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In a volcano, friction can create its own bubbles. This rewrites our eruption models.
ByKaif
For decades, volcanologists believed that the primary trigger for explosive eruptions was a drop in pressure as magma rises toward the surface. When pressure falls, gases dissolved deep within the molten rock separate out and form bubbles, making the magma lighter and causing it to rise faster.
Under the right conditions, this process can tear the magma apart and violently release energy. But this explanation has never fully matched what scientists observe in the real world. Several volcanoes known to contain gas-rich and potentially explosive magma have still produced gentle, slow-moving lava flows.
A new study published in Science and conducted by an international team of researchers, including ETH Zurich volcanologist Olivier Bachmann, offers a key missing piece of the puzzle. Their findings reveal that friction within the magma itself, specifically the shear forces created as magma moves through a volcanic conduit, can also generate gas bubbles. This mechanism helps explain why some volcanoes fail to erupt explosively even when the magma should, by all conventional expectations, blow apart.
How shear forces create bubbles
The researchers show that bubbles can form in rising magma not only because of pressure changes but also because of shear, the internal friction that occurs when different parts of the magma move at different speeds. The scientists compare this process to stirring a jar of honey. When honey is stirred, it flows faster around the spoon and more slowly along the edges of the jar, where friction is greatest.
Something similar happens inside a volcanic conduit. The magma near the center moves more freely, while the magma in contact with the conduit walls encounters resistance and slows. This contrast in movement effectively kneads the molten rock, creating the physical conditions needed for gas to separate and form bubbles even when pressure remains constant.
“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,” Bachmann explains. According to the study, these bubbles tend to appear near the conduit walls, where shear is strongest.
This mechanism grows more efficient as the gas content in the magma increases. As Bachmann notes, “The more gas the magma contains, the less shear is needed for bubble formation and bubble growth.” Existing bubbles amplify the effect further because bubbles create local zones where gases can accumulate and expand.
Why explosive magma sometimes behaves gently
This new understanding helps solve a long-standing mystery. Volcanoes such as Mount St. Helens in the United States and Chile’s Quizapu have sometimes released slow, calm lava flows despite containing magma that was highly viscous, gas-rich, and therefore expected to erupt explosively. The study shows that shear-driven bubble formation can allow gases to escape early, before pressure builds to dangerous levels.
When shear forces generate bubbles deep inside the conduit, the bubbles can merge into channels that allow gas to rise and vent gradually. These degassing pathways reduce internal pressure and prevent the rapid acceleration of magma that leads to violent eruptions. In other words, shear can act as a safety valve, enabling even potentially explosive magmas to reach the surface in a controlled, non-destructive manner.
An article on the ETH Zurich website describes how the 1980 eruption of Mount St. Helens illustrates this process. Although the magma feeding the volcano was extremely gas-rich, the eruption began with the slow emplacement of a lava flow. Strong shear forces inside the conduit helped create additional bubbles early on, allowing gas to escape.
Only later, when a landslide removed much of the volcano’s flank and caused a sudden drop in pressure, did the eruption transition into the catastrophic explosion that became one of the most well-known volcanic events of the 20th century. According to the study, many volcanoes with viscous magma likely release gases more efficiently than previously assumed, thanks to these shear-driven mechanisms.
Inside the experiment that revealed the process
To uncover this phenomenon, the research team designed a laboratory experiment that mimicked the interior conditions of a volcano. They used a thick, lava-like liquid infused with dissolved carbon dioxide gas.
By setting this liquid in motion and increasing shear forces, they observed that bubbles suddenly appeared once the shear exceeded a threshold. The higher the starting gas content, the easier it was for new bubbles to form. The presence of existing bubbles encouraged additional bubble growth nearby, creating a feedback loop.
These experimental observations were then combined with computer simulations of real volcanic systems. The simulations confirmed that shear-driven bubble formation is especially likely in zones where viscous magma scrapes along the conduit walls and encounters strong friction.
A new chapter in volcano modeling
The study’s findings introduce an important new factor into the understanding of volcanic eruptions. Until now, pressure drop has dominated models of bubble formation and eruption behavior. The discovery that shear forces can create bubbles independent of pressure changes means that models must be updated to capture the full physics inside a volcanic conduit.
“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,” Bachmann says. The research, conducted by the international team and credited in part to ETH Zurich, offers a clearer picture of what happens inside active volcanoes and helps explain why eruptions can vary so dramatically even within the same volcanic system.
By showing how friction and movement inside magma influence degassing, the study brings scientists a step closer to reliably predicting when a volcano will explode, and when it won’t.
Original:
https://interestingengineering.com/science/hidden-forces-stop-explosive-volcanic-eruptions
