Scaling of Gas–Melt Decoupling in Silicic Eruptions: Magma Ascent, Connected Permeability, and Atmospheric Confinement

Gas escape relative to magma ascent is a primary control on degassing, yet its governing parameters are difficult to compare across eruptions and planetary environments. This study develops a reduced-order dimensionless framework for diagnosing local gas–melt separation in connected, pre-fragmentation magma. The formulation combines phase mass conservation, porous gas transport, bubble-matrix permeability, independently constrained connected pathways, and inertial resistance. The resulting regime map separates the competition between magma advection and matrix gas segregation from changes caused by sealing, crystallization, fractures, and tuffisite pathways. It shows that crystallization has no unique effect. Matrix obstruction reduces gas escape, whereas brittle pathway formation can enhance it. Global sensitivity analysis indicates that ascent rate and bubble number density account for 98.4% of the variance in the matrix-controlled transport competition over the investigated ranges. When connected permeability is independently specified over the same logarithmic range as ascent rate, both contribute equally to the calculated variance. A literature-constrained calculation for the climactic Chaitén eruption places the bubble-matrix endmember in the advection-dominated regime without adjusting permeability to reproduce the observed outcome. Atmospheric scaling indicates strong near-surface suppression of bubble expansion on Venus, but its magnitude remains highly sensitive to the assumed mechanical overpressure scale. The framework provides a physically testable basis for comparing local degassing states without imposing an arbitrary eruption classifier. It does not predict eruption onset or fragmentation. These require time-dependent dynamics, mechanical failure criteria, and independent validation data.