Volcanism Of New Zealand

The volcanism of New Zealand has been responsible for many of the country's geographical features, especially in the North Island and the country's outlying islands.

While the land's volcanism dates back to before the Zealandia microcontinent rifted away from Gondwana 60–130 million years ago, activity continues today with minor eruptions occurring every few years. This recent activity is primarily due to the country's position on the boundary between the Indo-Australian and Pacific Plates, a part of the Pacific Ring of Fire, and particularly the subduction of the Pacific Plate under the Indo-Australian Plate.

New Zealand's rocks record examples of almost every kind of volcanism observed on Earth, including some of the world's largest eruptions in geologically recent times.

None of the South Island's volcanoes are active.

Major eruptions

New Zealand has been the site of many large explosive eruptions during the last two million years, including several of supervolcano size. These include eruptions from Macauley Island and the Taupō, Whakamaru, Mangakino, Reporoa, Rotorua, and Haroharo calderas.

Two relatively recent eruptions from the Taupō Volcano are perhaps the best known. Its Oruanui eruption was the world's largest known eruption in the past 70,000 years, with a Volcanic Explosivity Index (VEI) of 8. It occurred around 26,500 years ago and deposited approximately 1,200 km³ of material. Tephra from the eruption covered much of the central North Island with ignimbrite up to 200 metres (650 feet) deep, and most of New Zealand was affected by ash fall, with even an 18 cm (7 inch) ash layer left on the Chatham Islands, away. Later erosion and sedimentation had long-lasting effects on the landscape, causing the Waikato River to shift from the Hauraki Plains to its current course through the Waikato to the Tasman Sea. New Zealand's largest lake, Lake Taupō, fills the caldera formed in this eruption.

Taupō's most recent major eruption, the Taupō or Hatepe eruption, took place around 232 CE, and is New Zealand's largest eruption since Oruanui. It ejected some 120 km³ of material (rating 7 on the VEI scale), with around 30 km³ ejected in just a few minutes. It is believed that the eruption column was high, twice as high as the eruption column from Mount St. Helens in 1980. This makes it one of the most violent eruptions in the last 5000 years (alongside the Tianchi eruption of Baekdu at around 1000 and the 1815 eruption of Tambora). The resulting ash turned the sky red over Rome and China.

Mount Tarawera's eruption around 1310 CE, while not nearly as large, was still substantial, producing 2.5 km³ of lava and 5 km³ of tephra (VEI 5). Because its deposits, stretching from Gisborne to the Bay of Islands, were emplaced around the time that Māori permanently settled New Zealand, they have provided a useful archaeological marker. Tarawera erupted again on 10 June 1886, spewing ash and debris over , destroying the Pink and White Terraces and three villages, including Te Wairoa, and claiming the lives of perhaps 120 people. Approximately 2 km³ of tephra was erupted (VEI 5).

Hazards

As well as the direct effects of explosions, lava, and pyroclastic flows, volcanoes pose various hazards to the New Zealand populace. These include tsunamis, break-out floods and lahars from volcanically dammed lakes, ashfall, and other far field effects.

For instance, the Tangiwai disaster occurred on 24 December 1953 when the Tangiwai railway bridge across the Whangaehu River collapsed from a lahar in full flood, just before an express train was about to cross it. The train could not stop in time, and 151 people died. This was ultimately caused by Ruapehu's 1945 eruption, which had emptied the crater lake and dammed the outlet with tephra.

Effects can be widespread even for eruptions of only moderate size. Ash plumes from Ruapehu's 1996 eruption forced the closure of eleven airports, including Auckland International Airport.

Insurance against volcanic damage (along with other natural disasters) is provided by the country's Earthquake Commission.

Cultural references

The Māori had many myths and legends regarding the land's volcanic mountains. Perhaps the most well known regards the relative locations of Taranaki, Tongariro and Pihanga. It holds that the two first-named volcanoes competed for the love of the beautiful Pihanga and, after Tongariro succeeded, the defeated Taranaki moved to its lonely location near New Plymouth.

