The force of a volcanic eruption is closely tied to how many gas bubbles appear within rising magma, as well as the timing of their formation. For many years, scientists believed that most bubbles developed only when magma ascended and the surrounding pressure fell. In deeper layers, high pressure keeps gases dissolved in the molten rock, but once that pressure decreases, these gases escape and create bubbles. As bubbles accumulate, the magma becomes more buoyant and rises faster, sometimes tearing apart and erupting explosively.
This idea is often compared to opening a bottle of champagne. When the bottle is sealed, the carbon dioxide stays mixed into the liquid because it is under pressure. Removing the cork reduces that pressure, allowing gas to separate into bubbles. These bubbles rise rapidly, carrying the liquid upward and producing a sudden spray.
However, this classic explanation does not account for certain volcanic behavior. Some volcanoes, including Mount St. Helens in Washington state and Quizapu in Chile, have occasionally released slow, gentle lava flows even when their magma was rich in gas and considered highly explosive. An international team of researchers that includes a scientist from ETH Zurich has now identified a new factor that helps explain this long-standing puzzle.
Shear Forces as a Second Bubble-Forming Mechanism
In a recent paper in Science, the researchers report that gas bubbles can also appear in rising magma due to shear forces, not only from pressure drops. When bubbles form deep inside a volcanic conduit and begin to grow, they can merge into larger pathways that act as escape channels. This early release of gas can allow magma to emerge quietly at the surface.
To visualize shear, imagine stirring a thick jar of honey. The honey closest to the spoon moves faster, while the honey touching the walls of the jar moves more slowly because of friction. Something comparable happens in a volcanic conduit: magma near the walls travels more slowly than magma in the center. This uneven motion effectively kneads the molten rock and helps generate 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,” says Olivier Bachmann, Professor of Volcanology and Magmatic Petrology at ETH Zurich and one of the study’s co-authors. According to the team, bubbles form most readily near conduit walls where shear is strongest. Once some bubbles form, they make it easier for additional bubbles to appear. “The more gas the magma contains, the less shear is needed for bubble formation and bubble growth,” Bachmann explains.
Why Some Explosive Volcanoes Release Gentle Flows
The new findings show that even magma with relatively little dissolved gas can produce a powerful blast if shear produces a sudden surge of bubbles that pushes the magma upward rapidly.
On the other hand, shear can also create bubbles early in magma that already contains large quantities of gas. When these bubbles merge into wider channels, gas escapes before pressure can build. “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.
Mount St. Helens in 1980 illustrates this process. Although its magma held a high amount of gas and was capable of a major explosion, the eruption initially produced a slow-moving lava flow inside the cone. Strong shear in the rising magma generated extra bubbles, allowing early degassing. Only after a landslide opened the vent and triggered a rapid pressure drop did the volcano unleash its famous explosive phase. These results indicate that many volcanoes containing viscous magma may vent gas more effectively than previously assumed.
Laboratory Experiments Reveal How Shear Creates Bubbles
To explore how these internal processes develop, the research team designed a laboratory setup using a thick liquid similar to molten rock and infused it with carbon dioxide.
When the liquid was put into motion by shear, bubbles appeared suddenly once the force surpassed a specific threshold. Liquids with greater initial gas saturation needed even less shear to create more bubbles. The team also observed that existing bubbles encouraged the formation of additional bubbles nearby.
The scientists combined these experimental results with computer simulations of volcanic behavior. Their analysis showed that this bubble-forming effect is particularly active where viscous magma rubs against conduit walls and experiences strong shear.
Improving Volcano Forecasts With New Insights
Together, these findings provide an important new perspective on how active volcanoes behave internally and how eruptions begin. “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 Bachmann.
By incorporating shear-driven bubble formation into forecasting models, scientists may be able to evaluate eruption risks more accurately and understand why some volcanoes erupt violently while others release lava far more quietly.
