EXPLOSIVE VOLCANIC-ERUPTIONS .9. THE TRANSITION BETWEEN HAWAIIAN-STYLE LAVA FOUNTAINING AND STROMBOLIAN EXPLOSIVE ACTIVITY

The commonest eruption styles of basaltic volcanoes involve Hawaiian l ava fountaining or intermittent Strombolian explosions. We investigate the ways in which magma rise speed at depth, magma volatile content a nd magma viscosity control which of these eruption styles takes place. We develop a model of the degree of coalescence between gas bubbles i n the magma which allows us to simulate the transition between the two extreme styles of activity. We find that magma rise speed is the most important factor causing the transition, with gas content and viscosi ty also influencing the rise speed at which the transition occurs. Cou nter to intuitive expectations, a decrease in gas content does not cau se a transition from Hawaiian to Strombolian activity, but instead cau ses a transition to passive effusion of vesicular lava. Rather, a chan ge from Hawaiian to Strombolian style requires a significant reduction in magma rise speed.