Magnetic Reversals

Geophysics: Geomagnetic Reversals and the Earth's Mantle
Science Week, December 10, 1999

The earliest demonstration that the geomagnetic field of the Earth changed polarity in the past was provided by P. David and R. Brunhes, who in 1904-1906 described the magnetic properties of young lava flows in the Massif Central region of France. They found that clays baked by the lava flows had the same direction of remanent magnetization as the lavas, and that when the magnetization direction in the lava was opposite to that of the present-day field, the same was the case in the baked clay. They interpreted the opposite polarities as evidence that the geomagnetic field can reverse its polarity. M. Matuyama (1929) was the first to associate the polarity of remanent magnetization in lavas with their age as determined stratigraphically. Matuyama reported finding young Quaternary lavas with magnetization directions close to the present-day field direction, whereas the directions of older Quaternary and Pleistocene lavas were clustered about an antipodal direction. He also found that one of three samples of Miocene basalt was magnetized oppositely to the other two. Matuyama's interpretation was that geomagnetic polarity had changed several times during the Late Tertiary time-frame.

Although generally accepted today, the idea that Earth's geomagnetic polarity could change was controversial in the early part of this century, and for many years skeptics sought alternative interpretations of the data. Alternative explanations, however, have not been successful, and the phenomenon is now considered real and is studied as a special branch of geophysics. Geomagnetic polarity reversal is an inversion of the geomagnetic dipole. It is a global event, experienced simultaneously all over the Earth, and such reversals, apart from their intrinsic interest, provide a convenient means of stratigraphic correlations and stratigraphic dating. The paleomagnetic record indicates that the dipolar part of the Earth's magnetic field, which is the dominant structure of the geomagnetic field outside the core, has reversed its polarity several hundred times during the past 160 million years. The reversal durations (i.e., the periods during which the reversals are accomplished) are relatively short (typically 1000 to 6000 years), compared with the constant polarity intervals between reversals. Another feature of the reversal period is that the intensity of the magnetic field apparently decreases significantly during this time-frame.

Unlike the nearly constant periods of the solar magnetic cycle, geomagnetic polarity intervals evidently vary from a few tens of thousands of years to "superchrons" of the order of tens of millions of years. The duration of a superchron is roughly the timescale required for significant changes in the thermal structure of the Earth's mantle to occur as a result of subduction of tectonic plates and mantle convection, and this observation and some noted correlations between plate tectonics, geomagnetic field intensity, and reversal frequency have led to speculations that structural changes in the mantle may be influencing convection and magnetic field generation in the fluid outer core (the "geodynamo"). In particular, it has been suggested that changes in both the total heat flow and the pattern of heat flux over the core-mantle boundary may affect the geodynamo.