  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) {\phi}. When {\phi}
increases, Q* decreases and the degassing regime changes. Non-Newtonian
properties of magma might therefore play a crucial role in volcanic eruption
dynamics.
