  Magma viscosity is strongly temperature-dependent. When hot magma flows in a
conduit, heat is lost through the walls and the temperature decreases along the
flow causing a viscosity increase. For particular values of the controlling
parameters the steady-flow regime in a conduit shows two stable solutions
belonging either to the slow or to the fast branch. As a consequence, this
system may show an hysteresis effect, and the transition between the two
branches can occur quickly when certain critical points are reached. In this
paper we describe a model to study the relation between the pressure at the
inlet and the volumetric magma flow rate in a conduit. We apply this model to
explain an hysteric jump observed during the dome growth at Soufri\`ere Hills
volcano (Montserrat), and described by Melnik and Sparks [1999] using a
different model.
