Rebecca Priestley: Long-term hazards of Taupo's supervolcanoes

by Rebecca Priestley / 13 May, 2016
New research will help uncover the truth about the long-term hazards.
A painting of the Pink Terraces by Charles Blomfield. Photo/Alexander Turnbull Library /G-597
A painting of the Pink Terraces by Charles Blomfield. Photo/Alexander Turnbull Library /G-597

The Pink and White Terraces of Lake Rotoma­hana were once celebrated as one of the wonders of the natural world. European tourists bathed in the terraced pools and Maori revered the waters as a taonga with therapeutic properties. In June 1886, a violent eruption of Mt Tarawera destroyed the terraces, killing more than 100 people and forever changing the landscape. Over the next 15 years, the crater filled with water and Lake Rotomahana grew to five times its original size.

The lake is one of seven that sit above the Okataina caldera, a “supervolcano” responsible for six violent eruptions over the past 10,000 years. The caldera is one of two in the Taupo Volcanic Zone – the other is Taupo – formed where plate movements are causing the Earth’s crust to stretch and thin. As the crust is pulled apart, hot molten rock is flowing up from the mantle. Geophysicists know the size of the caldera and the temperature of this hot basalt rock, and have calculated the amount of heat flowing into the caldera from the mantle.

For the past five years, a team of geoscientists led by Cornel de Ronde, from GNS Science, have been surveying Lake Rotomahana, using a range of techniques including high-resolution bathy­metry, magnetics, seismic surveying, underwater photo­graphy and heat-flow measurement. The results have just been published in a special issue of the Journal of Volcanology and Geothermal Research. Although some findings – including the discovery of remnants of the Pink and White Terraces – have already captured the public imagination, de Ronde is more interested in what the research has revealed about what’s happening beneath the lake floor.

I spoke to him after he and his team had spent a day surveying from a boat on Lake Tarawera. He thinks lakes are the key to understanding more about the Okataina caldera.

At Rotomahana, lake-bed measurements show that heat flow is up to 132 megawatts, comparable to the output of the Wairakei geothermal power station. His colleagues suspect there is a magma body southwest of the lake floor, beneath Waimangu, with a fault providing easy passage for hot, buoyant fluids and volcanic gases to travel between the magma and the bottom of the lake.

“Ultimately, we want to survey the seven lakes above the caldera,” says de Ronde. Lakes cover 20% of the surface area of the ­Okataina caldera, so the total heat coming out of it can be calculated – by multiplying the lake heat output by five – and compared with the heat coming into the caldera.

“We suspect there’s more heat coming in than out. That means the heat has been stored and maybe there’s a big magma chamber building up.”

Okataina, like Taupo, will even­tually erupt again – though almost certainly not in our lifetimes – and knowing more about magma chambers beneath the volcano is an essential part of understanding any long-term hazard.

This is the first time lakes have been investigated with this combination of geological techniques, but de Ronde says this “proof of concept” is likely to be widely applied.

On land, vegetation cover means that geologists are usually only able to access the geology along road and stream cuttings and rocky outcrops.

On the lakes, “there’s no impediment to access, and you get this fantastic, super-detailed coverage of the lake floor”. Although the lakes were once big blanks in the geolo­gical maps, “we’re filling them in with far more detail than their on-land equivalent”.

To be able to pioneer this multi­disciplinary approach on Rotomahana, a lake with such historical, spiritual and geological significance, has been a highlight for de Ronde. “I think we’ve really added to the legacy of this region.”

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