Magnetic Anomalies over Oceanic Ridges

Abstract

Vine and Matthews¹ suggest that the pattern of local magnetic anomalies on the flanks of a mid-oceanic ridge is strongly lineated parallel to the ridge, and that these magnetic ‘stripes’ represent strips of material in the upper mantle the directions of permanent magnetization of which are alternately parallel and anti-parallel to the present local geomagnetic field. Vine and Matthews suggest that mantle material cools as it rises convectively under a ridge and then spreads² horizontally outward. As the material cools through its Curie point it is magnetized parallel to the contemporary local geomagnetic field. Because this field reverses quasi-periodically^3,4 with a period 2T, T being of the order of 0.5–1.0 million years, stripes of alternate permanent magnetization are produced the width of which is vT, v being the local horizontal velocity with which material at the surface of the mantle spreads away from the centre of the ridge. The stripes are observed¹ to have widths of the order of 20 km. If T is 0.5 million years, v is 4 cm/yr. Convective velocities of this order are also indicated by palæomagnetic data⁵.