Tonga Volcano: Why was it so big, and are there others we need to take care of?

When the Hunga Tonga volcano Hunga Ha’apai erupted in Tonga on Saturday, it sent a huge flag 30 kilometers up into the sky and a literal shock wave around the world.

Every week, there are about 20 volcanoes “showing some signs of unrest” across the globe, but most are fairly mild, says Scott Bryan, a volcanologist at Queensland University of Technology.

So what made this volcano so explosive, was the eruption predicted, will it soon erupt again, and are there other volcanoes we need to keep an eye on?

Why was it so explosive?

Graphics of volcanic activity and Earth's tectonic plates.
A graphic showing volcanic activity and Earth’s tectonic plates.(Encyclopaedia Britannica / UIG via Getty Images)

The Hunga Tonga-Hunga Ha’apai volcano is what is known as a subduction volcano.

Subduction zone volcanoes occur along tectonic plate boundaries, where one plate is forced down below another.

Subduction volcanoes tend to have double personalities, according to Professor Bryan.

On the one hand, they have slow, passive eruptions that build the classic conical shape of volcanoes like Mt Fuji, and on the other hand, they are violently explosive, as we have seen in Tonga.

There are two factors that could lead to a very explosive outbreak, and although it is still too early to say definitively, it appears that both factors may have been at stake on Saturday.

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Satellite images show the magnitude of the volcanic eruption

The first is a high concentration of pressurized water vapor and gases in the magma.

When that magma hits the surface from deep subsurface, there is a sudden release of pressure “like opening a champagne bottle”, causing the gases to expand explosively and burst the magma or lava apart in the process.

In the case of Hunga Tonga-Hunga Ha’apai, scientists had previously looked at the chemical composition of the volcano’s lava sediment from an eruption in 2009.

They found that the lava had ingredients for a powerful explosion, according to Heather Handley, a volcanologist from Monash University who was involved in that research.

“We could see from the chemistry of the rocks that the magma from that eruption moved rapidly to the surface and also held on to its gas,” said Dr. Handley.


The other factor that made the Tongan volcano so explosive was that it was an underground volcano – its lava valve was under the sea.

When magma hits water, it causes an explosive interaction between the two as the water quickly flashes to steam, Professor Bryan said.

“It’s the external addition of the water and the heat from the magma that comes in contact with it,” he said.

The volcano’s vent was above water before disappearing below sea level just days (or hours) before Saturday’s catastrophic eruption, according to Dr. Handley.

Tonga underwater volcanic eruptions
The volcano is seen smoking on January 7th. Sometime after this, an eruption or collapse caused it to go underground.(Planet Labs PBC via AP)

“The satellite images, if one compares from January 6 to two hours before the eruption, somewhere in that time frame, the center cone had disappeared,” she said.

A smaller eruption may have blown the cone apart, allowing seawater to enter the opening, which then catalyzed the larger eruption.

But Professor Bryan suspects that an underwater landslide or collapse may have magnified the eruption and caused the tsunami that followed.

“You have to displace seawater to make tsunamis,” he said.

“It’s more than just the eruption. Something else has happened underwater that has triggered this explosion.”

Was the outbreak predicted?

The Hunga Tonga-Hunga Ha’apai volcano was in an approximately 1,000-year eruption cycle, according to high-temperature geochemist Oliver Nebel of Monash University.

But that does not mean that we could have designated with any real accuracy when it should break out.

“We know… it’s due [to erupt], but it could mean yesterday, or it could be in 100 years, “said Dr. Nebel.

But there were some signs that Hunga Tonga-Hunga Ha’apai was becoming more active, he said.

Tonga volcano
The eruption on Saturday was made bigger after the volcano went underground.(AFP: Tonga Geological Services / Eyepress)

Dr. Handley said the volcano entered a phase of increased activity sometime around the 19th-20th. December.

“In the last few weeks, you’ve seen what we call phreatomagmatic eruptions, where water and magma come in contact,” she said.

“You get those dark feathered tabs coming out.”

According to the Global Volcanism Program database, gas vapor and ash flags had risen at least 12.2 kilometers in the air by the end of December, but activity had “dropped significantly” by the beginning of January.

The difficulty with evacuating people when volcanoes enter more active phases is that they can often settle again without a catastrophic eruption, said Dr. Fog.

Evacuating people every time a volcano showed signs of activity would not only be costly, but it would lead to an erosion of public confidence in scientists, he added.

Will it soon erupt again?

When a volcano like this erupts, it often happens as a series of eruptions, rather than a single one.

Records from the Global Volcanism Program show that the last period of activity on Hunga Tonga-Hunga Ha’apai, which began in December 2014, lasted just over a year.

During that period, a new island was formed, about 120 meters high and about 2 kilometers long.

Dr. Nebel said he suspected there were likely to be more outbreaks in the near future.

“It’s really difficult, slash impossible, to predict whether it will be of the same degree of difficulty.”

A combination of satellite images showing the Tongan volcanic eruption.
A combination of satellite images taken by Himawari-8, a Japanese weather satellite operated by the Japan Meteorological Agency, shows the eruption.(NICT via AP)

A huge eruption like this can mean that any subsequent eruptions will be less intense; however, there are again no guarantees.

The problem is that the magma chamber can be tens of thousands of kilometers deep, and there is no way of knowing how much more magma is still in the chamber.

“The only thing we can say is that it has erupted now, so the likelihood of there being much more down below is low, but we have seen [multiple large eruptions] before in the past, “said Dr. Nebel.

In the same way, Dr. Handley that it was “impossible to say” at this point whether we had seen the biggest eruption or whether there were more on the way.

Professor Bryan said that if there was an underwater landslide that triggered the eruption, it could actually be good news in terms of future eruptions.

“Hopefully, if there was a landslide or what happened last Saturday, it has stabilized the slopes to some degree,” he said.

“[In that case] we may have some explosions or high pillars, but we will not get the tsunamis. “

Although the fallout and the potential death toll are still unknown, Professor Bryan said the previous eruptions at least gave the people of Tonga a warning of what might be on the way.

He also said it was a bit positive that it happened in daylight.

“At the sound of it, most people saw the early signs. It was clear that there was a sonic boom and they had seen the pillars from the weeks before.

“If this happened like three hours later in the middle of the night [when] people are sleeping, it could have been a lot worse. “

Are there other volcanoes like this to watch out for?

Volcanoes like Hunga Tonga-Hunga Ha’apai are part of the Pacific Ring of Fire – an approximately 40,000 km long line around the Pacific Ocean that traces the edge of tectonic plates, where much of the world’s volcanoes are found.

But these have what is referred to as an “independent magmatic plumbing system,” said Dr. Fog.

What this means is that their magma chambers and any channels and vents are in no way connected to other volcanoes, and the eruption of one does not trigger the eruption of anyone else.

Worldwide, there are more than 1,300 active volcanoes, but active does not mean they are erupting now, according to Dr. Handley.

“To be active, we say they erupted in the last 10,000 years,” she said.

Each week, there are about 20 volcanoes showing signs of activity, according to Professor Bryan.

The Global Volcanism Program listed 46 volcanoes as “in continued eruption status” per year. December 9, 2021.

A number of these are in Australia’s Pacific neighbors, including Papua New Guinea, the Solomon Islands and Tonga.

As we have seen, it can prove extremely difficult to predict which volcanoes could be an imminent threat to life, but volcanic experts say more surveillance and tsunami warning systems could help.

“All of these volcanoes need monitoring because our ability to predict these eruptions is sometimes in the order of hours,” said Dr. Fog.


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