Geomagnetic Field, Polarity Reversals

Abstract

Bernard Brunhes (1906) was the first to measure magnetization directions in rocks that were approximately antiparallel to the present Earth’s field. Brunhes (1906) recorded magnetizations in baked sedimentary rocks that were aligned with reverse magnetization directions in overlying Miocene lavas from central France (Puy de Dome). In so doing, Brunhes (1906) made first use of a field test for primary thermal remanent magnetization (TRM) that is now referred to as the “baked contact” test. Matuyama (1929) was the first to attribute reverse magnetizations in (volcanic) rocks from Japan and China to reversal of geomagnetic polarity, and to differentiate mainly Pleistocene lavas from mainly Pliocene lavas based on the polarity of the magnetization. In this respect, Matuyama (1929) was the first person to use the sequence of geomagnetic reversals as a means of ordering rock sequences. The reality of geomagnetic reversals was then progressively established with the studies of Hospers (1951, 1953) in Iceland, and Roche (1950, 1951, 1956) in the Massif Central of France. The work of Hospers on Icelandic lavas was augmented by Rutten and Wensink (1960) and Wensink (1966) who subdivided Pliocene-Pleistocene lavas in Iceland into three polarity zones from young to old: N-R-N. Magnetic remanence measurements on basaltic lavas combined with K/Ar dating, pioneered by Cox et al. (1963) and McDougall and Tarling (1963a, b, 1964), resulted in the beginning of development of the modern geomagnetic polarity timescale (GPTS). These studies, and those that followed in the mid-1960s, established that rocks of the same age carry the same magnetization polarity, at least for the last few million years. The basalt sampling sites were scattered over the globe. Polarity zones were linked by their K/Ar ages, and were usually not in stratigraphic superposition. Doell and Dalrymple (1966) designated the long intervals of geomagnetic polarity of the last 5 Myrs as magnetic epochs, and named them after pioneers of geomagnetism (Brunhes, Matuyama, Gauss, and Gilbert). Then, the discovery of marine magnetic anomalies confirmed seafloor spreading (Vine and Matthews 1963), and the GPTS was extended to older times (Vine 1966; Heirtzler et al. 1968; Lowrie and Alvarez 1981). Since then, the succession of polarity intervals has been extensively studied and used to construct magnetostratigraphic timescales linking biostratigraphies, isotope stratigraphies, and absolute ages (see Opdyke and Channell 1996, “Magnetic stratigraphy”, for a review).