The eruption of Hunga Tonga Hunga Ha‘apai, just north of Tonga’s main island, surprised scientists. The underwater volcano had erupted a few times in recent years – but only small, localised outbursts. What happened in January was on a whole other scale – blanketing the Tongan archipelago in ash, and sending tsunami waves across the Pacific.
Dr. Yaxiong Shen, Dr. Colin Whittaker and Natalia Lipiejko in the University of Auckland Flume Lab. Photo: Ellen Rykers
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“When I heard about this one, I wasn't expecting anything near as big as what actually happened,” says Dr. Emily Lane, a tsunami expert at NIWA.
The tsunami in particular was unprecedented: most tsunamis are caused by earthquakes, not volcanoes. Plus, those tsunamis that are caused by volcanoes tend to only have effects within a few hundred kilometres. But Hunga Tonga Hunga Ha‘apai’s waves radiated as far as Japan and South America, and tsunami activity was even detected in the Caribbean and Mediterranean. What made this tsunami go global?
Hunga Tonga Hunga Ha’apai’s secret superpower lies in the air pressure shockwave it produced, which circled the globe. This shockwave supercharged the existing tsunami waves, giving them the energy and staying power to travel further than usual.
Plus, the air pressure wave can travel over land, allowing it to sweep across distant oceans like the Mediterranean and Caribbean, warping the sea surface and instigating detectable tsunami activity in these far-flung places.
But there are more complexities underlying volcanic tsunamis, and we don’t understand them – or the risk they pose – particularly well.
Enter Dr. Colin Whittaker and his research team at the University of Auckland. In a warehouse filled with giant aquarium-esque tanks, Whittaker’s team is unravelling the secrets of tsunamis generated by volcanoes.
Dr. Yaxiong Shen starts an experiment to investigate volcanic tsunamis. Photo: Ellen Rykers
One of the ways a volcano can make waves is through the sheer force of the explosion. This is what Dr Yaxiong Shen is investigating, by using a steam jet in a large tank to simulate an underwater eruption. By changing different parameters, Shen can figure out which conditions will lead to the biggest waves.
PhD candidate Natalia Lipiejko is probing another mechanism: hot, fast-flowing landslides of volcanic debris and gas called pyroclastic density currents. Lipiejko mimics these pyroclastic density currents by injecting compressed air into teeny volcanic beads – which makes the beads act like a fluid – and sending them sliding down a ramp into a tank of water.
Ultimately, the experimental results, combined with mathematical modelling and real-life data from Hunga Tonga Hunga Ha’apai, will help us understand these rare but deadly natural disasters – and perhaps prepare for them better in future.
The volcanic tsunami research project is a partnership between NIWA, the University of Auckland, GNS Science, and the University of Otago, and is supported by a Marsden grant from the Royal Society Te Apārangi.
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