Rheology of bubble-bearing magmas

The physical effects of air or argon bubbles on the rheology of a calcium aluminosilicate melt have been measured at temperatures ranging from 830 degrees to 960 degrees C, at 1 bar pressure. The melt composition is SiO2:64, Al2O3:23, and CaO:13 (wt%), while bubble volume fractions are: 0, 0.06, 0.13, 0.32, 0.41 and 0.47. Measured Newtonian Viscosities range from 10(10) to 10(14) dPa s. Melts with bubble fractions of 0.06 and 0.13 show with increasing temperature (T) an increasing relative viscosity for T < 850 degrees C. However at T > 850 degrees C, for all bubble fractions the viscosity decreases markedly with temperature. The observed maximum decrease of the relative viscosity is 75% for a bubble fraction of 0.47 at 907 degrees C. At all bubble fractions the viscosity is independent of the applied stress, which ranged from 11 to 677 bars. No clear indications were observed of non-Newtonian rheological behavior. Under our experimental conditions the relative viscosity of the two phase liquid depends primarily on the bubble fraction. Physical and volcanological implications of these measurements are discussed. (C) 1999 Published by Elsevier Science B.V. All rights reserved.