Some 1,396 volcanoes are currently active around the world, according to the database of the Global Volcanism Program of the Smithsonian Institution. Of these, about 70, including Cumbre Vieja on La Palma, have erupted so far this year.
“A volcano is considered to be active if it has erupted in the last 10,000 years,” the petrologist tells SINC Maria Jose Huertas. And the Canary Islands are very active, although it is very difficult to predict when a volcano will start to wake up. The signs of volcanic reactivation, between very long quiet times – which vary from years to decades – are not always so evident.
Except in the Cumbre Vieja area in La Palma, where several seismic swarms since October 2017 could mark the beginning of the resurgence of the Canarian volcano after 46 years of calm. They were possibly the first evidences of volcanic activity after the last eruption in 1971, that of the Teneguía volcano.
The signs of volcanic reactivation, between very long quiet times – which vary from years to decades – are not always so evident
The series of earthquakes recorded were nothing more than an indication that an intense supply of magmatic fluids was taking place at a depth of 25 km. This was revealed by the team of Vicente Soler, geologist at the Institute of Natural Products and Agrobiology (IPNA-CSIC), in the Journal of Volcanology and Geothermal Research.
Before the first series of earthquakes occurred, scientists recorded a change in emissions of gases, an increase in the concentration of hydrogen and radioactive chemicals such as radon in the vicinity of earthquakes, indicating “deep gas entry.”
With the second swarm, increases in radon and in the concentration of thoron, an isotope of radon, produced by the disintegration of another radioactive element – thorium -, continued in the soil. With all these data, the experts deduced the presence of a stagnant magmatic intrusion a few km deep.
Earthquakes, deformation and gases: the eruption indicators
Like a boiling living thing, magma twists between the rocks that contain it in a surface chamber located in the crust (the magma chamber) under the volcano. In constant imbalance, the pressure is high due to the presence of gases, which makes this liquid material formed by molten rock at more than 1,200 ºC an unstable entity.
“Its nature is to try to get to the surface, but for that it has to break these solid structures. That is why it looks for areas of fragility within the crust to be able to migrate,” the scientist illustrates.
Magma, less dense and lighter than its surroundings, tends to escape towards areas of lower pressure and less depth, that is, towards the surface. Thanks to its compounds and the gases that accompany it, which lubricate and alter the rocky environment making it more fragile and soft, volcanic material seeks an outlet to the outside.
The pressure is high due to the presence of gases, which makes the magma formed by molten rock at more than 1,200 ºC an unstable entity
It is for this reason that earthquakes occur, more numerous, followed and different from those caused by the movement of the Earth’s plates. They are the first evidence that volcanic activity could be generated. “Without earthquakes, eruptions do not develop,” says Huertas.
“They are fundamental, but so is the emanation of gases,” recalls the specialist, a professor in the department of Petrology and Geochemistry at the Faculty of Geological Sciences of the Complutense University of Madrid.
“If suddenly there is an increase in the emission of gases, it is known that it indicates something. Maybe it will come to nothing: the magma is quietly degassing as it emits them. Or very fresh pulses of magma with its gases may be arriving and these are released, “he continues.
“If there are earthquakes, unusual gas activity and a rising or bulging of the surface of La Palma, then obviously it does seem that there are precursors of an eruptive activity,” he emphasizes. To do this, it is essential to know the base levels of the volcano, that is, the average number of earthquakes that occur, the amount of gases that are emitted, etc. “.
“It is necessary to measure as many observables as possible. When the average levels that are normally registered become anomalous, for example, a greater number of earthquakes are registered, the volume of gas emitted increases, and if those anomalous observables remain in time, then one could speak of a revival, or unrest in English “, points to SINC Janire Prudencio, Professor of Earth Physics at the Andalusian Institute of Geophysics of the University of Granada (UGR).
Seismicity, deformation and volume of gases emitted are the main indicators of the current state of the volcano. “There has to be a combination of several to predict a volcanic eruption,” says Huertas.
