Why do persistently active volcanoes erupt?

Many volcanoes like Stromboli or Hawaii are persistently active, emitting vast quantities of gas on a continual basis. Given that the majority of gas can escape these volcanoes passively, why do eruptions occur at all? The answer to this puzzle lies partially in the dynamic interactions between gas and liquid magma and partly in the crystallization of the magma.  We currently work on developing a new, multi-scale framework for understanding basaltic volcanism that captures critical thresholds and can be tested against field data–a challenge recently identified as one of the three grand challenges in modern volcanology by the National Academies of Sciences, Engineering, and Medicine.