Magnetism of Pillow Basalts and Their Petrology

Abstract

The average intensity of magnetization of a layer of pillow basalt depends on (l) variations in the intensity of the earth's field as the lava is formed, (2) variations in the direction of magnetization within the layer, (3) the percent of nonmagnetic pore space between pillows, and (4) variations in the thermoremanence of the basalt due to variations in composition and in the degree of crystallization. To evaluate (4) we made a centimeter-by-centimeter examination of submarine pillow fragments and found that the remanence increases from almost zero in the glassy crust to high values of .04 emu/cc in the interior. The main control on remanence is the degree of crystallization. In very large pillows the remanence reaches a peak and decreases toward the center, the decrease apparently being due to an increase in grain size. The magnetization of the basalts resides in titanomagnetite grains possessing natural remanences in the range 1 to 2 emu/cc. The variation of natural remanence with grain size in the range 3 to 6 microns suggests pseudo-single domain behavior. Our best estimate of the average magnetization of a layer of submarine pillow basalt is .014 emu/cc. A layer less than 1 km thick with a value of remanence this high is adequate to account for most marine magnetic anomalies.