Is the Earth's Magnetic Field Young?
October 18, 2003
Updated August 10, 2008*
One of the more intriguing arguments put forth by young earth creationists for the age of the earth has is related to the strength of the magnetic field. The notion that the earth could not be older than ~10,000 years on the basis of magnetic field strength was forwarded by Thomas Barnes (1971, 1973). Barnes, using the known decay of the dipole moment over this century, argued that the magnetic field has decayed via free decay since creation no more than 10,000 years ago. Barnes' analysis relied, at least in part, on fitting an exponential curve to the observed decay whereas a linear decay model would give a maximum age on the order of 100 million years (Brush, 1983). Barnes rejected the idea that the earth's magnetic field had reversed polarity. Reversals were originally suggested in the early 1900's (David, 1904; Bruhnes, 1906) although it was unclear if this was a real feature of the magnetic field or a rock magnetic artifact. Subsequent work has demonstrated that reversals of the main field have taken place (Jacobs, 1994).
A magnetic reversal, in simple terms, occurs when the force between the two poles reverses direction. In a reversed magnetic field (by definition, today's field is 'normal') a compass will point to the south geomagnetic pole and the inclination (dip of the magnetic needle) will also invert itself at all locations on the globe with the exception of the magnetic equator (Figure 1). However, even this is a simplistic definition of a reversal. A more precise definition requires that the geocentric axial dipole term changes sign on a global basis, the change in sign must have some stability (typically unchanged for a few thousand years), Thus, strictly speaking there can be at most only a few reversals if the time scale favored by young earth creationists is correct. Shorter term variations in directions are described as 'excursions' or 'events' (Jacobs, 1994)**.
Figure 1: Schematic of a field reversal. The magnetic field lines reverse
direction and a north-seeking compass becomes a south-seeking compass.
As noted, Barnes (1973) did not accept that magnetic field reversals have taken place in the past. One of the reasons for rejecting field reversals was that reversals provided some evidence that the main field could be generated by a dynamo process. It should be noted that a dynamo does not require a reversal as there are reversing and non-reversing dynamos. However, if reversals do occur, then the decay of the dipole field cannot, in and of itself, be used to measure the age of the earth. A few decades later, it became apparent that reversals of the magnetic field have taken place and creationists had to move quickly to accomodate these new observations. Humphreys (1988,1993), a self-professed 'fan' of Barnes took up the gauntlet of trying to rescue the young magnetic field argument once reversals became undeniable. Whenever rates are discussed amongst ye-creationists, there is a nearly universal decree that says "the rates implied by conventional geology are wrong" and the real rates are faster. Humphreys quickly adopted the 'rapid reversal' explanation, but made several crucial mistakes along the way.
In an attempt to demonstrate that the reversal time scale could be fit to the young earth scale, Humphreys (1993) argued in two Impact articles (Impact 242 and Impact 188) that the reversals all took place during the flood and that the magnetic field has decayed since the time of the flood. Strictly speaking none of these could be considered true reversals since the time scale for stability is too short to meet the definition given above. Humphreys provided the following 'schematic' diagram (Figure 2) to support his hypothesis:
Figure 2: Humphreys (1993) ICR Impact article 242 (http://www.icr.org/pubs/imp/imp-242.htm).
Note the zero-line crossings in the diagram and the overall shape of the curve given by Humpreys.
Although he argued that this diagram was
schematic, he described the data that
led to this diagram as follows:
"Unfortunately, the archaeomagnetic data do not support that assumption. Instead, the data show that the field intensity at the earth's surface fluctuated wildly up and down during the third millennium before Christ (see figure 1). A final fluctuation slowly increased the intensity until it reached a peak (50% higher than today) at about the time of Christ. Then it began a slowly accelerating decrease. By about 1000 A.D., the decrease was nearly as fast as it is today." (Humphreys ICR Impact 242).
Intrigued, I decided to check reference number  to see if it supported the claims made by Humphreys. It comes from Merrill and McElhinny's book (1983, 1998 edition included Phil McFadden as co-author) on the Earth's Magnetic field and the pages referenced discuss the archeomagnetic data. One of the diagrams on the referenced pages comes from a paper by Barbetti (1983). Barbetti estimated the field strength based on aboriginal fireplaces and baked clays in southeastern Australia. His curve is shown below in Figure 3:
Figure 3: From Barbetti (1983). Note that there is no zero line on the curve. The line drawn
through the data is to represent the present day field strength and is not a zero line!
