Why Tonga’s volcanic eruption was so violent and what to expect next

The Kingdom of Tonga doesn’t often attract global attention, but a violent eruption of an undersea volcano on January 15 sent shockwaves, literally, around half the world.

The volcano is usually not much to see. It consists of two small uninhabited islands, Hunga-Ha’apai and Hunga-Tonga, located about 100 m above sea level, 65 km north of Tonga’s capital, Nuku’alofa. But hidden under the waves is a massive volcano, about 1800 m high and 20 km wide.

A massive underwater volcano sits adjacent to Hunga-Ha’apai and Hunga-Tonga islands.
Author provided

The Hunga-Tonga-Hunga-Ha’apai volcano has erupted regularly over the past decades. During the 2009 and 2014/15 events, hot jets of magma and steam exploded through the waves. But those eruptions were small, overshadowed by the events of January 2022.

Our research on these earlier eruptions suggests that this is one of the massive explosions the volcano is capable of producing approximately every thousand years.

Why are volcano eruptions so explosive when seawater should cool the magma?

If magma rises slowly in seawater, even at temperatures around 1200℃, a thin film of vapor forms between the magma and the water. This provides a layer of insulation to allow the outer surface of the magma to cool.

But this process does not work when the magma is expelled from the ground full of volcanic gas. When magma rapidly enters water, all layers of vapor are quickly disrupted, bringing the hot magma into direct contact with the cold water.

Volcano researchers call this the “fuel-coolant interaction” and it’s akin to weapons-grade chemical explosions. Extremely violent explosions tear through the magma. A chain reaction begins, with new fragments of magma exposing the cool, hot interior surfaces to water, and the explosions repeat, eventually spewing volcanic particles and causing explosions at supersonic speeds.

Read more: A volcano’s ‘pulse’ can be used to help predict its next eruption

Two scales of Hunga eruptions

The 2014/15 eruption created a volcanic cone, joining the two former Hunga Islands to create a combined island approximately 5 km long. We visited in 2016 and found that these historic eruptions were just curtain raisers for the main event.

By mapping the seabed, we discovered a “caldera” hidden 150 m below the waves.

A map of the seafloor shows the volcanic cones and the caldera.
A map of the seafloor shows the volcanic cones and the massive caldera.
Author provided

The caldera is a crater-like depression about 5 km in diameter. Small eruptions (as in 2009 and 2014/15) mostly occur at the edge of the caldera, but very large ones originate from the caldera itself. These large eruptions are so large that the top of the erupting magma collapses inward, deepening the caldera.

Looking at the chemistry of past eruptions, we now believe that small eruptions represent the magma system slowly recharging to prepare for a big event.

We have found evidence of two huge past eruptions of the Hunga caldera in deposits on the ancient islands. We compared these chemical deposits to volcanic ash deposits on the largest inhabited island of Tongatapu, 65 km away, then used radiocarbon dating to show that large caldera eruptions occur approximately every 1000 years, the last at AD1100.

With this knowledge, the January 15 eruption appears to be right on time for a “big one”.

Read more: Why White Island erupted and why there was no warning

What we can expect now

We are still in the middle of this major eruptive sequence and many aspects remain unclear, in part because the island is currently obscured by ash clouds.

The two previous eruptions on December 20, 2021 and January 13, 2022 were moderate in size. They produced clouds up to 17 km high and added new land to the 2014/15 combined island.

The latest eruption escalated in magnitude in terms of violence. The ash plume is already about 20 km high. More remarkably, it extended almost concentrically for a distance of about 130 km from the volcano, creating a plume with a diameter of 260 km, before being deformed by the wind.

This demonstrates tremendous explosive power – which cannot be explained by magma-water interaction alone. Rather, it shows that large amounts of fresh, gas-laden magma erupted from the caldera.

The eruption also produced a tsunami across Tonga and neighboring Fiji and Samoa. The shock waves traveled several thousand kilometres, were seen from space and recorded in New Zealand about 2000 km away. Shortly after the eruption began, the sky was blocked over Tongatapu, with ash beginning to fall.

All these signs suggest that the great Hunga caldera has awakened. Tsunamis are generated by coupled atmospheric and oceanic shock waves during an explosion, but they are also easily caused by undersea landslides and caldera collapses.

It is not known if this is the high point of the eruption. It represents a significant release of magmatic pressure, which can decant the system.

A warning, however, lies in the geological deposits from the volcano’s previous eruptions. These complex sequences show that each of the major 1000-year caldera eruption episodes involved many separate explosion events.

Therefore, we could be for several weeks or even years in major volcanic unrest of the Hunga-Tonga-Hunga-Ha’apai volcano. For the sake of the people of Tonga, I hope not.

About Kristina McManus

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