Why some gas-rich volcanoes erupt gently ^{CEFR B1}

Researchers report in the journal Science that gas bubbles in magma can form not only from falling pressure but also from shear forces inside the conduit. When magma rises and pressure drops, dissolved gases form bubbles and can make eruptions violent. The new work explains why some gas-rich magmas still produce slow, calm flows.

Shear is the difference in flow speed between the conduit centre and the walls. The study shows that shear can create bubbles, and if these bubbles grow and join deep in the conduit they can form channels that let gas escape early. In laboratory tests the team used a viscous liquid like molten rock and saturated it with carbon dioxide. Above a shear threshold, bubbles formed suddenly. Higher initial gas supersaturation meant less shear was needed, and existing bubbles helped form more bubbles nearby.

The researchers combined experiments with computer simulations and found the effect is likely where viscous magma flows along conduit walls. One example is the 1980 Mount St. Helens eruption, which began with a slow lava flow while shear released gas; a later landslide opened the vent, pressure dropped rapidly, and the volcano exploded. Olivier Bachmann of ETH Zurich says models should include shear forces to improve hazard assessment.