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Why was Tonga's volcanic eruption so severe, and what could we anticipate next?

The Kingdom of Tonga isn't often in the news, but a powerful eruption of an undersea volcano on January 15 sent shockwaves around half the world.

Normally, the volcano is unappealing. It is made up of two small uninhabited islands, Hunga-Ha'apai and Hunga-Tonga, which rise about 100 metres above sea level 65 kilometres north of Tonga's capital, Nuku'alofa. But under the seas sits a gigantic volcano, 1800m high and 20km broad.

Over the last few decades, the Hunga-Tonga-Hunga-Ha'apai volcano has erupted on a regular basis. Hot jets of lava and steam erupted through the waves during occurrences in 2009 and 2014/15. However, these eruptions were minor in comparison to the January 2022 occurrences.

According to research into previous eruptions, this is one of the tremendous explosions the volcano is capable of creating around every thousand years.

Given that seawater should temper the magma, why are the volcano's outbursts so explosive?

When magma slowly rises into seawater, even at temperatures about 1200°C, a thin layer of steam develops between the magma and the water. This acts as an insulator, allowing the magma's outer surface to cool.

However, when magma containing volcanic gas is blasted out of the earth, this process fails. When magma rapidly meets water, any steam layers are swiftly destroyed, putting hot magma into direct contact with cold water.

This is referred to by volcano scientists as "fuel-coolant interaction," and it is similar to weapons-grade chemical explosions. The magma is ripped apart by extremely powerful explosions. A chain reaction ensues, with new magma pieces exposing fresh hot inner surfaces to water, and the explosions continue, eventually jetting out volcanic particles and creating supersonic booms.

Hunga eruptions on two scales

The 2014/15 eruption formed a volcanic cone, which connected the two former Hunga islands to form a 5km-long merged island. When we arrived in 2016, we realised that the past eruptions were only preludes to the main event.

A secrete "caldera" was uncovered 150m below the waves while mapping the sea bottom.

The caldera is a 5km-wide crater-like depression. Smaller eruptions (such as those in 2009 and 2014/15) occur mostly along the crater's rim, but larger ones occur within the caldera itself. These massive eruptions are so powerful that the erupting magma's top falls inward, widening the caldera.

Scientists now believe that modest eruptions indicate the magma system steadily recharging itself in preparation for a large event, based on the chemistry of previous eruptions.

In deposits on the former islands, scientists discovered evidence of two massive previous eruptions from the Hunga caldera. Scientists linked them chemically to volcanic ash deposits on Tongatapu, the biggest inhabited island 65 kilometres away, and then used radiocarbon dates to prove that massive caldera eruptions occur every 1000 years, with the most recent one occurring in AD1100.

With this information, the January 15 eruption appears to be on target for a "big one."

What can we anticipate to happen now?

We're still in the midst of this large eruptive series, and many details are still unknown, partially because the island is hidden by ash clouds.

The two previous eruptions, which occurred on December 20, 2021, and January 13, 2022, were of modest scale. They created clouds with elevations of up to 17 kilometres and added additional territory to the 2014/15 merged island.

The most recent eruption has increased the level of violence. The ash cloud has already reached a height of roughly 20 kilometres. Most notably, it expanded virtually concentrically across a distance of around 130km from the volcano, forming a plume with a circumference of 260km before being deformed by the wind.

This exhibits enormous explosive force, which cannot be explained just by magma-water contact. Instead, it demonstrates that enormous volumes of new, gas-charged magma have erupted from the caldera.

The eruption also caused a tsunami that swept into Tonga, as well as neighbouring Fiji and Samoa. Shock waves travelled thousands of kilometres, were observed from orbit and were recorded in New Zealand, 2000 kilometres distant. The sky in Tongatapu was shut out shortly after the eruption began, and ash began to fall.

All of these indicators point to the enormous Hunga caldera waking up. Tsunamis are triggered by coupled atmospheric and ocean shock waves created by explosions, but they can also be caused by undersea landslides and caldera collapses.

It's uncertain whether this is the eruption's apex. It signifies a significant magma pressure release, which may help to settle the system.

However, geological deposits from earlier eruptions of the volcano serve as a warning. These intricate patterns demonstrate that each of the 1000-year main caldera eruption periods comprised several independent blast events.

As a result, the Hunga-Tonga-Hunga-Ha'apai volcano could cause considerable volcanic disturbance for several weeks or possibly years. I hope not, for the welfare of the people of Tonga.