Science

The explosion of the Hunga Tonga volcano equivalent to 10 megatons of TNT

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On January 15, the underwater volcano Hunga Tonga, located near the Tonga Islands in the South Pacific, erupted again. This large-scale event triggered a tsunami warning along the Pacific coast. The power of the explosion was such that the shock wave was felt as far away as Alaska. And for good reason: NASA scientists estimate that it was equivalent to 10 megatons of TNT, a power 500 times greater than that of the nuclear bomb dropped on Hiroshima!

Since its first eruption in 2009, the Hunga Tonga underwater volcano has evolved quite a bit: the accumulation of volcanic materials has gradually led to the emergence of a new small island in the Pacific. In 2018, its area is estimated at 2 km²; flora and fauna take possession of the place. A recent series of eruptions, triggered in December, had further increased its surface by 45%, according to experts. But last week’s explosion finally overcame the young volcanic island, of which almost nothing remains today.

The power of the event was nothing short of exceptional: the equivalent of 10 megatons of TNT according to James Garvin, chief scientist at NASA’s Goddard Space Flight Center. The explosion was heard as far away as Alaska, more than 9,000 kilometers from the volcano! “This may be the strongest eruption since [l’éruption du volcan indonésien] Krakatoa in 1883,” US Geological Survey geophysicist Michael Poland told NPR.

“A turning point in volcanology”

The Krakatoa eruption had caused tens of thousands of casualties and the volcanic ash plumes ejected into the atmosphere had even lowered average global temperatures by 0.5°C in the year after the event. In addition, the explosion of August 27, 1883 is considered the loudest noise ever heard in history: 160 km away, its sound level still reached 180 decibels (the sound level of a rocket taking off)!

If the Hunga Tonga eruption does not reach this magnitude, it is no less dramatic for the inhabitants of the Tonga Islands, who remain isolated from the world to this day after the rupture of the submarine communication cables. A tsunami inundated the entire west coast of the main island of Tongatapu, causing extensive damage. The capital Nuku’alofa and its airport are covered in ash, which does not facilitate relief intervention. The outer islands could have been even more affected, in particular Mango Island, where all the houses were devastated. In addition, some communities no longer have access to clean water.

According to a statement from the New Zealand Foreign Office, the death toll remains at three as of January 21: two Tongans and one British citizen. “Although the potential for future eruptions remains, they are unlikely to be as large as the January 15 eruption,” the document says. If the event is obviously terrible for the local populations, it greatly intrigues the experts. “It could be a landmark event, a kind of turning point in volcanology,” says Michael Poland.

Explosivity increased by sea water

The island of Hunga Tonga-Hunga Ha’apai began to emerge from the ocean during an eruptive phase that occurred in late 2014, early 2015. First two small islands appeared on each side of the caldera, then the accumulation of volcanic materials eventually created a platform that connects the two. Today almost nothing remains of these volcanic lands: everything was pulverized in the explosion, including the two oldest small islands.

Several observation satellites collected data during and after the eruption. A large plume of material appears in the images, which ended up creating what volcanologists call “an umbrella cloud,” accompanied by arcing shock waves and a record number of lightning strikes. According to volcanologist Simon Carn, the cloud reached 500 kilometers in diameter. “It is comparable to Pinatubo […]. However, the participation of water in the Tonga eruption could have increased the explosiveness compared to a purely magmatic eruption like that of Pinatubo”, specifies the specialist.

A hypothesis supported by James Garvin, who thinks that the event was undoubtedly caused by a sudden change in the level of the magma chambers located under the island, which caused the infiltration of seawater. cubic kilometer of liquid rock, things go bad quickly,” he explains. But what intrigues experts is that despite its explosive power, the eruption itself was relatively small. This lasted less than an hour and should not lead to short-term climate change, unlike other major eruptions, such as Tambora (1815), Krakatoa (1883) or Pinatubo (1991).

“The impact was disproportionate, far beyond the area one would have expected if the eruption had taken place over water. That is what puzzles us,” Poland said. To understand how such a “small” eruption could have caused such a large explosion and tsunami, scientists plan to conduct further studies in the area surrounding the caldera as soon as possible.

NPR

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