upper mantle) and ascending rapidly to this magma reservoir with only brief storage there. The relative uniformity of the eruptives (99% high-SiO2 rhyolite), suggests the Oruanui magma body had been vigorously convecting by the time of the eruption. Nonetheless composition analysis shows that three different rhyolites contributed, with the initial two phases of the eruption having contributions from a leak of biotite-bearing rhyolite, presumably along dykes at more than 2 km depth, associated with tectonic faulting from a magma chamber to the north. The biotite-bearing rhyolite composition is like that found within the Maroa Caldera adjacent to the Taupō Volcano.

These initial stages were from magma at relatively low overpressure and if stored and matured in a shallow magma chamber had a temperature of about 780 ± 20 °C, with between a week to two weeks ascent of magma before eruption. It is possible that if the later majority of the magma formed deeper, the maturing temperature was about 900 °C. About 0.5% of the eruptives was low-SiO2 rhyolite believed to have been tapped from isolated pockets in the underlying crystal mush. Two distinct mafic magmas were involved in the eruption, and a total volume of 3-5 km3 of mafic magma is atypically high compared to other nearby rhyolitic eruptions.

The timescales involved in the final eruption priming appear to be only decades long at most. The eruption itself lasted only a few months, with most of the stages as described below being continuous. The location of the eruptive vents are only known for the first four stages of the eruption. Vents during stage 1 and 2 were in the north-east portion of present Lake Taupō, a third vent (or more likely several vents) was closer to the eastern alignment of the later Hatepe eruption, and the 4th vent was more central. The later stages of the eruption may have had venting from much of what is now the northern part of Lake Taupō.

While pyroclastic density currents were generated throughout the eruption, the peak distance reached in ignimbrite deposits was about 90 km during phase 8. This phase, as well as several others, before phase 10, were not that much smaller than the later Hatepe eruption of the Taupō Volcano. Ash (Kawakawa tephra) distributed during the various stages created a stratigraphic layer found over much of New Zealand and its surrounding sea bed as wind direction varied, the eruptive columns were so high, and the volumes of ash were so large.

Local impact
Tephra from the eruption covered much of the central North Island and is termed Kawakawa-Oruanui tephra, or KOT. The Oruanui ignimbrite is up to 200 m deep. Ashfall affected most of New Zealand, with an ash layer as thick as 18 cm deposited on the Chatham Islands, 850 km away. The local biological impact must have been immense as 10 cm of ash was deposited from just south of Auckland over the whole of the rest of the North Island, and the top of the South Island, both of which were larger in land area as sea levels were considerably lower than present. The pyroclastic ignimbrite flows destroyed all vegetation they reached.

Later erosion and sedimentation had long-lasting effects on the landscape and may have caused the Waikato River to shift from the Hauraki Plains to its current course through the Waikato to the Tasman Sea. Less than 22,500 years ago, Lake Taupō, having filled to about 75 m above its current level, and draining initially via a Waihora outlet to the northwest, cut through its Oruanui ignimbrite dam near the present Taupō outlet to the northeast at a rate which left no terraces around the lake. About 60 km3 of water was released, leaving boulders of up to 10 m at least as far down the Waikato River as Mangakino. The impact has been summarised as:
 * 1) A new landscape with up to hundreds of metres thick ignimbrite that ponded in valleys around the volcano. The actual area of the ignimbrite is less than the subsequent smaller Hatepe eruption presumably because the later generated a more intense pyroclastic flow but much less accumulative tephra fall.
 * 2) The volume created by the caldera collapse acted both as a sedimentation sink for the local catchment and as the basin in which a new Lake Taupō accumulated.
 * 3) The former Lake Huka that had extended to the north and partially occupied the older Reporoa Caldera was destroyed and filled in with ignimbrite, which also created a temporary barrier between the Taupō and Reporoa watersheds that had to be eroded before a stable drainage of the new Lake Taupō was established.
 * 4) Destruction of vegetation over most of the central North Island.
 * 5) Remobilisation of the pyroclastic material as alluvium with rainfall changed the drainage pattern of the Waikato River. The large amount of material mobilised particularly impacted the Waikato Plains and Hauraki Plains.

Distal impact
The Oruanui eruption ash deposits from the final (tenth) phase have been geochemically matched to Western Antarctic ice core deposits 5000 km away and they provide a convenient marker for the last glacial maximum in Antarctica. This ash cloud has been modelled to have taken about two weeks to encircle the Southern Hemisphere. Diatoms from erupted lake sediments have been found in the volcanic ash deposits about 850 km downwind on the Chatham Islands.

Afterwards
The first characterised eruption from the Taupō Volcano after the Oruanui eruption took place about 5000 years later. The first three eruptions were dacitic as was the Puketarata eruption. The other twenty-four rhyolitic events until the present, including the major Hatepe eruption, dated to around 232 CE came from three distinct magma sources. These have had geographically focussed vent locations, and a wide range of eruption volumes, with nine explosive events producing tephra deposits.