Caldera volcanoes – areas where a volcanic explosion causes the ground to collapse in on itself to form a basin – are found around the world. Usually, eruptions like this only happen every 50,000 years.

But in New Zealand 240,000 years ago, scientific evidence suggests that 2 of the caldera volcanoes in the middle of the North Island – Rotorua and Ōhakuri – erupted just weeks apart.

Together, the eruptions at Ōhakuri and Rotorua were massive, throwing more than 300 cubic kilometres of ash and pumice into the sky and across the land (pyroclastic flows) – enough to cover all of New Zealand in more than 1 metre of ash and pumice . Some of the ash blew overseas, but most of it remains in New Zealand as rock deposits (ignimbrite).

Noticing the clues

Volcanologist Dr Darren Gravley from The University of Canterbury, (previously The University of Auckland), was investigating the rocks around Rotorua when he noticed something interesting. They didn’t look the same as the rocks that had erupted from the caldera volcano where Lake Rotorua is today. Where did these different rocks come from? Why were they lying directly on top of the rocks that did come from the Rotorua caldera volcano? He wondered if there were two eruptions from two different volcanoes. If so, where was the other volcano? Darren just had to investigate.

The first stages of Darren’s research were conducted around Lake Rotorua and the wider volcanic area to the south towards Taupo where volcanic activity is well known. He scoured the area, collecting rocks and looking carefully at how these rocks formed different layers. One thing he noticed was there were two layers on top of each other that had come from two different eruptions but there was no evidence of weathering or sedimentation in between the layers.

What does this mean?

The patterns of rocks tell geologists a lot about timescales and the sequence of events. Darren was looking for signs that would tell him how long it had been between the two eruptions.

When volcanic ash first falls from the sky, it is soft and can easily be washed away by rain. This leaves small channels in the ash caused by little rivers of water. In the rock layers that Darren found, there were no channels, no indentations, no evidence of rain. Even 250,000 years ago, to have weeks without rain would have been unusual, so this evidence told Darren that he had stumbled on something remarkable.

After a great deal of investigation, Darren concluded that there was not just one large caldera eruption but two eruptions each of 150 cubic kilometres of ash and pumice. Even more remarkable was the fact that they seemed to have occurred within days or weeks of each other.

New Zealand is the only place in the world where two caldera eruptions have been recorded to occur so closely together. Imagine it – 150 cubic kilometres of ash and pumice released into the atmosphere above Rotorua. The skies are dark, the country is in darkness. Just as the eruptions start to settle and the earthquakes stop, suddenly there is another equally violent eruption. The countryside rocks, ash plumes high into the air, pyroclastic flows race across the landscape and the countryside is coated in another huge layer of smothering ash and rocks. The landscape is changing, violently.

What next?

What Darren doesn’t know yet is why. Did one explosion trigger the other? If so, how did this trigger effect happen? Darren’s research will continue to find out why these eruptions were so special and what it means for our landscape today.

Nature of science

Scientists observe carefully. Darren noticed things out of the ordinary during his observations and asked questions that he then investigated. An important part of any scientific investigation is to ask questions. Such questions can lead scientists to make discoveries they didn’t expect.

    Published 9 April 2010