Hazards at the origin of many disasters, tsunamis are caused by the sudden movement of a large volume of water in an ocean, a sea or a lake. While earthquakes are one of their well-known sources, volcanic eruptions can also be the cause. However, rare are the situations where the collapse of a caldera, the ejection of a pyroclastic flow or the triggering of an underwater explosion is able to generate a transoceanic wave.
How, then, can the astonishing events of Saturday, January 15 be explained? On that day, the sudden explosion of an underwater volcano was, for the first time, followed by a series of tsunamis which were not only detected in all the seas of the globe, but disturbed their surfaces for a exceptional duration of four to five days. “Starting from the Tonga archipelago, a string of islands located to the east of New Caledonia, the first of these tidal waves goes, moving at a speed of more than 1,000 km/h, i.e. 1.5 to 2.5 times higher than normal, manage to cross the Pacific, Indian and Atlantic oceans in less than twenty hours”says Robin Lacassin, CNRS research director at the Institut de physique du globe de Paris (IPGP). “It will hit the coasts of Portugal and Italy even faster than if it had managed… to take the direct route by jumping over the continents!” »
Another oddity: the phenomenon diminished somewhat with distance, and continued to generate waves thousands of kilometers away. Thus, in certain remote regions of the Pacific rim, the rise in sea level recorded on the coast was almost the same as that recorded at the source. If waves of several meters were observed on certain parts of the coast of the island of Tonga, causing the death of four people, in other places of the archipelago, the rise in water remained limited to 1.14 meters . A value equivalent to that which was soon measured 10,000 kilometers away, in Chile, by the hydrographic stations of Arica (1.18 meters) and Coquimbo (1.43 meters)!
Gigantic column of dust
This astonishing tsunami on a global scale is the result of a series of phenomena, some of which had not yet been studied with modern means and others which were completely unknown. The nature of the rash itself is problematic. The latter involved a volcano, Hunga Tonga, which entered into activity in December 2021. “This important 1,800-meter-high underwater structure, of which only the upper part of the caldera emerges to the surface in the form of two small islands, will experience, from January 13, a series of explosions, the one, paroxysmal, on January 15 at 4:10 UTC, will be so strong that it will be heard 10,000 kilometers away, in Alaska: it will produce the highest plume ever measured! »explains Raphaël Grandin, teacher-researcher at the IPGP.
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