The Reversal of the Earth's Magnetic Field

Sigma Xi, The Scientific Research Society

The Reversal of the Earth's Magnetic FieldAuthor(s): Mike Fuller, Carlo Laj and Emilio Herrero-BerveraSource: American Scientist, Vol. 84, No. 6 (NOVEMBER-DECEMBER 1996), pp. 552-561Published by: Sigma Xi, The Scientific Research SocietyStable URL: http://www.jstor.org/stable/29775784 .

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The Reversal of the Earths Magnetic Field

Observations of transition fields may provide a better understanding of the

operation of the geodynamo and of the fluid motion in the outer core

Mike Fuller, Carlo Laj and Emilio Herrero-Bervera

Ninety years ago, Bernard Brunhes

reported the discovery of rocks from a roadcut in the Massif Central of France that were magnetized in exactly the opposite direction to the local geo?

magnetic field. Brunhes was attracted to these particular rocks?baked clays over which molten lava had flowed? because he knew from the work of

Guiseppe Folgerhaiter that ancient pot? tery carried a particularly strong and stable magnetization. Brunhes reasoned

Mike Fuller was educated at Christ's Hospital and

Gonville and Caius College, Cambridge. He came to

the United States in 1961 on a postdoctoral fellowship at Scripps Institution of Oceanography at La folia,

California. Subsequently, he worked at the Gulf Oil

Company Research Laboratory at Pittsburgh, tlte

University of Pittsburgh and the University of

California at Santa Barbara. He will shortly join the

third author at the University of Hawaii. His princi?

pal research interest has been the magnetism of rocks

and the paleomagnetic record of the magnetic fields of the earth and the moon. He has also worked on the

paleomagnetism and tectonics of Southeast Asia and

on biomagnetism. Carlo Laj is joint director of the

Centre des Faibles Radioactivites, Laboratoire mixte

CNRS-CEA, and director of the Laboratoire de

Modelisation du Climat et de l'Environnement of the

French Atomic Energy Commission. He received his

B.A. in physics and chemistry, his M.S. and his Ph.D.

in solid-state physics, all from the University of Paris.

His fields of interest, in addition to reversals of the geo?

magnetic field, are the intensity of the geomagnetic

field, environmental magnetism and radiometric dat?

ing. Emilio Herrera-Bervera is a research professor of

geophysics at the University of Hawaii at Manoa

within the School of Ocean Earth Sciences and

Technology at the Hawaii Institute of Geophysics and

Planetology. He recdved his Ph.D. from the

Department of Geology and Geophysics at the

University of Hawaii. His interests include the paleo?

magnetic record of the earth's magnetic field and also

the study of the anisotropy of magnetic susceptibility of igneous and sedimentary rocks. Address for Fuller:

Department of Geological Sciences, University of

California, Santa Barbara, CA 93106. Internet:

[email protected].

that because the clay had been fired by the lava, like pottery, it should retain an excellent record of the magnetic field. At one site, he chiseled two samples out of the flow. He found that they also had reversed magnetic polarity.

The reversed magnetization of the rocks convinced Brunhes that the earth's magnetic field had been re? versed when the lava erupted a few million years ago. Had modern com?

passes been present at the time, they would have pointed to the south rather than to the north, as they do now.

It took more than 50 years, including a period of intense debate in the late 1950s and early 1960s, for Brunhes's

suggestion of field reversals to gain gen? eral acceptance. Part of the problem was that in the late 1950s, Seiya Uyeda and Takesi Nagata from Tokyo University demonstrated that certain rocks, when cooled in a magnetic field, acquired a

magnetization just opposite to that field. For some this explained the reversely magnetized rocks. However, by skillful

age determination of young volcanic rocks whose polarity they determined, Allan Cox, Richard Doell and Brent Dal

rymple in the United States and Don

Tarling and Ian McDougall in Australia demonstrated that the polarity of these flows defines a simple age pattern. All rocks younger than a certain age, which

we now know to be approximately 780,000 years, are normally magnetized, whereas the older rocks fall into alter?

nating periods of reversed and normal

polarity. This was convincing evidence that reversals are the manifestation of

geomagnetic field phenomena and not

simply a bizarre tendency of some rocks to become magnetized in the opposite direction to the field in which they were formed. Meanwhile, Christopher Har? rison and Brian Funnell, at Scripps In

stitution of Oceanography in La Jolla, California, had discovered reversals in Pacific Ocean sediment cores, and their work was soon extended by John Fos?

ter, Neil Opdyke and others at the Lam ont Geological Observatory (now Lam

ont-Doherty Earth Observatory) of Columbia University.

