The ascent of magma during the May 18, 1980, Mount St. Helens eruption was modeled by means of a nonequilibrium two-phase flow model which accounts for the magma composition and crystal content. The model takes into consideration the flow of magma from the magma chamber to the surface which includes the conduit region below the gas exsolution level, the region between the exsolution and the magma fragmentation levels, and the region above the fragmentation level. The eruption was subdivided into four main phases which span from 0900 hours to its waning at about 1715 hours on the same day. These phases include three Plinian phases (I, II, and IV) and one pyroclastic flow phase (III). The input data to the model include the mass flow rate, conduit length, composition and crystal content of magma, initial temperature and pressure of magma in the magma chamber, and initial water content for each of the four modeled phases. These data were obtained on the basis of a large number of volcanological, petrological, and geophysical studies of the May 18 eruption. By employing the model to search for the flow conditions which produce a choked flow at the conduit exit, the modeling equations were numerically solved for pressure, gas volumetric fraction, and gas and pyroclasts velocity distributions.

The results provide an eruptive scenario which is consistent with the eruptive style of each of the modeled phases of the eruption and with some relevant stratigraphic characteristics of the related pyroclastic deposits. In particular, a change in style from the Plinian to the pyroclastic flow activity at the onset of phase III is explained by this phase being characterized by a larger conduit diameter, larger exit pressure, lower gas and particle exit velocities, and lower exit gas volumetric fraction with respect to the Plinian phases. All these conditions work in favor of the formation of a dense column above the vent which does not rise in the atmosphere but spreads laterally and collapses to form pyroclastic flows. This change in the eruptive style occurs without a significant decrease in the initial water content of the erupted magma, which was previously suggested to explain it. For constant magma composition and mass flow rate, a progressively lower initial water content of phase III produces larger conduit diameters, lower exit pressures, and larger exit gas volumetric fractions, which may result in the formation of a more buoyant column above the vent. All of the results reveal that the magma viscosity is a fundamental quantity in determining the flow patterns and exit conditions of the erupted magma. Variations in the magma characteristics which cause a decrease in the magma viscosity produce conduit exit conditions which may favor the transition from Plinian to pyroclastic flow activity. It is suggested that this transition, very often observed during explosive eruptions or inferred from the resultant deposits, may not reflect the tapping of a progressively water-depleted magma as generally assumed but at least in some cases may reflect the withdrawal of a more mafic and less viscous magma and that an eventual decrease in the water content may favor the formation of a less dense and more buoyant eruptive column.