Taupō Volcanic Zone

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The Taupō Volcanic Zone (TVZ) is a volcanic area in the North Island of New Zealand. It has been active for at least the past two million years and is still highly active.

Mount Ruapehu marks its south-western end and the zone runs north-eastward through the Taupō and Rotorua areas and offshore into the Bay of Plenty. It is part of a larger Central Volcanic Region that extends to the Coromandel Peninsula and has been active for four million years. The zone is contained within the tectonic intra-arc continental Taupō Rift and this rift volcanic zone is widening unevenly east–west, with the greatest rate of widening at the Bay of Plenty coast, the least at Mount Ruapehu and a rate of about per year at Taupō. The zone is named after Lake Taupō, the flooded caldera of the largest volcano in the zone, the Taupō Volcano and contains a large central volcanic plateau as well as other landforms.

Activity

thumb|left|[[Mount Ngauruhoe]]

There are numerous volcanic vents and geothermal fields in the zone, with Mount Ruapehu, Mount Ngauruhoe and Whakaari / White Island erupting most frequently. Whakaari has been in continuous activity since 1826 counting steaming fumaroles, but the same applies to say the Okataina volcanic centre. The Taupō Volcanic Zone has produced in the last 350,000 years over material, more than anywhere else on Earth, from over 300 silicic eruptions, with 12 of these eruptions being caldera-forming. Detailed stratigraphy in the zone is only available from the Ōkataina Rotoiti eruption but including this event, the zone has been more productive than any other rhyolite predominant volcanic area over the last 50,000 odd years at per thousand years. Comparison of large events in the Taupō volcanic zone over the last 1.6 million years at per thousand years with Yellowstone Caldera's 2.1 million year productivity at per thousand years favours Taupo.

The last major eruption from Lake Taupō, the Hatepe eruption, occurred in 232 CE. It is believed to have first emptied the lake, then followed that feat with a pyroclastic flow that covered about of land with volcanic ash. A total of of material expressed as dense-rock equivalent (DRE) is believed to have been ejected, and over of material is estimated to have been ejected in just a few minutes. The date of this activity was previously thought to be 186 AD as the ash expulsion was thought to be sufficiently large to turn the sky red over Rome and China (as documented in Hou Han Shu), but this has since been disproven.

Taupō erupted an estimated of DRE material in its Oruanui eruption 25,580 years ago. This was Earth's most recent eruption reaching VEI-8, the highest level on the Volcanic Explosivity Index.

The Rotorua caldera has been dormant longer, with its main eruption occurring about 225,000 years ago, although lava dome extrusion has occurred within the last 25,000 years.

Extent and geological context

thumb|left|[[Lady Knox Geyser, Waiotapu geothermal area]]

The Taupō volcanic zone is approximately long by wide. Mount Ruapehu marks its southwestern end, while Whakaari / White Island is considered its northeastern limit.

It forms a southern portion of the active Lau-Havre-Taupō back-arc basin, which lies behind the Kermadec-Tonga Subduction Zone. Mayor Island and Mount Taranaki are recently active back arc volcanoes on the New Zealand extension of this arc. Mayor Island / Tūhua is the northern-most shield volcano adjacent to the New Zealand coast, and is believed to have been active in the last 1000 years. It is formed from rhyolite magma. It has a quite complex eruptive history but only with one definite significant Plinian eruption.

Associated with the Taupō volcanic zone, intra-arc extension is expressed as normal faulting within a zone known as the Taupō Rift. Volcanic activity continues to the north-northeast, along the line of the Taupō Volcanic Zone, through several undersea volcanoes in the South Kermadec Ridge Seamounts, then shifts eastward to the parallel volcanic arc of the Kermadec Islands and Tonga. Although the back-arc basin continues to propagate to the south-west, with the South Wanganui Basin forming an initial back-arc basin, volcanic activity has not yet begun in this region.

South of Kaikōura the plate boundary changes to a transform boundary with oblique continental collision uplifting the Southern Alps in the South Island. A subduction zone reappears south-west of Fiordland, at the south-western corner of the South Island, although here the subduction is in the opposite direction. Solander Island / Hautere is an extinct volcano associated with this subduction zone, and the only one that protrudes above the sea.