Geomagnetic field reversals played a

key role in the revolution that trans? formed the geological sciences in the 1960s. Fred Vine and Drum Matthews from Cambridge University, and inde?

pendently the Canadians Lawrence

Morley and Anare Larochelle, com? bined Harry Hess's idea of seafloor

spreading with the geomagnetic anom? alies that had been found over the oceans. They suggested that when new crust is formed at a mid-ocean ridge, it becomes magnetized and records the

normal, or reversed, polarity of the am? bient geomagnetic field. In effect, the seafloor acts as a giant magnetic tape recorder. So, in a curious symbiosis, the reversals demonstrated the truth of seafloor spreading, and the paleomag netism of the sea floor finally settled

any lingering doubts about reversals. The record from the seafloor gave the

history of reversals back to the age of the oldest ocean floor, which turned out to be about 150 million years.

Studies of reversals then took off

along two different paths. One, led by Allan Cox of Stanford University until his premature death, was concerned with the recurrence intervals of rever? sals. The early work showed that the

lengths of the polarity intervals fol? lowed Poisson statistics, the statistics used to describe rare and independent events, so that the field evidently had no important memory of previous events. The present rate is between one and two per million years, but the rate

552 American Scientist, Volume 84



Figure 1. Cooling lava can preserve a record of the earth's magnetic field as it passes below the Curie temperature and the magnetization of the magnetic grains aligns with the field lines. By studying such lavas, shown here on the island of Hawaii, as well as slower-cooling igneous rocks and sediments, geophysicists have constructed a history of the earth's magnetic field. Prominent in this record are numerous reversals of the

geomagnetic poles, the last of which took place 780,000 years ago. Although the reversal record is relatively well established, much less is known about what happens during a geomagnetic pole reversal. The authors review the implications of data about such transitions, including the possibility that the poles may follow preferred paths during reversals. (Photograph courtesy of Gordon W. Tribble.)

has varied. For example, it appears that from roughly 120 million to 80 million

years before present, in the Cretaceous

period, reversals stopped, leaving the field in normal polarity. And for 50 mil? lion years in the Permian, about 300 mil? lion years ago, it was reversed. As Ron

Merrill of Washington University and Phil McFadden of the Bureau of Miner? al Resources in Canberra, Australia,

pointed out, however, there is nothing special about the Cretaceous normal state or the Permian reversed state. Rather, the reversal rate slowed until it

finally stopped, by chance leaving the field in one or the other polarity. Nu

merous suggestions have been made to account for changes in the reversal rate, most of which invoke changes in the conditions at the core-mantle boundary or the outer core-inner core boundary to change the vigor of the convection in the liquid-iron outer core.

The second path, which is the princi? pal concern of this article, has been the

attempt to find out what happens to the

geomagnetic field during a field rever? sal. In the mid-1960s the first paleo?

magnetic record of a field reversal was described by J. S. V. Van Zijl, K. W. T. Graham and Anton Hales from a se?

quence of lavas in Africa. The reversal

has recently been shown by Michel Prevot of the Unversite de Montpellier, France, to have taken place roughly 180 million years ago, and the intermedi? ate, or transitional, fields were recorded

by the magnetization of lavas erupted as the field changed polarity. During the reversal the field intensity de?

creased, and the field direction fluctuat? ed anomalously. Numerous records of reversals have now been obtained, and the main features of the process have been established.

Recently, a surprising aspect of the field behavior during reversals has

emerged. It appears as though the field

1996 November-December 553



^^R^^^^Hfet> mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm l^H^^^B^^Btat^

" mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm

Figure 2. Earth's interior can be roughly divided into four regions: the crust (black), the man?

tle (reddish-brown), the liquid-iron outer core (orange) and the solid-iron inner core (yellow). The geomagnetic field is generated in the liquid-iron outer core. (Adapted from a publica? tion of the American Geophysical Union.)

may maintain a relatively simple geom? etry during the reversal, so that the cen? ters of inward and outward radial field lines seem to be preferentially found in the longitude band over the Americas and the antipodal band over eastern Asia and Australia.

The new results have revitalized studies of reversals and generated a vig? orous debate on three issues. First, how robust are the statistical analyses sug? gesting the preferred paths? Second, how reliable are the records?could

they be artifacts of the magnetization process or of the uneven distribution of sites? Third, what precisely do they tell us about the core and the mantle? As has been very clear at recent meetings, we are still in the middle of the debate. Readers should therefore be aware that in parts of this article we shall be de

scribing an ongoing debate that is far from neatly and tidily settled. If s a sto?

ry not quite ready for the textbooks.

