Lake Taupō, in the centre of New Zealand's North Island, fills the caldera of the Taupō Volcano, a large rhyolitic supervolcano. This huge volcano has produced two of the world's most powerful eruptions in geologically recent times.
The volcano is in the Taupō Volcanic Zone within the Taupō Rift, a region of rift volcanic activity that extends from Ruapehu in the south, through the Taupō and Rotorua districts, to Whakaari / White Island, in the Bay of Plenty.
History Taupō began erupting about 300,000 years ago. The main eruptions that still affect the surrounding landscape are the dacitic Mount Tauhara eruption 65,000 years ago, the Oruanui eruption about 25,500 years ago, which is responsible for the shape of the modern caldera, and the Hatepe eruption, dated 232 ± 10 CE. There have been many more eruptions, with major ones every thousand years or so (see timeline of last 10,000 years of eruptions).A continent on the move: New Zealand geoscience into the 21st century. Graham, Ian J. et al.; The Geological Society of New Zealand in association with GNS Science, 2008. . page 66, 168."Taupo the volcano" (a single sheet pamphlet), C.J.N. Wilson and B.F. Houghton, Institute of Geological & Nuclear Sciences, c2004. The Oruanui eruption in particular destroyed or obscured much evidence of previous eruptive activity.
Taupō Volcano has not erupted for approximately 1,800 years; however, with research beginning in 1979 and published in 2022, the data collated over the 42-year period shows that Taupō Volcano is active with periods of volcanic unrest and has been for some time. Some volcanoes within the Taupō Volcanic Zone have erupted more recently. Mount Tarawera had a moderately violent VEI-5 eruption in 1886, and Whakaari/White Island is frequently active, erupting most recently in December 2019. Geologic studies published in 1888 following the eruption of Mount Tarawera first raised the possibility that there was a volcano under Lake Taupō, rather than the more obvious volcanoes near Mount Tongariro, to explain the likely source of the extensive surface pumice deposits of the central North Island.
Geology
The Taupō Volcano erupts rhyolite, a viscous magma, with a high silica content, a feature associated with the middle portion of the Taupō Volcanic Zone within the Taupō Rift. This is an intra-arc rift in the eastern part of the continental Australian Plate, resulting from an oblique convergence with the Pacific Plate in the Hikurangi subduction zone. In this region the Moho discontinuity starts about beneath the surface beyond the modern Taupō Rift boundaries to the west and east, but the interpretation of changes in seismic velocity at shallower depth has led to disagreement. Intermediate velocities occur down to between and are believed to be due to mainly quartzo-feldspathic crust. Between 15 and 25 km more mafic rock compositions are indicated by the P-wave velocities. Studies show large areas of partial melt below with a brittle-ductile rock transition at approximately beneath the surface. For unknown as yet reasons, possibly associated with the present high rate of rift spreading and the recent subduction of the Hikurangi Plateau this area is very productive in its surface volcanism.
If the magma does not contain much gas, rhyolite tends to just form a lava dome, and such eruptions are more common. However, when mixed with gas or steam, rhyolitic eruptions can be extremely violent. The magma froths to form pumice and ash, which is thrown out with great force. Such eruptions tend to be earlier in any given eruption cycle.
If the volcano creates a stable plume, high in the atmosphere, the pumice and ash are blown sideways, and eventually fall to the ground, draping the landscape like snow.
If the material thrown out cools more rapidly and becomes denser than the air, it cannot rise as high, and suddenly collapses back to the ground, forming a pyroclastic flow, hitting the surface like water from a waterfall, and spreading sideways across the land at enormous speed. When the pumice and ash settle, they are sufficiently hot to stick together as a rock called ignimbrite. Pyroclastic flows can travel hundreds of kilometres an hour.
Earlier eruptions
Earlier ignimbrite eruptions occurred further north than Taupō. Some of these were enormous, and two eruptions around 1.25 and 1.0 million years ago were big enough to generate an ignimbrite sheet that covered the North Island from Auckland to Napier.
While Taupō has been active for about 300,000 years, explosive eruptions have been more typical in the last 42,000 years.
Oruanui eruption
The Oruanui eruption (also known as the Kawakawa event) of the Taupō Volcano was the world's largest known eruption in the past 70,000 years, with a Volcanic Explosivity Index of 8. It occurred around 25,500 years ago and generated approximately of pyroclastic fall deposits, of pyroclastic density current (PDC) deposits (mostly ignimbrite) and of primary intracaldera material, equivalent to of magma.
Modern Lake Taupō partly fills the caldera generated during this eruption.
Tephra from the eruption covered much of the central North Island with ignimbrite up to deep. The ignimbrite eruption(s) were possibly not as forceful as that of the later Hatepe eruption but the total impact of this eruption was somewhat greater. Most of New Zealand was affected by ashfall, with an ash layer left even on the Chatham Islands, away which included diatoms from erupted lake sediments. Later erosion and sedimentation had long-lasting effects on the landscape, and caused the Waikato River to shift from the Hauraki Plains to its current course through the Waikato to the Tasman Sea.
