Thursday 20 January 2022

Tonga devastated by tsunami following eruption on the Hunga Ha'apai volcano

Three people, including a British citizen, have been confirmed dead and many more are missing following a tsunani that swept across the island nation of Tonga on Saturday 15 January 2022, following an eruption on the Hunga Ha'apai volcano. The volcano, which is usually underwater and is about 65 km to the north of the nation's capital, Nukuʻalofa, began erupting on 20 December 2021, producing a small island by the first week of January. However, on 15 January the centre of this island sank abruptly beneath the sea about two hours before the eruption. Vulcanologists have hypothesised that the eruption was caused by the collapse of a magma chamber, allowing seawater to come into contact with the hot magma, and triggering a phreatic explosion (rapid expansion of gas as a large volume of water turns to vapour more-or-less instantly).

Devastation on Tonga caused by the 15 January 2022 tsunami. Consulate of the Kingdom of Tonga.

Following the explosion a huge wave swept across the islands of Tonga, then out across the Pacific Ocean, reaching Australia and New Zealand in four hours, Japan in eight hours, California in ten hours and Alaska in eleven. Ths islands of Tonga suffered extensive damage, which destroyed much of the nation's communications infrastructure, with the extent of the destruction only becoming clear as ships from the New Zealand Navy reached the area several days later. As well as the obvious problems caused by the devastation of the nation's infrastructure, much of the area has been covered by sulphur-rich volcanic dust, which can be harmful if breathed in, and can react with fresh water to form sulphuric acid, threatening drinking water supplies. 

How a submarine phraetic explosion can cause a tsunami. BBC.

Hunga Ha’apai lies on the Tonga/Kermadec Ridge, and is fed by the subduction of the Pacific Plate beneath the Australian Plate along the Kermadec/Tonga Trench. As the Pacific Plate sinks into the Earth, it is warmed by the heat from the planets interior. This leads to partial melting of the Pacific Plate, with some of the melted material rising through the overlying Australian Plate as magma, fuelling the volcanos of the Kermadec/Tonga Ridge.

Diagram showing subduction along the Tonga Trench, and how this feeds the volcanoes of the Tonga Volcanic Arc. York University.

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