Dynamics of explosive degassing of magma Observations of fragmenting twophase flows

Liquid explosions, generated by rapid egassing of strongly supersaturated liquids, have been investigated in the laboratory with a view to understanding the basic physical laX•sses operating during bubble nucleation a d growth and the subsequent behavior fthe expanding two-phase flow. Experimenkq are carried out in a shock tube and ,me monitored by high-speed photography and pressure trm•sducers. Theoretical CO2 supersaturations up to 455 times the ambient saturation concentration ,are generated bya chemical reaction; K2CO3 solution is suddenly injected into an excess of HC1 solution i such a way as to mix the two solutions rapidly. Immediately after file injection event, abubble nucleation delay of a few milliseconds is followed by rapid nucleation ,mid explosive expmlsion f CO2 bubbles forming a highly heterogeneous foam. Enhanced diffusion due to advection i the 11ow coupled with continuous mixing of tile reactants, and hence on-going bubble nucleation after injection, generates an increasingly accelerating flow until the reactants become depleted at peak accelerations of around 150 g and velocities of about 15 m s -•. Stretching of the accelerating two-phase mixture nhances the mixing. Liberation of CO2 vapor is spatially inhomogeneous leading to ductile fragmentation ccurring throughout the flow in regions of greatest gas release as the consequence of the collision and stretching of lluid streams. Tile violence of the eruptions is controlled by using different concentrations of tile HCI and K2CO 3 solutions, which alters the CO2 supersaturation and yield and also file efficiency of the mixing process. Peak acceleration isproportional to theoretical supersaturation. Pressure tneasurements at the base of the shock tube show an initial nucleation delay and a pressure pulse related to the onset of explosive bubble fortnation. These chemically induced explosions differ t3⁄4om liquid explosions created in other experiments. Inexplosions caused by sudden depressurization f C02-saturated water, the bubbles nucleate uniformly fl•roughout the liquid in a single nucleation event. Subsequent bubble growth causes the two-phase mixture to be accelerated upward at nearly constant accelerations. Explosively boiling liquids, in which heterogeneous nucleation is suppressed, experience an evaporation wave which propagates down into the liquid column at constant average velocity. Fragtnentation ccurs at the shin'ply dellned leading edge of the wavefront. The chemical f ows effectively simulate highly explosive volcanic eruptions as they are comparable in terms of flow densities, velocities, accelerations, and in the large range of scales present. The lm'ge accelm,'ations cause su'ong extensional strain and longitudinal deformation. Comparable delbrmation rates in volcanic systems could be sufficient toapproach conditions for brittle l•,tgmentation. Tube pumice is a major component of plinian deposits and ignimbrites and preserves vidence of accelerating llow conditions.