Scientists have discovered that shear forces inside rising magma can create gas bubbles long before pressure drops occur.
The intensity of a volcanic eruption is shaped by how many gas bubbles develop in the magma and at what point they appear. Until recently, scientists believed that most bubbles formed mainly when rising magma experienced a drop in surrounding pressure.
At deeper levels, higher pressure keeps gases dissolved, but once the pressure decreases, these gases separate out and create bubbles. As bubble numbers increase, the magma becomes less dense and moves upward more quickly, which can cause it to fracture and erupt explosively.
A simple comparison can be made with a bottle of champagne. While the bottle is sealed and under pressure, carbon dioxide stays dissolved in the liquid. Opening the bottle lowers the pressure, allowing the gas to form bubbles. These bubbles lift the liquid toward the neck of the bottle and can cause it to spray out forcefully.
This view, however, does not tell the whole story – because some volcanoes, including Mount St. Helens in Washington, USA, and Chile’s Quizapu, have occasionally released lava gently even when the magma contained large amounts of gas capable of producing a powerful explosion. An international research group that includes a scientist from ETH Zurich has now offered a new way to understand this long-standing puzzle.
Shear as a new factor
In a recent study published in Science, the team reports that bubbles can also appear in rising magma when it is subjected to shear forces, not just when pressure decreases. When these bubbles grow within deeper parts of the volcanic conduit, they can merge and create pathways that allow gas to escape. If gas is released early through these channels, the magma can reach the surface in a much calmer flow.
We can imagine the shear forces in the magma as being like stirring a jar of honey: the honey moves faster where it is being stirred with the spoon. At the edge of the jar, where the friction is higher, it moves slower. A similar process is taking place in volcanic conduits: the magma moves more slowly at the edge of the conduit, where the friction is greatest, than it does in the interior. This essentially “kneads” the molten rock, producing bubbles of gas.
“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. The researchers’ experiments show that bubbles are formed primarily near the edges of teh conduit, where the shear forces are strongest. Existing bubbles further strengthen this effect. “The more gas the magma contains, the less shear is needed for bubble formation and bubble growth,” says Bachmann.
Why explosive volcanoes sometimes don’t explode
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 pronounced shear and the magma therefore shoots upwards quickly.
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.
One example 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 flow inside the volcanic cone. The strong shear forces acting on the magma produced additional gas bubbles that initially allowed a release of gas. It was only when a landslide opened the volcanic vent further and there was a rapid drop in pressure that the volcano exploded. The study’s results suggest that many volcanoes with viscous magma allow gases to escape more efficiently than previously thought.
Special laboratory experiment
In order to visualize the processes inside a volcano, the researchers developed a special experiment: they took a viscous liquid resembling molten rock and saturated it with carbon dioxide gas.
Then they observed what happened if the lava-like liquid was set in motion by shear forces. As soon as the shear forces exceeded a certain threshold, gas bubbles suddenly formed in the liquid. The higher the initial gas supersaturation, the less shear was needed to form further gas bubbles. The researchers also found that the presence of existing bubbles favored the formation of further bubbles in their immediate environment.
The researchers combined these observations with computer simulations of volcanic eruptions. By doing so, they showed that the effect is particularly likely to occur in areas where viscous magma flows along the walls of a conduit and therefore experiences strong shear forces.
With their work, the researchers provide a vital new piece of the puzzle when it comes to better understanding processes taking place inside active volcanoes and more precisely, assessing how volcanoes will erupt. “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,” says study co-author Bachmann.
Reference: “Shear-induced bubble nucleation in magmas” by Olivier Roche, Jean-Michel Andanson, Alain Dequidt, Christian Huber, Olivier Bachmann and David Pinel, 6 November 2025, Science.
DOI: 10.1126/science.adw8543
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