Explained: Why volcanic eruption in Tonga was so violent, what to expect next

Explained: Why volcanic eruption in Tonga was so violent, what to expect next

The Kingdom of Tonga doesn’t often garner global attention, but a violent eruption of an underwater volcano on January 15 sent shock waves across nearly half the world.

Volcanoes are usually not much to see. It consists of two small uninhabited islands, Hunga-Hapai and Hunga-Tonga, which are about 100 meters above sea level, 65 km north of Tonga’s capital Nuku’alofa. But hidden under the waves is a huge volcano, which is about 1800 meters high and 20 kilometers wide.

The Hunga-Tonga-Hunga-Hapai Volcano has erupted regularly over the past few decades. Hot jets of magma and steam burst through the waves during the 2009 and 2014/15 events. But these eruptions were small, largely dwarfed by the January 2022 events.

Our research into these earlier eruptions suggests that this is one of the largest eruptions that volcanoes are capable of producing approximately every thousand years.

Why are volcanic eruptions so explosive, given that ocean water cools the magma? If magma rises slowly in ocean water even at a temperature of about 1200 °C, a thin film of steam is formed between the magma and the water. It provides a layer of insulation to allow the outer surface of the magma to cool.

But this process does not work when magma erupts from a volcanic gas-laden ground. When magma rapidly enters the water, any layers of steam are quickly disrupted, allowing the hot magma to come into direct contact with the cold water.

Volcano researchers call this a “fuel-coolant interaction” and it equates to weapons-grade chemical explosions. Extremely violent eruptions tear the magma apart. A chain reaction begins, new magma fragments exposing freshly heated interior surfaces to the water, and the eruption repeats, eventually ejecting volcanic particles and erupting with supersonic speed.

Two Scale Hunga Eruption The 2014/15 eruption created a volcanic cone that joined the two older Hunga islands to form a combined island about 5 km long. We visited in 2016, and found that these historic eruptions were only curtain-raisers for the main event.

Mapping the ocean floor, we discovered a hidden “caldera” 150 meters below the waves. The caldera is a crater-like depression about 5 km across. Smaller eruptions (such as in 2009 and 2014/15) occur mainly at the edge of the caldera, but much larger ones come from the caldera itself. These large eruptions are so large that the top of the erupted magma collapses inward, deepening the caldera.

Looking at the chemistry of past eruptions, we now think that the smaller eruptions represent a magma system slowly recharging itself in preparation for a larger event.

We found evidence of two giant past eruptions from the Hunga Caldera in deposits on the older islands. We chemically matched these to volcanic ash deposits on the largest inhabited island of Tongatapu, 65 km away, and then used radiocarbon dates to show that large caldera eruptions occur in about 1000 years, the last of which occurred in AD1100. it happens.

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

What we can expect to happen now We are still in the middle of this major eruption sequence and many aspects remain unclear, partly because the island is currently covered with clouds of ash.

The two earlier eruptions, on 20 December 2021 and 13 January 2022, were of medium size. They formed clouds up to a height of 17 km and added new land to the combined island of 2014/15.

The latest eruption has increased the scale of the violence. The ash pile is already about 20 km high. Most notably, it spread almost concentrically over a distance of 130 km from the volcano, forming a plume with a diameter of 260 km, before being distorted by the wind.

It displays an enormous explosive force – which cannot be explained by magma-water interactions alone. Instead it shows that a large amount of fresh, gas-charged magma has erupted from the caldera.

The explosion also caused tsunamis in Tonga and neighboring Fiji and Samoa. The shock waves lasted for several thousand kilometers, were seen from space, and were recorded in New Zealand about 2000 km away. Soon after the eruption began, the sky over Tongatapu was blocked, causing ash to fall.

All these signs suggest that the Greater Hunga Caldera has woken up. Tsunamis are generated by coupled atmospheric and ocean shock waves during an eruption, but they are also easily caused by submarine landslides and caldera collapses.

It is not clear whether this is the climax of the explosion. This represents a major magma pressure release, which can settle the system.

One caveat, however, lies in geological deposits from the volcano’s past eruptions. These complex sequences suggest that each of the 1000-year major caldera eruption episodes consisted of many different eruption events.

So we can be from Hunga-Tonga-Hunga-Hapai volcanism to several weeks or even years of major volcanic unrest. I do not hope for the sake of the people of Tonga.

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