Although no one predicted Saturday’s powerful eruption of the Hunga Tonga-Hunga Haʻapai volcano, it was not a bolt from the blue. The Pacific volcano was already reawakened to life on 20 December 2021, when lava from the volcanic vent began to flow beneath the sea surface. The result of the lava meeting seawater was a powerful eruption, the formation of volcanic ash and a cloud several kilometres high hovering over the volcano. The calm did not come until 5 January 2022, when the volcano briefly went quiet. But on Friday, 14 January, it awoke again, spewing a volcanic cloud some 20 kilometres high. And then came Saturday’s eruption, the strongest yet, whose effects were felt by people thousands of kilometres away. Not only did the volcano spew volcanic material probably as high as 30 kilometres into the Earth’s stratosphere, but it also caused a tsunami to spread across the Pacific Ocean. Its height reached about 1,2 metres on the nearest inhabited islands.
However, it is important to remember that what we see above sea level, or have seen – the first satellite images show that the volcanic island was almost entirely destroyed by the eruption – is not all. In fact, there is a cone of submarine volcano approximately 20 kilometres wide below the surface. And at the top of that volcano is a 6 kilometre wide crater, a volcanic caldera. We don’t know yet whether the powerful eruption was the result of an eruption of all or most of this volcanic caldera or whether it was just a partial eruption somewhere in the caldera. It may seem like a minor detail, but this knowledge may bring us closer to understanding why the tsunami that devastated the coastal areas of the Pacific was generated. For its formation requires the sudden displacement of huge masses of water, and a volcano can do this either by a huge eruption – which doesn’t happen all that often – or by the collapse of part of its slope (as was the case with the Anak Krakatau volcano in the Sunda Strait in Indonesia on 22 December 2018). And this knowledge could give us a clue as to what to expect from the volcano in the future.
Indeed, if we look back in time, volcanic eruptions have been observed six times in the last 120 years at Hunga Tonga-Hunga Haʻapai. They were usually relatively weak, so they posed only a local risk and did not have a major impact. However, Saturday’s event shows that a volcano can produce a much more powerful explosion. And probably repeatedly. In fact, studies of rocks on nearby islands suggest that this might not have been its first powerful eruption – volcanic rocks can be found on nearby islands that were brought there by a volcanic eruption that occurred somewhere nearby. The Hunga Tonga-Hunga Haʻapai volcano is suggested as a possible source of this material, but we are not 100% certain that this was the case.
This brief excursion into the geology of the islands and their eruption history shows us that the Hunga Tonga-Hunga Haʻapai volcano has been fed by magma for a long time. This is because it lies near the point where the Pacific Oceanic Plate slides under a pair of lithospheric plates – and this volcano is on one of them. The retreating Pacific plate carries a lot of water, which is bound up in the minerals and rocks that make it up. The subsidence causes the rocks and water to go deeper into the Earth’s interior, where pressure and temperature increase. The increase in pressure and temperature causes the water to be released from the rocks, breaking the bonds between the atoms that make up each mineral. Once this disruption occurs, the rocks need only be at a lower temperature than normal to begin to melt. This produces huge quantities of magma at the insertion points of the lithospheric plates, which are capable of feeding volcanoes on the surface for long periods of time.
Thus, the Hunga Tonga-Hunga Haʻapai volcano has certainly not yet said its last volcanic word. But since the ability to predict volcanic activity has its limits, especially in the case of submarine volcanoes in sparsely populated areas, only time will tell what form the next eruptions of this volcano will take and when exactly they will occur.