Another legend recounts the exploits of Ngātoro-i-rangi, a tohunga (priest) who arrived from the ancestral Māori homeland, Hawaiki, on the Arawa waka (canoe). Travelling inland and then looking southward from Lake Taupō, he decided to climb the mountains he saw there. He reached and began to climb the first mountain along with his slave Ngāuruhoe, who had been travelling with him, and named the mountain Tongariro (the name literally means 'looking south'), whereupon the two were overcome by a blizzard carried by the cold south wind.
Near death, Ngātoro-i-rangi called back to his two sisters, Kuiwai and Haungaroa, who had also come from Hawaiki but remained on Whakaari / White Island, to send him sacred fire, which they had brought from Hawaiki. This they did, sending the fire in the form of two taniwha (powerful spirits) named Te Pupu and Te Haeata by a subterranean passage to the top of Tongariro. The tracks of these two taniwha formed the line of geothermal fire that extends from the Pacific Ocean and beneath the Taupō Volcanic Zone, and is seen in the many volcanoes and hot springs extending from Whakaari to Tokaanu and up to the Tongariro massif. The fire arrived just in time to save Ngātoro-i-rangi from freezing to death, but Ngāuruhoe had already died by the time Ngātoro-i-rangi turned to give him the fire. For this reason the hole through which the fire ascended, the active cone of Tongariro, is now called Ngauruhoe.

Eruptions in the North Island, such as the Kaharoa eruption of Mt Tarawera in 1314, have been used to help determine the approximate date of arrival of the early Polynesian colonists to about 1280. Fossilized footprints of perhaps second- or third-generation Polynesian colonists have been found in volcanic ash on islands in the Hauraki Gulf.

Volcanic areas

While there are remnants of volcanic activity throughout most of New Zealand, there are several areas where they are more obvious, and somewhere activity continues. Since Taranaki's last activity in 1854, all eruptions have been in the Taupō Volcanic Zone or the Kermadec Arc.

Kermadec Arc

The Kermadec Islands are an active volcanic island arc stretching north-northeast from New Zealand's North Island towards Tonga. While only a few volcanoes in the arc are tall enough to form islands, it includes about 30 sizeable submarine volcanoes with many in the South Kermadec Ridge Seamounts at the New Zealand end of the chain. The largest island, Raoul Island, produced a large eruption around 2200 years ago with a VEI of 6. Its activity has continued intermittently since, with its latest eruption occurring in 2006.

Northland

The Northland region contains two recently active volcanic fields, one centered around Whangarei, and the other is the Kaikohe-Bay of Islands volcanic field. The latest activity in the Kaikohe-Bay of Islands field, around 1300 to 1800 years ago, created four scoria cones at Te Puke (near Paihia).

Earlier, during the Miocene, a mainly andesitic volcanic arc ran through Northland and neighbouring regions (including the Three Kings Ridge and northern Coromandel Peninsula), with western and eastern belts active between 25 and 15 million years ago and 23–11 million years ago respectively. Although this produced substantial volcanic edifices, including New Zealand's largest known stratovolcano, the Waitakere volcano, most of these have been eroded away, buried, or submerged, especially in the west, where a series of volcanoes buried offshore stretches south almost to New Plymouth. This is called the Northland-Mohakatino Volcanic Belt. Remnants of these two ancient volcano belts are still exposed in many places, including Whangarei Heads, the Hen and Chickens Islands, around Whangaroa Harbour, Waipoua forest, and the Waitākere Ranges.

Auckland volcanic field

The basaltic Auckland volcanic field is a monogenetic volcanic field underlying much of the Auckland metropolitan area. The field's many vents have produced a diverse array of explosion craters, scoria cones, and lava flows. The largest and most recent is Rangitoto in the Hauraki Gulf

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