<!---EI from https://volcano.si.edu/database/search_eruption_results.cfm See also other Timeline on this page

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at:31/10/1973 shift:(190,-2) fontsize:XS text: 31 Oct 1973 Ruapehu

bar:test at:31/10/1973 mark:(line,red)

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</div>

Scientific study

Tectonics

In the North Island rifting associated with plate tectonics has defined a Central Volcanic Region, that has been active for four million years and this extends westward from the Taupō volcanic zone through the western Bay of Plenty to the eastern side of the Coromandel Peninsula. The dominant rifting axis associated with the Central Volcanic Region has moved with time, from the back-arc associated Hauraki Rift to the intra-arc Taupō Rift. As there is presently no absolute consensus with regard to the cause of the Taupō Rift's extension or its exceptional current volcanic productivity, some of the discussion on this page has been simplified, rather than all possible models being presented.

Recent scientific work indicates that the Earth's crust below the Taupō Volcanic Zone may be as little as 16 kilometres thick. A film of magma 50 kilometres (30 mi) wide and 160 kilometres (100 mi) long lies 10 kilometres under the surface. The geological record indicates that some of the volcanoes in the area erupt infrequently but have large, violent and destructive eruptions when they do. Technically the zone is in the continental intraarc Taupō Rift, which is a continuation of oceanic plate structures associated with oblique Australian and Pacific Plate convergence in the Hikurangi subduction zone. At Taupō the rift volcanic zone is widening east–west at the rate of about /year, while at Mount Ruapehu it is only /year and this increases at the north eastern end at the Bay of Plenty coast to /year. The rift has had three active stages of faulting in the last 2 million years with the modern Taupō rift evolving in the last 25,000 years after the massive Oruanui eruption and now being within two essentially inactive rift systems. These are the surrounding limits of the young Taupō Rift between 25,000 and 350,000 years and old Taupō Rift system whose northern boundary is now located well to the north of the other two being created before 350,000 years ago. Further the distinctive Waiteariki ignimbrite that erupted 2.1 million years ago in a supereruption, presumably from the gravity anomaly defined Omanawa Caldera, is within the postulated borders of the old Taupō Rift.

Faults

The multiple intra-rift faults are some of the most active in the country and some have the potential to create over magnitude 7 events. The fault structures are perhaps most well characterised related to the Ruapehu and Tongariro grabens. The recent deposits from major eruptions and lake features mean many potentially significant faults are uncharacterised, either completely (for example the 6.5 MW 1987 Edgecumbe earthquake resulted in the mapping of the Edgecumbe fault for the first time) or frequency of events and their likely magnitude are not understood. It can not be assumed that just because the rate of expansion of the rift is greatest near the coast that this is where most significant tectonic earthquakes in terms of human risk will be. The Waihi Fault Zone south of Lake Taupō and associated with the Tongariro graben has a particular risk of inducing massive landslips which has caused significant loss of life and appears to be more active than many other faults in the zone.

Volcanism

thumb|left|In 1886, Mount Tarawera produced New Zealand's largest historic eruption since European colonisation

The north (Whakatane Graben – Bay of Plenty) part of the zone is predominantly formed from andesitic magma and represented by the continuously active Whakaari / White Island andesite–dacite stratovolcano. Although Strombolian activity has occurred the explosive eruptions are typically phreatic or phreatomagmatic. The active emergent summit tops the larger, × , submarine volcano with a total volume of .

The central part of the zone is composed of eight caldera centres the oldest of which is the Mangakino caldera which was active more than a million years ago (1.62–0.91 Ma). Other than the now buried Kapenga caldera there are five caldera centres, Rotorua, Ohakuri, Reporoa, Ōkataina and Taupō. These have resulted from massive infrequent eruptions of gaseous very viscous rhyolite magma which is rich in silicon, potassium, and sodium and created the ignimbrite sheets of the North Island Volcanic Plateau. The detailed composition suggests subduction erosion might play a predominant role in producing this rhyolite, as later assimilation and fractional crystallization of primary basalt magma, is difficult to model to explain the composition and volumes erupted. This central zone has had the largest number of very large silicic caldera-forming eruptions recently on earth as mentioned earlier. Mount Tauhara adjacent to Lake Taupō is actually a dacitic dome and so intermediate in composition between andesite and rhyolite but still more viscous than basalt which is rarely found in the zone.

