Rheology of Basalt in the Melting Range

Experimental data have been obtained for viscosities of tholeiitemelts at temperatures from 1300 to 1120 °Cat 1 atm, usinga concentric cylinder viscometer. The apparent viscosity increasesmore than two orders of magnitude between 1200 and 1120 °C(0–25 per cent crystallization) for shear rates of about10 sec^-1 and even more for lower shear rates. Non-Newtonianbehaviour of ‘pseudo-plastic type’ becomes extremelypronounced at temperatures below about 1130 °C. At thesetemperatures, differences of less than 5 °C can producechanges in apparent viscosity amounting to orders of magnitude.These observations have led to the conclusion that the heatof deformation must itself influence rheological behaviour inthe melting range. An equation for thermal energy balances andtheir rates of change is constructed and placed in a non-dimensionalform that has been given published solutions by I. J. Gruntfest(1963) relating the shear stress, rate of strain, and temperaturethrough the temperature dependence of viscosity. The resultsshow that in an adiabatic system the heating rate increaseswith time so that the temperature eventually runs out of bounds,a process termed ‘thermal feedback’ by Gruntfest.A hypothesis of shear melting is derived on the basis of a simplifiedviscosity function extrapolated to the solidus temperature.The hypothesis is applied to magma generation in the earth onthe basis of dimensional arguments. It is also suggested thatthermal instabilities give rise to a sort of viscous failureresponsible for deep-focus earthquakes, and that the two phenomenahave the same cause relating ultimately to a gravitational energysource.