The variation of hardness with temperature was measured for olivine on a number of crystal faces by Vickers diamond pyramid technique (up to 800¿C) and by a mutual indentation technique (for temperatures up to 1500¿C). A comparative review of hardness data and compressive creep measurements obtained under large confining pressures confirms the hypothesis of Rice <1971> that single-crystal hardness measurements, corrected for elastic effects, can be correlated to the fully ductile yielding of a polycrystal by intragranular dislocation mechanisms, including dislocation climb and glide. The computed differential yield stresses, &sgr; (in gigapascals), which empirically correspond to a strain rate ? of 10-5 s-1, were well represented by an equation of the form &sgr;=9.1(¿0.3)-0.23(¿0.01)T 1/2, where T is the absolute temperature (in degrees Kelvin), and the quoted variances are for 1 standard deviation. The olivine data therefore predict a high-stress polycrystalline flow law that may be expressed as ?= 1.3¿1012exp-<60†103)/T> <1-(&sgr;/9.1)>2 where ? is the strain rate in s-1. A similar functional dependence of strain rate on stress is indicated for Al2O3 for temperatures below 900¿C but is contraindicated for MgO and NaCl. Using a semiempirical method of dislocation rosette analysis, the critical resolved shear stress on the {110} <001> slip system was estimated (to 20%) over the temperature range 20¿C to 780¿C as 1.2 GPa and 0.3 GPa, respectively. These data are useful in providing an upper bound to the yield stress in a region of stress and temperature space not easily accessible by other experimental methods. |