Open conduit volcanoes, of which Stromboli is an emblematic example, erupt with the highest frequency on Earth1. The activity at Stromboli is characterised by frequent small explosions intercalated by larger events that pose a significant risk to locals, tourists, and scientists2,3. Thus, identifying the signs of an impending larger explosion is of outmost importance for the mitigation of volcanic hazard. Here we show that the interaction between CO2-rich fluids and magma spontaneously leads to the accumulation of volatile-rich, low density and gravitationally unstable magma at depth, without the requirement of a permeability barrier4. CO2-flushing forces the exsolution of water and the increase of magma viscosity5, which proceeds from the bottom of the magma column upward. This rheological configuration unavoidably leads to the progressive thickening of a gas-rich and low density (i.e. gravitationally unstable) layer at the bottom of feeding system. Our calculations account for gas monitoring data and provide a base to trace the approach to deeply triggered large or paroxysmal eruptions and estimate their size from continuous gas monitoring data. The model we propose for Stromboli can be applied to any other open conduit volcano globally and offers a framework to anticipate the occurrence of unexpectedly large eruptions.