Tonga: what is an underwater volcano like and why was the eruption so violent that it triggered a tsunami in the South Pacific?

The eruption of the volcano near Tonga on January 15 put the entire planet on alert.

The volcano Hungarian Tonga–Hunga Haʻapai, in the South Pacific, caused an explosion that was heard as far away as the United States, and caused waves of more than a meter that hit the coast of Tonga.

In its first statement on the damage, the government on Tuesday described the situation as a “unprecedented disaster” and reported three confirmed deaths, that of two locals (a 65-year-old woman and a 49-year-old man) and a British citizen. This is Angela Glover, 50, who according to her family was trying to save her dogs when the water took her away.

Communications in the area are at a standstill after the eruption and tsunami severed an undersea cable, making it difficult to assess the scale of the destruction.

But the consequences were not only felt in the archipelago. 10,000 km from the volcano, in Peru, the death of two women was reported due to abnormally high waves.

And in other countries, including Japan and Chile, they generated tsunami alerts.

What are submarine volcanoes like the one in Tonga like and how do they manage to unleash such powerful events?

What is an underwater volcano?

A submarine volcano is a volcano located wholly or mostly below sea level.

They form in places where magma from inside the Earth seeps through vents or fissures in the Earth’s crust onto the ocean floor.

Slide the button to observe the changes in the northern part of the main island of Tonga

Underwater volcanic eruptions are characteristic of the rupture zones where the plates of the earth’s crust are formed, according to the US National Office of Oceanic and Atmospheric Administration (NOAA, for its acronym in English).

In these areas of high seismic activity the magma rises and accumulates between the cracks in the rocks of the volcano, until there is no more space and it explodes.

According to NOAA, three quarters of the volcanic activity on the planet corresponds to submarine eruptions.

Some oceanographers estimate that there are a million volcanoes on the floor of the Pacific Ocean alone, according to the UK’s National Maritime Museum.

What is the Hunga Tonga-Hunga Ha’apai volcano like?

The volcano consists of the union of two uninhabited islands, Hunga Tonga and Hunga Ha’apai, located 65 km north of Nuku’alofa, the capital of Tonga.

This is an area of ​​high seismic activity.

It protrudes 100 meters above sea level, but below it extends 1,800 meters long and 20 km wide, as explained in an article in The Conversation volcanologist Shane Cronin, a professor of Earth sciences at the University of Auckland in New Zealand.


In 2009, 2014 and 2015 the volcano had magma and steam eruptions, but much smaller than the one in January 2022.

According to Cronin, this volcano is capable of producing eruptions as powerful as this every thousand years.

The volcano is inside what is known as a “caldera”, which is a crater-shaped depression that gets deeper with each eruption.

How violent was the eruption?

The 8-minute eruption was so powerful that it could be heard over 800 km away.

The column of smoke and ash reached 20 km in height and 260 km in diameter.

Explanatory infographic of the violence of the eruption.


Cronin maintains that this was one of the strongest eruptions in the Tonga region in the last 30 years.

The Tonga volcano is of the basaltic type, like those of Hawaii or the Canary Islands, that is, its eruptions are not as violent compared to other types of volcanoes.

Basaltic eruptions are characterized by an outpouring of flowing magma.

The difference is marked by the contact of the magma with the sea waterAccording to the geologist Daniel Melnick, a researcher at the Institute of Earth Sciences of the Austral University in Chile, tells BBC Mundo.

The expert refers to the fact that when the magma, which can be around 1,000℃, enters sudden contact with water, an extremely violent reaction occurs that fragments the magma.

This phenomenon is known as “fuel-refrigerant interaction”.

There a chain reaction begins, in which the new magma fragments come into contact with the water, generating new explosions that launch volcanic particles and detonations with supersonic speeds, according to Cronin.

The volcanologist, however, indicates that the violent explosion cannot be explained solely by the interaction of the magma with the water.

According to him, “this explosion shows that large amounts of fresh, gas-laden magma erupted from the caldera.”

The expert mentions that through the analysis of ash from previous eruptions and radiocarbon techniques, he has been able to establish that these eruptions in Hunga Tonga-Hunga Ha’apai occur every 1,000 years.

The last one had occurred in the year 1100, so “the January 15 eruption appears to be just within the time frame to be ‘large,'” Cronin writes.

Melnick, for his part, warns that it is too early to know how the dimension of this eruption compares with those of other recent eruptions of other volcanoes.

What caused the tsunami?

According to Melnick, there could be two main causes.

The first was the eruption itself. The expert refers to an “explosive bombardment” that pushes large amounts of water.

Y the second is that the eruption caused the collapse of the volcano’s caldera. With that, an underwater collapse is produced that also pushes the water.

“That’s why after the eruption there was only a small piece of the island left,” explains Melnick.

Explanatory infographic of how tsunamis work.


In any case, Melnick and other experts agree that more studies are still needed to better understand the causes of this tsunami.

What can be expected in the coming days?

“Right now the area of ​​the eruption sure looks like Mordor,” says Melnick, referring to the hellish-looking country from the Lord of the Rings saga.

The expert explains that the smoke and ashes that spread thickly through the air interact with each other and with the atmosphere, creating electrical storms and strong winds in the area.

It may be weeks before it returns to normal, says the expert.

He also maintains that there may be more volcanic activity, “but there will not be an eruption like this, no way.”

“What can happen is a new collapse because all the material is unstable, there could be a submarine collapse and generate another tsunami, but with more local effects in Tonga or Fiji.”

Satellite image of the volcano eruption.


Cronin, for his part, adds that although this was an event that released great magma pressure, it is not known if it was the climax of the eruption.

The expert indicates that other eruptions of the caldera of this volcano have included several separate explosions.

“We could be several weeks or even years away from major volcanic upheaval from the Hunga-Tonga-Hunga-Ha’apai volcano,” says Cronin.

“For the sake of the people of Tonga, I hope not,” concludes the volcanologist.

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