Constraints on the rates of degassing and convection in basaltic open-vent volcanoes

Variations in gas emissions of open-vent volcanoes are investigated using a model of magma convection in narrow conduits. Laboratory experiments with both vertical and inclined conduits and dimensional analysis show that for Grashof numbers lower than 100 the volumetric rate of magma ascent is a simple function of equivalent conduit radius, density difference between the magmas, and viscosity of the degassed magma that descends back to the reservoir. The rate of magma ascent depends on the flux coefficient, estimated as 0.1 and 0.2 for vertical and inclined conduits, respectively. The equivalent radius parameter accounts for the dimensions of the conduit(s) regardless of its geometry, thus extending the treatment by previous models that used flow in pipes. The volume flow rate of convection increases with higher density difference and conduit size, but is also highly influenced by the large variations in viscosity of the degassed magma as volatile content and crystallinity change. The model presented here can be used to constrain the degassing and ascent rates of volatile-rich magma when combined with petrologic data on magmatic volatile content. Application of the model to Villarrica volcano (Chile) reveals that the background degassing levels observed (similar to 3 kg s(-1) SO(2)) are associated with convective ascent of a relatively degassed magma (0.04 wt% S, similar to 0.5 wt% H(2)O), while episodes of higher SO(2) emissions (measurements up to 15 kg s(-1)) can be explained by the ascent of magma with higher volatile content (up to 0.09 wt% S, similar to 1.5 wt% H(2)O).