Caldera1 volcanoes – areas where a volcanic explosion causes the ground to collapse in on itself to form a basin2 – are found around the world. Usually, eruptions like this only happen every 50,000 years.
But in New Zealand 240,000 years ago, scientific evidence3 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 pumice4 into the sky and across the land (pyroclastic5 flows) – enough to cover all of New Zealand in more than 1 metre6 of ash and pumice . Some of the ash blew overseas, but most of it remains in New Zealand as rock deposits (ignimbrite).
Ōhakuri
A caldera formed around 240,000 years ago. Unusually, this erupted only weeks after the explosion that formed Lake Rotorua, located 30 km to the north-east of Ōhakuri. Ōhakuri is part of the Taupō volcanic zone.
Acknowledgement: Vicki Mundoo, Creative Commons 2.0
Noticing the clues
Volcanologist7 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 weathering8 or sedimentation9 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 atmosphere10 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.
Hunga Tonga-Hunga Haʻapa
Researchers discovered that the large underwater Hunga Tonga–Hunga Haʻapa volcano, close to Tonga, had a hidden caldera 150 m below the waves. On 15 January 2022 this volcano exploded, creating a tsunami11 that travelled around the Pacific, a giant cloud of ash that reached 20 km into the atmosphere and an explosion that was heard thousands of kilometres away – the result of shock waves.
Activity idea
The Rotorua geothermal12 area was formed by a massive caldera. Watch the Rotorua caldera formation animation then go outside and conduct the student activity Calderas in the sandpit.
Related content
Magma13 Drillers Save Planet14 Earth uses humour and teamwork15 to dive into the serious issues of geothermal energy and sustainability. This article provides background information about the project. Instructions and supporting resources are found in this activity.
- caldera: When a volcano erupts, a large volume of magma (lava) can spew onto the Earth’s surface. When this large volume of material is removed from beneath the volcano, it can cause it to collapse into the emptied cavern to form a depression. This depression or basin is called a caldera. Some calderas are several kilometres deep and over 25 kilometres wide.
- basin: In geology, this means a depression of large size that may be caused by erosion or earth movements. Often you can’t see a basin on the surface as it has become filled in with other sediments or full of water.
- evidence: Data, or information, used to prove or disprove something.
- pumice: A type of rhyolite rock found in New Zealand, particularly in the central North Island. It is light in colour, not heavy and often filled with holes.
- pyroclastic: Material from a volcano vent that falls back to Earth travelling at great speed, often for some distance. Composed of rock (tephra) and gas, it can move at supersonic speed. Once it cools, it stops moving and solidifies.
- metre: The base unit of length in the International System of Units (SI).
- volcanologist: A scientist who studies volcanoes (alternative spelling is vulcanologist).
- weathering: Chemical, mechanical (including freezing and thawing) and biological processes that break down rocks into smaller pieces. Weathering does not include the transport away of broken-down material.
- sedimentation: The process of depositing sediment. Sedimentation rates can change by altering land use, e.g. deforestation tends to increase sedimentation, and exposed sediments can be carried along by wind and surface water and into rivers for eventual deposition in the sea.
- atmosphere: 1. The layer of gas around the Earth. 2. (atm) A non-SI unit of pressure equivalent to 101.325 kPa.
- tsunami: A series of massive waves generated in the ocean usually by earthquakes, volcanic eruptions or submarine and coastal landslides, but they can also be caused by the impact of meteorites from outer space.
- geothermal: Geothermal energy is energy produced from the heat of the Earth.
- magma: Molten rock that is found under the Earth and has not reached the surface. Formed from the Earth’s mantle and forms the lava that erupts from volcanoes.
- planet: In our Solar System, a planet is defined as an object that orbits the Sun, is big enough for its own gravity to make it ball-shaped and keeps space around it clear of smaller objects.
- teamwork: The co-operative or co-ordinated effort on the part of a group of individuals working together as a team to achieve a common goal, cause or purpose. Teams are usually structured in a defined way with a leader and group members, rules and roles, specific outcomes and timeframes.
