Direct observation of degassing during decompression of basaltic magma

Transitions in eruptive style during volcanic eruptions strongly depend on how easily gas and magma decouple during ascent. Stronger gas-melt coupling favors highly explosive eruptions, whereas weaker coupling promotes lava fountaining and lava flows. The mechanisms producing these transitions are still poorly understood due to a lack of direct observations of bubble dynamics under natural magmatic conditions. Here, we combine X-ray radiography with a novel high-pressure/high-temperature apparatus to observe and quantify in real-time bubble growth and coalescence in basaltic magmas from 100 MPa to surface. For low-viscosity magmas, bubbles coalesce and recover a spherical shape within 3 s, implying that, for lava fountaining activity, gas and melt remain coupled during the ascent up to the last hundred meters of the conduit. For higher viscosity magmas, recovery times become longer, promoting connected bubble pathways. This apparatus opens new frontiers in unravelling magmatic/volcanic processes, leading to improved hazard assessment and risk mitigation.