Why do some volcanoes erupt violently and others not?

Why do some volcanoes erupt violently and others not?

Whether a volcanic eruption is violent with high dust emissions or calm depends, among other things, on the composition of the melt (liquefied rock) which then forms the lava. It also depends on additional factors, e.g. whether this alloy will come into contact with water on its way, says geologist Prof. Ewa the weak.

In Iceland, it began on March 19 this year. volcanic eruption in the Fagradalsfjall massif, approx. 40 km from Reykjavik. Since then, the lava flows quietly there from cracks in the ground. In turn, in Sicily in mid-December 2020, Mount Etna woke up. Since then, there have been over a dozen eruptions there, during which lava and dust clouds emerge from the crater. After each such phase of awakening, it is necessary to sweep streets and roads of volcanic dust in the surrounding villages.

Why is the Icelandic volcano in Fagradalsfjall now calmer than the Sicilian Mount Etna? This was explained in an interview with PAP by geologist Prof. Ewa Sły, director of the Institute of Geological Sciences of the Polish Academy of Sciences and president of the European Mineralogical Union.

"In order for an eruption to occur, a volcano must have an alloy that is nothing other than liquefied rock. The composition of this alloy determines what the eruption will look like" - says Prof. Weak.

Volatile substances are a component of the alloy, which plays a significant role in violent eruptions, the researcher explains. The mantle and the crust may contain minerals that contain volatile groups, e.g. compounds of sulfur, fluorine, chlorine, CO2, and OH groups. When an alloy containing such components and high temperature, it is homogeneous, and these substances are built into its structure. However, when its temperature drops, these substances are released in the form of more or less toxic gases. Their release from the alloy is accompanied by a sharp increase in pressure as the alloy rises to the surface and in the volcano's chimney, it breaks into small fragments that escape from the volcano in the form of a huge cloud of dust and larger magma fragments (these volcanism products are called pyroclastics).

Volcanoes are formed in most cases at the junction of tectonic plates. And whether these plates push against each other or, on the contrary, diverge, is precisely related to the presence of volatile compounds, and thus the type of volcanism and the nature of their eruption.

Volcanic feet usually contain more volatile substances where tectonic plates press against each other. This is the case, for example, in the Sicilian Mount Etna. This is why this volcano emits large amounts of ash and coarser fractions.

"Meanwhile, volcanoes, from which lava is emitted calmly, usually arise in places where continental or oceanic plates are stretched and, as a result of a long process, break and separate in these places, it should be noted that oceanic plates - being thinner - are more susceptible to this process. We call it the process of the rift "- says Prof. Weak. He explains that under the rift zone, especially in the ocean, the Earth's mantle, which does not contain too many volatile substances, is melting. The rift zone runs through the entire Atlantic Ocean, including Iceland. Hawaii is another site of steady eruptions of low-volatile alloys. The genesis of these volcanoes, however, is different, not related to the rift.

For example, the most recent activity of an Icelandic volcano in the Fagradalsfjall massif are crevices, during which the alloy is emitted very calmly. Prof. Poor points out that there are some amounts of volatile substances in the foot (that is why residents of the surrounding areas were advised to close their windows). However, there are so few of them that a violent eruption could not occur in this environment.

"In places where tectonic plates are stretched, volcanic eruptions are calm. However, they may be accompanied by additional conditions that may also make these floods dangerous" - says Prof. Weak. “Such was the case, for example, in Iceland in 2010, when the Eyjafjallajokull volcano erupted, which temporarily paralyzed air traffic in Europe. In this case, the problem was that the volcano was covered with a glacier cap. Ice was melted by the temperature of the lava (around 1000 degrees C), and the combination of water and hot magma gave an incredibly strong explosion effect. This reaction of the melt with water is called phreatomagmatic. The alloy was then shattered violently, turned into a cloud of dust. The dust column rose about 12 km and encountered horizontal currents in the atmosphere, which spread this cloud over a large area of ​​Europe, paralyzing air traffic. It is a paradox that the nearest airport in Reykjavik was not affected by this phenomenon "- describes Prof. Siwy.

He adds that Icelanders were also worried in 2014. Then the volcano Bardarbunga woke up, which is partially under the glacier and has a very dynamic system of as many as five magma chambers at great depth. "An eruption would be so dangerous for the local inhabitants that not only would there be a strong emission of lava, dust and toxic gases, but also severe floods would occur. Fortunately, the volcano found a side outlet for its alloy and calmly emitted it without the rapid melting of the glacier. No. So there was a catastrophe "- says, Prof. Weak.

A geologist, when asked whether it is possible to predict volcanic eruptions, explains: "no; at this stage of science development, we can identify and model the mechanisms leading to the formation of the alloy, predict its composition. We are also able to predict what type of eruption it will be, taking into account the location of the volcano's activity. , we can predict what the lava flow or the flow of pyroclastic materials (dust, coarser fractions) will look like and what its composition will look like, what morphological forms, what structures. and able to predict a very precise time of the eruptions themselves. However, using the knowledge derived from the study of contemporary volcanoes, we can reconstruct the origin of the alloys and their path of activity for volcanoes from millions of years ago ".

So it is not possible to accurately forecast an eruption. The PAP interviewee points out that, for example, the eruption of the Japanese Ontake volcano in 2014, which was a popular destination for weekend trips, was a complete surprise. More than 60 people were killed then. "No one could have foreseen that it would come to this. The day was beautiful and a large number of people went for a walk to its slopes. The cloud of dust that appeared quite suddenly caused the people within it to stop breathing." - he comments. He also gives the example of the White Island near New Zealand. “The local Whakaari volcano was and still is a tourist attraction and many companies organized tours, crater hikes, and helicopter flights over the volcano there. In 2019, there was a violent, unexpected eruption and more than 20 people were killed.

“There is no way to control volcanic activity. These are powerful forces, and man is only able to increasingly recognize these phenomena without being able to control them. To a very limited extent, it can direct them a bit, "he says. He only gives an example of micro-scale actions that man is able to undertake - noting that it has nothing to do with the possibility of significant control of the powerful forces of nature that lead to the formation of phenomena. volcanic.

"Let's go back to Mount Etna. During one of the eruptions, the inhabitants of a village near this volcano dug ditches for the discharge of lava to prevent their homes from being destroyed. This action had a positive effect. However, please note what one small stream on a large scale means. eruptions that can emit millions of m3 of lava (e.g. Etna in 2018, 3-6 million m3). This best shows the scale of the phenomenon and our position as a human, observer and researcher of this type of phenomena "- says the researcher. (PAP)

Author: Ludwika Tomala