The Earth's Magnetic Field Reversals are part of the bigger puzzle of the earth's magnetic field. As a re? sult of the work of theoreticians at mid

century, led by Walter Elsasser and Sir Edward Bullard, it became clear that the geomagnetic field is maintained by processes going on in the electrically conducting fluid of the outer core. The

general principles of such regeneration of magnetic fields are now well estab?

lished, and they are evidently a com? mon phenomenon in the universe. Two

processes are involved.

The first process is the creation of new magnetic fields from the ambient

geomagnetic field by the motion of the

fluid in the outer core. This is admit?

tedly a little mysterious and comes about because magnetic field lines are

trapped in electrical conductors, such as the fluid of the outer core. Although this trapping of magnetic lines has im?

portant technical applications, we are not familiar with it in everyday life be? cause the atmosphere in which we live is a poor electrical conductor. The ef? fect follows from Michael Faraday's celebrated law of electromagnetic in?

duction, which tells us that when a

magnetic field changes, an electromo? tive force is set up, giving a current to

oppose that field change. Hence the movement of magnetic field lines with

respect to the molten iron of the core is inhibited by the current induced in this

highly conducting medium. The field line is trapped in the fluid?the frozen field effect. The magnetic field is there? fore carried along with the fluid as it

moves in response to the forces im?

posed on it. In so doing, the field lines are stretched and twisted, and a new

magnetic field is created. Two cases of

special interest are illustrated in Figure 3. They are the a and the co effects. The latter describes the stretching of lines of force into a toroidal shape as they penetrate the core, which would be caused by an increase in angular rota? tion. The former describes the lifting and twisting of the toroidal field lines

by cyclonic convection, which could be driven by thermal or compositional buoyancy.

The second process involved in re?

generating magnetic fields is the diffu? sion of the fields. It is at least superfi? cially less puzzling than the first. Just as a drop of colored dye in a swim?

ming pool soon diffuses throughout the pool, so a concentration of magnet? ic field lines diffuses throughout the outer core. Yet this diffusion must take

place against the frozen field effect. It is the balance between these two compet? ing processes that determines the time

dependence of the magnetic field? whether the field decays away or is re?

generated. On the larger scale of astro

physical or planetary bodies, the field lines are caught up in the fluid motion and distorted and generate new mag? netic field before they diffuse away. In the earth's core, the natural decay time of the magnetic field appears to be about 15,000 years.

It is clear that some form of dynamo, or generator in the familiar American

usage, operates in the fluid outer core




of the earth. One such scheme is illus? trated in Figure 3. As is often the case in

geology and geophysics, however, it is easier to develop models explaining na? ture than it is to demonstrate that a par? ticular model has anything to do with

what happens in nature. The actual re?

generative processes that maintain the

geomagnetic field are obscure, and, un? less some observation can be made that

distinguishes between the many possi? ble models, the details may, as Walter Elsasser once noted, elude us forever.

Much of the interest in reversals arises because, as a dramatic aspect of geo? magnetic field behavior, it is tempting to think that they might provide some ob? servation that could be a key feature of the dynamo process. There is no diffi?

culty in fitting reversals into the frame? work of the geomagnetic dynamo?the fundamental equations governing the

phenomenon are insensitive to the sign of the field. Moreover, numerical models of the field-generation process and even electromechanical devices that simulate the field both exhibit field reversals. Re? versals are thus probably internally dri? ven, as they are in the solar dynamo, but the geomagnetic dynamo is different from the solar dynamo in that it does not reverse so regularly or so frequently

Various reversal models have ap? peared for different dynamos. One such model developed for the scheme illus? trated in Figure 3 depends on the

buildup of reverse flux by precipitation of cyclonic action within restricted lati? tude bands, which give rise preferen? tially to opposed flux. These studies took a giant leap forward recently when a full magnetohydrodynamic (MHD) solution for the core was presented that exhibited stability over some 40,000 years, and then executed excursions, a

field-intensity decrease and a reversal. This remarkable achievement by Gary Glatzmaier of Los Alamos National

Laboratory and Paul Roberts of the Uni?

versity of California at Los Angeles re?

quired a solution of the nonlinear MHD

equations for a thermally convecting fluid in a rotating shell. The reversal is illustrated in Figure 4. To see what actu?

ally happens during a reversal and to

compare these results with the models and simulations, we now turn to paleo

magnetism.

