The laundry burns: from 40 to 300 degrees

Between September 19 and December 13, 2021,
the volcano of La Palma, unofficially called Tajogaite, released through its six mouths more than 200 million cubic meters of burning lava that covered 1,219 hectares of the Aridane Valley and created two 48-hectare strips of land reclaimed from the sea. The lava, then, flowed from Cumbre Vieja to
about 1,200 degrees Celsius.

Now, nine months after the eruption, this terrain is made up of badlands (AA-type lava) and pahoehoe (cordate lava with a more fluid and cushioned appearance)
it is still hot and, at certain points, continues to burn.

The geologist Juan Carlos García of the Geological and Mining Institute of Spain (IGME) reports this. Since the start of the eruption, together with his colleagues from this public research body, he has been monitoring the paths of the lava and is now trying to establish a
cooling process pattern.

"In the center of the lava flows, that is, from the base of the cone to the strips, we have
a general temperature between 40 and 60 degrees on the surfacebut this zone is not continuous and
there are specific points where we can find up to 300 degrees on the surface», explains the IGME geophysicist who works identifying these points, above all, to make this information available to those responsible for road construction works.

This work has made it possible to identify the areas where there is
risk due to high temperatures or emission of volcanic gases on the road that connects La Laguna and Las Norias, where drivers are strictly prohibited from stopping and getting out of the car.

In this way, García and his colleagues have identified twelve
hot spots.

In addition, the IGME teams have provided the technicians who work on the coastal road -which will connect Tazacorte with the upper area of ​​Puerto Naos- with the cooling maps of the lava flows and have instructed them in carrying out thermal measurements to identify them. . In this work -competence of the Ministry of Transport, Mobility and Urban Agenda (Mitma)- up to 30 hot spots have been identified. These enclaves
They represent a real danger.

To demonstrate its existence in a practical way, they show us one of these points located on the same edge of the road that connects the south and north of the Aridane Valley through which vehicles have been traveling without time restrictions and in two directions since last month. of August.

In a crack located in the middle of an artificial slope of the road, just over a meter from the surface, he inserts the metal rod of a thermocouple - a sensor to measure temperature. The heat radiating from the laundry is felt in the environment. In a few minutes, the sophisticated thermometer reaches
559.4 degrees centigrade.

What happens at this point? Why is such a temperature reached?

“Lava cools very quickly on the surface in contact with the air. That allowed us to get closer to the edge of the laundry because it is sealed by that layer of cold laundry that made the heat stay inside and not radiate out, except in
points of discontinuity of that crust» explains Garcia.

«The lava, in general, in the center of the lava flows -the area between the base of the volcanic edifice and the coast-
on the surface it is around 40 and 50 degrees. What varies is the potential for that to remain hot. When in depth we have an area through which an active channel passes, which radiates heat towards the surface,
if it has a lot of power inside, it's going to take a long time to cool down», explains the geologist about those underground lava tubes where the incandescent lava still rests, cooling more or less quickly, depending on its depth and the number of superimposed flows that cover them, isolating them from the atmosphere. «
If we have less power, less thickness and fewer channels underneath, the laundry takes less time to cool», comments the scientist.

But the layer that covers these lava channels is not homogeneous. At some points there are discontinuities, small cracks that allow heat, accompanied by volcanic gases, to escape to the surface. This is where you reach
300 degrees centigrade on the surfaceGarcia says.

visual clues

The scientist knows how to identify even visually the areas likely to hide these hot spots. They are usually located on the edges of the valleys through which the lava rivers have flowed. «
The discontinuities appear at the limit between the canals and the levees» -a kind of lateral elevations that are formed by the cooling of the magma on the edges of the channels-, indicates the geologist.

«When the lava flows through a channel and, suddenly, the cone emits less lava, the flow is reduced and what has cooled on the edge remains as an elevation of the banks while the channel continues to flow in the middle» Garcia abounds.

"The area between these raised edges, which we call 'levés', and the channel, where the lava flowed, is a
rupture Zone, of dynamics, because there is something that is moving and something that is stopped. In that contact zone there is usually a lot of temperature that comes from below, from the base”, says García, pointing to one of these linear ridges that run towards the coast that give clues to the location of the highest temperatures of the lava flows.

In addition, other visual elements indicate the presence of these discontinuities that connect with the channels where the incandescent lava remains and where the highest temperatures are detected.

Where the fractures contact the surface air, small
yellow, ocher and white spots, exactly the same tones that stain the volcanic crater. These colors arise from the precipitation of volcanic gases when they come into contact with the atmosphere, forming
sulfur crystals and chlorine, fluorine and ammonium salts.

But beyond these visual clues, what has been key to identifying the hot spots of the lava flows has been the previous work carried out during the eruption, with special participation of the
dronesa new and key tool in monitoring this volcanic emergency.

Daily flights and maps against the clock

“Throughout the entire eruption, we were doing a
early morning flightna, with the first hours of light, to discriminate the incandescent channel and the edge of coladas with the spatial references that the houses, the cultivated fields and other elements gave us. These data allowed us to make a
mapping of the progress of the castings», explains the IGME scientist,

«By noon that cartography was already digitized thanks to the work of our colleagues in Madrid, who made it available to the emergency managers so that they could know which area had been occupied by the lava flows during the night and a forecast of where it could continue progressing the laundry”, comments García about this daily task carried out against the clock.

These flights allowed them to have exhaustive information on where the most important channels had been active. «Then the Cabildo de La Palma made a
thickness calculation», points out the geologist about a study that identified a maximum thickness of the flows of 70 meters at a specific and central point at the base of the cone. To this information, they added the data regarding the dates on which the channels appeared and when they began to cool down.

“What we did with all that data is a
cooling mapor from the surface castings that we quickly made available to the administrations that were going to plan the roads”, explains García about the final destination of the information collected.
This model is being validated with measurements made at the foot of the roadwhile locating the most problematic points of the road.

“Until now we have taken data on the surface, but
we are starting to do polls that they are giving us how the temperature is in depth, ”says the geologist accustomed to the heat of lava flows after having monitored some 150 locations.

The ultimate goal is to know the rhythm of the cooling process, something that, at the moment, is still unknown.

The volcanic cone, a reservoir of heat for years

The temperatures recorded in the
Tajogaite volcanic cone they are higher than in the coladas and will be for much longer, perhaps years.

There, according to the geologist of the Geological and Mining Institute of Spain (IGME) Juan Carlos García, the surface of the 200 meter high volcanic building presents
temperatures between 60 and 80 degreeyes

In addition, the
fractures are wider and they go directly to the vertical fissures that connect with the magma chamber. So in some cracks
you can see the lava still incandescent.

In one of them, with a mouth four meters wide, the temperature reached
830 degrees. To measure it, they used a thermocouple sensor with a rod ten meters long.

Normally, they calculate the temperature variations of the crater with
drones equipped with thermal cameras.