2023-08-16 12:05:57
While it was thought that the main driver of explosive volcanic eruptions was the presence of water in the magma, a new study shows that in the case of certain volcanoes, it is rather the CO2 which should be blamed.
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What happens when magma heated to over 1,000°C meets water? The latter will suddenly vaporize and pass into gaseous form, causing an explosion. This phenomenon is called hydromagmatism. It is responsible for some of the most powerful explosive eruptions in history and is usually observed in the case of volcanoes located at shallow depth below the surface of the sea or whose crater is occupied by a lake or located under a glacier.
Water, engine of explosive eruptions?
But hydromagmatism is not just regarding that. Water is indeed a volatile element among others which is present in the magma in variable proportion. If the mixture is stable at depth, the decrease in pressure with the rise of the magma will gradually cause the water to pass into gaseous form. This is the exsolution process. The amount of water acquired by the magma in contact with the rocks will thus determine the intensity of the explosions, which will allow the magma to reach the surface. Water has therefore long been considered one of the main drivers of explosive eruptions.
A new study published in the review PNAS has just demonstrated that it would not, however, be the only one.
The special case of explosive basaltic volcanoes
Researchers at Cornell University in the United States have indeed looked into the mechanisms behind explosive basaltic eruptions. Usually, basaltic volcanoes, whose lavas are rather fluid, are not considered very explosive volcanoes. Unlike highly active volcanoes found in subduction zones, such as those in the Pacific Ring of Fire, basaltic volcanoes are located in the intraplate domain. They are therefore positioned not on the edge of the tectonic plates but somewhere above. Due to the thickness of the crust on which they rest, these volcanoes are therefore fed by a magma produced at great depth, around 20 to 30 kilometers below the surface and are associated with mantle plumes (hot spots). The typical example that is often used to characterize this type of volcano is Hawaii. However, this generalization of the Hawaiian type seems to have obscured certain properties of these basaltic volcanoes, in particular their capacity to produce explosive eruptions.
Magma produced at great depth and very rapid ascent through the crust
The Fogo volcano thus appears to be an extreme case of a basaltic volcano exhibiting explosive activity. Located on the oceanic crust of the African plate, it forms an island of the same name which is part of Cape Verde, west of Senegal, in the Atlantic Ocean. To determine the cause of this volcanic behavior, the scientists were interested in the composition of the volcanic rocks produced by the Fogo, and more particularly in the presence of volatile elements. They thus analyzed microscopic bubbles of gas trapped in the crystals at the time of the eruptions. This method revealed a high concentration of CO2. This observation suggests that the magma was stored deep within the mantle, and not within the crust.
A rise in magma thanks to CO2
However, the magma is loaded with water at the time of its passage and, from a stronger, from its temporary storage in the crust. In the case of Fogo, the magma would cross the crust much too quickly for its water content to be the engine of the eruption here. For the researchers, everything would therefore depend on the carbon dioxide present in the magma and its exsolution during the ascent.
These new results completely revolutionize our vision of the functioning of this type of volcano and in fact have an impact on the understanding of the associated volcanic risk.
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