Laboratory simulations of sustained volcanic eruptions

Many violent eruptions are driven by rapid exsolution of dissolved volatiles within liquid magma. The accelerating two-phase mixture emerges at the vent as a sustained quasi-steady discharge lasting for periods from hours to days (refs 1, 2). The initial growthin discharge rate commonly observed, subsequent fluctuations and discrete pulses and shocks remain largely unexplained. We have simulated volcanic conduit flows by producing sustained, quasi-steady explosions in a liquid undergoing rapid exsolution of a gas. This was done by rapidly decompressing large volumes of CO_2-saturated water. The results reveal fluctuations in discharge rate that reflect heterogeneities in the two-phase mixture that form spontaneously as a consequence of the size and geometry of the experimental system. An initial transient with a growing discharge rate is observed in experiments in which material is erupted from a spherical flask up a narrow neck that mimics the magma-chamber/conduit assembly of volcanic systems. The fragmentation region propagates down the neck during the initial transient until it reaches a stable position at the top of the flask, at which point a quasi-steady discharge ensues