When do volcanic eruptions stop? Predictions are hardly possible

Illuminated fountains jut out from the crater, incandescent lava flows roll down the cliffs to the sea, and the night sky shines red. For nearly 80 days, La Palma’s Cumbre Vieja emits rocks, gas, and ash. It is indeed the largest volcanic eruption since records began over 500 years ago on the Canary Island, which is popular with tourists. At the moment, La Palma attracts volcanologists and other researchers from many countries. They investigate what is going on above the ground, but above everything below.

“La Palma is now one of the best-watched volcanic regions in the world,” says Thomas Walter of the Center for Geographical Research Potsdam (GFZ), who has visited the island several times since September. “But there are still many unanswered questions.” Above all, this includes forecasting activity – both in the short and long term. Because the end is not yet in sight. On the contrary: On November 30 the number of earthquakes there reached 371 in 24 hours, the highest point so far.

Lava creates 50 hectares of head

Such tremors occur when rocks fracture under the pressure of the rising magma and the gases they contain, explains Thor Hanstein of the Geomar Helmholtz Center for Ocean Research in Kiel. In mid-September, magma first penetrated the Earth’s surface, and there are now half a dozen new craters in the area.

7,000 residents have had to move to safety since the outbreak began. By the beginning of December, lava had destroyed nearly 2,800 buildings and more than 70 kilometers of roads in the southwestern half of the island. More than 1,150 hectares are covered with a meter-thick layer of lava – an area the size of about 1,600 football fields. Off the west coast, the lava created a new cape of approximately 50 hectares.

La Palma is particularly active from a volcanic point of view

“In the past, outbreaks in La Palma usually ended after about two to four weeks,” Walter says. “We now assume that this time, even after two months, could take much longer.”

All of the Canary Islands except for La Gomera are volcanically active, but none is quite like La Palma. For Hans-Ulrich Schmincke, old director of the department of volcanology and petrology at the then Leibniz Institute for Marine Sciences (IFM-Geomar) in Kiel, this activity is not surprising. Most of the oceanic islands were based on volcanoes that grew from the ocean floor. In this regard, La Palma is nothing but the summit of a huge massif rising from a depth of thousands of meters from the Atlantic Ocean.

Rota de los Volcanes is popular with tourists

Cumbre Vieja is a testament to recent volcanic activity: the volcanic chain, about 2,000 meters high, stretches more than ten kilometers over the southern half of the island along the ridge to the southern tip. Above it is the Ruta de los Volcanoes – the volcanic road – popular with hikers, passing through dozens of potholes and offering panoramic views of the neighboring islands of Tenerife, La Gomera and El Hierro.

In general, according to Walter of the GFZ, the Canary Islands showed a whole range of possibilities in terms of intensity: in 1909, for example, the last eruption of Tenerife in Chinero, a cinder cone at Pico del Teide, ended a few days later. days. In turn, another eruption occurred about 180,000 years ago that destroyed the entire island, which is three times the size of La Palma.

The outbreak can change quickly

Current eruptions in Cumbre Vieja are of two types: geyser-like eruptions called Strombolian eruptions are quite regular: every few minutes, the rising gas bubbles in the magma erupt. With a thud, the mountain spits out fountains of lava.

On the other hand, sudden changes leading to eruptions, as powerful ash clouds rise for kilometers above the island, can hardly be expected. “The outbreak can currently completely change its face in just a few hours,” Walter says. “We don’t yet know what will lead to this shift.”

There are hardly any possible predictions in Cumbre Vega

The formation of new craters is also puzzling: since September, eruption sites have moved hundreds of metres. By the beginning of December, at least seven craters had formed in the area of ​​the summit of the new volcanic cone, the most recent of which were 1,124 meters high, which look like a crescent-shaped sickle and sometimes overlap.

In the journal “Science,” volcanologist Marc-Antoine Longebre of the City University of New York describes how a volcanic eruption developed. It also underscores how difficult it is to predict Cumbre Vieja’s activity. In the case of continuously active volcanoes such as Mount Etna in Sicily or Kilauea in Hawaii, special patterns can be created that portend an imminent eruption. With Cumbre Vieja, with long breaks, this is impossible.

The outbreak was announced a week ago

Accordingly, there have been six outbreaks since 1500, with interruptions ranging from 24 to 237 years. The last time Teneguía erupted at the southern tip of the island was in 1971. It is clear that the current activity has developed over several years. After a long period of rest, the ground under La Palma shook from October 2017: at that time, 128 tremors were recorded within eight days, and another 84 tremors followed a few months later.

Longpré wrote: “In retrospect, these single earthquakes at a depth of 15 to 30 kilometers may have been a sign of the first signs of volcanic disturbances.” Then the actual outbreak was announced about a week ago. He writes: “Since September 11, 2021, the patterns of unrest have changed dramatically.” “The number of shivers recorded rose to several hundred per day. They concentrated at shallow depths and the average force was greater than before.” In addition, the surface began to deform, possibly due to rising magma flows.

There is no magma chamber under Cumber Vega

A few days later, on the afternoon of September 19, the volcano erupted to an altitude of about 1000 meters – at first through two cracks, each 200 meters wide. “The magma takes the path of least resistance and then penetrates a weak point,” says Hanstead.

But what exactly happens underground is largely unclear. Unlike many other volcanoes, there does not appear to be a magma chamber a few kilometers deep below Cumber Vieja. Data from the Spanish National Geographic Institute (IGN) in early December indicated that most of the tremors are concentrated in two areas: more than 30 kilometers deep, that is, still in the Earth’s mantle, and about 12 kilometers deep. Evidently, large amounts of ascending magma had accumulated there. “The system is refilled periodically,” says Hanstein. At the end of the week, the tremors moved to the region at a depth of 10 to 15 kilometers.

Researchers travel to La Palma

The path of the channels – vertical chimneys or horizontal fractures in the rock – is currently not entirely clear. “It seems the upheavals in my La Palma area cross each other,” Walter says. “And in the flatter areas, we hardly see any tremors. There is radio silence to a depth of eight kilometres. It is very difficult to explain this by the activity of this eruption.”

In order to gain more knowledge, a team around Walter traveled to La Palma again at the beginning of December – with plenty of equipment in their luggage: seismographs should record the smallest vibrations and help create a model of underground activities as accurate as possible. Satellite images and GPS ground stations record small vertical and horizontal changes in the Earth’s surface, and so-called inclinometers detect the smallest changes in tilt.

Explosions can turn

From the distortions – for example the angle of elevations – conclusions can be drawn about the depth of the causal changes. The main question is about how magma rises from the earth’s interior to the craters. This can provide information about when and where new exit gaps can be opened. This, in turn, will have an effect on the trajectory of more lava flows.

“I wouldn’t be surprised if the bangs changed again,” Walter says. It was not until the beginning of December that the lava flow emerging from a new fissure flowed into the previously avoided place of La Laguna at an altitude of up to 800 metres.

Expert: The outbreak will end at some point

Another concern for experts: A new crater could open on top of or to the east of Cumbre Vieja’s main ridge. Then the lava flows will not only flow westward as before, but also down the eastern side of the island. If they were to reach the sea, southern La Palma would be completely cut off from the rest of the island by land – as happened in the 1949 eruption.

“We’ve become very careful with our expectations,” says GFZ expert Walter. “You can’t rule anything out in La Palma.” But at some point this outbreak will end. And then it will be given the name of the newly created cone which was not previously named.