Intensity of the Earth's Magnetic Field in the Geological Past

Abstract

RECENTLY, Krs1,2 has obtained two estimates of the Permo-Carboniferous geomagnetic field intensity from cassiterite, and one estimate of the Permian field intensity from ignimbrite, in central Europe, using the method developed by Thellier and Thellier3. Both the Permo-Carboniferous values (0.50 oersteds and 0.60 oersteds at lat. 50.12° N., long. 12.80° E.; maximum error ±10 per cent) and the Permian value (0.50±0.03 oersteds at lat. 50.63° N., long. 16.30° E.) were similar to the present field intensity in the same region (0.482 oersteds), the implication being that the Earth's field intensity has not changed appreciably since the Permo-Carboniferous. Furthermore, because the cassiterite was obtained from a greisen body. Krs considered that its thermoremaneiit magnetization was acquired over a period long enough to average out secular variations in intensity. The quoted Permo-Carboniferous field intensity estimates were thus thought to represent the mean geomagnetic field rather than the instantaneous field corresponding to a certain point on the secular variation curve.