Lava effusion in mountainous terrain generates flow backup and excess inundation.

Most volcanic eruptions occur in high-relief terrains, however the impact of mountainous topography on lava emplacement is poorly constrained. Here we use analogue materials (syrup, glycerol) to investigate lava effusion where changes in channel width form constrictions, leading to fluid/lava backup. Backup magnitude positively correlates with high volumetric fluid/lava fluxes, high fluid/lava viscosities, and narrow constrictions. Viscosity modulates the impacts of slope: steep slopes produce the largest backups when viscosity is high ( ~ 50 Pa s), while shallow slopes produce the largest backups at low viscosities ( ~ 1.5 Pa s). Scaling experimental results to natural systems, we propose lava backup occurs due to three primary reasons: (i) obstacle-flow interaction, (ii) constriction-induced flux imbalance and (iii) choked flow caused by critical flow transition. Lavas inundating constrictive topographies are highly susceptible to backup, causing excess upstream lava inundation and prolonged downstream effusion, increasing the inundation area and extending the time that communities are at risk.