Likelihood of Tongan volcano erupting again 'hard to predict' due to lack of active monitoring

Sunday, 16 January 2022

The eruption at Hunga-Tonga-Hunga-Ha'apai covered a huge area in ash and smoke, and caused tsunami waves to hit Tonga.

The underwater volcano that erupted on Friday causing tsunami waves to hit Tonga could erupt again, a geological expert says, but with no active monitoring of the volcano, the likelihood of another eruption cannot be determined.

The underwater volcano Hunga-Tonga-Hunga-Ha'apai, located about 30 kilometres south-east of Fonuafo'ou island in Tonga, erupted sending ash, steam and gas 20 kilometres into the air.

Tsunami waves swamped low-lying areas of the Pacific Island nation on Saturday, forcing Tongans to flee to higher ground as the water crashed into homes.

Marco Brenna, senior lecturer at the School of Geology at the University of Otago, said the impact of the latest eruption has been “relatively mild”, but another eruption with a much bigger impact could not be ruled out.

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Tsunami waves that followed the eruption sparked “screams everywhere”, one local says.

The Hunga volcano is part of the string of volcanoes forming the Tonga Arc, formed by the subducting Pacific tectonic plate beneath the Indo-Australian tectonic plate.

Its feature is a summit caldera - a large depression formed when a volcano erupts and collapses - which was formed during large eruptions about 900 years ago. The volcano erupted several times over the past century including in 1912, 1937, 1988, 2009 and 2014.

Little is known of the early 1900s ones, but erupted volumes during eruptions in 1988, 2009 and 2014 appeared to have increased, Brenna said.

This satellite image taken by Himawari-8, a Japanese weather satellite, shows an undersea volcano eruption at the Pacific nation of Tonga.

“There is a magma reservoir at some 5 to 6km depth that has been feeding the previous eruptions, and it is likely that the ongoing event is being fed by the same reservoir,” he said.

But because the volcano was not being actively monitored, it was not possible to say with any certainty whether another eruption was imminent.

“[Because of a lack of monitoring] it is hard to know how much magma has been input into the shallow magmatic reservoir beneath the summit. Usually on volcanoes that are being monitored there are tiltmeters and other instruments that allow evaluation of how much the volcano is swelling to estimate how much magma has been injected into the shallow magma chamber. But without that information, it is really hard to predict what might happen next.”

Brenna said reasons why the volcano was not being monitored could be due to a lack of funding and resources in Tonga, or because earlier volcanic eruptions were relatively minor, it was not considered a priority to monitor it.

“There are not yet any satellite images of what has remained after Friday’s event, but given its magnitude it is possible that maybe even the whole island has disappeared. And that might be it, or it might lead on to something even bigger.”

Brenna said satellite imagery showed the eruption plume spread over a vast area, including Tonga, which meant an ash fallout was likely.

Despite the name, volcanic ash is not similar to ash from a fire but rather a dense mixture of ground, rock and mineral particles. “If it blankets a landscape and a few centimeters of ash end up on the roofs of buildings it can be quite a weight on structures that cause them to collapse.”

The chemicals in the ash are toxic and can destroy vegetation and contaminate water sources, while ash in the air can cause respiratory problems.

Brenna said it was vital for the Tongan government to inform people of the risks, including the possibility of further tsunamis and the risk of ash inhalation or consumption of contaminated water.