Some Volcanoes Don’t Explode When Erupting — A Hidden Force Helps Pressure to Escape
One of the most explosive volcanoes in U.S. history began its eruption with a trickle, not a blast. Mount St. Helens’ gas-laden magma oozed into the cone before the mountain finally erupted in 1980. Similar behavior has been seen at Chile’s Quizapu volcano and others with thick, magma seemingly destined to explode.
This pattern has confused volcanologists for decades. According to long-standing models, gas bubbles should form only when rising magma experiences a drop in pressure, much like uncorking a bottle of champagne. More bubbles should mean faster ascent and a greater chance of an explosive blast. But that explanation never fully accounted for why volatile-rich, magma sometimes emerges quietly.
In a new study published in Science, researchers have identified the missing mechanism behind these calm eruptions: shear forces inside the volcanic conduit can trigger gas bubbles deep underground, allowing pressure to escape before it builds to explosive levels.
“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,” said Olivier Bachmann, a co-author, in a press release.
How Magma Movement Sets Up a Volcano Eruption
Gas bubbles play a major role in how eruptions unfold, but most models have treated them as forming mainly when pressure decreases during magma ascent. What these models don’t capture is how magma actually moves: it slows along conduit walls and stretches through the center, creating strong shear forces.
The study shows that shear force is common throughout volcanic systems and could supply the energy needed to trigger new bubbles. This matters because some thick, gas-rich magmas that should be explosive occasionally erupt quietly, hinting that another process inside the conduit helps gas escape before pressure builds.
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Testing How Shear Forces Change Magma Behavior
To mimic conditions inside a volcano, the team worked with a lava-like liquid saturated with carbon dioxide. When they set this material into motion, the experiments revealed exactly how shear force could trigger gas release.
“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,” said Bachmann in the press release. Most bubbles formed along the outer, high-shear regions of the liquid, and they often appeared close to earlier bubbles. “The more gas the magma contains, the less shear is needed for bubble formation and bubble growth,” Bachmann continued.
The researchers also noticed that bubbles tended to cluster and merge, strengthening the effect even further. This helped them identify where bubble growth and coalescence were most likely to occur inside a real volcanic conduit — especially along the walls, where shear force is strongest.
They then paired these experiments with computer simulations of volatile-rich magma. The simulations showed the same pattern: once shear forces passed a critical level, small volatile-rich domains rapidly merged into new bubbles. The results suggest that shear-induced bubble formation is likely to occur deep inside volcanic conduits, where magma experiences steep velocity differences as it rises.
Changing How We Model Eruptions
The study adds an important missing mechanism to eruption physics, showing that shear forces can trigger bubbles in a conduit and influence whether magma explodes or releases gas gently. The authors note that this process needs to be built into future hazard assessments.
“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,” said Bachmann.
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