Models that inform how magma moves and volcanic eruptions unfold may need an update, according to a new study. It reports that gas bubbles in magmas can form through the mechanical forces of shear as magmas flow and deform– a new physical mechanism for magma bubble nucleation that challenges conventional degassing models. The formation of gas bubbles within magma – also known as nucleation – is a fundamental process that shapes how volcanic eruptions unfold. The timing and rate at which these bubbles appear and expand influences key magma features, including its buoyancy, viscosity, and explosive potential. Understanding nucleation is therefore vital for building accurate models of magma movement and predicting eruptive styles. Traditionally, bubble nucleation has been attributed mainly to depressurization as magma rises to the surface, which causes dissolved gases like water vapor and carbon dioxide (CO2) to separate from the mix. Although these bubbles can form spontaneously, it's thought that they usually form more easily on tiny mineral crystals within the magma, which act as microscopic catalysts for nucleation within magmatic reservoirs and conduits.
Here, Olivier Roche and colleagues investigated a new way that gas bubbles might form in magma. Instead of focusing on decompression as the trigger, Roche et al. propose that the mechanical energy from shear forces, created as magmas move, can also drive bubble nucleation. In a series of laboratory experiments, the authors observed bubble formation in a pressurized molten polymer infused with dissolved CO2 – an analog for magma – as different shear rates were applied to the liquid. They found that, while some bubbles formed naturally, most bubbles formed after shear was applied, especially in regions where shear stress was the strongest, and that the required shear stress needed to trigger nucleation decreased as dissolved CO2 content within the liquid increased. Overall, the findings demonstrate that viscous shear – a mechanical force common in flowing magmas – can supply the energy needed to trigger bubble formation without a reduction in pressure. Moreover, sudden mechanical shocks to the liquid resulted in rapid, widespread nucleation, further illustrating that deformation and motion within magma can actively drive bubble formation. Roche et al. use theoretical and computational models to show that shear-induced nucleation can occur naturally in volcanic conduits, especially in high-viscosity magmas, and that it may be enhanced by magma decompression during ascent. According to the authors, shear could promote efficient degassing and explain why highly-viscous, gas-rich magmas can sometimes erupt quietly and effusively, and without explosive fragmentation.
