Volcanic eruption in La Palma: Why did it happen and how long will it last?

Residents of the Spanish island were aware this day could come, and scientists had been closely following the tremors that preceded an event that could go on for weeks or even months

A live feed of the volcano in La Palma, which erupted on Sunday.
Javier Salas

Around 5,000 people were evacuated from their homes this weekend after a volcano erupted on the Spanish island of La Palma, part of the Canary Islands off the northwestern coast of Africa. The event had been widely anticipated following thousands of tremors in the space of a week.

Residents also knew something like this could happen because the entire archipelago is volcanic in origin and activity continues to be recorded in the area. There was another eruption in La Palma in 1971, as well as a submarine eruption off the island of El Hierro in 2011.

Why was an eruption expected?

A series of small tremors began to take place on September 11 in southwestern La Palma, under a mountain range known as Cumbre Vieja, leading scientists to believe there could be magma pushing under the surface of the earth. This seismic activity gradually migrated to the surface: at first the earthquakes were recorded at a depth of 20 kilometers, but in the last two days they were felt only 100 meters underground. In another sign that magma was forcing its way up, the area had experienced surface displacement, with an accumulated vertical uplift of 15 centimeters on Sunday.

Why did it happen now?

The last eruption in La Palma took place almost exactly 50 years ago, in October 1971, when the Teneguía vent spewed lava for around three weeks. That was the last recorded volcanic eruption on land in Spain. The next time there was any significant seismic activity was in 2017, which marked the beginning of a series of tremors (known as earthquake swarms) over the next years. But this past week’s activity has been much more intense, and together with the sudden uplift of the ground, it heralded an upcoming eruption.

How many earthquakes took place before the eruption?

According to the National Geographic Institute (IGN), the latest earthquake swarm may have contained nearly 7,000 low-intensity tremors. The current activity is high-intensity and has already released more energy in a few days than what the 2011 underwater eruption of El Hierro released in two months, said IGN director for the Canaries María José Blanco.

Why did it happen in that area?

The island of La Palma is very young in geological terms, just around two million years old, although it began forming underwater four million years ago. But there are two clearly differentiated areas on the island: the north is older and more solid, while the south is younger and still forming. The magma continues to expand the island’s land surface on its southern side. All the volcanic eruptions of recent centuries have taken place in the south: San Juan in 1949 and Teneguía in 1971.

Why isn’t there just one volcano?

Eruptions in the Canary Islands tend to take the form of fissures: the earth cracks open and lava, gases and other matter start to come out from several points along it. David Calvo, from the Canary Islands Volcanology Institute (Involcan), explained that these fissure eruptions are known as “racimadas” in Spanish. “We’ve counted eight for now, but there could be more in the coming hours while others may die down,” he said. It would be normal to see more points, generally running along a line; as the eruption enters a mature phase, some of these fissures should lose energy and get blocked, and at that point all the lava would come out of a single opening. That is what happened with Teneguía in 1971, and more recently during the underwater eruption of El Hierro.

How did scientists know the eruption would take place at that spot?

The scientific community has had measuring stations in the area for years, run by IGN, the Geology and Mining Institute (IGME), local universities and Involcan. That is why it was perfectly possible to measure the earthquake swarms and the ground uplift that preceded the eruption. In recent days a team of scientists had been brought in with additional equipment to analyze the event. The European Union’s earth observation satellite program Copernicus also helped determine the surface deformation, and the central government of Spain sent support aircraft to the Canaries to monitor the volcanic activity.

How long will the eruption last?

It is hard to say for certain. Historical precedent and volcanic activity in the area suggest it could last several weeks, perhaps even months. The submarine eruption of Tagoro, off the island of El Hierro, lasted five months, and the 1971 eruption of Teneguía was active for over three weeks. But it is still too early to know how this latest eruption will develop, how much energy will be released through its various vents, or how many cubic meters of lava are pushing to get out.

English version by Susana Urra.

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