Why some gas-rich volcanoes erupt gently ^{CEFR B2}

New research published in the journal Science explains a mechanism that helps to resolve a long-standing puzzle: why some volcanoes with gas-rich magma do not erupt explosively. Previously scientists attributed bubble formation mainly to a drop in ambient pressure as magma rises, a process often compared to opening a bottle of champagne. The new work shows that shear forces inside the volcanic conduit can also create and amplify bubbles.

Shear arises because magma flows faster in the conduit centre and slower near the walls. The team found that shear produces bubbles, especially near the walls where deformation is strongest. If these bubbles grow and coalesce deep in the conduit, they can form persistent channels that allow gas to escape early. Gas loss through such channels can keep the magma flowing quietly rather than fragmenting into an explosive eruption.

In laboratory tests the researchers used a viscous fluid similar to molten rock, saturated it with carbon dioxide, and applied shear above a threshold. Bubbles formed suddenly; higher initial gas supersaturation required less shear to trigger bubble formation, and existing bubbles promoted further nucleation nearby. Computer simulations combined with the experiments indicate the effect is important where viscous magma slides along conduit walls and experiences strong shear. The findings explain different outcomes: low-gas magma can still explode if shear makes many bubbles and the magma rises rapidly, whereas shear can also create degassing channels in gas-rich magma and reduce early explosion risk. The 1980 Mount St. Helens eruption began with a slow lava flow that allowed gas escape by shear-produced bubbles, but a later landslide opened the vent, the pressure fell rapidly, and the volcano exploded. The researchers, including Olivier Bachmann at ETH Zurich, say volcano models should be updated to take shear forces in conduits into account for better hazard prediction.