The first explanations for the eruption of a volcano in Tonga

• On January 15, 2022, the most violent volcanic explosion since Pinatubo (Philippines, 1991) took place in the Pacific Ocean, according to our partner The Conversation.

• Unlike those of Vesuvius and Mount Pelee which dominated cities, the 2022 eruption took place on a desert island;

  there was therefore no direct death due to this eruption.

• This analysis was conducted by Pierre Thomas, Emeritus Professor of Earth Science at the École Normale Supérieure in Lyon.

On January 15, 2022, at 5:10 p.m. (local time), the most violent volcanic explosion took place since that of Pinatubo (Philippines) in 1991. Its Volcanic Explosivity Index (VEI in English) is estimated between 4 and 5 (on a logarithmic scale which ranges from 0 to 8).

This places this eruption between that of Mount Pelée (Martinique, 1902; VEI of 4) and that of Vesuvius-Pompeii in the year 79 (VEI of 5).

But unlike Vesuvius and Mount Pelee which towered over cities, the 2022 eruption took place on a desert island, and there were no direct deaths from the eruption.

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Until 2014, there were two small islands in this sector of the Pacific Ocean: the islands of Hunga Tonga and Hunga Ha'apai (see fig 3 – 1).

These two islands corresponded to the remains of the barely emerged summit of an old large volcano dominating the bottom of the sea by about 2000 meters, a large volcano whose summit had been destroyed in the 11th century by a violent eruption with explosion, collapse and formation of a caldera (underwater).

A caldera corresponds to a volcanic depression (a crater) due to an underground collapse.

A map of the ocean floor shows volcanic cones and the central caldera © P. Thomas (via The Conversation)

Since this 11th century, it seems that there have been only small underwater eruptions giving rise to ephemeral volcanic islands quickly destroyed by marine erosion.

The last of these small eruptions took place in 2009. This volcanic activity is to be linked to that of the "Pacific Ring of Fire", where the Pacific Ocean sinks by subduction under a continent (case of the Andes Cordillera ) or under another sector of the Pacific such as Tonga.

The lavas and ashes emitted by these Tongan volcanoes are mainly andesitic (a lava called andesite with 60% silica and very common in subduction zones such as the Andes).

In December 2014 and January 2015 there was a bigger eruption than usual.

A new volcano (more than 100 m high) appeared in two months between the two islands (previously 2000 meters apart) and joined them.

The unique island of Hunga Tonga – Hunga Ha'apai was born (see fig 3 – 2).

The eruptions that built this new volcano were of the phreato-magmatic type (also called surtseyen).

The seabed was made up of ash and other porous rocks soaked in seawater (water table).

The arrival of very hot magma in this water table caused the water to boil, making it difficult to escape.

The pressure increased and this generated relatively "moderate" explosions (like a pressure cooker with a clogged valve).

A more or less continuous arrival of magma led to a series of explosions which followed one another.

The erupting Hunga Tonga-Hunga Ha'apai volcano © Eyepress News / AFP (via The Conversation)

After a rest of almost 7 years, the activity of Hunga Tonga – Hunga Ha'apai resumed on December 20, 2021. These were still phreato-magmatic eruptions, more violent than those of 2014-2015, and which have " enlarged and enlarged” the new volcano of 2014-2015 (see fig 3-3).

With ups and downs, the phreatic eruptions followed one another until January 11th.

Tongan authorities issued a danger advisory to airlines, and an alert for possible tsunamis.

The explosions ceased on January 11, and the alerts were lifted.

The eruption resumed with more violence on January 14 around 4 a.m. (local time), with eruptive columns several kilometers wide and high.

The Tongan authorities raised the alerts.

A satellite image taken on January 15 at 3:25 p.m. (see fig. 3-4) shows that a large part of the 2014-2015 building (enlarged between December 20, 2012 and January 11, 2022) had disappeared.

This same January 15, at 5:10 p.m., began a hyper-violent explosive phase.

The atmospheric shock wave of this explosion was recorded all over the planet.

This paroxysmal phase lasted eight minutes before calming down a bit, then stopping.

The Plinian plume (so called because well described by Pliny the Younger during the eruption of Pompeii in 79) reaches 15 to 20 km, spreads over several hundred kilometers.

The next day (January 16),

the ashes reached an altitude of 31 km.

A satellite image from January 17 shows that all of the 2014-2015 edifice, the majority of the former island of Hunga Tonga and 30% of the former island of Hunga Ha'apai have disappeared.

A radar satellite image taken on the evening of January 15 shows that these disappearances took place in the minutes (or hours) following the 5:10 p.m. explosion.

Montage of satellite images showing the course of eruptive events from 2014 to 2022.

– The two islands of Hunga Tonga and Hunga Ha'apaien in 2013, before the eruption of 2014-2015

– The islands united by the eruption of 2014-2015

– The eruption of 2021-2022 had begun, and the “new volcano” of 2014-2015 had doubled in surface

– The strong explosions which had resumed the day before had already destroyed part of the volcano of 2014-2015 enlarged the month before

– The plume caused by the explosion of January 15, 5:10 p.m., 20 to 30 minutes after this explosion

– Satellite image of what remains of the Hunga Tonga volcano and Hunga Ha'apaien on January 17, two days after the explosion © GoogleEarth / NASA – Montage P. Thomas (via The Conversation)

Motion picture showing a series of images taken by a geostationary satellite on January 15, 2022 between 5:10 p.m. and 6:50 p.m., time-lapse.

At its largest, the plume is about 500 km in diameter © Japan Meteorological Agency / NASA SPoRT / Wikimedia CC BY-SA 4.0 (via The Conversation)

What happened on January 15, 2022 at 5:10 p.m.?

We can only make plausible hypotheses, but which will need to be confirmed, modified, invalidated... after field studies carried out in the weeks and months to come, in particular after bathymetric studies to know the new morphology of the seabed.

An andesitic magma very rich in dissolved gas rose slowly under the volcano.

Its internal pressure decreased as it approached the surface;

the solubility of the gas decreased;

the gas separated from the magma, forming vesicles which propelled the magma towards the “exit”.

All this reduced the density of the magma, which reduced the pressure in the column of magma... A Surtseyan explosion perhaps more violent than the others would have brutally destroyed the solid "cap" covering the magma,

The phenomenon would then have been "packed" by "chain reaction" and "snowball effect", and the cataclysmic explosion occurred.

The gas + magma mixture ejected upwards would have incorporated a lot of hot air, which lightened it and turned into a powerful ascending plume: a Plinian eruption.

A huge amount of magma therefore came out in a few minutes, which would have created a "vacuum" under the volcano, a void which would have collapsed.

Explosion and collapse completely destroyed the 2014-2015/2021-2022 volcano, destroyed the majority of the two old 11th-century islands, and arguably enlarged the 11th-century caldera.

Our “VOLCANOES” file

The ashes emitted by the explosion covered the surrounding islands causing great damage.

The explosion and the probable formation of the caldera led to the formation of a giant wave, a tsunami, which spread throughout the Pacific Ocean.

It quickly reached the populated islands of Tonga, then the more distant islands and coasts.

Fortunately, the tsunami warning had been issued and there were only a few casualties.

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This analysis was written by Pierre Thomas, Emeritus Professor of Earth Science at the École Normale Supérieure in Lyon.

The original article was published on The Conversation website.

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