La Palma: the announced eruption of Cumbre Vieja

It was only a matter of time before La Palma's volcano would erupt. Most people have never heard of the Cumbre Vieja - literally, the old mountain ridge - yet it is one of the most active volcanoes in the Canaries, and in Europe. And with each eruption it becomes painfully clear how destruction and renewal, and fascination and terror, are inextricably linked. In this article we look at what is actually happening on La Palma, what preceded the eruption and how these events compare with other volcanoes.

Author: Kathelijne Bonne

The last eruption of Cumbre Vieja was 50 years ago in 1971. Also then, lava destroyed properties, yet the entire economy of this beautiful island depends on the fertile soils that develop on basalt (solidified lava), as well as on the beautiful landscapes.

Earthquake swarm

After a dormant phase of 46 years, La Palma started to rumble in the depth in 2017. Seismic shocks, tremors, were recorded during a four-year period until June 2021. In January of this year, a group of scientists published an article in the journal Nature about the potential risks associated with this volcanic unrest. They were proved right.

On 11 September, a stronger earthquake swarm shook La Palma. About 22 thousand tremors were recorded during the week before the eruption of Cumbre Vieja. The depths of the earthquakes gradually migrated upwards, from a depth of 20 kilometers to only 100 meters below the surface and they had a magnitude of up to 4.2 on the Richter scale. The land surface also started to deform, which was clearly visible on radar images taken by satellites, detecting the slightest changes in the earth's surface. This was a clear signal that magma was moving upwards from the depths, accompanied by a lot of infrasound rumbling and crackling. There were also some landslides as a result of the seismic shocks.

On 13 September, the inhabitants of the municipalities of Los Llanos de Aridane, Fuencaliente, El Paso and Villa de Mazo were warned of a possible imminent eruption and the alert level was set to yellow. Around 40,000 people had to pack their bags and prepare to leave their homes.

Cumbre Vieja erupts

On 19 September, the volcano erupted. In the forested area of Cabeza de Vaca, in the municipality of El Paso, lava and ash began to emerge from two fissures. Eight small craters, also called vents, formed along the fissures.

Cumbre Vieja is not a steep cone-shaped stratovolcano with one large crater at the top, like Vesuvius, Mount St Helens or Fuji. Cumbre Vieja is a broad, flattened shield volcano. Shield volcanoes are usually not lethal and the eruptions always start at a new location. On a relief map you can see that this type of volcano is dotted with craters. There can be hundreds of them. They are typical of Hawaii, Etna and Mount Cameroon.

Lava flows down the slopes

Lava from Cumbre Vieja flows down in several tongues towards the inhabited, lower areas. It has reached the sea and destroyed more than 1,000 buildings in its path.

Lava is super-hot rock with a temperature of around a thousand degrees, which is why it is liquid. It immediately begins to solidify on contact with the air. This is why an advancing lava flow looks like a black, metre-high crumbly front, with glowing red rock that can be seen in fissures beneath the surface. The flows don't kill because they move rather slowly, a few hundred metres per hour, so you can walk away from them, but they inexorably slide downwards, destroying everything in their path: houses, churches, schools, roads, fields, trees. Once the flows have cooled, they become part of the island, and by interacting with water, air and life, they eventually form soils that are among the most fertile in the world.

As the days pass by on Cumbre Vieja, the speed of the lava changes; sometimes they flow at 700 metres an hour, while others slow down to a few metres an hour, or they stop moving altogether, only to be set in motion again. All this is driven by the volume of lava produced from the craters, per unit of time.

Ash column

Days later, more vents opened. Spectacular lava fountains are observed, providing an unparalleled spectacle of terrifying beauty. The volcanic activity also became more explosive, i.e., smaller stones, gas and ash are thrust into the atmosphere via an ash column, which became more intense on 25 September and rose six kilometres into the sky. As a result, La Palma's air traffic was temporarily suspended. The ash column causes disruption, but not death (unless you are really close). The superfine ash that dwindles down deposits a thin blanket, but it is incomparable with the deadly pyroclastic clouds and surges from Mount St Helens, Vesuvius, or Campi Flegrei, which belong to a completely different category of volcano.

Cumbre Vieja's current activity can be classified as a strombolian eruption, which produces lava fountains and flows arising from fissures and small craters, and is distinct from the Plinian eruptions, which are highly explosive and have a much larger, abrupt and catastrophic impact.

Living on a volcano?

The current eruption on La Palma is not the first and probably not the last, and for the population it is a toss-up as to whether it is worthwhile continuing to live on a volcano. But as history shows, it pays off, because a very large proportion of the world's population lives on, near or not far from an active volcano. Because the benefits outweigh the risks, in the past and today. Volcanic soils are favourable for agriculture and the diverse volcanic landscapes are prime tourist attractions.

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Read our next article about the geological history of La Palma, how it fits into the bigger picture of tectonic plates and why there is a volcanic archipelago in this place in the Atlantic Ocean:

• The geology and hazards of La Palma

And how different from La Palma is Mount Saint Helens on the westcoast of the United States! Or Campi Flegrei, the unknown but devastating super volcano of Naples! Click on the links to read about them.

Title picture: Eduardo Robaina (WikimediaCommons license).

Sources: 

Global Volcanism Program, 2021. Report on La Palma (Spain). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 22 September-28 September 2021. Smithsonian Institution and US Geological Survey. https://volcano.si.edu/volcano.cfm?vn=383010

Fernández, J., Escayo, J., Hu, Z. et al. Detection of volcanic unrest onset in La Palma, Canary Islands, evolution and implications. Sci Rep 11, 2540 (2021). https://doi.org/10.1038/s41598-021-82292-3

https://english.elpais.com/science-tech/2021-09-20/volcanic-eruption-in-la-palma-why-did-it-happen-and-how-long-will-it-last.html

https://es.wikipedia.org/wiki/Erupci%C3%B3n_volc%C3%A1nica_de_La_Palma_de_2021

Article by Kathelijne Bonne, geologist & soil scientist. Editor of GondwanaTalks. 

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