A plume of steam and ash rising above Mount St Helens in Washington in the aftermath of the May 18, 1980, eruption. (Photo by UPI/Bettmann Archive/Getty Images)
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How and if a volcano explodes depends on how and when bubbles of water vapor and carbon dioxide form in the rising magma.
This process can be likened to a bottle of champagne: while the bottle is closed and therefore pressurized (like molten rock in a magma chamber), the carbon dioxide remains in solution. When the cork is removed from the bottle, the pressure drops and the carbon dioxide forms bubbles. These bubbles draw the liquid upwards with them, forming foam and cause it to spray out of the bottle explosively.
Until now, it was thought that a gas bubbles driving a volcanic eruption were formed primarily when the ambient pressure dropped while the magma was rising. However, this explanation is incomplete and can not explain the eruption style of some volcanoes.
Now, an international research team including a scientist from ETH Zürich has simulated how magma moves, providing a new explanation for how it erupts to the surface.
The researchers show that gas bubbles can form in the rising magma not only due to a drop in pressure, but also due to shear forces in the molten rock.
To simulate what happens in a volcano, the researchers took a viscous polymer resembling the physical properties of molten rock and saturated it with carbon dioxide gas. Then they observed what happened if the lava-like liquid was set in motion.
The researchers’ experiments show that bubbles are formed primarily near the edges of the conduit, where the liquid is dragged along the conduit walls. The friction generates shear movements that deform the viscous material and promote the nucleation of bubbles.
“Our experiments showed that the movement in the magma due to shear forces is sufficient to form gas bubbles – even without a drop in pressure,” explains Olivier Bachmann, Professor of Volcanology and Magmatic Petrology at ETH Zurich and one of the co-authors.
According to the new findings, magma with a low gas content that seems not to be explosive could nevertheless lead to a powerful explosion if a large number of bubbles form due to shear forces in the rising magma.
Conversely, shear forces can also cause bubbles to develop and combine at an early stage in gas-rich and potentially explosive magma, leading to the formation of “degassing channels” in the magma that bring the gas pressure down.
“We can therefore explain why some viscous magmas flow out gently instead of exploding, despite their high gas content – a riddle that’s been puzzling us for a long time,” says Bachmann.
Diagram summarizing the study’s findings, showing how shear forces in the magma flowing through the volcanic conduit can control how and when gas bubbles form.
Roche et al. 2025/Science/ETH Zürich
A real-life test for this model is the eruption of Mount St. Helens in 1980. Although the magma was gas-rich and therefore potentially explosive, the eruption began with the emplacement of a very slow lava intrusion inside the volcano, slowly deforming the mountain for months. The strong shear forces acting on the magma produced additional gas bubbles that initially allowed a release of gas, explaining high gas levels measured on the surface. It was only when a landslide opened the volcanic vent further and there was a rapid drop in pressure that the volcano exploded.
“In order to better predict the hazard potential of volcanoes, we need to update our volcano models and take shear forces in conduits into account,” concludes Bachmann.
The full study, “Shear-induced bubble nucleation in magmas,” was published in the journal Science and can be found online here.
Additional material and interviews provided by ETH Zürich.