Taupo's explosive volcano zone
Tongariro is in the Taupo Volcanic Zone, where most of New Zealand’s volcanism of the past 1.6 million years has occurred.
GNS Science said the zone, which extends from White Island to Ruapehu, was extremely active on a world scale.
It included the three frequently active cone volcanoes of Ruapehu, Tongariro/Ngauruhoe and White Island, along with two of the most-productive calderas in the world: Okataina and Taupo. Calderas are large, deep craters with extremely violent origins.
GeoNet said Tongariro was a massive complex of volcanic cones and craters formed by eruptions from at least 12 vents over more than 275,000 years. Erosion during the last Ice Age had worn away what was once a substantial mountain into the hiking destination it was today.
Five eruptions had been recorded at Tongariro between 1855 to 1897.
The Te Mari craters, about 2km east of Ketetahi hot springs on the north side of Mt Tongariro, were the last craters confirmed as active on Tongariro. In 1869, a large eruption accompanied by an earthquake at Tongariro formed the upper Te Mari Crater during an explosive eruption. Maori descriptions talked of "bright red flame through the smoke that would burst and fall like snow", GeoNet said.
In November 1892 Te Mari again belched forth an immense quantity of steam, mud and boulders. The ejected material rose 600 to 900 metres before rushing down the mountain side. The last eruption began in November 1896 and continued until the end of that year.
Ruapehu last erupted in September 2007, producing two lahars and a moderate eruption column to about 4600 metres, with ash fall and rock falls across the summit of the volcano. The explosive eruption was accompanied by an earthquake that lasted eight minutes, while the explosive part of the eruption lasted for no more than a minute.
In March 2007 the tephra dam at Ruapehu's crater lake collapsed, as had long been predicted. It sent a lahar down the Whangaehu River. Alarms and an emergency response plan were effective and the lahar passed down its natural path to the sea with minimal property damage.
A lahar was the cause of the 1953 Tangiwai Disaster. The lahar rushed down the Whangaehu River and damaged the rail bridge, derailing a train and killing 151 people.
Ruapehu is the highest peak in the North Island, rising to 2797 metres.
It began erupting at least 250,000 years ago, with three summit craters having been active in the past 10,000 years, GNS Science said.
In recorded history major eruptions had been 50 years apart, in 1895, 1945 and 1995. Minor eruptions were frequent with about 60 since 1945.
The last major eruption started on September 23, 1995 with an explosion beneath the Ruapehu crater lake which threw water, rocks and scoria bombs onto the snow covered summit area. Lahars flowed down the slopes, reaching the Whakapapa skifield, which had closed for the day, only 90 seconds after the eruption started. Eruptions continued through October with traces of ash falling up to 250km from the mountain.
Further eruptions started in June 1996 and continued until August. Millions of tonnes of fine grey were carried by wind, mostly northeast to the coast and across the Bay of Plenty.
Ngauruhoe is the largest and youngest of the cones in the Tongariro complex.
GNS Science said its cone started to build up at least 2500 years ago, although recent research suggested it may be up to 4500 years old.
While Ngauruhoe had typically erupted at least every nine years there had been no eruption since 1975, with the last lava flow in 1954. In 1973, red-hot blocks of lava were ejected, while during 1974 and 1975, ash eruptions had continued with lava blocks thrown as far as 3km away.
Small scale eruptions are under way at White Island, with confirmation coming from webcam footage taken early on Sunday.
About 48km from the Bay of Plenty coast, and 2km in diameter, White Island marked the northern end of the Taupo Volcanic Zone, GNS Science said.
It had been active for at least 150,000 years, with continual low level activity and small eruptions since human settlement in New Zealand.
Sulphur mining was carried out at intervals from the 1880s to the 1930s, with 11 miners being killed by a debris avalanche in 1914, when part of the crater rim collapsed.
White Island was the country's largest volcano by volume. Seventy per cent of the cone was under the sea, while the highest point reached 321 metres.
Taupo itself is a "supervolcano", which erupted 1800 years ago in the most violent eruption known in the world in the past 5000 years.
The eruption plume reached 50km into the air, with areas near the lake being buried in more than 100 metres of scorching hot pyroclastic flow, which spread up to 90km from the vent and covered all local features except Ruapehu, GNS Science said.
It was possible ash from the eruption was the cause of red sunsets recorded by the Romans and Chinese at the time.