An 'unusual' and unexpected eruption, certainly different from those we are used to seeing on the small and big screens. Not a very high lava fountain like the ones that Etna gives us or that paint the nights of Stromboli with fire. Not a red, incandescent tongue crawling sinuously on the ground as happens in Iceland, or licking the sides of the volcano as happened on the Cumbre Vieja of La Palma, in the Canary Islands.
The eruption of the Hunga Tonga-Hunga Haʻapai volcano, in the Polynesian kingdom of the Tonga Islands, was a very violent explosion, which for days cut off the small archipelago from the world and which made itself seen (and heard) in several points of the planet with a thirty kilometer high eruptive cloud, tsunami waves, echoes of very strong rumbles and atmospheric pressure shocks.
To find out more about what happened in the middle of the Pacific Ocean, off the coasts of Australia and New Zealand, we interviewed INGV volcanologist Boris Behncke, who guided us to discover one of the most explosive submarine volcanoes in the world and he also told us something more about Marsili, the local giant lying on the bottom of the Tyrrhenian Sea.
Boris, what's going on in Tonga?
At the moment, little seems to be happening, in the sense that, judging by satellite images, the volcano appears to be calm at the moment. After the violent eruption on Saturday 15 January there were only a few small emissions of ash and steam.
So can we say that the eruption is over?
It would seem so, at least as regards the main eruptive phase, which ended a few hours after the main event.
What kind of volcano is Hunga Tonga-Hunga Haʻapai and what kind of eruption was the January 15 eruption? How did it develop?
Well, as far as the general geodynamic context is concerned, we can start by saying that the Hunga Tonga-Hunga Haʻapai volcano is located in a subduction zone where the Pacific plate pushes under the Australian one, "sliding" underneath it. In contexts like this, the formation of volcanic buildings is particularly common: think of the many volcanic archipelagos present in the Pacific Ocean or even the Aeolian Islands, for example. Well, the volcanoes present in the subduction zones are characterized by a magma very rich in silica and water vapour, which usually generates particularly explosive eruptions.
Furthermore, the Hunga Tonga-Hunga Haʻapai is largely a submerged volcano: a volcano characterized by a caldera of about 5 kilometers in diameter on its summit, or by a large depression generated by the collapse of part of the volcanic cone. Of this caldera, only a few small parts of the rim emerge from the sea waters. This means that the eruption of this volcano took place in direct contact with sea water, significantly increasing its explosiveness. It was in fact a very voluminous eruption, among the largest explosive eruptions in the world recorded in recent decades.
From the first moments following the eruption, the authorities of various countries bordering that sector of the Pacific Ocean issued a tsunami warning. How many and what damages have been reported following the arrival of the tsunami waves?
As we know and as we have seen from the news circulated since the first hours following the eruption, the event generated tsunami waves that crossed the entire Pacific Ocean, reaching, among others, the coasts of Japan and the Americas ( both Northern, Central and Southern). The damage was considerable in the Tonga Islands, at least as far as we learn from the little news that manages to arrive: a submarine cable for telecommunications was in fact damaged by the eruption and the first planes with supplies and basic necessities were able to land on the island only five days after the eruption, due to the thick layer of ash that covered the airport runway. Three deaths are also reported in the Tonga islands. Elsewhere, however, the damage was fortunately much lighter but, unfortunately, we are talking about two victims in Peru caused by the tsunami wave.
The eruption also generated sound waves that were heard up to thousands of kilometers away (even in Alaska, just under 10.000 kilometers from the site of the event) and an atmospheric shock wave, i.e. an alteration of the atmospheric pressure recorded by barometers around the world.
Were there any other episodes preceding this eruption?
The last eruption of Hunga Tonga-Hunga Haʻapai, much smaller but equally spectacular, was in 2014/2015. That eruption "created" the island as we knew it until January 15th: in fact, as I said, of this volcano we only see a few pieces of the edge of a caldera that emerge from the waters of the sea in the form of islets. Well, until the eruption 7 years ago, there were two islets, Hunga Tonga and Hunga Haʻapai. The products erupted in 2014 created new "soil" that came to connect the two islets, giving life to the Hunga Tonga-Hunga Haʻapai cone.