The Magnetization of Rocks The paleomagnetic record of the geo? magnetic field carried by rocks has made fundamental contributions to the

/ nonuniform rotation

?

r

cyclonic convection

Figure 3. Dynamo processes proposed by Eugene Parker of the University of Chicago and

Eugene Levy at the University of Arizona at Tucson depend on fluid flow of the outer core to

act on a dipole field. In the first stage (upper right), faster angular rotation deep in the outer core shears dipole field lines to give a toroidal field. In the second stage (lower right), the toroidal lines are uplifted by convection of the liquid iron and twisted by Coriolis effect.

Finally, diffusion of the field gives its initial geometry.

earth sciences. Without it, we might still be arguing about whether the con? tinents drift. Yet the presence of this record is somewhat fortuitous, requir? ing magnetic mineral grains of just the

right grain size to be good magnetic recorders.

The geomagnetic field is weak in the sense that it is small compared with the

magnetic field required to set, or switch, the magnetization of particles such as those that carry the paleomagnetic record. This ensures that subsequent changes in the field (after the initial

magnetization of the rock) will not af? fect that magnetization. There is, how? ever, a paradox: How can the earth's weak field initially set the magnetiza? tion of the particles?

The magnetization of many sedi? ments, such as those laid down on the ocean floor, is readily explained by the

alignment of the detrital magnetic parti? cles in the geomagnetic field. This pref? erential alignment in the sediment is locked in as water is lost. The details of the process, however, are less clear. For

example, the depth at which the magne? tization is locked in, the degree to which

the record averages the field values and the lower limits of the field that can be recorded all remain poorly known.

Igneous rocks, whether they cool on the surface as lavas or beneath the sur? face as intrusions, acquire their magne? tization as they cool. In all but some rare

examples the rock solidifies before the

magnetization is acquired. As it contin? ues to cool after solidification, the rock

passes through the Curie points of its

magnetic minerals, and they become

magnetic. The stable magnetization of the fine particles is acquired a little be? low the Curie temperature in a manner first explained by the French Nobel lau? reate Louis Neel. He showed that at

temperatures immediately below the Curie point, thermal energy dorninates. At these temperatures, although the

magnetization exhibits a statistical bias in the direction of any field present, there is no stable remanent magnetiza? tion when the field is removed. He then demonstrated that the relaxation time of the magnetization increases with

falling temperatures, so that the statisti? cal bias in the field direction eventually becomes fixed, or blocked, as a stable




Figure 4. Intermediate state of the field predicted by the Glatzmaier-Roberts model, the first

full magnetohydrodynamic solution for the earth's core. Yellow flux lines are directed out?

ward from conventional positive regions, and blue lines are directed inward toward negative

regions. The field is shown to be more complicated than a dipole field, with its single center

of outward-directed flux at the geographic south pole and antipodal inward-directed flux at

the north pole. The motion of the positive and negative flux bundles as they are carried

along by core flow is, of course, independent of our sign convention for flux lines.

(Reproduced courtesy of Gary Glatzmaier.)

lished important aspects of the reversal

process. But it gave little idea of how

long the reversal took, because the de? tails of the history of the volcanic ac?

tivity were unknown. It was a record from ocean sediments studied by B. L. K. Somayajulu and Christopher Harri? son that first demonstrated that the

change in direction of the field took a few thousand years. The sedimenta? tion rate was estimated from the sedi?

ment thickness between known time markers, so that the time represented by the sediments carrying the reversal could be calculated as a simple ratio.

Following a suggestion from Haruaki Ito from Shimane University in Japan, one of us (Fuller), Vic Schmidt, Rich Dodson and Bob Dunn at the Universi?

ty of Pittsburgh obtained records from intrusions. These records indicated that the decrease in intensity took longer than the directional change, that dur?

ing the reversal the field continued to show patterns of change similar to, but

stronger than, its usual secular varia? tion, and that the reversal was accom?

plished by changes in the field of a

stop-and-go nature.

The late Norman Watkins had mean? while studied an impressive lava pile at Steens Mountain in Oregon. This was later resampled by Sherm Gromme and Ed Mankinnen from the U.S. Geo?

logical Survey, Rob Coe from the Uni?

versity of California at Santa Cruz and Michel Prevot. This group's work

yielded for the first time a convincing field intensity record during a reversal, as well as details of the directional

changes. The results suggested that the

field-intensity decrease was more syn? chronous with the directional change than the intrusion results had indicated but that very strong secular variation? like changes persisted. The work also demonstrated some astordshingly rapid variations in the field, a result that re? mains controversial.

