Observations of Explosive Exsolution in Liquids

Abstract

Explosive two-phase (gas-liquid) flows generated by rapid degassing of supersaturated liquids have been investigated experimentally to simulate conditions in violent volcanic eruptions. Two configurations were examined: i) Decompression of CO₂-saturated (P_(CO₂) ≤0.7 MPa) water causes explosive bubble growth and foaming. The large volume increase accelerates the two-phase mixture upward to velocities as large as 14 m/s at nearly constant accelerations up to more than 200 g. Constant acceleration implies that the bubble growth rate is proportional to t^(2/3), i.e., bubble size increases with time more rapidly than the t^(1/2) law expected for simple diffusive growth of spherical bubbles in an infinite medium, ii) Rapid mixing of concentrated K₂CO₃ and HCl solutions generates CO₂ supersaturations up to a few MPa. In this case, reaction and degassing generate an increasingly accelerating flow until the reactants become depleted at peak accelerations of about 100 g and velocities up to 45 m/s. In both experiments the rapid expansion causes the foam to fragment into a heterogeneous spray. The experimental flows are comparable to explosive volcanic flows in terms of decompression ratio, velocities, accelerations and in the large range of scales present. In addition, because the flow is confined to a conduit, the large accelerations cause strong extensional strain and longitudinal deformation of the foam. We speculate that widespread tube pumice in Plinian pyroclastic deposits and ignimbrites could preserve evidence of analogous flow conditions.