Geomagnetic reversals

The possibility that the geomagnetic field reverses polarity was first suggested during the early part of the 20th century, when it was noted that reversed magnetizations were present in some rock samples, and that the low amplitudes of magnetic anomalies observed over

Geomagnetic Reversal

Figure 4.2 Magnetic lineations either side of the mid-Atlantic ridge south of Iceland. Positive anomalies in black (after Heirtzler et al., 1966, in Deep Sea Research 13,428, with permission from Pergamon Press. Copyright Elsevier 1966).

Figure 4.2 Magnetic lineations either side of the mid-Atlantic ridge south of Iceland. Positive anomalies in black (after Heirtzler et al., 1966, in Deep Sea Research 13,428, with permission from Pergamon Press. Copyright Elsevier 1966).

certain volcanic sequences were explicable in terms of a reversed magnetization vector.

By the early 1960s the concept of geomagnetic field reversals was being revived, both because of the large number of paleomagnetic measurements revealing reversed magnetization, and the demonstration that self-reversal, whereby a reversed magnetization can originate from interaction with normally magnetized material, was a very rare phenomenon. By the mid 1960s, following the work of Cox et al. (1964, 1967) on lava flows erupted within the past few million years, the concept was widely accepted. More recently, paleomag-netic studies of rapidly deposited sediments, lava flow sequences, and slowly cooled igneous intrusions have shown that a magnetic reversal occurs over a time interval of about 5000 years. It is accompanied by a reduction in field intensity to about 25% of its normal value which commences some time before the reversal and continues for some time afterwards, with a total duration of about 10,000 years.

There is no general theory for the origin of the geomagnetic field. However, it is recognized that the main part originates within the Earth, and must be caused by dynamic processes. A magnetostatic origin appears impossible as no known material is sufficiently magnetic to give rise to the magnitude of the field observed at the surface, and subsurface temperatures would be well in excess of the Curie point, even given that its dependence upon pressure is largely unknown. The temporal variation of the internally generated field would also be inexplicable with such a model.

The geomagnetic field is believed to originate by magnetohydrodynamic processes within the fluid (outer) part of the Earth's core, magnetohydrodynam-ics being that branch of physics concerned with the interaction of fluid motions, electric currents, and magnetic fields. Indeed, this process is also believed to be responsible for the magnetic fields of other planets and certain stars. The process requires the celestial body to be rotating and to be partly or completely composed of a mobile fluid which is a good electrical conductor. The turbulent or convecting fluid constitutes a dynamo, because if it moves in a pre-existing magnetic field it generates an electric current which has a magnetic field associated with it. When the magnetic field is supplied solely by the electric currents, the dynamo is said to be "self-excited." Once "excited," the dynamo becomes self-perpetuating as long as there is a primary energy source to maintain the convection currents. The process is complex, and analytic solutions are only available for the very simplest configurations, which cannot approach the true configuration in the core. The field is thought to be maintained by convection in the outer core, which is thermally or gravitationally driven, either by heat sources in the core, such as potassium (40K), or the latent heat and light constituents released during solidification of the inner core due to the slow cooling of the Earth (Section 2.9) (Merrill et al., 1996).

A mathematical formulation of the geodynamo has not been possible because of the complexity of the physical processes occurring in the Earth's fluid core. Consequently theoreticians have had to resort to numerical modeling. Initially these simulations were severely limited by the computing power available for the very large number of numerical integrations involved. Large-scale, realistic simulations of the dynamo model had to await the advent of the so-called

Geomagnetic Reversal

Figure 4.3 Interpretation of a magnetic anomaly profile across the Juan de Fuca ridge, northeastern Pacific Ocean, in terms of normal and reversed magnetizations of two-dimensional rectangular blocks of oceanic layer 2. The arrow marks the ridge crest (redrawn from Bott, 1967, with permission Blackwell Publishing).

Figure 4.3 Interpretation of a magnetic anomaly profile across the Juan de Fuca ridge, northeastern Pacific Ocean, in terms of normal and reversed magnetizations of two-dimensional rectangular blocks of oceanic layer 2. The arrow marks the ridge crest (redrawn from Bott, 1967, with permission Blackwell Publishing).

supercomputers in the 1990s. The first results of numerical integrations of full three-dimensional, nonlinear, geodynamo models were published in 1995 (e.g. Glatz-maier & Roberts, 1995). These, and other comparable simulations through to 2000, were reviewed by Kono & Roberts (2002). The models simulate many of the features of the Earth's field, such as secular variation and a dominant axial dipole component, and in some cases magnetic reversals. Some of the latter are very similar in duration and characteristics to those deduced from paleomagnetic studies (Coe et al., 2000).

The rates at which geomagnetic reversals have occurred in the geologic past is highly variable (see Figs 4.4, 4.13). There has been a gradual increase in the rate of reversals during the Cenozoic, following a period during the Cretaceous when the field was of constant normal polarity for 35 Ma. Paleomagnetic studies reveal a similar prolonged period of reverse polarity in the Late Carboniferous and Permian (McElhinny & McFadden, 2000). This seems to imply that the geodynamo can exist in either of two states: one which generates a field of constant polarity for tens of millions of years, and one during which the field reverses in polarity at least once every million years. This is surprising in that convective overturn in the core is thought to be on a timescale of hundreds of years. It is difficult to imagine processes or conditions in the core that could account for two different states, which, once attained, persist for tens of millions of years. This timescale is characteristic of convection in the mantle. Changes in the pattern of convection in the mantle could produce changes in the physical conditions at the core-mantle boundary on the appropriate timescale. Small changes in seismic velocities in the mantle, revealed by seismic tomography, are interpreted in terms of temperature variations associated with convection, although they could in part be due to chemical inhomogeneity (Section 12.8.2). This raises the possibility that the heat flux at the core-mantle boundary is nonuniform, and changes significantly over periods of 10-100 Ma. The low viscosity and relatively rapid overturn in the outer core will ensure that the temperature at the core-mantle boundary is essentially uniform. The inferred temperature differences in the lower mantle, however, will give rise to a nonuniform distribution of heat flux at the core-mantle boundary. Anomalously cold material near the boundary will steepen the temperature gradient and increase the heat flow, whereas hotter material will decrease the gradient and heat flow. The new advances in computer simulations of the geodynamo make it possible to explore this possibility. The initial results of such computations (Glatzmaier et al., 1999) are very interesting and encouraging in that different heat flow distributions do produce significant changes in the reversal frequency and might well explain the variations observed in Fig. 4.4.

The results obtained from numerical simulations of the geodynamo since the mid 1990s represent remarkable breakthroughs in our modeling and understanding of the possible origin of the Earth's magnetic field. However one has to bear in mind that, although the physical formulation of these models is thought to be complete, the parameters assumed are not in the range appropriate for the Earth. This is because the computing power available is still not adequate to cope with the spatial and temporal resolution that would be required in the integrations.

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