Reversal records from oceanic sedi? ment cores were being studied at Lam

ont-Doherty by Neil Opdyke, Bill Lowrie, Dennis Kent and Brad Clement, and at the Hawaii Institute of Geo?

physics by one of us (Herrero-Bervera) and his colleagues. The Ocean Drilling Program sediment cores provide a nat? ural source of reversal records. Unfor?

tunately, the sedimentation rates at

many sites are so low that the resolu? tion is poor. Nevertheless, the careful

work of Brad Clement has generated multiple records of several of the last

remanent magnetization that can sur? vive over the geological eons.

Paleomagnetic records from lavas do not smooth field behavior, as do sedi? ments, because the cooling time of lavas and hence the time during which their

magnetization is acquired is for the most

part short compared with the field

changes of interest. Single lavas, howev? er, sometimes exhibit multiple directions that are hard to interpret. The paleo

magnetism of intrusions, which cool more slowly beneath the earth's surface, records the passage of the Neel blocking temperature into the body and so com? bines the more reliable magnetization of an igneous rock with the continuous record of the field changes of a sedimen?

tary record. Petrological complexities in the long cooling histories of intrusions, however, can produce magnetizations that are tricky to interpret.

Paleomagnetic records of transitional fields are hard to obtain and test our

techniques to the limit. The geomagnetic field has spent most of the few billions of

years of earth history in a stable normal or reversed state and relatively little time in the process of reversing. To get a re? versal record, one must therefore first find rocks whose magnetization was ac?

quired during these rare times. More? over, to get a useful record, one needs to determine a sequence of field directions

throughout the reversal?the more indi? vidual readings of the field during the reversal, the more we are likely to learn about the process. This often involves re?

lying heavily on the magnetization of in? dividual or small numbers of samples in reversal records, which makes establish?

ing their validity very difficult. We do not have the luxury of many samples from which to make estimates of the

mean and variance of populations of di? rections, as we do in tectonic applica? tions of paleomagnetism.

Paleomagnetic Records of Reversals The reversal record discovered in the South African lava sequence provided spot readings of the field and estab




reversals. These have included records of the same reversal that reveal com? mon features between sites separated by more than a thousand kilometers, thereby attesting to the repeatability of such records.

These studies and others provided a

good picture of the field behavior at in? dividual sites during a reversal. Al?

though some aspects remain contro?

versial, it is generally accepted that the reversals took a few thousand years, that they were accompanied by a de? crease in field intensity of about a fac? tor of 10, and that directional changes

were more rapid and greater than those associated with the nonreversing field. The important question of the

global configuration of the field, how? ever, remained untouched.

The Field During Reversals To describe the global form of the geo? magnetic field during a reversal, one needs records of a single reversal from numerous sites distributed over the earth's surface. Carl Friedrich Gauss, in his celebrated analysis of the field

published in the memoir Allgemeine Theorie des Geomagnetismus of 1838, used 84 "observations." He actually generated a regular grid of held values from the available observations. His

analysis allowed him to describe the field mathematically and to prove that, as William Gilbert had surmised 300

years earlier, the earth's magnetic field did have an internal source.

Jack Hillhouse and Allan Cox took the first small step in analysis of the transition field during reversals by com?

paring records of the last reversal from two sites?one in Japan and the other in the U.S. They chose to compare the

paths traced out by the pole during the reversal. When the field is not revers?

ing, the geometry of the field lines ap? proximate those of the field of a giant bar magnet at the center of the earth, or the equivalent giant current loop (see

Figure 2). These define two poles, which are the centers of mcoming and outgo? ing radial field lines, as shown. In a

pure dipole field the declination and in? clination angles define a single center of

radially inward field lines. This magnet? ic south pole is called the Virtual Geo?

magnetic Pole (see Figure 5). If, on the other hand, the field has contributions from sources other than the central di?

pole, the calculated virtual geomagnetic poles (VGPs) from different sites would be dispersed. The greater the ratio of

geographic north

c d

Figure 5. Virtual geomagnetic poles (VGPs) are constructed by observing declination and

inclination at different sites on the earth's surface (a). The declination reveals the direction

toward the pole, and the inclination gives the angular distance 0 through a simple trigono? metric relation (b). In a pure dipole field, such as the field of a bar magnet or of a current

loop, the declinations and inclinations from sites anywhere on the earth's surface give a sin?

gle axis, the south magnetic pole of which we call the VGP (c). In a field that is not a pure

dipole, the VGPs are dispersed, because at each site the nondipole field contributes to the

local field and thus affects the local declination and inclination id).

nondipole to dipole field, the greater the

dispersion of the VGPs. The compari? son of VGP paths is a very effective way to compare two reversal records, be?

cause if the paths are dissimilar, the transition field must be nondipolar. The

comparison made by Cox and Hill house demonstrated that the two VGP tracks were quite different, so that ac?

cording to the testimony of these two

records, the field during the last reversal could not be dipolar. Reanalysis of both records has been undertaken. And, al?

though neither of the original records has survived unscathed, the conclusion that the field could not have been pure? ly dipolar remains.

