Stability of Remanent Magnetization of Rocks under Compression-Its Relation to the Grain Size of Rock-forming Ferromagnetic Minerals

Abstract

Stress stability of the remanent magnetization of basalt under compression was experimentally tested. It was also examined, using a thick lava of basalt, whether the effect of compressive stress on remanent magnetization depends upon grain size of rock-forming ferromagnetic minerals. Intensity of IRM of the rocks shows an irreversible and remarkable reduction under axial compression. TRM and NRM decrease gradually as axial force is increased. Change of remanent magnetizations with respect to pressure is well expressed in a hyperbolic form: M(P)=Mo/(1+/Br.P), in the same manner as that of the initial magnetic susceptibility of volcanic rocks. TRM is more stable than IRM against pressure.For IRM, the stronger the applied magnetic field, the more stable the intensity and direction of remanent magnetization become with respect to pressure. It was found that the magnetically effective grain size of ferromagnetic minerals controls the stress stability of NRM as well as the intensities of TRM, the coercive force and the initial magnetic susceptibility.

Intensity of the Earth's Magnetic Field in Geological Time

“…The uppermost ten meters of this lava is exposed on the surface. The magnetic properties of the rocks show a considerable change with depth from the top (Ohnaka, 1969). Samples were collected at different depths in the lava in order to investigate if the same results can be obtained from rocks which cooled at the same time and place but with different magnetic properties.…”

“…According to Ohnaka (1969), changes of magnetic properties such as x and H~ are observed at about 7 meters in this lava. However, it is not certain if these changes are related to the failure of intensity experiments.…”

Synfolding magnetization in the Jurassic Preuss Sandstone, Wyoming‐Idaho‐Utah Thrust Belt

“…Experimental acquisition of PRM has been conducted by applying deviatoric stress, for short time periods, sufficient to produce only elastic strain. Under these test conditions the proportion of a preexisting thermoremanent magnetization reset by stress was small (typically <20%, [Ohnaka and Kinoshita, 1968;Ohnaka, 1969]). However, little experimental evidence is available to assess the effects of long-term stress that might lead to permanent deformation of magnetite grains.…”

“…Stress-enhanced acquisition of IRM has been termed piezoremanent magnetization (PRM [Domen, 1962]). Whereas PRM acquisition has been modeled theoretically and produced experimentally [e.g., Domen, 1962; Ohnaka and Kinoshita, 1968;Nagata and Carleton, 1968;Ohnaka, 1969;Nagata, 1970], it has not commonly been recognized in paleomagnetic studies of natural rocks. This is not surprising because theoretical studies indicate that stress resets only the least stable portions of a preexisting magnetization [Kern, 1961], and therefore it may easily be mistaken for or overprinted by a VRM.…”