By Amy Barrett

Published: Wednesday, 19 January 2022 at 12:00 am

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The eruption of the Hunga-Tonga-Hunga-Ha’apai volcano on Saturday, 15 January 2022 has left thousands people without homes, supplies and internet. On Tuesday 18 January, the Tongan government confirmed the eruption had resulted in three deaths, two of which were local people, and one British national.

The volcano’s explosion sent shockwaves as far as Alaska and the UK, and caused a tsunami that affected the shores of Australia, the US and Russia. The volcano is said to have been responsible for nearly 400,000 lightning events in the hours following the explosion on Saturday, according to meteorologist Chris Vagasky.

Around 10 hours after the eruption, people in Miami, USA – over 7,000 miles away from the volcano – saw pressure ‘waves’ at speeds of 695 mph – acoustic ripples in the air which, according to atmospheric scientist Brian McNoldy on Twitter, were essentially traveling at the speed of sound through the atmosphere.

The shockwaves continued to travel around the globe, and were still being recorded on Tuesday 18 January. Seismology student Felix Eckel said readings made it “clearly visible that [the wave] has circled the planet twice at this time”.

The volcano’s activity has dropped since the eruption, however it continues to release gases into the atmosphere. These include sulfur dioxide and nitrogen oxide, which volcanologist Shane Cronin told Reuters news agency could interact with water and oxygen in the air to create acid rain over Tonga. Additionally, the amount of ash released at the time of the eruption could impact local waters, causing marine life to die or migrate to cleaner waters, and impacting the Tongan fishing industry.

Read more about volcano eruptions:

• In pictures: The volcanic eruption devastating La Palma
• What would happen if all volcanoes on Earth erupted at once?

As a submarine volcano, Hunga-Tonga-Hunga-Ha’apai is less studied than land volcanoes. This made it difficult to predict how big the eruption would be when volcanologists noticed activity in the area two days before the event, said Dr Samuel Mitchell.

Mitchell, a volcanologist at the University of Bristol, specialises in submarine volcanoes. He has worked on several volcanoes around, though not including, the Hunga-Tonga-Hunga-Ha’apai volcano, and spoke to BBC Science Focus about the recent disaster. 

Why couldn’t scientists predict the Tonga eruption?

While some volcanic eruptions can be predicted, not all will show show warning signs, says Mitchell.

“For example, at the recent La Palma eruption in the Canary Islands, we had a classic signal that something’s going to happen. There was a scattering of earthquakes slowly moving up towards the surface to a particular spot.

“There also could be gas emissions, or increased temperatures of surface water, or the ground inflating slowly­ – these things can be detected by satellite.”

But most of the data that serve to warn scientists of an impending eruption are collected by instruments around the volcano, and Mitchell says gathering that data is very difficult in the case of submarine systems.

The preceding activity also depends on the cause of the eruption. Scientists still don’t know why the Hunga-Tonga-Hunga-Ha’apai volcano erupted, but as it is a submarine volcano, it’s likely to have been an instantaneous reaction, not something that had been bubbling under the surface for some time.

“You have magma that is 1,000°C interacting with the cold seawater, as opposed to an eruption [on land] fuelled by magma trying to get to the surface.”

There are two islands on the edge of the volcanic crater: Hunga-Tonga and Hunga-Ha’apai. Following the eruption, satellite images indicate these islands are a lot smaller, meaning the explosion likely destroyed a large part of the land.

How did the Tonga volcano cause a tsunami?

Unfortunately, scientists say that instances of volcanic-induced tsunamis aren’t well recorded. And until more data comes through, it’ll be hard to say what exactly caused the Tonga tsunami, says Mitchell.

“There could be a few different factors, but the main thing needed to create the tsunami is a huge displacement of water.

“That could be from an explosion under the water that basically propels water out and away from the volcano. There is a case for that here, because we’ve seen the size of the physical explosion. The shockwave it generated could be a way to suddenly move water outwards.”

The tsunami could also have been caused by large amounts of land moving in the ocean. If the Hunga-Tonga-Hunga-Ha’apai volcanic crater has collapsed, it would have caused a huge movement of water below the surface.

What do scientists know about the Tonga volcano?

All that can be seen above the water surface are the two islands, Hunga-Tonga and Hunga-Ha’apai. But underneath the waves, it looks more like a “classic volcano”, says Mitchell, with a large caldera-like crater.

“In the large [volcanic] systems you don’t tend to get big eruptions happening in the middle of the volcano. Normally, they happen around the edges.

“On the edge of a crater you get small cones and small vents which is what happened on this volcano in the past. You might get a small explosion, and if it’s shallow, they’re visible above the surface. But we’re still trying to understand exactly why this eruption was so explosively formed.”

The volcano is one of many to be found in the Tonga area. From Fiji, through Tonga and down to New Zealand there is a long arc of submarine volcanoes, explains Mitchell. Many of these have erupted in relatively recent times, including Hunga-Tonga-Hunga-Ha’apai, which saw a small eruption in 2014, and again in December 2021.

How many submarine volcanoes are there?

Submarine volcanoes can be found around the globe, at a variety of depths, from shallow waters to deep on the seafloor.

However, scientists don’t yet know how many there are.

“Our seafloor hasn’t been mapped in sufficient detail to tell us exact numbers, explains Mitchell. “But estimates currently think around 70 to 80 per cent of all volcanic activity actually happens on the seafloor.”

According to Mitchell, submarine volcanoes are “an unfamiliar part of volcanology”.

“The irony is that in one sense, this area is unknown because it’s normally not a very impactful part of volcanology. The activity of an underwater volcano usually stays on the seafloor, it has very little impact on land.

“Sometimes you have events that impact society. Back in October 2021, around Japan, an eruption there caused pumice rocks to float and collect in a harbour.”

Mitchell says these volcanoes are not as well monitored because they’re not considered particularly dangerous.

“We don’t know a lot about them, but there is little funding [for scientists] to be able to get out there and put out instruments down that can help tell us more and potentially give that warning signs.”

This meant that the Tongan disaster was unpredictable. But, it will help volcanologists like Mitchell better understand and predict future events.

“Every event that happens enables us to do better for the future.”

What happens next in Tonga?

As of Tuesday 18 January, volcanologist have begun to see signs that the explosion was “relatively short-lived”.

“It doesn’t look like there’s as much activity there now, as the satellite images look quite clear,” says Mitchell.

“Once we’re able to collect data from the seafloor, we’ll be able to find out more about why, when and how this explosion actually occurred.”

In response to the eruption and subsequent tsunami, governments have offered relief and aid to Tongan citizens. Made up of more than 170 islands, Tonga is home to around 100,000 people. Evacuations in the islands hit worst by the disaster are underway, and aid that was initially hampered by the huge covering of ash can now be organised. However, the Tongan government has expressed concern that aid may also bring COVID-19 into the country again. Tonga currently has a COVID-free status.

About our expert, Dr Samuel Mitchell

Samuel is a volcanologist and science communicator at the University of Bristol. He researches submarine volcanoes and the dynamics of explosive eruptions. He has a PhD in Geology and Geophysics from the University of Hawai’i where he studied the styles and dynamics of large volcanic eruptions on the seafloor.

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