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[EN VIDÉO] 8 Things You Should Know About Volcanoes Volcanoes, objects of both fascination and terror, are among the most indomitable forces of nature. From the mythological origin of her name to the eruptions that shaped history, here are 8 things to know about her.
Volcanic eruptions are common on Earth and represent one of the most important manifestations of our planet’s intense geological activity. There are currently more than 1,000 active volcanoes on Earth, and between 50 and 70 would erupt each year. The intensity, type or duration of each eruption is extremely variable and depends mainly on the tectonic context.
In the course of its history, the earth has experienced an incalculable number of volcanic eruptions, some of which were particularly significant. Supereruptions are characterized by their intensity, an extremely long activity of the volcanic system (on the order of tens of millions of years) and the enormous amounts of magma associated with them, but the formation of this type of volcanism and the associated mechanisms are still poorly understood .
Magma on the surface but also at depth
While surface supereruptions are characterized by vast amounts of volcanic debris, they are also associated with the emplacement of large plutons. Plutons are formed when magma crystallizes deep in the earth’s crust. Plutonic (or intrusive) rocks are thus distinct from volcanic (or effusive) rocks, which crystallize on the surface. Plutonic and volcanic rocks can have the same mineralogical composition because they come from the same magma, but differ in their appearance and especially in the size of the crystals, which vary depending on the cooling time of the magma. If the study of volcanic rocks allows understanding the evolution of the magmatic system over time, the analysis of plutonic rocks allows a better understanding of the formation of the magmatic chamber and the overall dynamics of the system responsible for the eruption.
Because at depth, the formation of a pluton depends on many parameters: fractional crystallization, differentiation of the magmatic fluid, injection of new magma…
Based on the study of certain volcanic rocks in northern Chile associated with a series of four supereruptions, a team of scientists were able to track the onset of this type of eruption to better understand the magmatic dynamics involved.
A fast eruption, but a very long deep magma storage
Indeed, their results, published in Nature, show that when the placement of plutons within the crust occurs over millions of years, attesting to a continuous supply of magma over very long periods of time, the final ascent of the magma occurs towards the shallow magma chambers and the Initiation of an eruption occurs extremely quickly, in just a few decades.
The longevity of volcanic systems associated with plutons is therefore in stark contrast to the very short time it takes to fill magma chambers in the upper crust just before an eruption. Indeed, the study shows that certain crystals remain at temperatures around 470°C (the temperature that allows liquid magma to be maintained) for a very long time before being incorporated into the effusive magma. The magma would therefore be stored long-term in a hot mid-crustal zone before being expelled episodically and rapidly to produce supereruptions. In this magmatic “incubator” certain minerals begin to crystallize to form and grow the plutons. The researchers believe that the magma at the origin of the eruptions came from an unstable pocket located on top of this incubator. This data provides a better understanding of how supereruptions occur and could help identify volcanoes likely to produce such eruptions.
However, supereruptions remain rare. Even if they only occur every 20,000 years on average, they remain extremely destructive phenomena that can have a significant impact on climate and the environment.
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