The undersea volcano had been burbling since late December 2021, shaking the seas near Tonga with a series of outbursts. Things kicked into higher gear this month, with powerful blasts on Jan. 13 and Jan. 14 and then an even bigger eruption on Jan. 15 that sent ash and dust 25 miles (40 kilometers) into the Pacific sky.
Tevita Fukofuka was in the Tongan capital Nuku’alofa on January 15, the fateful day the Hunga Tonga-Hunga Ha’apai volcano erupted and seared itself into his memory. The young father and local government worker took to Facebook to post an emotional diary entry he penned last week, 24 hours after his country’s harrowing ordeal.
“I thought it was a big lorry’s blown tyre or something,” Tevita recalled. “I looked around the road confused, then a second bang; I thought it sounded like cannons going off close by. But the third explosion was much louder and sounded like it was just above my head; I knew it was that damn volcano and something was very wrong.”
The Tonga volcanic blast created a vast ash cloud and led to a tsunami (two people drowned across the Pacific ocean in Peru). Explosions of volcanic gas were audible more than 2,000km away in New Zealand and plumes of gas rose more than 20km into the sky. In a biblical flourish, pebbles rained down on the Tongan capital, Nuku’alofa, located some 65km south of the volcano.
UK scientists examining weather satellite data calculate it to be around 55km (35 miles) above the Earth’s surface. The most powerful eruption in the second half of the 20th Century came from Mount Pinatubo in 1991. Its plume is thought to have climbed to roughly 40km.
A volcanic cloud extended to cover all of the country’s roughly 170 islands, according to Tonga’s Prime Minister, impacting the entire population of more than 100,000 people.
“This is a preliminary estimate, but we think the amount of energy released by the tonga volcanic eruption was equivalent to somewhere between 4 to 18 megatons of TNT,” Jim Garvin, chief scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said in a statement.
The Tonga volcanic eruption and tsunami killed at least three people, destroyed hundreds of homes and left remote islands cut off from the world with no communications.
A key factor, he said, was the depth below the ocean surface at which gas-rich magma came into contact with seawater – at just 150-250m.
To work out the position in the sky of the plume from Tonga’s Hunga-Tonga Hunga-Ha’apai volcano, data from three weather satellites – Himawari-8 (Japan) GOES-17 (USA) and GK2A (Korean) – was used.
“Because they’re all at different longitudes, we can use the parallax between their views of the tonga volcanic eruption to determine altitude. This is a pretty well established technique for storm cloud heights, and should actually work better here as the altitude [and hence parallax] is greater,” Dr Proud told BBC News.
For perspective: The 1980 eruption of Mount Saint Helens in Washington released about 24 megatons of TNT equivalent, and the famous 1883 explosion of Indonesia’s Krakatau is estimated to have unleashed 200 megatons or so, NASA officials said in the same statement.
The atomic bomb that the United States dropped on the Japanese city of Hiroshima in July 1945 released the energy of roughly 15 kilotons of TNT. There are 1,000 kilotons in a megaton, so the high end of the Tonga volcano estimate is equivalent to about 1,200 Hiroshima bombs.
The mid-January tonga volcanic eruptions undid this island-building work, however, blasting away the recently created land and leaving small, separated remnants of Hunga Tonga and Hunga Ha’apai. Tonga volcanic eruptions provides more data for Garvin and other researchers to analyze, to help them better understand volcanoes here on Earth and on other worlds as well.
Early data suggests the Tonga event could have measured as high as five on the volcanic explosivity index (VEI). This would certainly make it the most powerful eruption since Pinatubo, which was classified at six on the eight-point scale.
The Philippines volcano famously dropped Earth’s average global temperature by half a degree for a couple of years. It did this by injecting 15 million tonnes of sulphur dioxide into the atmosphere. SO2 combines with water to make a haze of tiny droplets, or aerosols, that reflect incoming solar radiation.