During the final year of the grant, we have investigated (1) why the strengths of rocks decrease with increasing temperature and in the presence of water through study of the fracture process in Westerly granite and Sioux quartzite specimens deformed in extension (some in true tension), (2) frictional strengths of rocks at high temperatures, (3) the stability of boreholes in fractured rock, and (4) slip in biotite single crystals (in that biotite is probably the weakest and most ductile of the common constituents of crystalline rocks.

This report summarizes the research performed to gain a fundamental understanding of the mechanical and transport properties of rocks under confining pressure and elevated temperature. There have been many contributions to our understanding of the mechanical behavior or rocks at high temperatures and pressures, but perhaps the three most outstanding contributions are the data which: (a) have helped to demonstrate the scientific feasibility of energy extraction from buried magma by assessing the likelihood of the rock mass to support stable boreholes at the pressures, temperatures (to partial melting), and aqueous conditions apt to occur in crystalline rocks above buried magma chambers; (b) have demonstrated that crystalline rocks deform primarily by brittle fracture when deformed at effective confining pressures to 200 MPa and temperatures to partial melting (to>1000°C), water-saturated or room-dry, and in constant strain rate tests (e dot = 10−4−1°sup -7//sec) or in creep tests; and (c) have shown that under these same conditions the time-dependent behavior of the rocks in the quasi-steady state regime is well described by the flow law: e dot = Asigma(superscript n)exp( -Q/RT) - a formulation previously thought to be applicable to rocks deforming primarily by crystal plasticity. This result suggests that fracture is also a time-dependent, thermally-activated process.