The generation of gas overpressure in volcanic eruptions

Observations of natural eruption products show that different parts of a si ngle magma batch may experience different degassing histories during ascent in a volcanic conduit. In non-explosive eruptions, lava issuing from a vol canic vent may contain overpressured gas bubbles. These important features of volcanic eruptions cannot be accounted for by existing flow models, whic h rely on simplifying hypotheses for the relationship between pressures in the gas phase and in the bulk flow. Volcanic flows involve highly compressi ble material which undergoes large viscosity variations as degassing procee ds. We show that these properties may lead to large gas overpressures in er upting lava. The magnitude of this overpressure depends on the initial vola tile content of magma and is largest for relatively volatile-poor magmas, d ue to the extreme viscosity variations at water contents less than 1 wt%. W e develop a simple analytical model to illustrate the main features of comp ressible viscous flows: (1) at any level, gas pressure is larger near the c onduit axis than at the walls, (2) gas overpressure is an increasing functi on of mass discharge rate. (C) 1999 Elsevier Science B.V. All rights reserv ed.