Volcanology: The Tonga Eruption That Will Make History


But the magma of January 2022 is different. Shane Cronin, a volcanologist at the University of Auckland in New Zealand, and his colleagues have analyzed ash collected by disaster workers on Tonga’s largest island. According to Cronin, the fresh magma rose conspicuously quickly – too fast for the typical changes in the magma chamber.

Surprisingly little ash

Geologist Taaniela Kula and his colleagues at the Tonga Geological Survey in Nuku’alofa have collected ash samples from islands throughout Tonga, which are being analyzed by Cronin and others. The height of the layers and their distribution also provide information about the course of the eruption. The fact that the volcano emitted comparatively little ash compared to the size of the eruption could be due to the fact that it was submerged, but at a relatively shallow depth.

Volcanic eruptions in deep water rarely manifest themselves above the surface as large explosions. The pressure of the overlying water prevents gas bubbles from expanding with explosive force. However, the volcanic vent that erupted at Hunga Tonga-Hunga Ha’apai on January 15 was only 10 to 250 meters deep. That’s shallow enough that the water doesn’t suppress the force of the explosion, but deep enough that the erupting magma encounters plenty of water.

Then the heat can turn the water into steam in a flash, which expands enormously. The thermal energy of magma is converted very efficiently into the kinetic energy of an eruption, says Michael Manga, a geoscientist at the University of California, Berkeley: “Some of the strongest eruptions have been water-triggered.” Another important factor is how much volcanic gas mixed with the magma before it erupts. A gas-rich magma rise may have fueled the Jan. 15 eruption by providing a large number of bubbles for the explosion, says Raymond Cas, a volcanologist and professor emeritus at Monash University in Melbourne.

force from water vapour

© NASA / Visible Earth

Impact from water vapor | Because the eruption occurred at a relatively shallow water depth, the steam generated by the hot magma was able to unfold its full effect.

Strange simultaneities

The eruption of Hunga Tonga-Hunga Ha’apai is special in that unusual things happened together, says Cas. Volcanologists know of other examples of eruptions that took place underwater or under snow and ice, in which water consequently also played a key role. Extremely high eruption columns have also been seen before. However, Hunga Tonga-Hunga Ha’apai is the first known case in which both phenomena occurred together. It could possibly even serve as a prototype for a new type of eruption, says Cas.

In submarine eruptions, there are “relatively few instances where we see large plumes,” says Kristen Fauria, a volcanologist at Vanderbilt University in Nashville, Tennessee. For example, the massive deep-sea eruption of the Havre volcano north of New Zealand in 2012 mainly created a huge carpet of pumice on the ocean. This eruption took place at a depth of more than 900 meters.



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