The effect of stress cycling and inelastic volumetric strain on remanent magnetization

Abstract

The relation between remanent magnetization (natural and thermal), volumetric strain, and stress was determined for five rock types. Twenty different samples (fine‐grained intrusive, gabbro, basalt, tuff, and andesite) were repeatedly cycled to 95% of their fracture stresses at confining pressures of up to 1250 bars. At the completion of the first cycle there was always a permanent demagnetization of the rock. Subsequent stress cycles show a progressively smaller demagnetization, and the changes of remanence with loading become nearly reproducible by the fourth cycle. The onset of dilatancy produced changes in the rate of magnetization change. For two rock types the magnitude of the remanence increased up to the onset of dilatancy and then decreased as the stress was augmented. For these two rock types the remanent vector first rotated toward the axis of applied stress up to the onset of dilatancy; continued loading caused a rotation away from the applied stress axis. Typically, the changes in remanent intensity were of the order of 5%, which is about 3 times smaller than the changes in the individual components parallel and perpendicular to the axis of the greatest compressive stress. In fact, it was not uncommon for one component to increase while the other decreased. As a consequence, large rotations, up to 10°, of the remanent moment were observed, and at differential stresses in excess of half the compressive strength the remanent vector always rotated away from the axis of greatest compression. In addition, time dependent changes in remanent magnetization of several percent accompanied time dependent increases in crack porosity during constant stress creep tests at high differential stresses.Table 2 is available with entire article on microfiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D.C. 2006. Document J78‐003; $1.00. Payment must accompany order.