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Volcanic activity represents the natural force par excellence to which the history of our planet is intimately linked. What do we mean by volcano exactly?? According to the excellent definition given by Futura Sciences, “A volcano is an orifice in the earth's crust from which, during eruptions, lava (made up of magma) and gases (sulphur dioxide, carbon dioxide, nitrogen, for example) escape. and ashes“. These particular geological structures shape the continents, influence the climate and fertilize the land. Stratovolcanoes, calderas, submarine volcanoes or rift volcanoes, there are a multitude of them, classified according to their shape, their composition or their types of eruptions.
Today there are 1 670 active volcanoes around the world, most of which lie along tectonic plate boundaries. These regions are the theater of dynamic interactions between the internal forces of our planet and its surface. Beneath these stone giants lie complex processes governing eruptions, invisible to the naked eye. What are they and how are they formed??
The rise of magma, the engine of eruptions
All volcanic eruptions have one thing in common: the presence of magma, a molten rock that originates in the lithosphere . The latter is the “ zone forming one of the concentric envelopes of the terrestrial globe […]d’thickness varying from less than 100 km (under the oceans) to sometimes more than 200 km (under the continents) » according to Larousse. There is infernal heat, sometimes exceeding 1 200° C.
The magma drops, less dense than the rocks that surround them, then begin their ascentat the heart of the lithosphere. When it travels to the surface, magma is naturally stored in what are commonly called magma chambers, located between 20 and 50 kilometers below the surface. Today, we prefer the term reservoir, because it is more evocative of the diffuse nature of magma storage.
Indeed, we should not imagine caves filled with magma, but rather a network of fractures, cracks and voids where liquid and solid elements are mixed. These reservoirs play an important role in the eruptive process, since it is here that the chemical and structural composition of the magma evolves. Its density changes, its viscosity, its temperature and also its capacity to contain gases.
Higher in its ascent, the magma reaches a superficial reservoir, located only a few kilometers below our feet. This is where it all comes down to! Due to the ambient supersaturation of gases and the movements of tectonic plates, a phenomenon of overpressure can occur. If this reaches a critical threshold, the pressure accumulated inside the reservoir then becomes powerful enough to cross the resistance threshold of the rocks surrounding it.
The earth's crust ruptures, cracks are created and the magma and gases stored until then suddenly escape towards the surface. This is the eruption. It can be of several types, depending on the quantity of gas contained, local geological characteristics, chemical composition and viscosity of the magma: effusive or explosive. Note that this is a simplified description, and that other complex phenomena can be the cause of an eruption , such as the interaction between water and magma or the decompression linked to the rise of the latter.
Below, we can clearly see the violence of the eruption of Krakatoa on October 17, 2018, a volcano with highly explosive eruptions. This is the same volcano, which on August 27, 1883, erupted and produced the most intense noise ever heardin the history of humanity. This was 10,000 times more intense than the bomb dropped on the city of Hiroshima and the power of the eruption was such that the resulting shock waves traveled < strong>seven times the globe.
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Volcanoes: creators and destroyers of worlds
If we call volcanoes the “ architects of our planet ”, it’s not for nothing. They are not just spectacular phenomena to observe, but also play a key role in creating conditions favorable to life.
Today it is estimated that volcanoes have shaped more than 80% of the earth's surface. A process that spanned millions of years during which solidified lava accumulated, ashes and pyroclastic materials (fragments of rocks and magma resulting from volcanic eruptions). This activity thus formed mountains, plateaus, islands and contributed to modifying the terrestrial landscape.
Thanks to them, the first livable atmospheres were able to be born by the release of gassuch as carbon dioxide, nitrogen and water vapor.
Volcanic activity is also at & #8217;origin of the birth of some of the most fertile soils on our planet. Volcanic rocks are very rich in essential minerals, and when they decompose, they release nutrients essential to plant life, such as calcium, phosphorus or potassium. Without these elements, the first civilizations would never have been able to take advantage of arable lands to develop agriculture.
Despite the splendor of the spectacle they offer and their ecological role, the destructive potential of volcanoes is enormous. When Mount Tambora (Indonesia) exploded in 1815, 92,000 people died directly or indirectly as a result of the eruption. Mont Pelée (Martinique) completely destroyed the town of Saint Pierre in 1902, killing more than 30,000 people in a few minutes. More recently, the eruption of Nevado del Ruiz (Colombia) in 1985 caused enormous lahars (volcanic mud flows) which carried away 23,000 people.
Even though modern monitoring tools have evolved, volcanoes remain largely unpredictable, and accurately predicting an eruption is still impossible. There will never be technology that will protect us from the force of these colossi. The only means we have to mitigate volcanic risk remains early warning systems coupled with good evacuation strategies.
- The central process leading to a volcanic eruption is the rise under pressure of magma from deep in the lithosphere.
- Volcanoes played a key role in the formation of the Earth and participated in the emergence of life.
- They are also very destructive, certain past eruptions may have caused the death of several tens of thousands of people each.
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