When you look at the future of La Palma (almost) everything is unknown. Although there are many precedents for eruptions and it is generally known what will happen, each volcano is a world and the specificity of how each one is going to behave is particular. It is unknown precisely how long the eruption will last, how long it will take for the lava to cool and solidify, and what will happen next to these lands.
What will happen when the lava from the La Palma volcano reaches the sea?
The first thing, obviously, is to wait for the eruption to stop. When will this happen? Basically, it is unknown, although experts maintain that the volcano can continue to pour lava for weeks or even months.
Once the eruption stops, the next thing is to keep waiting: now, for the laundry to cool and solidify. And this process can be extended over time, warns Manuel Nogales, a researcher at the IPNA-CSIC and delegate of the Higher Council for Scientific Research in the Canary Islands. “How long it takes cannot be answered, each volcano is very individual from the point of view of knowledge and that is why the information is so important” that is being collected these days on La Palma, he explains.
Pablo J. González, a volcanic geophysicist at the CSIC in Tenerife, dares to make an estimate and explains that “it depends on the thickness” of the runoff, “but if it is around four meters on average, it will cool down in a few months or a year. If it is more important, we will see fumaroles in some places for several years, “he predicts wisely. At some points, the lava right now is around 8-10 meters high, which would lengthen the forecasts of this expert.
The cooling will also be conditioned by the type of lava that the volcano expels. In this case, a Strombolian volcano, it is a basaltic composition, “which normally comes from a deeper and hotter reservoir, around 1,100 or 1,200 degrees”, explains González, and which will logically lengthen the process. “Other andesitic compositions, with more silica, reach about 800 degrees and those 300 degrees of difference do a lot,” he adds.
Another characteristic of basaltic lava is that it is less viscous, continues the geophysicist, and the viscosity of a stream determines the behavior of the gases it carries. “That difference in viscosity makes it easier or harder for the gas in the magma to escape. If it can’t, it will find its own way. When you have low viscosity, gas bubbles can flow through the magma, which they can’t do when the silica is high, they get trapped. Then they just grow in size and you’ll have explosions, “he elaborates.
The Canarian almagre
In any case, no matter how long it takes to solidify, the ground will be practically unusable for a fairly long period of time. “The lands that the lava gains are impracticable for cultivation in the short term. You have to wait 15 or 20 years, they are not useful for practically anything before,” explains Nogales.
Also, lava changes the ground it touches. “The lava passes over the developed organic soil and alters its chemical composition. The first exposed layer, if it is clayey soils, will cook and quite impermeable clays will form,” González describes. In the Canary Islands, where they have experience with the subject, this roasted soil is called almagre and it is a layer, with its characteristic reddish color, that can be observed on any slope of the islands, says the expert.
The experience with volcanic soils also helps to know what to do with the terrain gained by lava. “In the Canary Islands it is very typical for lava platforms near the sea to be covered with soil from midland soils and to plant on it”, explains the geophysicist. Many banana trees in coastal areas fit this description.
And the destroyed houses? Can they be recovered? Again, a question without a definitive answer. Removing the lava, once solidified, is possible, but so expensive that it is not worth it, experts say. González recalls that after the 1949 explosion “the roads were redone, sometimes by drilling (the lava), at other times by modifying the route.” But it is early, “you have to see the magnitude of the lava field to see if it is feasible or not.”
And build on lava? Like the above, yes and no. “The Canary Islands are solidified lava,” recalls Esteban García, a civil engineer with a master’s degree in Geology. All the Canarian soil has come out of some volcano. But, continues the geotechnical expert, “you have to wait for it to cool down completely, and I remember walking in Iceland through a wash of less than 20 years that was still hot.”
And then there is the legal side. The Canary Islands soil law does not contemplate what happens to the soil in the event of an eruption. The President of the Government of the Canary Islands, Ángel Víctor Torres, explained two days ago that his Executive is preparing “a draft decree law that allows us to requalify the land and rebuild the houses.” Garcia hopes it is not literal. “Rebuilding where a wash has destroyed the houses does not seem like a good idea,” he values.
Just in case, neighbors evacuated from their homes these days rush to collect the deeds of their houses when the authorities allow them to return to rescue something, although those who did not arrive on time can rest easy because the information is in the registry of the property. José Luis Barrera, volcanologist from the College of Geologists, points out that part of the problem may be “reconstructing the cadastral map of each plot” in a sea of lava, while González ventures that the areas flooded by lava could become a natural setting that prevent development on them, although if this happened there would be legal consequences and the land would have to be expropriated.