The eruption of Tonga's Hunga-Tonga-Hunga-Hunga-Ha'apai volcano sent shock waves throughout the planet.

It is believed to have been the most explosive eruption in 30 years, generating a tsunami larger than geologists anticipated.

Much about the eruption remains unknown, and geologists fear it is far from over.

However, it has already prompted scientists to reconsider their assumptions about the potential impact of such a volcanic eruption.

The Hunga-Tonga-Hunga-Hunga-Ha'apai volcano is quite active. Hunga, located approximately 65 kilometers from Tongatapu, Tonga's largest island, erupted numerous times over the twentieth century, most recently in 2009 and then again in December 2014 and January 2015.

On December 20, the most recent eruption began. The explosion was audible from 170 kilometers distant, and a massive cloud could be seen from the Tongan capital of Nuku'alofa.

Early January activity ceased but resumed on January 13, when a massive ash column was launched 17 kilometers into the stratosphere.

On January 15, two days later, a far greater eruption occurred.

Eruptive explosion

According to Professor Shane Cronin, a vulcanologist at the University of Auckland, the eruption's force was extraordinary.

"Judging by the explosivity, it appears to have been one of the most intense eruptions in the recent thirty years," Cronin added.

"On the other hand, it was a relatively brief eruption that lasted less than 10 minutes, therefore the volume of material and magma was not enormous on a worldwide scale."

Cronin compared the Hunga-Tonga-Hunga-Ha'apai eruption to the 1991 Mount Pinatubo explosion in the Philippines and the 1883 Krakatoa eruption in terms of explosiveness.

Cronin stated that this was based on satellite photos and reports of the locations of shock and sound waves.

The pressure wave was detected by barometers as far away as 17,000 kilometers distant in Switzerland. At the same time, the explosion's sound was heard throughout New Zealand and as far away as Alaska, more than 9,000 kilometers away.

Cronin stated that the eruption's volume may, and most likely would, increase in the following days and weeks.

He predicted that it might eventually rival the magnitude of the Pinatubo eruption.

Contextualization

This magnitude of eruption is to be expected.

Cronin's research into the Hunga-Tonga-Hunga-Ha'apai volcano, as well as ash deposits on surrounding islands, indicates that it experiences a large eruption approximately every 900 to 1000 years.

"There were two eruptions that bear a strong resemblance to this volcano," Cronin explained. "One was around 1100AD, while the other was around 280AD.

"Those previous events were greater in total and included a succession of eruption episodes. As a result, this may not be the end."

Tsunami

The eruption triggered a powerful tsunami that surged through Tonga's islands.

Dr. Emily Lane, a Niwa hydrodynamics scientist, said the tsunami reached a height of nearly two meters on several Tongan islands.

However, she stated that the tsunami's force did not dissipate as expected.

"Before this, I would have answered, yes, a volcanic tsunami may inflict damage up to 200 or 300 kilometers from the volcano," Lane explained. "However, we're seeing boats destroyed in New Zealand and Japan, as well as automobiles and boats being washed away and destroyed in the United States.

"These are all thousands of kilometers from the volcano, so an enormous amount of energy has been thrown out to generate this."

Tonga's undersea eruption location

Hunga Tonga-Hunga Ha'apai volcano is located approximately 60 kilometers north of Tongan capital Nuku'alofa and about 2000 kilometers north of Auckland.

According to the Washington Post, a 1.2-meter tsunami rocked Port Saint Luis in California, and a tsunami measuring slightly less than a meter was recorded at King Cove, Alaska.

According to Al Jazeera, Peru closed 22 ports as a precaution, and Japan's Pacific Coast registered a 1.2-meter tsunami.

However, the tsunami was not limited to the Pacific.

The weather service in Trinidad and Tobago recorded a 12cm tsunami on Mona Island, Puerto Rico, and smaller tsunamis were observed around the Caribbean.

What was the cause of the tsunami?

Dr. Lane stated that several things might have caused the tsunami.

"Earthquake tsunamis are pretty straightforward. You get an earthquake, it moves the ground under the sea, which carries the seawater and radiates out, and the big ones of those are 50km wide by 200km long, so it's a huge area.

"With volcanoes, it's a far smaller area, so actually getting out enough energy to get all the way across the Pacific," she said. "It's quite flabbergasting, to be honest."

Although the caldera – the chamber containing magma – at Hunga-Tonga-Hunga-Ha'apai is just around 5 kilometers across, Lane said various mechanisms could be at work in triggering the tsunami.

The first is a simple explosive eruption, while the second is a phreatomagmatic eruption. Lane explained that when magma reacts with water, it can vaporize the water above it, resulting in a tsunami.

A tsunami is also possible due to the caldera and seafloor collapsing.

Sentinel-1 Synthetic Aperture Radar images show that a large portion of the Hunga Tonga and Hunga Ha'apai islands on each caldera side have been obliterated by the eruption, implying a possible collapse.

Lane added that pyroclastic flows and the shock wave from the explosion might generate a tsunami.

"Some of the damage in the Caribbean was undoubtedly caused by air pressure waves, as [the tsunami] could not have reached the Caribbean so quickly," Lane explained. "As a result, what we're seeing is probably a synthesis of a number of these distinct mechanisms."

With additional eruptions likely, scientists may never fully understand why such a massive tsunami was generated.

Fall of ash

Cronin predicted that up to 20cm of heavy ash might fall on Tongatapu and the Ha'apai island group based on prior significant eruptions.

He stated that the ash should not be particularly harmful and devoid of volatile chemicals like sulfur or fluorine. However, the ash can still contribute to acid rain and leachates, rendering the water unfit for human consumption.

Cronin stated that the islands would require assistance to restore drinking water.

Volcanic lightning was also observed in the hours following the eruption and may reappear if other eruptions occur.

According to the Washington Post, in the 15 minutes following the initial eruption, more than 60,000 lightning strikes were reported - 70 per second.

Volcanic lightning typically occurs in the ash column emitted by a volcano and is unlikely to pose a hazard to humans but may affect communication systems.