The next step in interpretation of transitional fields was undertaken by

Ken Hoffman, now at California Poly

technic University in San Luis Obispo, California. He noticed that VGP paths that had been observed for the last re? versal could be explained by interme? diate, or transitional, fields that are

symmetric about the rotation axis, as shown in Figure 6. These are the fields of zonal harmonics, in the terminolo?

gy of spherical harmonics used to de? scribe the field since the time of Gauss.

The suggestion was plausible. As

pointed out by Raymond Hide, T. G.

Cowling's famous theorem precludes dynamo action when the magnetic fields are perfectly axisymmetric, so it seemed natural that axisymmetric fields might be prominent as the field failed in the first stage of field reversals.

Moreover, some of the simplest axial fields are geometrically related to di




reversed

1

4

normal

Figure 6. Axial quadrupole (upper left) and octupole (upper right) fields, like dipole fields, are

constant around lines of latitude?that is, they are symmetric about the earth's rotation axis. In a

positive quadrupole field, flux emerges from the north and south poles, and there is a ring of

radially inward-directed flux around the equator. Anywhere on the equator, the inclination would be vertically down, and the VGP would appear to be directly beneath the site. In a posi? tive octupole field, flux emerges from the geographic north pole and intermediate southerly lati?

tudes, and flux reenters at intermediate northerly latitudes and the geographic south pole. If tran?

sition fields have such geometries, then as the reversal progresses (bottom) vertical inclination would always be observed at some stage during the reversal, no matter where the site is located.

pole fields, so that one could imagine energy from the processes generating the dipole field flowing into processes generating these fields. Indeed, this is

happening at present. Ian Williams, working with one of us (Fuller) at the

University of California at Santa Bar? bara (UCSB), developed a simple field simulation based on this flow of energy from dipole fields to simple axisym?

metric fields. Transition fields generat? ed by the model were studied and com?

pared with the observed fields. For a while it appeared that new data con

firmed the model, but as more and more nonaxisymmetric behavior was

discovered, the idea had to be aban? doned?the paleomagnetic data simply were not consistent with its predictions.

At about this time, the first results

began to emerge that led to the present excitement. The VGPs of records re?

ported by Brad Clement and by Dennis Kent appeared to be confined to longi? tudes over the Americas and their an

tipode. Results were obtained from Crete and from northern Italy by one of us (Lap and his colleagues at the Cen

tre des Faible Radioactivites at Gif-sur Yvette, France. The research of Jean Pierre Valet involved records of a suc? cession of reversals in sections of

blue-gray marls from the island of Crete, whereas that of Emmanuel Trie established a reversal in the sediments of the Crostolo River valley. Very de? tailed collections were made across these transitions. When the directions of magnetization were analyzed, it was discovered that the VGP paths of the various reversals were similar, being predominantly within a band of longi? tudes centered over the Americas, with some over their antipode (see Figure 7). Results from Herrero-Bervera were also consistent with this pattern.

The emergence of the new results was striking. The preponderance of

paths over the Americas first became clear at an international meeting in 1989 at Exeter, England. By the next

meeting, two years later at Vienna, oth? er results supporting the idea of pre? ferred VGP paths had accumulated, and a cover of Nature illustrating such VGP paths was much in evidence at the meeting.

At this time, a number of groups again took up the challenge of deter?

mining the morphology of the plane? tary wide field. Jeff Love and David Gubbins of the University of Leeds re?

ported an inversion of the field. Mean? while, Alain Mazaud from the Gif-sur Yvette group had published another inversion, and Shao Ji-Sheng at UCSB found a novel way of inverting direc? tional data. More than 150 years after the Gauss inversion, we have the ad?

vantage of computing power over

Gauss, but our data set is sparse and individual records are noisy. Neverthe? less, the calculated models reflect the

importance of the preferred VGP

paths, and we can expect better models to come along before too long.

