6 Minutes
Scientists have found a surprising new trigger inside volcanic conduits: shear forces that "knead" magma and produce gas bubbles long before pressure drops. This mechanism helps explain why some gas-rich eruptions stay calm while others explode. Here’s what the discovery means for our understanding of volcanic hazards and how researchers recreated the process in the lab and on the computer.
New research reveals that volcanic magma can generate gas bubbles not only as pressure drops during ascent, but also through internal shear forces that “knead” the molten rock. These shear-driven bubbles can either trigger explosive acceleration or open escape pathways that calm an otherwise violent eruption.
Rethinking bubble formation: more than a pressure story
For decades, the standard explanation for bubble formation in magma focused on decompression. As magma rises toward the surface, the surrounding pressure falls and dissolved gases exsolve—much like carbon dioxide popping out of an opened champagne bottle. When many bubbles form, magma becomes buoyant, accelerates upward, and can fragment explosively.
But field observations have long exposed exceptions. Some volcanoes with gas-rich, viscous magma—conditions typically associated with violent eruptions—have sometimes emitted slow, viscous lava flows instead. Volcanoes such as Mount St. Helens (USA) and Chile’s Quizapu have shown that gas content alone does not paint the whole picture.
The new research adds a crucial ingredient to the recipe: shear. When magma flows through a conduit, it moves faster in the center and slower along the walls, creating velocity gradients. Those gradients impose shear stresses on the molten rock and, as researchers now demonstrate, can directly induce bubble nucleation and growth.
Lab experiments and numerical models: watching bubbles form under shear
To visualize and quantify this effect, the team behind the study built a controlled laboratory analog. They used a viscous fluid that mimics silicate melt and saturated it with carbon dioxide. Then they imposed shear by moving the fluid relative to boundaries, simulating magma sliding along conduit walls.
As shear exceeded a threshold, bubbles appeared abruptly—without any change in pressure. Bubbles preferentially nucleated near the boundaries where shear rates were highest. Moreover, existing bubbles catalyzed further bubble formation nearby: the local disturbance in the flow and pressure field around a bubble makes additional nucleation easier.
Computer simulations bridged the lab scale to natural conduits. The models showed that shear-induced bubble nucleation is most efficient where viscous magma flows against conduit walls, creating a stratified pattern of bubbles and gas pathways. When these bubbles coalesce, they can form continuous channels that allow gas to escape before magma reaches the surface.
Olivier Bachmann, Professor of Volcanology and Magmatic Petrology at ETH Zurich and a co-author on the paper, summarizes the shift succinctly: shear can generate bubbles even in the absence of decompression. That insight reframes how scientists think about the timing of degassing and eruption dynamics.
Why shear can make an eruption calm—or suddenly violent
The new mechanism explains two seemingly contradictory behaviors.
- Early degassing and gentler eruptions: In gas-rich magmas, shear can trigger bubble growth and coalescence deep in the conduit. These bubbles can link up to form escape routes—degassing channels—that vent gas gradually. The result: pressure is relieved before rapid ascent, and lava may effuse rather than explode.
- Shear-driven explosive acceleration: Conversely, a magma that appears low in gas can still erupt explosively if strong shear forms a sudden abundance of bubbles. A rapid increase in bubble volume reduces the magma’s density and can accelerate it upward, leading to fragmentation and an explosive eruption.
Historical cases illustrate both paths. During the 1980 eruption of Mount St. Helens, an initial slow emplacement of viscous lava inside the crater gave the system time to degas locally. It was only after a landslide rapidly opened the vent—producing an abrupt decompression—that the eruption transitioned to a catastrophic explosion.
Implications for monitoring and hazard assessment
Incorporating shear-driven bubble dynamics into volcanic models could improve forecasts of eruption style and timing. Traditional monitoring focuses on gas flux, seismicity, ground deformation and other proxies for decompression-driven degassing. But shear depends on conduit geometry, magma viscosity, and flow rate—parameters that can vary rapidly and are not always captured by current observables.
Updating numerical eruption simulations to include shear-induced nucleation will help scientists assess whether a specific volcano is more likely to vent gas quietly or to accelerate toward an explosive event. That has practical consequences for evacuation planning, flight safety (ash hazards), and long-term risk mapping.
Technology and future studies
Future work will pair laboratory rheology experiments with higher-resolution conduit imaging and improved real-time monitoring. Advances in remote sensing, infrasound analysis, and fiber-optic strain meters could help detect the flow regimes where shear-driven degassing is likely. Likewise, laboratory experiments using natural melt compositions and volatiles other than CO2 will refine thresholds for bubble formation under shear.
Expert Insight
"This discovery fills a blind spot in how we interpret volcanic behavior," says Dr. Maya Reynolds, a volcanologist who collaborates with monitoring agencies. "Shear changes the timing and location of bubble growth inside a conduit. That can be the difference between a lava flow that allows people to stay near a volcano and a surprise explosive eruption that forces mass evacuations. Incorporating shear into predictive models could make early warnings more reliable."
Beyond improving hazard models, the finding is a reminder that Earth's internal processes often reflect the interplay between chemical physics and fluid mechanics. A seemingly subtle shift in how melt flows can cascade into major changes in eruption behavior.
As research teams expand laboratory experiments, refine simulations, and couple results to field measurements, volcano science will gain a richer toolkit to anticipate the next eruption. For now, the message is clear: pressure isn't the sole arbiter of bubble birth—magma's internal motion matters just as much.
Source: scitechdaily
Comments
skyspin
Wait how do they rule out pressure effects entirely? Lab fluids are neat but natural magma is messier... sounds plausible but I'm not convinced yet, need more field data
labcore
Wow didnt expect shear to be a game changer for eruptions. Mind blown, kinda scary. If shear does this deep down, monitoring needs to level up fast
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