The 'schematic' used by Humphreys is a near mirror-image of the Barbetti curve. I found the correlation extremely curious because it came directly from the pages used by Humphreys to support his 'rapid' reversal model at the time of the flood. I posted this correlation on several bulletin boards and it was picked up by several other scientists who confronted Humphreys. Humphreys vociferously denied the connection and stated that he used 'additional data' not included in the McElhinny book (see http://www.trueorigin.org/ca_rh_01.asp). He argued that he drew his curve based on the 500 year average data1 and 'other data I didn't cite'. These data, of course, were not presented and one can hardly construct Humphreys curve from the archeomagnetic data in the 500 year average (Figure 4). I maintain that Humphreys is guilty of at best misleading creationists with this schematic or, at worst, deliberately dishonest with his claim. However, until he produces the data, it's a matter of 'he said', 'no he did not say that'. I'll leave it to the reader to decide. If we look at the mean dipole moment data taken from the same pages referenced by Humphreys, we get the following averages (Merrill et al., 1998). Note, there is no point in the past 50 kyrs where the data (or even using the error bars) cross the zero line. The bottom figure expands the latest 10,000 years and we can see that the dipole moment of the present field is decreasing, it is still well above the dipole moment of times past. I tried several polynomial fits (up to degree 6) to these data and in no case could the curves be successfully modeled showing zero line crossings2. Humphreys also makes the claim that these averages neglect non-dipole contributions. That is not quite true. McElhinny and Senanayake (1982) argued that because non-dipole changes occur on shorter time scales than dipole changes, the averaging at 500-1000 year intervals averages out the variation in non-dipole contributions. However, they also went on to estimate an ~17.5% for the standard deviation of the non-dipole field. So we have Humphreys claim that he used uncited (to this day) data and an artificial rightward compression of the time scale to get his schematic Figure 2. One might ask, if we can simply invent data to fit our conclusions, then what is the point of doing basic research?
Figure 4: Dipole moment averages
archeomagnetic data presented in Merrill et al. (1998) pages
123-128. Note that the
dipole has not reversed in this interval. It should also be noted
that Humphreys claim that "During this period (e.g. creation about 10000 years ago), the field
would have been more than ten times stronger than it is today". All available data indicate it was weaker at creation no matter how one compresses the time scale!
Given that Humphreys 'evidence' for rapid reversals in the archeomagnetic data are totally unsubstantiated by ground truth, let us now turn to his assertion that paleomagnetic data from the Steens Mtn. sequence supports his claim of rapid reversals. I want to reiterate the fact that Humphreys has not documented a single reversal in the archeomagnetic record, so his entire discussion of the Steens Mtn data is predicated on an unsupported assertion. In 1989, Coe and Prevot published an article entitled "Evidence suggesting extremely rapid field variation during a magnetic reversal" which Humphreys seized on as evidence for rapid reversal. However, it is important to actually look at what Coe and Prevot (1989) were discussing. First, Coe and Prevot (1989) made no argument that the reversal process itself was rapid, only that the reversal period included evidence of rapid fluctuation in field intensity and direction. A subsequent publication in 1995 by Coe and co-authors was titled "New evidence for extremely rapid change of the geomagnetic field during a reversal". So, let's look at what Coe and co-authors were talking about and compare this to the assertions made by Humphreys. The time scale necessary for the Earth's magnetic field to complete a reversal is poorly known. Most estimates put the reversal interval on the order of 1,000 to 10,000 years (Merrill et al., 1998). However, if we compress all the reversals to the time of the year long flood, then the rate of reversal must be on the order of 1 per day or more frequent (and therefore not reversals according to the normal definition of a reversal)3. Humphreys argues for one reversal per week during the flood, but of course this is from a flood that has never been harmonized with the geologic record. Furthermore, since reversals are known from Archean rocks to rocks as young as 780 kyrs, then the flood must have started in strata conventional geologists call the Archean and persisted through the most recent Cenozoic. However, let's return to the reversal record and show why Humphreys is guilty of misusing the conclusions of Coe and Prevot (1989). We will also examine why the real conclusions of Coe and Prevot (1989) may be more problematic for ye-creationism than Humphreys is willing to admit. (ADDED 4/18/2004). Clement (2004) presents data that shows the average length of time required for a reversal averages about 7000 years. This is a near death blow to the Humphreys hypothesis.