The southern part of the zone contain classic volcanic cone structure formed from andesite magma in effusive eruptions that cool to form dark grey lava if gas-poor or scoria if gas-rich of this part of the zone. Mount Ruapehu, the tallest mountain in the North Island, is a andesite cone surrounded by a ring-plain. This ring plain is formed from numerous volcanic deposits created by slope failure, eruptions, or lahars. Northwest of Ruapehu is Hauhungatahi, the oldest recorded volcano in the south of the plateau, such as within the Taupō Fault Belt, but many faults will be uncharacterised as was the case with the 1987 Edgecumbe earthquake. Earthquakes can be associated with landslides and inland or coastal tsunami that can result in great loss of life and both have happened on the Waihi Fault Zone. The relative low grade volcanic activity of the andesite volcanoes at each end of the zone has resulted in recorded history in both direct loss of life and disrupted transport and tourism. The only high grade eruption in recorded history was atypically basaltic from Mount Tarawera and although very destructive is not likely to be a perfect model for the more typical and often larger rhyolitic events associated with the Taupō Volcano and the Ōkataina Caldera. As mentioned earlier the Ōkataina caldera complex is the highest risk volcanic field risk in New Zealand to man pyroclastic flows and surges, which rarely have covered large areas of the North Island in ignimbrite sheets, earthquakes, lake tsunamis, prolonged lava dome growth and associated block and ash flows with post-eruption lahars and flooding. The old zone almost certainly contains volcanoes in the Tauranga Volcanic Centre.

Tauranga Volcanic Centre – Bay of Plenty
Activity commenced here over two million years ago and is now extinct.
Whakatāne Graben – Bay of Plenty
Submarine Whakatāne Seamount
Mayor Island / Tūhua
Moutohora Island
Whakaari / White Island
Te Paepae o Aotea
Putauaki
Geothermal field
Kawerau Power Station
Rotorua Volcanic Centre
Rotorua Caldera, size: 22 km wide
Mount Ngongotahā
Lakes
Lake Rotorua
Mokoia Island
Geothermal fields
Tikitere/Hell's Gate
Whakarewarewa
Pōhutu Geyser (Te Puia)
Ōkataina Volcanic Centre: The Haroharo and Tarawera complexes impounded the lakes against the outer margins of the Ōkataina Caldera. The Ōkāreka Embayment and the Tarawera Volcanic Complex were placed inside the Haroharo Caldera which in turn is inside the Okataina Ring Structure, according to Newhall (1988), but this was reclassified by Cole (2009).
Haroharo volcanic complex, northern end of the Okataina Volcanic Centre
Puhipuhi Embayment
Ōkareka Embayment
Rotomā Caldera
Lakes
Lake Ōkataina
Lake Tarawera
Lake Rotokākahi (Green Lake)
Lake Tikitapu (Blue Lake)
Lake Ōkāreka
Lake Rotomahana
Lake Rotoiti
Lake Rotomā
Lake Rotoehu
Geothermal fields
Waimangu Volcanic Rift Valley
Frying Pan Lake
Maroa Volcanic Centre: The Maroa Caldera formed in the north-east corner of the Whakamaru Caldera, and the Whakamaru Caldera partially overlaps with the Taupō Caldera on the South. The Waikato River course follows roughly the northern Maroa Caldera rim on one side. The town of Whakamaru and the artificial Lake Whakamaru, on the Waikato River, have the same name too.
Reporoa Caldera, size: 10 x 15 km The course of the Waikato River crosses this area, between the artificial Lake Ohakuri (), the town of Mangakino () and Hamilton.
Artificial Lake Maraetai

Other important features of the TVZ include the Ngakuru and Ruapehu grabens.

Note

thumb|right|[[Craters of the Moon (geothermal site)|Craters of the Moon geothermal area]]

There is more recent, somewhat different classification, by some of the same authors, that uses the term caldera complex: