Effects of relative motion between gas and liquid on 1-dimensional steady flow in silicic volcanic conduits: 2. Origin of diversity of eruption styles

We investigate the origin of diversity of eruption styles in silicic volcanoes on the basis of a 1-dimensional steady conduit flow model that considers vertical relative motion between gas and liquid (i.e., vertical gas escape). The relationship between the assemblage of steady-state solutions in the conduit flow model and magma properties or geological conditions is expressed by a regime map in the parameter space of the ratio of liquid-wall friction force to liquid-gas interaction force (non-dimensional number epsilon), and a normalized conduit length Lambda. The regime map developed in the companion paper shows that when epsilon is smaller than a critical value epsilon(cr), a solution of explosive eruption exists for a wide range of Lambda whereas an effusive solution exists only when Lambda-1. On the other hand, when epsilon>epsilon(cr), an effusive solution exists for a wide range of Lambda Diversity of eruption styles observed in nature is explained by the change in epsilon accompanied by the change in magma viscosity during magma ascent. As magma ascends, the magma viscosity increases because of gas exsolution and crystallization, leading to the increase in epsilon. For the viscosity of hydrous silicic magma at magma chamber, epsilon is estimated to be smaller than epsilon(cr), indicating that an explosive solution exists for wide ranges of geological parameters. When magma flow rate is small, the viscosity of silicic magma drastically increases because of extensive crystallization at a shallow level in the conduit. In this case, epsilon can be greater than epsilon(cr); as a result, a stable effusive solution co-exists with an explosive solution. (C) 2008 Elsevier B.V. All rights reserved.