During uniaxial compression creep tests at 24°C, quartzite and granite specimens were subjected to stresses ranging from 73% to 95% of their uniaxial fracture strengths. Fracture times ranged from 10 to 1.6×106 s. Strains both parallel (εz) and perpendicular (εr) to the maximum stress direction were recorded continuously in order to compute volumetric strain. Tertiary creep is signified by a continuous increase of strain rate in either direction. The amount of time spent in tertiary creep prior to failure was different for the two rock types. Inelastic volumetric strain at the onset of tertiary creep was computed and found to be nearly constant, independent of stress level. The magnitude of inelastic strain at the onset of tertiary creep appears to be related to fracture strength and rock type. For quartzite the value is about 3×10−3, and for the granite it is about 1×10 −3. The hypothesis that the tertiary creep onset is dependent upon the induced dilatant volume formed by microcracking is examined in relation to critical crack density models of creep rupture.