The coordination team monitoring the eruption of a volcano on La Palma, in Spain’s Canary Islands, is working with all possible scenarios, from the starkest to the most benign.
No human loss has been reported, but around 6,000 people have been evacuated and 320 homes destroyed by towering lava flows rising up to 12 meters high. Scientists are also warning about the potential danger of toxic gas clouds if and when the molten rock reaches the sea.
The volcano’s behavior so far, together with the area’s history, suggest that for weeks to come there will continue to be an effusive rather than explosive eruption – that is, when magma flows out of the vent and oozes down the mountainside as lava, without sending rocks, ash or gas high in the air.
The Canary Islands, a volcanic archipelago located off the northwestern coast of Africa, have a history of eruptions. The longest one on record in La Palma took place in 1585 and lasted 84 days; the shortest one dates to 1971 and extended for 25 days. There was also an underwater eruption off the island of El Hierro in 2011.
But even when the lava stops coming out of Cabeza de Vaca, inside the crater-filled Cumbre Vieja natural park, evacuated residents will not be able to go back to normal, at least not immediately. First, the land will have to cool down and stabilize. Any infrastructure left standing must be checked for damage. Toxic gas levels and ash accumulation will need to be measured, among other parameters.
The danger maps establishing if and when displaced people may return to their homes will analyze the risks posed by lava flows, ash, pyroclastic flow (solid particles spewed out by the volcano), gas emissions, lahars (mudflow) and tsunamis. Earthquake activity and ground deformation will also be taken into account, as well as concentrations of carbon dioxide and sulfur.
Raúl Pérez, a geologist, seismologist and researcher at the Geological and Mining Institute of Spain (IGME), is part of the team monitoring the eruption in La Palma. He underscores the fact that the lava is now flowing slower on its way down to the sea, and also notes that a new eruptive vent that opened up near the village of Tacande, in El Paso, has followed the same pattern as previous fissures.
“The earthquakes that came with it are telling us that the magma keeps searching for new paths to the surface, but with the same eruptive style: the lava flows down the hillside without explosive eruptions.”
It is similar behavior to that displayed by the Teneguía volcano between October 26 and November 18, 1971, when only one person died from exposure to toxic gases after getting too close to the eruption zone. The lava did not seriously affect populated areas, and when it reached the sea it added two million square meters to the island’s surface area.
“In theory, any volcanologist will tell you that no two eruptions are the same and that each has its own peculiarities,” adds Pérez. “However, we are finding characteristics that are quite convergent with historical events in the Cumbre Vieja area: the lava flows favor the back flank and the magma takes advantage of the fractured terrain to find an easier way out. This, in a way, can help make the explosions less violent.”
Ana Crespo Blanc, a professor of geodynamics at Granada University, agrees that the history of the area suggests similar behavior in the coming days and weeks. “In principle, we expect to see the same pattern as in past centuries: eruptions that last between a month and a half to two, three months, and then, when the pressure in the magma chamber goes down, it’s all over.”
But measuring that key element will take some time. Geochemical control is essential to knowing the pressure inside the magma chamber, the cavity that pushes the magma to the surface. But Crespo, who studied in Lausanne (Switzerland) and Seville, insists that based on prior eruptions in the area “the most likely scenario is an accumulation of lava that will reach the sea, where it will cool down and perhaps even increase the size of the island.”
Worst-case scenarios are ruled out
Scientists are rejecting the worst-case scenario defended 20 years ago by Steven Ward of California University and Simon Day of University College London in the journal Geophysical Research Letters: “Geological evidence suggests that during a future eruption, Cumbre Vieja Volcano on the island of La Palma may experience a catastrophic failure of its west flank, dropping 150 to 500 cubic kilometers of rock into the sea. Using a geologically reasonable estimate of landslide motion, we model tsunami waves produced by such a collapse. Waves generated by the run-out of a 500 km³ (150 km³) slide block at 100 m/s could transit the entire Atlantic Basin and arrive on the coasts of the Americas with 10–25 m (3–8 m) height.”
Crespo Blanc firmly rejects this possibility and warns: “I think it’s important not to alarm people. In the worst of all cases, the magma chamber would empty out entirely and collapse, but this is not the case in La Palma. That would involve a very large eruption affecting the entire island, which is obviously not the case here. We can assume that once the current activity has ended, everything should go back to normal.”
The geologist Raúl Pérez also rejects Ward and Day’s apocalyptic scenario. “This eruption does not involve high-violence phenomena. As far as volcanism is concerned, this is a small phenomenon. In order to see a collapse of the caldera, there would have to be enormous volumes involved, and that is not the case.”
“What is being considered is the possibility that minor interaction between the lava and the water might produce some kind of explosion,” he adds.
However, when the lava stops oozing out there will be work to be done before things can go back to normal. Even if there is no lava flow, it could still be building up inside the volcano, notes Pérez. Evaluating which areas are outside the risk zone will require monitoring the lava’s movement until its activity ends. “After that, we’ll have to see which infrastructure has been cut off, and check the fields of lava and ash for toxic elements that may have fallen on crops and aquifers. Plus the land has to cool down and stabilize. We need to know if volcanic pipes are forming underneath and if the affected land is stable. It’s going to take a long time, I couldn’t say how long, but the return to normality is not going to be immediate.”
English version by Susana Urra.