Hunga Tonga-Hunga Ha’apai was, until January 2022, a minor entry in the catalogue: a submarine volcano in the Tongan arc whose 2014-15 eruption had joined two islets into one, a favourite of satellite-imagery stories about new land. Eruptions resumed in December 2021 and seemed to be winding down.
On 15 January 2022, at 17:14 local time, the volcano detonated. The proximate physics: a vast body of magma interacting with seawater at the ideal depth — shallow enough for the steam explosion to reach the surface unmuffled, deep enough for maximal water contact — in a runaway sequence that ranks as the largest volcanic explosion of the instrumental era (VEI 5-6, energy estimates in the tens to hundreds of megatons).
The records
The event rewrote several. The eruption column reached about 57 kilometres — through the entire stratosphere and into the mesosphere, the first volcanic plume ever observed there. The atmospheric pressure wave propagated at near the speed of sound and was recorded by barometers circling the Earth at least four times over six days; it generated meteotsunamis in the Mediterranean and the Caribbean — ocean basins with no line of sight to Tonga. The bang itself was heard as thunder-like booms in Fiji, New Zealand — and Alaska, some 9,700 kilometres away, likely the farthest a sound has been documented to travel since Krakatoa 1883. Lightning detection networks logged almost 400,000 strokes in the plume in six hours — the most intense lightning storm ever measured. And because the volcano was submarine, the blast lofted not ash so much as water: on the order of 146 million tonnes of vapour into the stratosphere, raising its global water content by roughly 10 per cent — a novel, years-long experiment on the climate system (net effect: slight warming, unlike the cooling of ash-rich eruptions, plus a measurable nibble at the ozone layer).
Tonga
For the Kingdom of Tonga, 65 km away, the eruption meant ashfall, a drinking-water crisis, and tsunami waves that reached about 15 metres on nearby islands, destroying every house on some. The islands’ single submarine fibre-optic cable was severed in two places; the country dropped off the internet and telephone network for days — a nation communicating with the world by ham radio and reconnaissance overflight in 2022 — with repairs taking five weeks. The confirmed death toll was mercifully small — four in Tonga, with two more drowned by the trans-Pacific tsunami in Peru — a testament to drilled evacuation and to the eruption’s timing before communal weekend gatherings.
The volcano blew its own islands apart; where the 2015 land stood, there is again sea. The scientific literature, meanwhile, has treated the eruption as a natural experiment without precedent — the one time modern instruments have watched the atmosphere ring like a struck bell.