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Influence of non‐Newtonian rheology on magma degassing

Many volcanoes exhibit temporal changes in their degassing process, from rapid gas puffing to lava fountaining and long‐lasting quiescent passive degassing periods. This range of behaviors has been explained in terms of changes in gas flux and/or magma input rate. We report here a simple laboratory experiment which shows that the non‐Newtonian rheology of magma can be responsible, alone, for such intriguing behavior, even in a stationary gas flux regime. We inject a constant gas flow‐rate Q at the bottom of a non‐Newtonian fluid column, and demonstrate the existence of a critical flow rate Q* above which the system spontaneously alternates between a bubbling and a channeling regime, where a gas channel crosses the entire fluid column. The threshold Q* depends on the fluid rheological properties which are controlled, in particular, by the gas volume fraction (or void fraction) ϕ. When ϕ increases, Q* decreases and the degassing regime changes. Non‐Newtonian properties of magma might therefore play a crucial role in volcanic eruption dynamics.