Volcanoes often occur at the boundary where two tectonic plates meet. In New Zealand, the Taupō Volcanic Zone (TVZ), extending from Mt Ruapehu through Rotorua to White Island, is the front of a wedge where the Australian and Pacific plates collide.
The volcanic zone is named after Lake Taupō – a caldera volcano – where a large eruption several thousand years ago formed a depression that has naturally filled with water to form a lake.
The mountains of the central plateau – Mt Ruapehu, Mt Ngāuruhoe and Mt Tongariro, form the southern edge of the volcanic zone. The Taupō Volcanic Zone continues up through lake Taupō, Rotorua (Lake Rotorua is also a caldera, like lake Taupō) and further north-east up to White Island, one of New Zealand’s most active volcanoes. This zone forms the southern tip of a row of volcanic activity called the Tonga-Kermadec arc and is part of the bigger Ring of Fire that circles the Pacific Ocean.
History of eruptions
The Taupō Volcanic Zone contains many extinct volcanoes as well as several that remain active today. One of the biggest eruptions in the area was the caldera explosion that created Lake Rotorua 240,000 years ago.
Also around this time – 250,000 years ago – Mt Ruapehu started erupting. This volcano remains one of the most active in the world with eruptions recorded as recently as 2007. Ash from Ruapehu covers much of the central plateau, and during recent eruptions, ash and mudflows have disrupted road and rail transport across the central North Island region.
It is estimated that Tongariro was also active around 250,000 years ago. Consisting of a collection of vents, Tongariro contains 12 separate cones. It has remained active over its history, with the latest eruptions occurring just 30–40 years ago. One of the vents of Tongariro has been active enough to form its own separate peak, which we refer to as Mt Ngāuruhoe. The earliest eruptions of Ngāuruhoe seem to date from 2,500 years ago, and the volcano remains active today.
These volcanoes and surface features result from the activity of the tectonic plates. Along the length of the Taupō Volcanic Zone, the Australian and the Pacific plates are colliding. Slowly, over long periods of time, the Pacific plate is being forced under the Australian plate. As the Pacific plate is pushed down into the mantletemperature and pressure increase. This melts the rock that makes up the plate, which forms the basis of the magma that eventually erupts from the volcanoes along the plate boundary.
The explosion that resulted in the formation of Lake Taupō changed New Zealand forever. Volcanologists estimate that this was one of the largest explosions in the world in the last 5,000 years. Scientists think that there were actually 6 different eruptions from 3 different volcanic vents under what is now the lake. The eruptions sent ash out across the Central Plateau, and pumice fell across the North Island.
The history of the Waikato River, New Zealand’s longest, is closely linked to the history of the Taupō volcano. The river drained the depression that filled with water after the explosions, and at some stage, the Waikato River used to go through the ‘Hinuera Gap’ to eventually drain in the Firth of Thames. The current course of the river is thought to have happened about 20,000 years ago, after the Kawakawa eruption from Taupō that was an even greater event than the one of 186AD. It is thought that the river became blocked with pumice until it eventually diverted westwards to follow its current path through Cambridge, Hamilton and out south of Auckland at Port Waikato.
The soil around Taupō today can be traced to the fallout from this eruption. While the surface layers look dark and thick, the layers underneath still contain pumice from this early eruption. This means that plants struggle to grow and the region is known for its low shrubs and tussock, which have adapted to both the soil and the climate of hot summers and cold winters.
Find out more about the Taupō Volcanic Zone on the GNS Science website.
This Oregon State University website explains how a crater lake forms.
A supervolcano erupts with at least 1015 kg of magma, creating about 1000 cubic km of pumice and ash, enough to bury the whole land area of New Zealand under nearly 4 m of debris. Lots more to discover in the video The life and times of supervolcanoes.