Geophysical and geological implications of the two types of fracture

It has been suggested in the previous chapter that when the compressive strength is equal to the frictional strength, low porosity dry silicate rocks do not always become ductile, as they would do if they obeyed the friction hypothesis for brittle-ductile transition ( Section 3.1.2 ), but remain in the brittle regime accompanied by a change in fracture mechanism. Such conditions appear at ~ 0.8 GPa confining pressure for Man-nari granite and MurotomisakI gabbro, at ~ 1.0 GPa for Horoman dunite, and at ~ 2.0 GPa for Akaishi eclogite at room temperature. Stesky et al. (1974 ) have suggested that similar conditions appear at 400 MPa and 600°C for Westerly granite, and at 400 MPa and 400°C for San Marcos gabbro.