Magnetization of unmetamorphosed varves and marine sediments

Abstract

Introduction—Knowledge concerning the range of its variation is essential to an understanding of the Earth's general magnetic field. Clearly, if, as some claim, the Earth's magnetic field has been completely reversed in past geologic ages our notions of its origin, and perhaps our entire conception of the Earth's interior, must be radically revised. On the other hand, if it can be shown that the Earth's general magnetic field is a rather static phenomenon and that the observed secular changes involve only a superficial portion of the field other types of causes are suggested.Direct observations of secular variation extend over four centuries at a few places on the surface of the Earth. Although some of the changes have been very large they suggest cyclic variations rather than continuous variations in one direction. Some of the more remarkable of these changes are a shift in declination from 11° east to over 24° west at London from 1600 to 1800, a decrease of 5900 gammas in horizontal intensity at Capetown from 1843 to the present time, and a decrease of 6000 gammas in vertical intensity off the Guinea Coast from 1885 to 1922. If changes as great as these have occurred in modern historic times how great have the changes been in thousands and millions of years of prehistory?