RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES3003, doi:10.2205/2005ES000175, 2005

The Behavior of Virtual Geomagnetic Poles (VGP) During Reversals

[13]  The background of the often disordered, abrupt changes in the VGP positions during reversals often shows more or less regular changes in the VGP positions which are usually not only interpreted, but also described in different ways. The principal difference in describing VGPs during their reversals is expressed in the terms of "location'' and "displacement''. The term "displacement'' implies some systematic movement, such as, for instance, the rotation of the dipole field axis. The term "location'' suggests that there is no systematic displacement. What actually takes place is the breakdown of the dipole field and its reconstruction with some change in the direction of its axis. The different VGP position in this case reflects the different direction of the instantaneous (virtual) geomagnetic field, averaged over the time during which the study rocks had accumulated, or over the time of their magnetization, to be more exact, recalculated to the VGP coordinates using the central dipole formulas. No matter how these two versions are described in words or formulas, we deal with the propositions which are formulated roughly above.

[14]  In this connection, before we pass to analyzing the VGP positions during reversals, I would emphasize the following. The position of a virtual geomagnetic pole is usually located using central dipole formulas, and makes sense only in the case of this field. The researchers who deal with the study of geomagnetic reversals realize this fact. At the same time, the representation of the results in the VGP form allows one to identify the dipole character of the magnetic field at hand, the duration or absence of a dipole field, the differences between the inversions of the same age studied in the same region, and to perform many other operations.

[15]  During the early studies of geomagnetic reversals, the geophysicists who performed them traced the paths of virtual geomagnetic poles by way of connecting 2 or 3 data points by a line. Later, after new data were obtained, including those for the transitional zones described in detail, that is, containing a great number of virtual geomagnetic poles (VGP) between 60o N and 60o S, a view was advanced concerning the longitudinal sector of the VGP movement, suggesting that these movements have a loop-type pattern, propagating along meridional and latitudinal loops.

[16]  Views differ as to the location of the longitudinal sectors. Some authors suggested the presence or obvious predomination of one [Laj et al., 1991, 1992] or two sectors in the area of 120-180o E and, approximately, in the opposite sector of 300-360o E [Clement, 1991]. The reality of this predomination, that is, any association with the geomagnetic field, was doubted by Langereis et al. [1992]. Another group of authors supported the idea of the "near'' and "distant'' sectors. The near sector was supposed to include the coordinates of the sampling site, the remote sector was supposed to be opposite [Fuller et al., 1979; Hoffman, 1977]. The researchers of both groups offered convincing arguments to prove that they were right, which proved not only the erroneous character of one view (or of both of them), but also the absence of any distinct concept during the data interpretation. Both groups of the researchers offered convincing illustrations of their correctness, this suggesting not only the fallibility of one or both of their views, but also the difficulty of the problem and the absence of any unique concise concept in terms of data interpretation. In one of my previous studies [Gurarii, 1988] I checked the agreement between the VGP trajectory distributions during the Late Cenozoic inversions in a region bordering the equator and their uniform distribution in terms of the Kuiper criterion [Kuiper, 1960; Stephens, 1965] for 32 inversions and found a significant probability of their uniform distribution, that is, my results did not prove the presence of any "predominant sectors''. The mathematical processing of the experimental data, based on combining, for a certain inversion, the VGP trajectories (geomagnetic field trends), located at statistically insignificant distances from one another, into one group (as it is done in cluster analysis), revealed the following regularity: two types of processes, chaotic and quasistationary ones, operate during inversions, at least during some of them. In the case of chaotic conditions the typical duration of a trend change is < 100 years. In the case of quasistationary conditions, the characteristic time periods of which show main spectrum variations (archaeomagnetic data), the virtual geomagnetic poles (VGP) remain in one limited area ~(20o times 20o), the field trend varying insignificantly at the measurement site. The coordinates of these quasistationary regions vary, after the chaotic conditions, both in one hemisphere and between the two of them [Vadkovskii et al., 1980]. This coincides perfectly with the results obtained as a result of studying most of the transitional zones in igneous rocks. The study of the inversion recorded in the Stin Mountain lavas, USA, [Coe and Prevot, 1989] revealed a very rapid change in the direction of the geomagnetic field between its quasistationary states without any chaotic state of the field between them. A similar result was reported by Leonhardt et al. [2002] and other researchers, and was also discovered during the study of the Twera-Gilbert inversion in the sedimentary rocks of East Georgia, Caucasus [Gurarii and Kudasheva, 1995a].

