Periodically the Earth’s magnetic field flips, so that its direction reverses. The signals of magnetic field reversals occur in well-dated continental lavas, and this chronology is one of the keys to understanding the more continuous magnetic signature preserved in surveys running at right angles to the oceanic ridge systems. They presented to Earth scientists the now familiar patterns of magnetic ‘stripes’ of normal and reversed polarity running parallel to the ridges, which characterise oceanic lithosphere. The ‘stripes’ permit the dating of the ocean floor, which increases more or less systematically in both directions away from the ridges. That pointed unerringly to the formation of oceans by sea-floor spreading, and underpins the theory of plate tectonics. That is a fine example of deduction from, in many respects, fortuitous information of an empirical kind, and has kept Earth scientists extremely busy since Vine and Matthews twigged its significance in the 1960s.
Why these magnetic upheavals take place has proved a tough nut to crack. Not long after Earth scientists began to speak of little else, theoretical geophysicists proposed that somehow the Earth contained a self-sustaining dynamo prone to inverting its magnetic effects. The only conceivable source was the almost certainly iron-rich core, with an outer liquid shell and a solid inner core, proven by analysis of seismic waves travelling through the Earth’s central parts. Motion within the core moves electrons, thereby simulating current flow, and from Maxwell’s law there must be a related magnetic field that would shift as the motion changed. The liquid outer core is clearly the part that undergoes the most complex motion, partly as a consequence of rotation, and partly because of heat transfer. Ideas on the nature of that motion have developed over the last 3 decades, importantly through analysis of the drift of the magnetic field itself. The key feature however, is that the mantle, outer and inner core are mechanically decoupled, at least partly, by the outer core’s fluidity. Discovering how the solid inner core moves is clearly important for more realistic models of the self-exciting dynamo.
Vidale and co-workers (25 May 2000 issue ofNature, vol. 405, p 445) show how they re-analysed 30 year old records of seismic wave arrivals from Soviet nuclear tests to ‘image’ inner-core motion from the scattering of these signals – one of very few useful outcomes of the Cold War, and hopefully one that will never be repeated! Their results are not definitive, but suggest that the inner core rotates on a different axis from that of the Earth as a whole.