Researchers have discovered that shear forces within volcanic conduits can trigger gas bubble formation in magma, independent of pressure drops. This mechanism helps explain why some gas-rich volcanoes produce gentle lava flows instead of violent eruptions. The findings, published in Science, could improve eruption forecasting models.
The intensity of volcanic eruptions depends on the number and timing of gas bubbles forming in rising magma. Traditionally, scientists thought bubbles emerged mainly as pressure decreased during ascent, similar to uncorking champagne. However, this model failed to explain instances where volcanoes like Mount St. Helens in Washington state and Quizapu in Chile released slow lava flows despite containing highly explosive, gas-rich magma.
An international team, including Olivier Bachmann from ETH Zurich, identified shear forces as a key alternative mechanism. In their study published in Science on November 21, 2025, they describe how magma movement in conduits—slower near walls due to friction—kneads the molten rock, nucleating bubbles even at high pressures. "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," Bachmann stated.
Laboratory tests using a carbon dioxide-infused liquid mimicking magma confirmed that shear exceeding a threshold rapidly generates bubbles, especially in gas-saturated mixtures. Simulations further revealed that bubbles form preferentially near conduit walls and can coalesce into escape channels, allowing early gas release. For high-gas magmas, this prevents pressure buildup, leading to gentle flows. "The more gas the magma contains, the less shear is needed for bubble formation and bubble growth," Bachmann explained.
Conversely, sudden shear in low-gas magmas can surge bubbles, accelerating ascent and causing blasts. The 1980 Mount St. Helens event exemplifies this: initial shear-induced degassing produced a slow lava dome, but a landslide later caused a rapid pressure drop and explosion. "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," Bachmann noted.
These insights suggest many viscous-magma volcanoes degas more efficiently than assumed. Incorporating shear into models could enhance hazard predictions. "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," Bachmann urged.