The geological processes that created New Zealand’s stunning mountain landscape and rugged terrain are also responsible for the country’s volcanic activity.
Monday’s deadly eruption on White Island, off the east coast of the North Island, is one event in a geological process that has been happening for over tens of millions of years.
“These geological forces have created New Zealand’s natural beauty, but they can also destroy,” said volcanologist Jan Lindsay from the University of Auckland.
Clustered together in the centre and north of the North Island are New Zealand’s active volcanoes.
Most of the country’s active volcanoes are in an area known as the Taupō Volcanic Zone (TVZ), which extends south-west from Whakaari / White Island in the Bay of Plenty to the Ruapehu volcano.
“The Taupō Volcanic Zone is known as one of the most active volcanic regions in the world,” Dr Lindsay said.
“This is why New Zealand has the landscape it has.”
NZ has three types of volcanoes
The volcano on Whakaari / White Island is an example of a stratovolcano or cone volcano, which Dr Lindsay says is one of the three types of volcanoes in New Zealand.
“They produce a magma called andesite, which is an intermediate composition of magma,” Dr Lindsay said.
On the magma spectrum this one is near the middle – not too runny, not too thick – but can still have quite explosive eruptions every decade or so.
New Zealand also has a couple of supervolcanoes, or calderas, which have rhyolite magma that’s more viscous.
“The eruptions [from calderas] are even more explosive because it’s harder for gases to escape the gluggy magma, so it blasts the magma apart,” Dr Lindsay said.
Taupō and Okataina on the North Island are calderas, as are Yellowstone in the US and Lake Toba in Indonesia.
A bunch of volcanic pimples on the landscape makes up the third type of volcano in New Zealand.
These are called volcanic fields and they have basaltic magma that is on the runnier end of the spectrum.
“The gases that are in the magma can escape quite easily, so they don’t tend to have big explosions,” she said.
Volcanoes form on plate boundary
The meeting of two continental plates in just the right way is the key to volcano formation, marine geoscientist Maria Seton, from the University of Sydney, said.
New Zealand is right on top of one of these boundary lines between two plates.
“The North Island has a convergent plate boundary, where the Pacific plate moves underneath the Australian plate,” Dr Seton said.
“That forms a whole lot of volcanoes right near the area where the crust is going down into the mantle.”
The volcanoes that formed on this convergent plate boundary are relatively young, geologically speaking, compared to other volcanoes around the world.
Some volcanoes on convergent plates formed around 25 million years ago, whereas the TVZ volcanoes are more likely to be under 10 million years old.
Volcanoes in the Andes in South America are another example of this type of formation, where the Nazca plate is going underneath the South American plate.
There aren’t active volcanoes on the South Island because the plate boundary is quite different, said Dr Seton.
“There you get the Pacific and Australian plates moving side by side, in a strike-slip motion, so you don’t get volcanoes,” she said.
“Whereas Australia isn’t on any plate boundaries, we’re in the middle of the Australian plate, so we don’t get many volcanoes.”
History of eruptions
On average, there are around two eruptions of stratovolcanoes (like Whakaari / White Island ) each decade.
“There have been a couple of eruptions at Taranaki in the last hundred years,” she said.
“There’s been several at Ngauruhoe. Tongariro erupted in 2012.
“In 1995, Ruapehu erupted quite significantly.”
On the other hand, the caldera volcanoes only erupt every couple of thousand years – but when they do, it’s significant.
“Taupo has had periods of unrest, which can mean increased seismicity,” Dr Lindsay said.
“Sometimes there is some ground deformation underneath Lake Taupō and the lake level changes.
“The Taupo volcano last erupted around 230AD.”