One of the most explosive volcanoes in U.S. history began its eruption with a trickle, not an explosion. Gas-laden magma from Mount St. Helens seeped into the cone before the mountain finally erupted in 1980. Similar behavior has been observed at Quizapu Volcano in Chile and other volcanoes with thick magma apparently destined to explode.
This trend has baffled volcanologists for decades. According to old models, gas bubbles should only form when the ascending magma experiences a drop in pressure, a bit like uncorking a bottle of champagne. More bubbles should mean faster ascent and a greater chance of explosion. But this explanation has never fully explained why volatility-rich magma sometimes emerges quietly.
In a new study published in ScienceResearchers have identified the missing mechanism behind these quiet eruptions: Shear forces inside the volcanic conduit can trigger gas bubbles deep underground, allowing pressure to escape before it reaches explosive levels.
“We can therefore explain why certain viscous magmas flow slowly instead of exploding, despite their high gas content, an enigma that has perplexed us for a long time,” explains Olivier Bachmann, co-author, in a press release.
How the movement of magma triggers a volcanic eruption
Gas bubbles play a major role in how eruptions unfold, but most models consider them to form primarily when pressure decreases as magma rises. What these models don’t capture is how the magma actually moves: it slows down along the conduit walls and expands through the center, creating strong shear forces.
The study shows that shear force is common in all volcanic systems and could provide the energy needed to trigger new bubbles. This is important because some thick, gas-rich magmas, which should be explosive, occasionally erupt gently, suggesting that another process inside the conduit is helping the gas escape before pressure builds.
Learn more: Activity at Alaska’s Mount Spurr suggests the volcano is about to erupt
Test how shear forces change magma behavior
To mimic the conditions inside a volcano, the team worked with a lava-like liquid saturated with carbon dioxide. When they set this material in motion, the experiments revealed exactly how the shear force could trigger the release of gas.
“Our experiments showed that movement in magma due to shear forces is sufficient to form gas bubbles – even without a pressure drop,” Bachmann said in the press release. Most bubbles formed along the outer regions of the high-shear liquid and often appeared close to earlier bubbles. “The more gas the magma contains, the less shear is required for bubble formation and growth,” Bachmann continued.
The researchers also noticed that the bubbles tended to cluster together and merge, further enhancing the effect. This helped them identify where bubble growth and coalescence were most likely to occur inside a real volcanic conduit, particularly along the walls, where the shear force is strongest.
They then combined these experiments with computer simulations of volatile-rich magma. Simulations showed the same pattern: Once shear forces exceeded a critical level, small volatility-rich domains quickly merged to form new bubbles. The results suggest that shear-induced bubble formation is likely to occur deep inside volcanic conduits, where magma experiences large differences in velocity as it rises.
Changing the way we model eruptions
The study adds an important missing mechanism to the physics of eruptions, showing that shear forces can trigger bubbles in a conduit and influence the explosion of magma or the gentle release of gas. The authors note that this process should be incorporated into future hazard assessments.
“In order to better predict the hazard potential of volcanoes, we need to update our volcano models and take into account the shear forces in the conduits,” Bachmann said.
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