One of the most explosive volcanoes in US history began its eruption with a trickle rather than a bang. Mount St. Helens' gas-rich magma seeped into the cone before the mountain finally erupted in 1980. Similar behavior was observed at Chile's Quizapu volcano and other volcanoes with thick magma that appeared destined to explode.
This pattern has baffled volcanologists for decades. According to long-standing models, gas bubbles should only form 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 explosion. But this explanation has never fully explained why volatile-rich magma sometimes arises unnoticed.
In a new study published in ScienceResearchers have identified the missing mechanism behind these quiet eruptions: Shear forces within a volcanic conduit can cause gas bubbles to form deep underground, allowing pressure to escape before it reaches explosive levels.
“We can therefore explain why some viscous magmas flow gently rather than explode despite high gas content – a mystery that has puzzled us for a long time,” said Olivier Bachmann, co-author, in the paper. press release.
How the movement of magma causes a volcanic eruption
Gas bubbles play an important role in how eruptions unfold, but most models view them as forming primarily as pressure decreases during magma ascent. What these models don't capture is how magma actually moves: It slows down along the walls of the conduit and is pulled through the center, creating strong shear forces.
The study shows that shear forces are common to all volcanic systems and can provide the energy needed to trigger new bubbles. This is important because some thick, gas-rich magmas that should be explosive sometimes erupt quietly, hinting that another process inside the conduit is helping the gas escape before pressure builds.
Read more: Activity at Mount Spurr in Alaska suggests the volcano is about to erupt
Testing how shear forces change the behavior of magma
To simulate the conditions inside volcanoThe team worked with a lava-like liquid saturated with carbon dioxide. When they set this material in motion, experiments showed exactly how the shear force could cause gas to be released.
“Our experiments showed that the movement of magma due to shear forces is sufficient to form gas bubbles – even without a drop in pressure,” Bachmann said in a press release. Most bubbles formed along the outer regions of the high-shear fluid and often appeared adjacent to earlier bubbles. “The more gas there is in the magma, the less shear is needed for bubbles to form and grow,” Bachmann continued.
The researchers also noticed that the bubbles tended to cluster and coalesce, further enhancing the effect. This helped them determine where bubble growth and coalescence within a real volcanic conduit is most likely, especially along the walls where the shear force is strongest.
They then combined these experiments with computer simulations of volatile-rich magma. The simulations showed the same pattern: once the shear forces passed a critical level, small volatile-rich domains quickly coalesced into new bubbles. The results show that shear-induced bubble formation likely occurs deep inside volcanic conduits, where magma experiences sudden changes in velocity as it rises.
Changing the way eruptions are modeled
Study adds important missing mechanism to eruption physicists showing that shear forces can cause bubbles to appear in the conduit and affect whether magma explodes or releases gas smoothly. The authors note that this process should be included in future hazard assessments.
“To better predict the potential hazards of volcanoes, we need to update our volcano models to take into account shear forces in conduits,” Bachmann said.
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