The eruption of the submarine volcano in the island state of Tonga last Saturday was particularly violent. The eruption caused a tsunami that reached from New Zealand to the United States and the air pressure in the Netherlands rose. Geologists do not expect the volcano in Tonga to cause a long global cooling, as has been the case with other violent volcanic eruptions.
The volcano lies underwater between the uninhabited islands of Hunga Tonga (0.39 square kilometers) and Hunga Ha’apai (0.65 square kilometers). Magma that erupted from smaller eruptions in 2009 and 2014 cooled and formed a connecting piece between the two islands.
Last Saturday morning, Hunga Tonga and Hunga Hapai became two separate islands again when the volcano erupted with a violent explosion. The run-up to the eruption started late last year with smaller explosions. In early January, activity waned and geologists believed the volcano had subsided. Yet it erupted again last Saturday, much more violent than the eruptions before. Just before this explosion, the kilometre-long connecting piece collapsed and disappeared under the water.
The eruption lasted eight minutes. The cloud of ash and toxic gas particles, such as sulfur oxide, that was released, reached a height of 30 kilometers and was about 500 kilometers wide. By way of comparison: the plumes from the recent volcanic eruption on La Palma reached a maximum height of about five kilometers. The capital of Tonga, sixty-five kilometers away, is covered in a thick layer of ash. This has consequences for the safety of drinking water in Tonga, for example.
An explosion of an underwater volcano like the one near Tonga is more violent than, for example, that eruption on La Palma. “This is because the volcanism here is caused by subduction: the Pacific plate sinks under Tonga into the Earth’s mantle and takes water down with it,” says professor of paleogeography Douwe van Hinsbergen of Utrecht University. “This water rises with magma as gas, as water vapour. This puts enormous pressure on the earth’s crust. Arwen Deuss, geoscientist at Utrecht University, compares it to a bottle of cola. “When you shake a bottle of Coke, the gas will come out with great force. That’s probably what happened in Tonga, but we don’t know exactly.”
Ten meters
The eruption was followed by a tsunami that reached several Pacific coast countries. Deuss: “How exactly this comes about has yet to be investigated. It almost never happens that a tsunami is caused by a volcanic eruption. The collapse of the volcano is believed to cause movement in the water, creating a wave. In the middle of the ocean, ships don’t notice that much. Only when the tsunami approaches the coast and the ocean is less deep do the waves get higher, but luckily that wasn’t too bad. In some places the waves were one to two meters, and on the coast of the United States only a few centimeters. That is not nearly as high as, for example, the tsunami in Sumatra in 2004, which was caused by an earthquake. There the waves reached a height of about ten meters in some places.”
The consequences were also noticeable in the Netherlands, but especially with measuring equipment. The seismic waves released by the eruptions were measured half an hour after the eruption in the Netherlands, and the air pressure here temporarily rose fifteen hours later. During the explosion, air in the atmosphere is pushed aside, causing the air pressure to increase, in De Bilt by three Hectopascals. As a result, the wind even stopped here temporarily, because a high-pressure area contains less moisture and fewer clouds, and therefore less wind.
Geologists do not expect long-term effects of the eruption on the weather. A violent volcanic eruption can cause the whole world to cool down temporarily. The particles that are pumped into the air during an eruption reflect and absorb sunlight and ensure that the sun is less able to warm up the earth’s surface. The volcanic eruption on Krakatoa in 1883 led to a worldwide cooling of about 1.2 degrees Celsius for a year. The Pinatubo in 1991 caused a temperature drop of about a quarter of a degree Celsius.
unpredictable
Such a cooling occurs especially when the particles are injected high into the atmosphere. They stay there longer. “The Tonga volcanic eruption was powerful and that is why the ash clouds were also high, but it lasted for a relatively short time,” says Elske de Zeeuw-van Dalfsen. She is a volcanologist at the KNMI. “The eruption lasted eight minutes, Pinatubo hours. The first measurements of the ash particles above Tonga show that they contain fifty times fewer sulfur particles than in the Pinatubo eruption. At first sight it is not enough to have a long-lasting effect, but more extensive analyzes still need to be done.’
Deuss: “Locally, above Tonga, the ash particles do affect the weather. For example, there will be bright red sunsets there.”
It is still uncertain when the volcano will erupt again. Deuss: “According to geological data, such a violent explosion only occurs once every thousand years. But it is also possible that the volcano erupts twice in a row. Whether he has settled down, we never know. It remains unpredictable.”
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