A computation of the average depth to the bottom of the Earth's magnetic crust, based on a statistical study of local magnetic anomalies

Abstract

It is recognized that the Earth's crust is composed of a sedimentary layer of from zero to perhaps ten miles in thickness, a magnetic granitic layer, and an underlying layer which is either non‐magnetic or is uniformly magnetic. The sedimentary layer may in general be considered non‐magnetic, since only relatively weak magnetic anomalies originate from this source. The lower layer may correspond to the basaltic layer and, from considerations of temperature and plasticity, this rock may be completely non‐magnetic or, on the other hand, since no reliable information is available, it is equally possible that it is homogeneously magnetic. The Curie point for basaltic material is about 475°C, according to Chevallier [see 1 of “References” at end of paper], and this temperature should be attained, according to Benfield [2], at an average depth of from 11.4 to 15.7 miles below the Earth's surface. On the other hand, gravity‐anomalies, apparently due to density‐contrasts at much greater depth, are not uncommon.