In December 2021 this cone was reactivated, causing the island to grow further, to then calm down in early January 2022. On January 14, one day before the great eruption, the activity resumed with an already very violent explosive eruption which produced an eruptive cloud about 20 kilometers high and destroyed the ground created by the eruptions of previous years. In the afternoon of January 15, local time (it was the first hours of dawn here in Italy), the major eruption definitively destroyed what little of the cone that still remained above sea level, restoring the two original islets and, indeed, making them even smaller according to the new satellite images of the area.
What future developments should we presumably expect from this eruption?
I always say that from volcanoes you have to expect everything and more: that they still erupt but also that, paradoxically, they go out! However we have some preliminary information deriving from studies on the eruptive history of this volcano which tell us that approximately every 1000 years the Hunga Tonga-Hunga Haʻapai produces an eruption similar to that of January 15th. Prior to this, the last documented one dated back to the late Middle Ages and, in that case, consisted of a series of different explosive events: for this reason, today we cannot exclude that other similar explosions may occur during the current activity.
What are the most famous underwater volcanoes in the world?
One of the most famous for international volcanology is certainly Loihi, the youngest volcano in the Hawaiian Islands: it is located off the island of Hawaii, near the Kilauea volcano and, presumably in about ten thousand years, it will emerge from the seabed (where today it lies at a depth of about 1000 meters) becoming another island of the Hawaiian archipelago.
Then there is a volcano
off Oregon, in the eastern sector of the Pacific, the Axial Seamount, which is the only undersea volcano that is constantly monitored. For the scientific community it is really a volcano-laboratory, on which various multi-parameter monitoring networks have been installed (from seismographs to instruments suitable for measuring soil deformations) and which is "visited" every time it erupts by shipments of ships research workers who go to the site to take samples to be analyzed in the laboratory.
On the other side of the Pacific, in the archipelago of the Solomon Islands, there is another, the Kavachi, which every few years gives rise - with the debris of its eruptions - to a small island which is then rapidly 'erased' by the sea erosion.
A bit like what happened in the XNUMXth century with the Ferdinandea Island in the Sicilian Channel…
Exactly, exactly like that, only that the Kavachi is always in business, while the Ferdinandea after 1831 has never re-emerged.
So what are the submarine volcanoes present in the Mediterranean?
There are several, some presumably active, such as the one that formed Isola Ferdinandea in 1831: in this case it is a so-called "volcanic field" in which each eruption produces a new coneto and every now and then one of these little cones pops out of the sea. And then we have the most famous, famous worldwide and not only in the Mediterranean, which is Marsili, whose peak is about 500 meters deep under the surface of the Tyrrhenian Sea, between Sicily and Calabria.
Here, in this regard it should be remembered that lately there is often talk, sometimes in a rather "mythical" and imaginative way, of Marsili. But is it likely to imagine our seas affected by scenarios like the one we have witnessed in the last few days on the other side of the world?
Marsili is a young and very large volcano, of which there is evidence of eruptions dating back even a few millennia ago and of which it has been repeatedly said that it could even generate tsunamis in the event of the collapse of one of its flanks. However, these are extreme scenarios, which, in general, occur very rarely.
To date, Marsili is not constantly monitored, but various scientific campaigns have been carried out on it - one in 2006 - during which various instruments have been installed, including submarine seismographs which tell us that this volcano is seismically active, giving us the typical signs of a "live" volcano, not erupting but live and potentially active.
However, there are significant differences with the Hunga Tonga-Hunga Haʻapai: first of all, the magma of the two volcanoes is different, since that of Marsili is a basaltic magma, more fluid, which could less easily produce such explosive eruptions as those of Hunga Tonga- Hunga Haʻapai, which instead has a much more viscous and gas-rich magma. The second important difference lies in the fact that the Tonga volcano erupted practically at sea level, producing a very violent interaction between the magma which was fragmenting and the water which was penetrating inside; conversely, as I said, the Marsili lies at a depth of 500 metres, therefore even a possible highly explosive eruption would not be felt on the surface in the same way as we have witnessed off the Pacific coast.
Link to the in-depth article on the INGVvulcani Blog: