The Permanent Magnetism of the Mull Lavas

Abstract

An analysis of the natural magnetic polarization of the basalt flows of Mull shows that many are adversely magnetized but the mean direction of magnetization of each flow, based on observations of a number of specimens, is not always significant. For such flows measurements of the magnetic stability, response to heat treatment, etc., suggest that the original intensities have been modified by later events. Finally, the observed natural intensities could be acquired by cooling in a field similar in magnitude to that of the Earth's present field.Introduction.-The detailed history of the Earth's magnetic field is only known from observations of the secular variation over a few centuries. Recently efforts have been made to extend this limited knowledge by a study of the natural residual magnetism of both sedimentary and igneous rocks. Thus the study of glacial varves (I) has suggested that the Earth's field has not changed significantly during very recent geological times. Similarly with igneous rocks, the examination of modern lavas has shown that the rocks acquire a residual magnetization in a direction corresponding to that of the Earth's field in which they cool (z), and observations on Quaternary Lavas (3) again indicate little change in the general direction of the Earth's field over the last million years, any small variation in direction comparing with the secular changes which have been observed directly. Many older igneous rocks, and also some older sedimentary rocks, show remarkable directions of magnetization, differing widely from the direction of the present field, and in most cases the direction of polarization is nearly opposite to that of the Earth's present field. Thus, a magnetic survey over the Pilandsberg Dyke System of South Africa (4), which is of age about zoo x 1o6 years, has yielded magnetic anomalies compatible with an adverse magnetization of the whole system. In many other places, e.g. Germany, Iceland, Japan, New Zealand, etc., other igneous rocks show similar characteristics. In particular, the suite of tholeiite dykes of the North of England (5) has been examined at a number of points over an area of some 200 miles by 80 miles. Here the investigations consisted of a determination of the magnetic anomaly in the vertical component of the Earth's field produced by the dykes at a number of sites, and at a selected number of sites oriented rock specimens were collected and their magnetic properties determined in the laboratory. These observations demonstrated that in general the whole of the system possessed adverse magnetization. It is significant here that the magnetic anomaly, predicted on the basis of the properties of the collected specimens, agreed closely with that observed, so that there could be no doubt of the representative nature of the specimens.

Rock magnetism: A new approach to some palaeogeographic problems

“…The mean field direction approximates to that which would be produced by a geocentric axial dipole whose axis is along the axis of spin. Further back in time Hospers (1955) has shown that from his own data and that of Roche (1951), Bruckshaw and Robertson (1949), Bruckshaw and Vincenz (1954) and Bancroft (1951), that this result holds for Miocene, Pliocene and Pleistocene and Recent rocks of Europe and Iceland. Hospers (1955);6, Oligocene;7, Eocene ;8, Triassic ;9, Permian ;10, ll, Carboniferous ;12, Devonian;13, Cambrian, 14, Longmyndian;15, Upper Torridonian;16, Lower Torridonian.…”

A paleomagnetic study of the Mull lava succession

“…This volcanic province was one of the first studied and is described in a series of classic papers and memoirs published during the late nineteenth and early twentieth centuries. The province also provided materials for a number of the early experiments in paleomagnetism (Bruckshaw & Robertson 1949;Bruckshaw & Vincenz 1954;Vincenz 1954;Leng 1955). Some of the important results from this re-examination are the presence of many groups of volcanic units with closely similar directions, representing either a high frequency of volcanic activity or periods of low secular variation, and the widespread occurrence of volcanic units with southerly directed, shallow remanence inclinations.…”

Analysis of Present Geomagnetic Field for Comparison with Paleomagnetic Results

“…( Fisher 1953;Bruckshaw & Vincenz 1954;Wilson 1959). For example, Wilson (1959) In more common use is the precision parameter k', an inverse measure of the dispersion, defined (Fisher 1953) In applying these methods of paleomagnetic analysis to the earth's present field, c hanges in field direction due to nondipole irregularities will be considered separately from those due to changes in orientation of the main inclined dipole.…”