SOME PERMANENT STRUCTURAL CHANGES IN ROCKS DUE TO PRESSURE AND TEMPERATURE.

Temperture to 400 degree C and confining pressure to 10 kbar produced permanent changes in the pore structure of rocks, as observed with the scanning electron microscope for ion-thinned sections. Confining pressure by itself produced changes starting at a pressure which depended on porosity; for Katahdin granite, about 0. 5 kbar was required, for Westerly granite, 1 kbar. For diabase, no changes were detected to 5 kbar. For granite, pressure collapsed tiny bridges between nearby low aspect ratio cavities, and enlarged some pores. These effects suggest that certain crack-dependent properties of rocks may depend slightly on pressure history; some experience with electrical conductivity bears this out. Temperature by itself caused fairly extensive cracking, particularly along grain boundaries. 5 kbar pressure suppressed the cracking in granite that would normally occur at 400 degree , 1 bar pressure.