Shear Forces Identified as Novel Trigger for Volcanic Magma Bubble Nucleation

A new study published in November 2025 by an international research collaboration, including scientists from ETH Zurich, has substantially revised the long-standing scientific consensus regarding the formation of gas bubbles that drive volcanic eruptions. Previously, the dominant theory attributed bubble nucleation exclusively to the reduction of ambient pressure as magma ascended through volcanic conduits. The current investigation establishes a more intricate mechanism, demonstrating that the mechanical action of deforming gas-saturated magma can initiate bubble formation even without a significant external pressure change.

The core finding centers on the influence of shear forces, which arise from differential magma flow velocities, becoming most pronounced near conduit walls where frictional forces are maximized. Researchers validated this concept using an analogue system involving pressurized molten polymer saturated with carbon dioxide (CO2) to simulate natural magma conditions. This experiment revealed the spontaneous nucleation of bubbles specifically within zones subjected to high mechanical stress when controlled shear was introduced, thereby establishing shear-induced nucleation as a mechanism that complements the established decompression process.

A critical observation from the research indicated that the necessary shear stress required to trigger bubble formation decreases proportionally as the concentration of dissolved gas within the magma increases. This newly identified mechanism carries significant ramifications for volcanology, potentially explaining instances where highly viscous, volatile-rich magmas result in effusive, relatively calm eruptions, such as extensive lava flows, rather than highly explosive events. Early bubble formation driven by shear facilitates a gradual, progressive release of gas, effectively lowering internal pressure before the magma reaches critical decompression zones and moderating subsequent eruptive behavior.

The study, which included contributions from Olivier Roche and Olivier Bachmann of ETH Zurich, suggests that current models used for volcanic hazard prediction must be updated to incorporate the mechanical effect of shear within volcanic conduits. This mechanism challenges the conventional interpretation of geological records, as prior estimations of magma ascent rates, derived from analyzing bubble textures, may be inflated if shear-induced nucleation is a significant factor. Traditional methods for estimating ascent rates, which rely on volatile diffusion into bubbles, have yielded varied results, with some estimates for rhyolitic magma ascent ranging from 0.05–0.35 m/s for the Oruanui eruption to faster rates of 40 m/s inferred from the Bishop Tuff.

Olivier Bachmann, Professor of Volcanology and Magmatic Petrology at ETH Zurich, noted that shear forces alone can generate gas bubbles without any pressure drop, with the strongest shear occurring near the conduit walls. This early degassing can account for the puzzling observation of quiet, effusive lava flows from gas-rich magmas, such as those seen at Mount St. Helens in 1980, where shear-induced bubble formation may have managed initial gas release before a catastrophic pressure drop. Incorporating this shear-driven process into forecasting models is now deemed necessary to more accurately assess eruption risks and distinguish between violent and quiescent eruptive styles.