The magma that shapes the crust
Although the activity may have started to show signs around five years ago at the Cumbre Vieja volcano on La Palma, its eruption was unpredictable. “It is very difficult to know that the eruption is imminent. The deformation, gases and seismicity may indicate that it is becoming more and more probable, but it is not known with certainty when and where the eruption will begin,” adds Prudencio.
In part, this is because most magmas never reach the surface and stay inside the earth’s crust. “Many times, despite the amount of gases, earthquakes, and that the surface is bulging, an eruption is not triggered,” Huertas comments.
The deformation, gases and seismicity may indicate that it is increasingly likely, but it is not known with certainty when and where the eruption will begin.
Janire Prudencio, professor at the UGR
As a result, the bark grows. Most of the time, even with an increase in activity, the eruption does not take place, the chamber cools and forms a plutonic rock at great depth: “The magma is forming and thus modifying the structure and composition of the crust”, he details the geologist of the UCM.
And how much magma accumulates in a chamber? How much could still emerge from the La Palma volcano? Although estimates have been made, current technology does not allow the volume of magmatic chambers to be calculated. One of the reasons is that they are continually recharging through the conduits that link them to the mantle. However, “scientists can ‘guess’ where they are by different techniques,” Huertas emphasizes.
In the case of the Canaries, which are “connected” with the earth’s mantle, the magma looks for areas that have already been previously fractured, in previous eruptions, in order to free itself. “The earthquakes can be very numerous and sometimes they occur in one area, then appear in another. The melt is migrating from one place to another to look for an area of weakness where to sneak in and reach the surface. This is where it will get its balance: where there is less pressure and less depth “, describes the expert.
How is a volcano “turned off”?
This is exactly what happened on Sunday, September 19. After more than two weeks of strombolian eruption, the lava flow, which has already reached the sea forming a fajana, has a width of more than a kilometer and a half and extends over more than 500 hectares, according to the Steering Committee of the Special Plan Civil Protection and Emergency Assistance for Volcanic Risk of the Canary Islands (Pevolca).
But the situation changes from day to day. “Even from hour to hour because as the amount of gases it emits changes, the eruption changes. The moment the magma begins to cool and the first mineral crystals form, the eruption will change as well. Everything evolves rapidly as time passes, “says the geologist.
Several scenarios are proposed: that the magmatic chamber empties after a few days and the eruption ceases; or for the chamber, which is connected to magma reservoirs deeper in the mantle, to recharge
For now, the eruption of the volcano, which has suffered several landslides throughout the weekend on the north face of the cone accelerating the speed of the runoff, continues. But several scenarios arise: that the magmatic chamber empties after a few days and the eruption ceases; or that the chamber, which is connected to magma reservoirs deeper in the mantle, is recharged with newer and more primitive magma and the eruption continues.
“Nobody knows how long it can last because it can be recharged with fresh material from the mantle or not,” warns Huertas, although the average duration of eruptions on La Palma is 27 to 84 days. The speed at which it comes out also influences. “You can do it very quickly or more slowly. They are unpredictable things, that no one at the moment would dare to quantify.”
For now, scientists from the UGR, together with researchers from INVOLCAN, the University of La Laguna and other foreign institutions, have extracted samples of the lava and tephras (pyroclasts, small rock fragments) from the volcano to understand, on the one hand , the conditions and processes that occur in its interior, and on the other, how the magmatic system is evolved.
The eruption will end when the magma stops rising from the chamber to the surface. But that does not mean that the camera is completely empty, it can lose much of the volume. “The upward movement of the magma stops and the supply of gases slows down. Little by little it is being extinguished”, says the petrologist.
The internal part of the laundry can be for months at temperatures between 200 ºC and 400 ºC
María José Huertas, petrologist
And when that happens, the whole process will come to a standstill – the washes will cool very slowly and retract. They will lose volume and we will enter a phase other than the eruption. “The internal part of the laundry can be for months at temperatures between 200 ºC and 400 ºC”, informs Huertas. After that, it will turn into solidified igneous rock.