Reversals of the earth's magnetic field

One of the most intriguing problems in geophysics to-day is why the Earth's magnetic field reverses. This review summarizes the progress in our knowledge of the subject since Bullard's [1] Bakerian Lecture to the Royal Society in 1967. Reversals have played a major role in the changed outlook in geological thinking through the development of the concept of plate tectonics. Polarity transitions also provide the geologist with horizons which may be applied to stratigraphic correlations and dating problems. The mean frequency of reversals has also shown marked changes in the past and these may well be related to major tectonic changes. In spite of all these increased benefits to the geologist, the physics of the reversal mechanism is still not well understood. The detailed behaviour of the magnetic field during a polarity transition is described as well as the more recently observed phenomenon of excursions of the field or aborted reversals. Physical and mechanical models of the geodynamo are examined in some detail in so far as they may affect reversals. The role of convection in the Earth's core is also discussed.