Society 21/09/2021

Eruption on La Palma: chronicle of a volcano foretold

A small volcanic cone about 100 metres tall will appear and ground temperature will not return to normal for two years

3 min
An image of the La Palma volcano erupting on Sunday night

BarcelonaThree weeks from now, if everything goes according to plan, Cumbre Vieja, on the island of La Palma, will be a new geographical feature that will barely exceed a hundred metres in height. It will be the product of the eruption that started last Sunday at around 3.18 p.m GMT and which will most probably continue to expel pyroclastic material and gases for between 1 and 2 months. As the eruption loses intensity, the lava will end up sealing the vents and the crater of the volcanic cone which will appear. We will have to wait a couple of years for the earthquakes to stop completely, and it will not be until then that the ground temperature will be back to normal and wild vegetation will begin to naturally repopulate what the rivers of lava have turned into devastation and desolation. This is the geological rhythm that repeats itself in the Canary Islands, which are of volcanic origin and where the third largest volcano in the world, the Teide, stands.

What has happened?

Cumbre Vieja area, located on the southwest of La Palma, has a particularly high seismic and volcanic activity. Since the Spanish conquest of the islands there have been up to eight eruptions of varying intensity - the first in 1480 - but all with a common pattern: the expulsion of incandescent basaltic rock which, once cooled, has left a landscape of small volcanic cones with fertile slopes due to the large amount of minerals that make up this type of rock. This is why, historically, there have been human settlements on the sides of the newly formed mountains. These are called Strombolian mountains, due to their shared characteristics with the Italian island of Stromboli. The same happens across the Canary Islands, except Tenerife.

Erupció del volcà de Cumbre Vieja

The behaviour of the new Cumbre Vieja volcano is similar, explains Joan Martí, a volcanologist at the CSIC's Barcelona Geosciences Centre. In essence, the accumulation of magma and gases has been increasing over time and at the same time has progressively increased the pressure of the internal geological layers. "The pressure had to be released," he explains. The increase in pressure is what has caused the earthquakes on the island – more than 25,000 according to Involcan, the centre for the study of volcanos in the Canary Islands – and, finally, the eight secondary vents through which the lava has begun to emerge, as well as the two main vents. "The crater of the volcano is forming," the researcher adds.

Could it have been foreseen?

The exact day of the eruption was impossible to predict, says Rosa Mateos, a researcher at Spain's Geological and Mining Institute. But there were "clear indications" of an "imminent" eruption. These are known as seismic swarms, which were detected "clearly" a dozen days before the eruption. These are usually low intensity but high frequency earthquakes, with up to a thousand earthquakes above 3.5 on the Richter scale on the day of the eruption, and up to 25,000 in the hours after the eruption, albeit of very low intensity.

Previously, Martí explains, seismic swarms had already been detected that suggested the possibility of an eruption. "It was a warning that something was brewing." The event happened three years ago in this same spot. "It was an aborted eruption," says Mateos, a relatively common phenomenon. The detection was possible thanks to the network of seismographs all across the islands and the use of GPS and satellites to measure changes in the terrain. As a result, it was discovered that the ground had risen by up to 15 centimetres in the days leading up to the eruption.

What happens next?

The magma chamber contains, according to indirect calculations, about 11 million cubic meters of incandescent material and is emitting 6,000 to 9,000 tons of gases, mainly water vapor and, to a much lesser extent, sulfur dioxide (SO2) and carbon dioxide (CO2), among others. The quantities are a quarter of those estimated for the eruptions of Teneguía (1971) and San Juan (1949), both on the island of La Palma. This suggests that a hundred-metre cone will form similar to those that already exist in the same area, and that both the fissures and the crater will have closed within "a few weeks," Mateos and Marti agree.

"It won't be a big volcano," the researcher explains, who also saw the eruption of the submarine volcano of El Hierro ten years ago. If history repeats itself, "the thermal effects will have disappeared within two years," says Martí, and the devastated land will be fertile again "in a few years". "Knowing what we know," he continues, "we should include volcanic risk prevention in future planning and land use". Both Mateos and Martí think that an eruption of this kind "is more than likely" in the coming years.

Could it happen in Catalonia?

The main volcanic areas in the Peninsula are in Ciudad Real (Calatrava), Almería (Cabo de Gata) and Garrotxa (Olot). The first two have been extinct for millions of years, and the third for about 11,000 years. They are not considered active volcanos, unlike the Canary Islands, where eruptions are frequent and, in the case of Teide, very high risk.

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