Hatepe eruption
The Hatepe eruption (also known as the Taupō or Horomatangi Reef Unit Y eruption) represents the most recent major eruption of the Taupō Volcano, and occurred about 1,800 years ago. It was the most powerful eruption in the world in the last 5,000 years."Taupo the eruption" (a single sheet pamphlet), C.J.N. Wilson and B.F. Houghton, Institute of Geological & Nuclear Sciences, c2004.Wilson, C.J.N. and Walker, G.P.L., 1985. The Taupō eruption, New Zealand I. General aspects. Philosophical Transactions of the Royal Society of London, A314: 199–228. The type of eruption that occurred is the most extreme volcanic hazard due to the pyroclastic flows very high mobility and heat content. It has been stated to have had an energy release equivalent to about 150 ± 50 megatons of TNT.
Stages of eruption The eruption went through several stages which were redefined in 2003 with at least 3 separate vents: A minor eruption occurred beneath the ancestral Lake Taupō lasting hours, and producing of fine ash. A dramatic increase in activity produced a high eruption column from a second vent, and dry ash. A vent erupted mainly wet phreatoplinian ash but some dry magmatic ash to a total of over tens of hours. Either a short break occurred or two vents became active at the same time with one producing a wet dark ash- and obsidian-rich fall deposit, the Rotongaio fine phreatoplinian ash. At the end of the last phase or beginning of this there was a period of heavy rainfall. A larger dry eruption ensued, which erupted ash/pumice over a huge area, over up to 17 hours, before partial column collapse with as many as eleven dry pyroclastic flow density currents resulting in of local ignimbrite deposits to the east of the present lake. The most destructive part of the eruption then occurred. Part of the vent area collapsed, as part of a process that unleashed about of material, that formed a fast-moving, pyroclastic flow lasting no more than 15 minutes. Rhyolitic lava domes were extruded some years later, helping form the Horomatangi Reefs and Waitahanui bank.Houghton, B.F. (2007). Field Guide – Taupo Volcanic Zone . These later smaller eruptions of unknown total size also created large pumice rafts and terminated within decades of the major eruption.
The main pyroclastic flow devastated the surrounding area, climbing over to overtop the nearby Kaimanawa Ranges and Mount Tongariro, and covering the land within with ignimbrite from Rotorua to Waiouru. Only Ruapehu was high enough to divert the flow.
The power of the pyroclastic flow was so strong that in some places it eroded more material off the ground surface than it replaced with ignimbrite. Valleys were filled with ignimbrite, evening out the shape of the land.
All vegetation within the area was flattened. Loose pumice and ash deposits formed lahars down all the main rivers.
The eruption further expanded the lake, which had formed after the much larger Oruanui eruption. Its new deposits also briefly created another large lake to the Taupō Volcano's north that extended to the Reporoa Caldera which in due course broke out into the Waikato River valley and released over a short period of water. The previous outlet of Lake Taupō was blocked, raising the lake above its present level, until shortly after the first smaller flood, it broke out in a huge flood, that released about of water.
Dating the Hatepe eruption Many dates have been given for the Hatepe eruption. One estimated date was 181 CE from ice cores in Greenland and Antarctica.Lake Taupō Official Site It is possible that the meteorological phenomena described by Fan Ye in China and by Herodian in Rome were due to this eruption, which would give a date of exactly 186. However, ash from volcanic activity does not normally cross hemispheres, and radiocarbon dating by R. Sparks has put the date at 233 CE ± 13 (95% confidence). A 2011 14C wiggle-matching paper gave the date 232 ± 5 CE. A 2021 review based on five sources reports 232 ± 10 CE.
New Zealand was unpopulated at that time, so the nearest humans would have been in Australia and New Caledonia, more than to the west and northwest.
Current activity and future hazards
Composition studies suggest the Taupō Volcano has had historic vents to the south and north of the present lake, and recent seismic activity does extend beyond the lake to its north and south. To the north the border with the Maroa Caldera is ill-defined but most of the seismic activity is likely related to structures related to this caldera. While studies have identified one Taupō composition vent to the north of Lake Taupō, this presumably resulted from a dyke extusion about 26,000 years ago. Recent activity to the north of the lake is assigned in terms of magma bodies, to the Poihipi volcano under Wairakei. it is possible that Taupō is in a state of internal instability that is susceptible to dynamic triggering by tectonic earthquakes, as the 2016 Kaikōura earthquake triggered a deformation event in the north-west portion of the volcano without seismic or deformation events being observed in closer volcanoes to that earthquake's epicentre.
From May through December 2022 there was increased earthquake activity with lakeside slumping and inundation from a small tsunami and ground deformation. The Volcanic Alert Level for Taupō Volcano was raised to Volcanic Alert Level 1 (minor volcanic unrest) on 20 September 2022. In retrospect analysis was consistent with the largest earthquake being of volcano-tectonic origin linked to magmatic overpressure.
While no witnessed eruptive event has been recorded from Taupō, there have been seventeen episodes of volcanic unrest since 1872, with the most recent being in 2019 and 2022–2023. This manifested as swarms of seismic activity and ground deformation within the caldera. The present-day magma reservoir is estimated to be at least in volume and have a melt fraction of >20%–30%.