The 1980 eruptions of Mount St. Helens, Washington. Fluid dynamics of the May 18 blast at Mount St. Helens.

The lateral blast of the May 18 eruption of Mount St. Helens devastated an area of approx 500km² N of the volcano. Two major, irregularly shaped zones can be defined: 1) an inner zone, termed the 'direct blast zone', in which the flow of the blast was approximately radial from the volcano and was relatively undeflected, in plan view, by large topographic features, and 2) an outer zone, termed the 'channelized blast zone,' in which the flow followed or was deflected, by the local topographic features. The blast was a supersonic expansion of a multiphase (vapor-solid-liquid) mixture from a reservoir at a pressure much greater than that of the atmosphere into which it expanded. The model for the flow presented here explains the existence of the two blast zones, characteristics of the transition between the devastated area and surrounding undamaged forest, and many of the eyewitness and instrumental observations of the blast. The model can easily be scaled with pressure, temperature, and dimensional ratios, but plausible initial conditions used here for illustration are: a reservoir pressure of 12.5MPa, a temperature of 600K, and a solid-to-vapor mass ratio of 25. About half of the mass of the blast was discharged through the vent in 10-20s at the initial velocity of 100m/s; the remainder followed at decreasing velocities. The maximum mass flux, during temporary steady flow at the vent, would have been 0.6X10⁴g/s/cm² and the thermal flux would have been 2.5M/cm². The total thermal energy released was 24Mt, of which 7Mt was released during propagation of the blast through the devastated area, and 17 Mt during penecontemporaneous condensation of the stream and cooling of the condensed water and entrained solids.-from Author