Significant technical developments have been made toward the
measurement of rheological and related physical properties
under high pressures and temperatures. Meade and Jeanloz
(1988a,b, 1990) used a diamond anvil cell to measure the
yield strength of MgO and NaCl up to 40 GPa (1 GPa = 10
Pa) at room temperatures. Tingle et al. (1993) modified a
piston-cylinder type (the Griggs) apparatus to improve the
resolution of stress measurements and to increase the maximum
confining pressure to 5 GPa (see also Gleason and Tullis,
1993). Bussod et al. (1993) modified the multi-anvil
apparatus for deformation experiments under high pressures
(up to 16 GPa) and high temperatures (to 1800 K). Bussod et
al. (1993) used the size of dynamically recrystallized grains
to estimate the differential stress. Weidner et al. (1994)
developed a new technique for measuring differential stress
at high pressures and temperatures using in-situ X-ray
diffraction. Rubie et al. (1993) developed a specimen
assembly for a multi-anvil apparatus in which small
differential stress and controlled chemical environment are
realized and applied it to measure the pressure dependence of
dislocation recovery (see Karato et al., 1993). These
technical developments constitute an important component
toward our continuing effort to understand the rheological
properties of the earth's deep interior, but precise
measurement of differential stress under high pressures and
temperatures remains a challenging goal.