Etna is a giant in slow but constant movement. For decades, experts have observed that the southeastern slope of this volcano slides progressively towards the sea. Many aspects of this geological phenomenon, also present in other volcanoes, remain to be explored. The first study on the submarine movements of this Italian volcano, published this Wednesday in Science Advances, has concluded that the process is mainly due to the weight of the forest itself, possibly in combination with the action of the plates on which the sea rests at the bottom. The previous hypothesis that the primary origin of this instability was the magma drive towards the sides of the crater is thus ruled out. The volcanologists do not exclude that the sliding of a slope can even evolve in the catastrophic partial collapse of a volcano. To know more in depth the dynamics of this phenomenon would help to better understand what could happen in the future on Etna and other volcanoes, they maintain.
The eastern slope of Mount Etna, the highest active volcano in Europe, is part of the Sicilian coast of the Ionian Sea. From the nineties, when the first measurements were taken, it began to register a movement of this side towards the sea of two or three centimeters of annual average, according to Alessandro Bonforte, researcher of the National Institute of Geophysics and Volcanology of Italy (INGV ). "Sliding is always present, both in the loading phases [es decir, cuando el magma sube hacia el cráter del volcán] as in the eruption phases, "says Bonforte, who is among the authors of the new study on the submarine movement of Etna,
The researcher explains that this phenomenon periodically generates "superficial" seismic effects, which can break some points of the ground. These cracks can cause damage to buildings and infrastructure if they occur in urbanized areas, explains Bonforte. The volcanologist remembers that this area of Sicily has a lot of population density, which increases the risk factors.
The scientific community has identified different factors that could be at the base of this geological phenomenon. But in many cases the current knowledge has not allowed to reach definitive conclusions. "We know well that the slopes of many volcanoes behave abnormally, and Etna is an example," says Morelia Urlaub, a researcher at the GEOMAR Institute in Kiel (Germany) and lead author of the study. "What was unknown is if and how the sectors of the volcanoes move under water," he adds.
The scientist assures that one of the reasons why she chose Etna as a case study is that "it is among the most monitored in the world" and "almost any type of data" is available on the surface part. Also the great dimension and the high population density of its hillsides have influenced to consider it as an ideal object of its investigation, he adds.
Urlaub and his team used a novel technology to face the challenge of measuring possible movements under the sea. "On the surface, the measurements are based on electromagnetic waves. But under water these do not work because of the high level of absorption ", the researcher explains. The German scientists, in collaboration with three members of the INGV, installed for 15 months on the seabed five instruments that use sound to detect movements. "Under water, sound waves work very well," explains the project leader. "If the distance between the instruments changes, it means that the seafloor has moved. The fact of having a network of these instruments has allowed us to get a clear idea, "he adds.
The weight of volcanoes
Previous studies had hypothesized that the instability of the south-eastern slope of Etna would depend on the lateral thrust of the magma during the rise phase to the crater. However, the observation of submarine movement has allowed to draw another conclusion. "We have discovered that 20 kilometers away from the coast the slope moves in the same measure as on the coast (approximately four centimeters in 15 months)," says Urlaub. "This means that a large part of the volcano moves, and not just the one near the magmatic chamber," he says.
Bonforte adds that when the magmatic activity increases, the slip can accelerate. But this does not mean that the main engine of the phenomenon has to be searched "much more in depth", he points out. This information, in the opinion of the Italian researcher, is important because it allows a better understanding of the "extension" of this movement and its possible consequences. "It is being discovered that volcanoes weigh a lot and can break or destabilize under their own weight," he explains.
The consequences of the progressive sliding of a volcano can be of enormous proportions, experts say. "The associated danger and risk are very high, since a collapse of these characteristics means the instantaneous mobilization of a large volume of rock," says the volcanologist at the Higher Council of Scientific Research Joan Martí. The collapse would entail "a great impact on the affected areas, very populated in the case of Etna", adds this expert. The effects could go much further. "Its consequences can be devastating not only where it occurs, but in remote areas precisely because it is a sector of the volcano that" looks "at the sea, which could result in tsunamis of gigantic proportions," maintains David Calvo, a researcher at the Volcanological Institute of Canary Islands.
The need for more studies
The conclusions of the study published on Wednesday imply that the risk of part of Etna collapsing is higher than previously thought, according to the authors. However, current knowledge in this field does not allow us to predict when and how an event of this type could occur, they highlight. "We would need to better understand how the entire south-east slope moves over a period of time of more than 10 years," Urlaub says. In past eras there have already been partial collapses of volcanoes, but none has been able to study direct, according to Bonforte.
David Calvo puts the nuance in that the calculation of time in geology takes into account periods much longer than what is usually considered in the common language. "Humans are usually in a hurry because things happen. The Earth, let's say, takes it at a different pace, "he says." For volcanologists, talking about a short period of time may well result in hundreds of thousands of years being the kind of landslides or collapses that can be derived These instabilities can occur once every many hundreds of thousands of years, "he continues.
The researcher of the Volcanological Institute of the Canary Islands recalls that the processes of movement of the walls of volcanoes have also occurred in this archipelago. "The valley of La Orotava, in Tenerife, is nothing more than the result of a gigantic gravitational slide that occurred half a million years ago," he says. The researcher explains that at the moment this process is not being verified in any island, but it does not exclude that in the future it can be produced.
Although there is still a long way to go before knowing in depth the slippage of the slopes of the volcanoes, Bonforte is convinced that the study in which he participated contributes a "bit of sand" to this cause. Martí agrees with this vision and notes the need to continue monitoring the phenomenon "continuously and in more detail". Calvo also highlights the relevance of the work published on Wednesday. "Now we have an overall vision, a more focused photo that will allow us to know the relationship of the different volcanic processes involved in the life of Etna with unprecedented precision," he says.