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Q&A: The eruption of Hunga Tonga-Hunga Ha’apai

  • Written by  Jacob Dykes
  • Published in Tectonics
Hunga sits on the rim of a five-kilometre-wide submarine caldera in the Tonga-Kermadec volcanic arc. Beneath the surface lurks a massive volcano, around 1,800 metres high and 20 kilometres wide Hunga sits on the rim of a five-kilometre-wide submarine caldera in the Tonga-Kermadec volcanic arc. Beneath the surface lurks a massive volcano, around 1,800 metres high and 20 kilometres wide Marco Brenna
25 Feb
A volcanologist unpicks the devastating eruption of Hunga Tonga-Hunga Ha’apai

On 15 January 2022, the submarine volcano Hunga Tonga-Hunga Ha’apai, which is located just 65 kilometres from the capital of the Kingdom of Tonga, violently erupted. The blast released shockwaves that sent a barrage of air and water thousands of kilometres around the world. Tonga Geological Services reported that the plumes of gas and ash released by the volcano reached a height in excess of 20 kilometres. A tsunami followed, hitting Tongatapu – Tonga’s main island – and devastating some of the Pacific archipelago’s low-lying islands. Three people have been confirmed dead in Tonga and many more injured. Meanwhile, 1.74-metre waves were recorded in Chanaral, Chile, more than 10,000 kilometers away.

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In 2015, a team of researchers visited Hunga with the goal of assembling a history of the area’s volcanic activity. They found evidence of two huge past eruptions that formed the existing caldera. Through radiocarbon dating, they showed that this type of big eruption occurs roughly every 1,000 years. Geographical caught up with volcanologist Marco Brenna from the University of Otago to find out more.

How did the eruption of Hunga unfold in January?

The eruption actually started on 20 December 2021 with some small-scale magma–water interactions. It started in the same fashion as the last few volcanic events at Hunga. Hunga Tonga-Hunga Ha’apai has erupted regularly over the past few decades; a notable one came in 2014, when hot jets of magma and steam broke through the waves at the surface and a volcanic cone emerged, joining the two Hunga islands to create a new singular island five kilometres in length.

But there was nothing to indicate that it would escalate to the very large, explosive eruption that we saw in January. When our team went to Hunga in 2016, we did notice that there was some indication of uplift. A volcano is basically formed when magma coming from the mantle reaches the surface. As magma enters the shallow plumbing system and magma-storage area, the volume of material and gas increases, and so the volcano swells. So there was an indication that the volcano was recharging. Our research shows that Hunga is prone to these large eruptions once every 1,000 years. We knew it would happen at some point in the future, but had no indication of when.

What do researchers think happened within the volcano to cause this enormous eruption?

It’s early days to know for sure, because we haven’t been able to gain access to and analyse any of the erupted ash as yet. The eruption may have been caused by fresh, hot, gas-rich magma recharging and pressurising the shallow reservoir, and in the process destabilising it to make it explode. Alternatively, some near-surface process, such as a flank instability and landslide, could have destabilised the upper edifice of the volcano, leading to depressurisation and eruption.

How does the scale of the eruption compare to other recent events?

Well, Hunga itself hasn’t erupted like this for 900 years. At the global level, large eruptions happen more commonly than many might assume, but not on the same scale as the recent Hunga eruption. Just to name a few of the historical eruptions comparable or larger, there was Mount Pinatubo in 1991, Anak Krakatoa in 1883, Mount Tambora in 1815. Every 70–80 years, roughly, there is one very large volcanic event. 

But volcanoes do erupt all the time, all over the world, and for some reason this one has grabbed our attention. Tsunamis and volcanoes cause many deaths every year, for instance in places such as Indonesia, but those events often pass unheard.

What caused the subsequent tsunami to take place?

It’s not clear what the exact mechanism that generated the tsunami was. The ‘proximal’ waves that hit Tonga, which is just 60 kilometres away from the volcano, were one to two metres in height. But the ‘distal’ waves that hit the other side of the Pacific – in California, Alaska and South America – hit with the same intensity. If you drop a stone in a pond, it makes a wave and the wave disperses. That wasn’t the case with Hunga. Certainly, however, the low-lying islands of Tonga would have taken the brunt of the tsunami’s impact. 

Can volcanic eruptions such as this be predicted before they occur?

These kinds of eruptions can be predicted with the right equipment in place. Seismometers, tilt meters, or gas-monitoring equipment can inform scientists as to what’s happening inside the volcano. For example, scientists managed to detect seismicity prior to the recent eruption of Cumbre Vieja in the Canary Islands, enabling a large area of the island to be evacuated. When the eruption started there, it destroyed houses and other properties, but the major impact to human life was averted. 

Some volcanic events can also be predicted by monitoring the volcano’s surface. For instance, a partial collapse of the volcanic edifice might drive the eruption. But the problem was that Tonga had no such monitoring. The local geologists and geological surveys basically relied on jumping in a boat to see what was happening. It’s very difficult to do predictive monitoring without the right equipment.

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