Why the Eruption in Tonga Was a ‘Once-in-a-Millennium’ Event for the Volcano

The blast, which sent tsunami waves across the Pacific, left thousands of Tongans without access to water and power

A satellite image of a mushroom cloud of ash from a volcano eruption in the pacific
The explosion was likely the biggest volcanic eruption recorded anywhere on the planet in more than 30 years. NOAA via Reuters

A massive underwater volcano near the South Pacific island nation of Tonga erupted on January 15, spewing smoke into the sky and triggering tsunami advisories across the Pacific. Now, thousands of Tongans are without power and water, and disrupted communication and air travel have made it difficult to assess where help is needed most. It is still too early to assess most of the damage on the ground. 

The blast and associated tsunami caused "significant damage" along the western coast of the main island of Tongatapu, according to the New Zealand High Commission in Nuku'alofa. "A thick layer of ash remains across Tongatapu," the Commission said in a statement.

The blast was a "once-in-a-millenium" event for the volcano, explains Shane Cronin, a professor in volcanology at the University of Auckland, New Zealand, for CNN. 

"It takes roughly 900-1000 years for the Hunga volcano to fill up with magma, which cools and starts to crystallize, producing large amounts of gas pressure inside the magma," Cronin writes for CNN. "As gases start to build up pressure, the magma becomes unstable. Think of it like putting too many bubbles into a champagne bottle — eventually, the bottle will break."

The blast has halted life in the Kingdom of Tonga, which includes more than 170 islands and is home to about 100,000 people, Helen Regan reports for CNN. The Hunga-Tonga-Hunga-Ha'apai volcano that erupted Saturday sits around 20 miles southeast of Tonga's Fonuafo'ou island. The top of Hunga-Tonga-Hunga-Ha'apai peaks just 330 feet above sea level but stands about 6,500 feet high from the seafloor.

When the volcano erupted, it spewed gas and ash up 12 miles into the atmosphere and set off an atmospheric shockwave that traveled at about 1,000 feet per second, according to reporters for BBC. The explosion was captured by the GOES West Earth-observing satellite operated by the United States National Oceanic and Atmospheric Administration.

In addition to local damage, the powerful explosion triggered large waves and tsunami advisories that hit Australia, New Zealand, Japan, and west coasts of North and South America. Since the eruption, those impacted by the tsunami waves have been sharing footage on social media. Part of Japan saw waves as high as nine feet tall, and the west coast of the United States received swells around three and four feet high. In Peru, two people died from the high water, per Reuters.

At least two Tongans have lost their lives to the recent eruption, and thousands more are battling the remaining debris. One major concern is access to safe drinking water, which can be contaminated by ash and smoke from the blast. Countries like New Zealand and the U.S. have already pledged aid, but the thick cloud of airborne ash has grounded air travel. The blast also disabled Tonga’s main undersea communications cable, making it challenging to assess the scale of the destruction and the help needed.

The volcanic eruption is just the latest natural disaster to hit Tonga. In 2018, a Category 5 tropical storm wiped out roughly 170 homes and claimed the lives of two people, and a cyclone in 2020 caused over $100 million in damage, according to reporters for the New York Times. The country shut its borders when the pandemic began in 2020, and it is one of the few places in the world to remain essentially free of Covid-19 despite struggling economically due to lost tourism. As foreigners arrive to provide aid, Tonga will have to battle another challenge: providing help to those that need it, without spreading the virus.

For more about the volcano's history and past activity, read this informative Twitter thread form the Smithsonian's Global Volcanism Program.

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