Doubts About Data While some groups were busy devel?

oping ideas based on the proposed pat? terns in the reversal record, reaction

questioning the reality of the patterns had also developed. Jean-Pierre Valet, Piotr Tucholka and Vincent Courtillot, from LTnstitut de Physique de Globe in Paris, questioned the robustness of the statistical analysis of the data and

suggested that the data could equally well be explained by random distribu? tions of the VGP paths. With the sim?

plest possible presentation of the zero




crossing of the VGP paths, one can see in Figure 8 that the paths are far from confined to the longitudes of the Americans and the antipodes. Yet there are clusters of crossings at these longi? tudes. The distribution can be shown to depart from a uniform distribution

by standard tests of circular distribu? tions, as was pointed out by Rao Jam

malamadaka from UCSB. Pierre Rochette at Faculte St-Jerome,

France, and Cor Langereis and Ton Van Hoff of Utrecht suggested that even if there is a statistically robust pattern of VGP paths, the bias could be an arti? fact of the process of sedimentary mag? netization. Xavier Quidelleur and oth? ers at U Institut de Physique du Globe demonstrated that some sediments are unable to record the directions of mag? netic fields with intensity as small as those proposed during reversals.

Meanwhile McFadden had argued that the distribution of VGPs was related to the site distribution because sediments had a systematic inclination error such that their magnetization was nearer horizontal than it should be. This then resulted in a tendency for the poles to be 90 degrees from the site. Thus sites in Europe gave poles over American and Eastern Asia.

The key question to be settled now is whether the process of sedimentary magnetization simply smooths the field record or whether it radically distorts the character of the field behavior to

give an artifact. In all probability, both situations will be encountered. So be? fore a reversal record can be accepted as a reliable record of field changes, it will have to be demonstrated that no more than simple smoothing has taken place.

The reexamination of the evidence for and against the reality of the pat? tern has produced intense controversy. In the face of these difficulties, the

pragmatic approach of demonstrating the presence or absence of internal con

Figure 7. VGP paths from Crostolo in Northern

Italy (top) and from Crete (bottom sequence) led one the authors (Laj) to suggest that there are preferred paths of VGPs during reversals.

Shaded areas in the vertical bar correspond to

periods of normal (dark) and reversed (light)

polarity. In the sequence of reversals from

Crete, the VGP is plotted for normal-to

reversed reversals, whereas the pole at the

opposite end of the magnetic axis is plotted for

reversed-to-normal reversals. This draws atten?

tion to the possibility that field line bundles of either sign may be transported along the same

path during all of the reversals.




180'

0"

Figure 8. Equatorial crossings of VGP paths show a wide distribution but a noticeable con?

centration between 45 and 90 degrees west longitude.

sistency of records of the same reversal from different rock types is the best

hope to test the reliability of the records. If lavas and ocean sediments, whose

magnetization necessarily arises from

quite different mechanisms, give the same result, an

explanation in terms of

magnetization artifacts strains credulity. Meanwhile, we are in the midst of one of those periods of intense and critical reexamination by which science keeps its house in order.

VGPs and the Core-Mantle Boundary The reader may well be asking why the

proposed preferred paths of the VGPs caused so much excitement. Principal? ly, this was because the same bands of

longitude apparently preferred by the transitional VGP paths turn out to be

important in three other geophysical observations.

First, the preferred longitudes have

particular significance in models of the

present radial field on the core-mantle

boundary. Methods that calculate the radial field on the core-mantle bound?

ary from the surface field were devel?

oped by David Gubbins, Jeremy Blox ham and their colleagues, then at

Cambridge, and Jean-Louis Le Mouel of L'Institut de Physique du Globe,

building on earlier studies of George Backus of the Institute of Geophysics and Planetary Physics at La Jolla. There are two prominent features in the

Northern Hemisphere that lie in the

preferred longitudes of the VGP paths, whereas in the Southern Hemisphere a

single feature extends from high lati? tudes along the longitude west of Aus? tralia (see Figure 9a).

Second, the location of the preferred VGP paths also correlates with an in?

triguing feature of the fluid flow in the outermost core. As we noted above, the keystone of dynamo theory is that in a good electrical conductor, the field lines are trapped in the conductor and are inhibited from moving with respect to the conducting material. If we make this frozen-field approximation, we can turn the argument around to infer flow directions on the surface of the core from changes in the field values on the core-mantle boundary. A flow pattern derived this was from the field is illus? trated in the Figure 9b. The VGP paths during reversals fall on the longitudes of north-south flow in the outer core.

