Hunga Tonga Eruption (2022): The Biggest Blast in a Century

The Hunga Tonga eruption of 15 January 2022 was the most powerful volcanic explosion recorded with modern scientific instruments. A largely submerged volcano in the South Pacific kingdom of Tonga blew apart, sending an ash plume higher than any ever measured, an atmospheric pressure wave that circled the Earth several times, and tsunamis across the Pacific. It killed at least six people and cut Tonga off from the world for weeks.

Background: a hidden volcano

Hunga Tonga-Hunga Ha’apai lies about 65 kilometres north of Nuku’alofa, Tonga’s capital, on the Tonga-Kermadec volcanic arc, where the Pacific tectonic plate dives beneath the Australian plate. Most of the volcano is under the sea. Only two small islands, Hunga Tonga and Hunga Ha’apai, showed above the surface, the remains of the rim of a large submarine caldera, or collapsed crater.

An eruption in 2014-2015 had built a new cone of ash that joined the two islands. Geologists knew the volcano had produced large explosive eruptions in the distant past, but it attracted little wider attention. Tonga is a nation of about 100,000 people spread across more than 170 islands, many of them low-lying.

The eruption

A new eruptive phase began on 20 December 2021, producing ash plumes and some disruption to flights. A strong explosion on 14 January 2022 gave a further warning. Then, at about 5:14 pm local time on 15 January 2022 (04:14 UTC), the volcano erupted with extraordinary violence. Scientists believe that seawater rushing into the hot magma chamber helped drive a series of huge explosions.

Satellites captured an umbrella-shaped cloud spreading over hundreds of kilometres within an hour. The plume rose about 57 kilometres, through the stratosphere and into the mesosphere, the highest volcanic plume ever recorded. Instruments detected hundreds of thousands of lightning flashes in the hours that followed, an unprecedented rate. Most of the newly joined island disappeared, leaving only small fragments above water.

A wave around the world

The explosion produced an atmospheric pressure wave that travelled around the globe at about the speed of sound. Weather stations everywhere recorded it, and it passed over each point on Earth several times as it circled the planet in both directions. The boom was heard as far away as Alaska, about 10,000 kilometres distant. Scientists compared the wave with the one produced by the eruption of Krakatoa in 1883.

The pressure wave also pushed on the ocean surface, generating tsunami-like waves that arrived in distant harbours earlier than ordinary tsunamis would. Sea-level changes were recorded in Japan, the Americas and the Caribbean. In Peru, unusual waves struck a beach north of Lima, drowning two people, and caused an oil spill at a refinery when a tanker was disturbed while unloading.

Impact on Tonga

In Tonga, tsunami waves reached up to about 15 metres on some islands. Several small islands of the Ha’apai group, including Mango and Fonoifua, had most of their homes destroyed. On the main island of Tongatapu, the waves damaged coastal areas and ash covered the land, contaminating water supplies and crops. At least four people died in Tonga, a toll limited by warnings and by people moving to higher ground after the earlier explosions.

The eruption cut Tonga’s only international undersea fibre-optic cable, along with the domestic cable. For days the outside world had almost no information about conditions in the country. Partial communication was restored by satellite, but full internet access returned only when a repair ship mended the cable in late February 2022. Australia, New Zealand and other countries delivered aid, using contactless drop-offs because Tonga had so far kept out COVID-19. Despite these precautions, the virus arrived in early February 2022.

Effects on the atmosphere

Unlike most large eruptions, Hunga Tonga released relatively little sulphur dioxide, the gas responsible for the cooling seen after the eruption of Mount Pinatubo in 1991. Instead it injected an estimated 146 million tonnes of water vapour into the stratosphere, increasing the amount there by about 10 per cent. Water vapour is a greenhouse gas, and scientists are still studying how this unusual injection affected temperatures and the ozone layer in the following years.

Timeline

• 2021, 20 December: A new phase of eruptive activity begins.
• 2022, 14 January: A strong explosion produces a large ash plume.
• 2022, 15 January, about 5:14 pm local time: The climactic eruption; tsunamis strike Tonga within the hour.
• 2022, 15 January: The pressure wave begins circling the Earth; tsunami warnings are issued across the Pacific.
• 2022, 15-16 January: Two people drown in Peru as waves strike the coast.
• 2022, 22 February: Repairs to Tonga’s international cable restore full internet access.

Why it matters

The Hunga Tonga eruption gave scientists their first chance to study an explosion of this size with satellites and global sensor networks, revealing new ways that volcanoes can affect the atmosphere and oceans. It also exposed how vulnerable small island nations are when a single cable links them to the rest of the world, a concern shared by communities descended from the seafarers who undertook the settlement of Tonga and Samoa.