[17]  The location of these quasistationary areas shows a kind of order. These areas are located usually in the longitudinal sectors which had been earlier assigned to the VGP trajectories. Petrova [1987] noted another regularity: the quasistationary regions surround global magnetic anomalies, being located at their slopes. Most of the quasistationary regions are located in the areas of the magnetic center projection on the Earth surface, this center, in turn, being located not far from the intersection of the third (equatorial) radius of the geoid. This could be associated with the asymmetry of the Earth core, namely, with its displacement which has long been discussed by geomagnetologists and gravity researchers, and is acknowledged now by seismologists. At the same time, proceeding from the conventional VGP position in the inversion state of the field, which has been mentioned above, this assumption should be treated with extreme care.

[18]  Hoffman [1992, 1993, 1996] supported the conclusion concerning the concentration of the largest number of virtual geomagnetic poles (VGP) during their inversions in the limited number of areas in the Earth ("patches''). Yet, he believes that these concentrations do not correlate with the global anomalies. On the other hand, B. M. Clement, who compared the data available for the VGP positions during the Matuyama-Brunhes inversion, which was studied in several different areas of the world ocean, and the data obtained in the course of studying several Early Pliocene reversals in remote territories (sedimentary rocks and lavas), emphasized their good agreement and restriction to the same longitudinal stripe, this suggesting, in his opinion, the significant role of a dipole during these reversals [Clement, 1991; Clement et al., 1998]. It should be noted that in his recent paper [Clement, 2004] he arrived at the opposite conclusion. The restriction of the VGP bulk to a certain longitudinal sector during some definite inversion, or during a few successive ones, is associated by some geoscientists [Constable, 1992; Gubbins, 1994] with the potential existence of some not axially symmetric parts of the geomagnetic field, which may not vary during some long period of time. The existence of such a field, which is comparable, in the simple case, with the field of some additional dipole, including the equatorial one, was recorded during some particular studies [Gurarii et al., 2000a; Rodionov et al., 1998]. Assuming that the field of such a dipole is preserved during inversions, the transitional virtual geomagnetic poles must be located in one longitudinal sector, or in two sectors, differing roughly by 180o, in the case of its inversion.

2005ES000175-fig02
Figure 2
[19]  The authors of the paper published by Gurarii et al. [2002] proved that the presence of such a nonaxial dipole was confirmed by the results of studying the rocks located in the direct vicinity of the Early Jaramillo transitional zone (West Turkmenia). They confirmed that nearly all of the main features of the field during this reversal could be explained by changes in the values and polarities of these dipoles (Figure 2). Of great interest is the fact that the results, coinciding in many respects with the results of our work in terms of the positions of two VGP crowdings, were obtained by Leonhardt et al. [2002]
2005ES000175-fig03
Figure 3
(Figure 3) as a result of studying a Middle Miocene inversion, using the magnetization of lavas. The data we obtained in our 2003-2004 studies suggest the existence of such an additional field in the territory of West Turkmenia throughout the Matuyama Chron (see the Table 1).

[20]  The assumed presence of an additional dipole of this kind suggests the following sequence of the field variations during the reversals: the magnetic moment of the main dipole, associated with the main system of convective movements in the core, declines to zero and then grows to its normal value again either in the opposite direction (inversion) or in the previous direction (unfinished inversion or excursion).

[21]  As the magnetic moment of the main dipole decreases at the Earth surface, an increasingly important role is played by the field of an additional dipole (or dipoles), the sources of which can be the rock material movements associated with the heterogeneities of the core-mantle boundary, or with those in the upper part of the core and in the lower mantle. The number of the additional dipoles and their dispositions and orientations control the distribution of the magnetic field elements at the Earth surface, as well as their variation from one inversion to another.

[22]  The use of additional sources provides a good explanation of differences in the magnetic field behavior during its excursions, studied at different sites of the ground surface, namely, from the field intensity decline, unaccompanied by any changes in the field direction, to a short time reversal. This model can be used to explain different relationships between the time periods, marked by a low magnetic field, and the periods marked by changes in the direction of the field in the course of studying one and the same reversal in different areas or studying reversals of different ages. This model can be used to explain a drastic change in the characteristics of the same reversal studied at the sites spaced less than a few hundred kilometers apart. The high efficiency of this model has been proved by the mathematical modeling of the field at the Earth surface using a central axial dipole and an additional differently oriented dipole located at the core-mantle boundary.

[23]  At the same time, changes in the field during the inversions of different ages, studied in the region discussed, changes in the duration of the inversions over long periods of time, changes in the characteristics of the inverted field from one region to another, as well as the character and scale of these variations, can be used as the indicators of the state and structure of a boundary between the core of the Earth and its lower mantle [Gurarii, 1988].

[24]  It is significant that Gubbins [1994] arrived at similar conclusions. It should be noted that a similar interpretation for the positions of the virtual geomagnetic poles (VGP) during the reversals was offered by Creer and Ispir [1970].


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Citation: Gurarii, G. Z. (2005), Geomagnetic field reversals: Main results and basic problems, Russ. J. Earth Sci., 7, ES3003, doi:10.2205/2005ES000175.

Copyright 2005 by the Russian Journal of Earth Sciences

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