How big was the eruption of the Hunga Tonga-Hunga Ha’Apai volcano? Decryption

A volcanic eruption is, by definition, an impressive natural phenomenon. But for a small-scale eruption, like that of Piton of the Fournaise in Reunion which has just ended, to an eruption that could cause global climatic disturbances, there is a big difference in explosiveness.

In order to be able to compare them, Newhall and Self created the Volcanic Explosivity Index (or VEI) in 1982, a scale which makes it possible to estimate the magnitude of one eruption from different parameters. This assessment is mainly based on the volume of tephras emitted at the time of the paroxysm, which corresponds to the magma fragmented during the most violent event of the eruption, and on the height of the associated volcanic plume. However, this VEI is not the result of a calculation, but rather of an estimate: the approach is therefore somewhat subjective. But this scale is logarithmic, that is to say that the index increases by one value when the volume of tephras emitted is ten times greater, which allows a certain largesse. Thus, two eruptions from the same VEI are not strictly equivalent, but this scale makes it possible to fairly reasonably classify the eruptions between them. For example, those of Piton of the Fournaise are classically of a VEI between 0 and 1, when the eruption of 600,000 years ago in Yellowstone in the United States is 8, the highest index of this scale!

Panache!

The eruptive column of January 14 above the Tongan volcano, the day before the paroxysm, reached 20 kilometers in altitude. This already made this event an eruption of a certain magnitude, since the limit between the troposphere and the stratosphere, which is called the tropopause and which is about 15 kilometers above sea level at this latitude, requires a certain flow eruptive to be crossed. But Wellington’s VAAC (the information center for emissions of ash in this sector of the globe) warns that the next day’s eruptive column reaches an altitude of about 30 kilometers! Satellite data confirmed this, some even suggesting a maximum altitude of 35 kilometers!

It is comparable to the 34 kilometers of the paroxysm of the eruption of Pinatubo in the Philippines, on June 15, 1991, just like the umbrella formed by the spreading of the plume in the stratosphere whose diameter reaches about 500 kilometers for these two eruptions. But the few centimeters of ash that fell on the Tonga archipelago are incomparable to the total darkness in which much of the island of Luzon found itself in broad daylight during thePinatubo eruption ! And if a large part of the ash from this eruption in Tonga fell back into the Pacific Ocean, thus annihilating the estimate of a reliable volume for this eruption, it appears that the VEI here is necessarily weaker than the 6 of the Pinatubo.

The little ash linked to this activity, relative to the significant height of the eruptive plume, is explained by the mechanism at work at the level of the eruptive mouth. This one being under water, the contact between lava and the sea water engenders the vaporization a large amount of water vapour. From gas therefore adds to the eruption! In short, volcanic eruptions under shallow water are complicated by seawater which makes them more explosive! So more explosive, but not necessarily more lava-emitting…

By comparing this eruption in Tonga with that of Chaitén in Chile, in May 2008, which is a VEI 4, we also realize there the deficit of ashes emitted by the volcano Hunga Tonga-Hunga Ha’Apai. Indeed, if the height of the eruptive plume is slightly less for the Chilean volcano, the thickness of the ash deposit of its paroxysm is 30 centimeters at 65 kilometers from the volcano! To go further, we can also estimate theenergy deployed at the time of the eruption. James Garvin, a scientist from the Nasa, thus estimated that theexplosion of January 15, 2022 had released an energy equivalent to 10 megatons, the equivalent of 500 Hiroshima bombs! This is comparable with the explosion of Mount Saint Helens in the United States, in May 1980, which is estimated at a VEI 5.

In view of these comparisons, the VEI of the eruption in Tonga is therefore very probably between 4 and 5, which makes it a very violent eruption, but not cataclysmic. And if on a human scale, it is an important event, on a planetary scale, it is much less so!

An impact over a large area

Unlike Chaitén or Mount Saint Helens, this eruption was felt several thousand kilometers away. The tsunami indeed spread throughout the Pacific Ocean and the sound of the explosion was heard as far as Alaska, more than 8,500 kilometers away! This is reminiscent of the eruption of Krakatau in Indonesia, in August 1883, estimated at a VEI 6 with an eruptive plume that reached about 50 kilometers in altitude. The sound of the paroxysm was heard almost throughout the Indian Ocean, from Australia to Rodrigues Island, a small island near The meeting. the tsunami was he gigantic, with waves several tens of meters high on the coasts of Java and Sumatra, which explains the very heavy toll associated with this eruption: 36,000 victims at least! This tsunami is the consequence of thecollapse of the volcano which peaked at about 800 meters high in the ocean, which is to date the most likely hypothesis to explain the tsunamis of the eruption in Tonga…

Finally, it is also appropriate to ask whether this eruption could be the cause of climatic disturbances. Because the volcanic eruptions scale release significant amounts of carbon dioxide sulfur, a gas that combines with water in theatmosphere to form micro-droplets ofsulfuric acid. This aerosol constitutes a kind of veil which reflects part of the solar radiation, which can modify the weather if these quantities of gas are significant. The eruptions of Krakatau in 1883 and Pinatubo in 1991 are for example associated with climatic disturbances for a few months or years. But in the case of the eruption in Tonga, the quantity of sulfur dioxide emitted was estimated at 400,000 tonnes, which is too little to have climatic consequences.