Why Tonga’s eruption has been so violent and what could happen next

The kingdom of Tonga does not usually attract world attention, but the violent eruption of an underwater volcano January 15th has sent shock waves literally across half the world.

Normally, the volcano doesn’t look like much to the naked eye. It consists of two small uninhabited islands, Hunga-Ha’apai and Hunga-Tonga, which rise about 100 m above sea level 65 km north of Nuku’alofa, the capital of Tonga. But under the water lies a huge volcano about 1,800 m high and 20 km wide.

The huge underwater volcano next to the Hunga-Ha’apai and Hunga-Tonga islands.

The Hunga-Tonga-Hunga-Ha’apai volcano has erupted regularly in recent decades. In 2009 and 2014/15, hot jets of magma and steam exploded over the waves. However, these eruptions were small compared to the January 2022 phenomenon.

Our research on previous eruptions suggests that the most recent is one of the massive explosions that the volcano is capable of producing approximately every thousand years.

Why are the volcano eruptions so explosive, if the sea water should cool the magma?

If magma rises through water slowly, even at temperatures of about 1,200 ℃, a thin film of steam forms between the magma and water. This provides an insulating layer that allows the outer surface of the magma to cool.

But this process does not work when the magma comes out of the ground filled with volcanic gas. As material rapidly enters the water, the vapor layers rapidly break up, bringing hot magma into direct contact with cold water.

Volcanologists call this “fuel-coolant interaction” and it is similar to weapons-type chemical explosions. Extremely violent explosions rip apart the magma. A chain reaction is then started, with new magma fragments exposing new hot interior surfaces to the water, and the explosions repeat themselves, finally throwing out volcanic particles and causing explosions with supersonic speeds.

The two scales of the Hunga eruptions

The 2014/15 eruption created a volcanic cone, joining the two former Hunga islands to create a combined island about 5 km long. We visited it in 2016 and discovered that these historical eruptions were nothing more than opening act for the main phenomenon.

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

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

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

After looking at the chemistry of past eruptions, we think the small eruptions represent the magma system slowly recharging itself in preparation for a big event.

We found evidence of two huge past Hunga caldera eruptions in the deposits of the ancient islands. We compared them chemically with volcanic ash deposits on the largest inhabited island, Tongatapu, 65 km away, and then used radiocarbon dates to show that major caldera eruptions occur about every 1,000 years, the last of which in the year 1100.

With this knowledge, the January 15 eruption appears to be right on schedule for one of the big ones.

What can we expect to happen now?

We are still in the midst of this major eruptive sequence and many things 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 17km high and added new land to the combined 2014/15 island.

The latest eruption has increased the scale in terms of violence. The ash plume is already about 20 km high. The most remarkable thing is that it spread almost concentrically to a distance of about 130 km from the volcano, creating a plume with a diameter of 260 km, before it was distorted by the wind.

This supposes an enormous explosive power, which cannot be explained solely by the magma-water interaction. Instead, it shows that large amounts of fresh, gas-laden magma have exited the caldera.

The eruption also produced a tsunami throughout Tonga and in neighboring Fiji and Samoa. The shock waves traveled many thousands of kilometres, were seen from space and were recorded in New Zealand some 2,000 km away. Shortly after the start of the eruption, the sky was obscured at Tongatapu, and ash began to fall.

All these signs suggest that Hunga’s great caldera has awakened. Tsunamis are generated by coupled atmospheric and oceanic shock waves during an explosion, but are also easily caused by submarine calving and caldera collapses.

It is not yet clear if this is the climax of the eruption. It represents an important release of magma pressure that can calm the system.

However, the geological deposits from the volcano’s previous eruptions contain a warning. These complex sequences show that each of the major caldera eruption episodes every 1000 years involved many separate blast events.

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Thus, the Hunga-Tonga-Hunga-Ha’apai volcano could cause major volcanic upheaval for several weeks or even years. For the sake of the people of Tonga, let us hope that this is not the case.

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