Third, the VGP paths correlate with

high-velocity regions in the lower man? tle. Seismologists have long used the elastic waves that are generated by earthquakes to probe the interior of the earth. Major advances have been made in the past decade by seismic-tomogra? phy studies. These techniques are analo?

gous to the scanned-projection radiog? raphy used in medicine to give images of sections through the human body. In the medical technique, a divergent fan

shaped beam of x rays is passed through the body and recorded with a ring of de? tectors. When the source is rotated around the body, the ring of detectors records x rays having taken different

paths through the body, so that a two-di? mensional image of the section can be built up. In the seismic studies, the glob? al distribution of detectors recording dif? ferent earthquakes establishes a three-di?

mensional image of the deep earth that demonstrates large-scale lateral inho

mogenities in the mantle. High- and

low-velocity regions deep in the mantle are interpreted in terms of low and high local temperatures, which in turn are as? sociated with descending cool mantle

material and ascending hot mantle ma? terial. The preferred VGP paths correlate

with the cold circum-Pacific regions. The interpretation of these diverse

phenomena linked by the VGP paths first recognizes that the low tempera? ture in the lower mantle could give rise to a low-temperature anomaly at the base of the mantle, in what seismolo?

gists call the D" layer. This could per? turb flow in the outermost core, perhaps by pinning upwelling sites as suggested by Keke Zhang and Gubbins. Thus, al?

though the fluid flow in the core has no intrinsic long-term memory, it could be controlled over long periods by the core-mantle boundary conditions.

During a reversal, the radial flux at

high northerly and southerly latitudes somehow exchanges positions. For ex?

ample, concentrations of outward-direct?

ed flux lines in the southern hemisphere associated with a normal held are re?

placed by inward-directed flux. As Gub? bins and Bloxham point out, this is tak?

ing place at the moment with a build up of inward-directed, reverse-field flux in

high southern latitudes at the longitude of the Americas. The north-to-south,

poleward flow under South America is thus destroying one of the major out?

ward radial-flux features of the present field. If this proves to be an important

mechanism of the reversal process, then




the concentration of radial flux at the lon?

gitude of South America would be re? flected in VGP paths. The linking of the flux transport to the regions of north south flow thus could be part of the ex?

planation of the observed VGP paths. Other models were put forward to ex?

plain the phenomena. Keith Runcorn of the University of Newcastle, and later of

Alaska, whose recent tragic death has stunned the geomagnetism community, favored a role for variations in electrical

conductivity in the DM layer to control the VGP paths. The proposed mechanism was an inhibition of flux penetration into the more highly conducting regions.

Thus it is actually the convergence of ideas in seismology, geomagnetism and

paleomagnetism that caused such excite? ment among earth scientists. Neverthe?

less, unless the preferred VGP paths emerge successfully from the present controversy, this approach will, like the zonal model, have to be abandoned.

Still Too Soon for a Conclusion As the reexamination of the data contin? ues, new records emerge. Some of these are consistent with the earlier proposed preferred paths, such as those obtained

by Brad Clement and colleagues from Fiji and the records from Chinese loess by Ri

xiang Zhu working with the second au? thor at Gif. Others, such as a group of six records from nearby sites in the North

Pacific, being studied by groups from UCSB and Davis, give puzzlingly incon? sistent results. Recent work by Hoffman on transition directions from lavas from a

variety of sites has shown clusters of VGPs that lie within the same longitudi? nal bands as the sediment VGPs. As he noted, the clusters may reflect particular states of the field determined by patterns of core convection controlled by temper? ature variation in the lower mantle, as

proposed by Zhang and Gubbins. Re? sults from lava sequences in Iceland ob? tained by the third author are also consis? tent with the preferred paths. In conflict

with these results, compilations of transi? tion poles from lavas by Prevot and Pierre Camps at Montpellier show no

preferred latitudes. So the debate continues. As is usually

the case when there is controversy in sci? ence, we lack the key data to settle the

argument. Developing many more-reli? able records of paleomagnetic reversals will help. But above all, we need some? one clever enough to come up with con?

vincing criteria to distinguish the reliable from the flawed records of reversals.

radial field at the top of the core

fluid motion at the top of the core

p-wave velocity at 2,300 kilometers in depth

Figure 9. Three features of the outer core/core-mantle boundary are coincident with the pre? ferred paths of VGPs. Radial field at the core surface (a) shows prominently paired features in

the eastern hemisphere, with inward flux (green) at northerly high latitudes and outward flux

(purple) in the south. Beneath the Americas in the north there is a similar strong inward-flux fea?

ture, but in the south the outward flux is being replaced by flux directed oppositely to the pre? sent field. Flow directions in the outer core (b) also are coincident with preferred VGP paths. The length of the arrows is proportional to the fluid-flow velocity, and the regions of the north

south flow correlate with the proposed VGP paths. Finally, seismic tomography (c) of the lower

mantle measures the velocity of seismic waves to develop a three-dimensional picture. Blue cor?

responds to regions where seismic waves travel faster, which are interpreted to be colder.




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The Reversal of the Earth's Magnetic Field