Figure 1 shows a schematic for a normal and reverse magnetic field. This reversal, by all accounts is not an instantaneous process. The Earth's field reversals are defined on the basis of the dipole component only. In mathematical terms, the magnetic field of the earth is made up of a dipole field and higher order harmonics (quadrupole, octupole and so forth) that contribute to the overall field. Every 5 years, the magnetic induction parameters for the dipole and higher order fields are evaluated and listed in the IGRF (more on this below). Paleomagnetists and geophysicists are interested in the processes that drive a reversal and also in the vector changes of the field during a reversal. One way to examine these changes is to examine the behavior of the field in rapidly deposited sedimentary rocks or via 'snapshots' taken by rapidly cooled lavas. Both types of analyses have problems. In sedimentary rocks, paleomagnetists are looking at an averaging that is inherent in slowly deposited sedimentary rocks and also problems due to inclination shallowing (the deflection of the inclination vector toward horizontal upon compaction and dewatering). Reversal signatures in igneous rocks present difficulties because of gaps in the eruption record. For example, although the Steens Mtn volcanic eruptions happened over a relatively short time period, we cannot be absolutely sure that we have a week to week (much less a day to day) documentation of the magnetic field behavior. This shortcoming is partially overcome in the thicker flows because conductive cooling will result in the recording of field behavior during the period it takes for the magnetic minerals to lock in the directions. Both the sedimentary record and the igneous records demonstrate that the reversal process involves large fluctuations in direction and intensity during the transitional period from one polarity to the next. Simply put, the magnetic dipole field strength does not drop to zero and then instantaneously recover in the opposite sense nor does the direction switch from north to south seeking (although this is mathematically feasible in the case of zero intensity). There is considerable controversy in the paleomagnetic community as to whether or not the field follows some preferred path (with multiple oscillations) or whether the transitional field directions and strengths are a purely stochastic process (see Merrill et al., 1998 for a comprehensive discussion). Nevertheless, let's look at what the Steens Mountain reversal record looks like.
The Steens Mountain volcanic sequence (Figure 5) that contains evidence for the transitional field is based on a nearly 1000 meter section of individual sub-areal basalt flows4. These 15.5 Ma flows (note they are older than 6000 years) do record a reverse to normal polarity interval. The transition was initially documented by Watkins (1965), but it was not until Mankinen et al. (1985) and Prevot et al. (1985) investigated the interval that several interesting observations came to light. The interval begins with a several thousand year interval of reverse polarity and is followed by two transitions. The first phase lasted some 550 years (+/- 150 years) where the field reached the normal field direction where it remained for perhaps 200 (+/- 100) years. The second phase consisted of a large deviation from either pole and the directional changes proceeded through a large counterclockwise loop that included occupation of both normal (N) and reverse (S) directions. This phase lasted 2900 (+/- 300) years. The record closes with several thousand years of normal polarity. Mankinen et al. (1985) estimated that the total duration of these transitional fields was 3600 +/- 700 years. There was considerable controversy regarding the exact definition of these two phases (some argued that the two phases were unrelated to each other (Valet et al., 1985). The paper by Coe and Prevot (1989) which forms the casus belli for Humphreys claims does not, in fact, document reversals of the field but rather excursional impulses of the field during a reversal.
Figure 5: Steens Mtn mid-Miocene reversal sequence. Individual flows are easily
recongizable especially in the left and top part of the photo. (Photo courtesy of
The original publication (Coe and Prevot, 1989) documented a major directional change in two of the basalt flows that lay between the normal polarity sequence and the reverse polarity sequence along with a variety of intermediate directions throughout the section (Figure 6). It is important to compare this record with the claim made by Humphreys. In his ICR article he states:
"In 1988, startling new evidence was found for the
prediction of my theory--very rapid reversals".
"I cited newly discovered evidence for rapid reversals (Coe and Pr�vot, 1989), evidence in thin lava flows confirming my 1986 prediction. Since then, even more such evidence has become known (Coe, Pr�vot, and Camps, 1995)."
Compare these statements to the statement made by the authors:
“This is not to suppose that geomagnetic
place much more quickly than the several thousand years
currently supposed, but
rather to suggest that polarity transitions may be punctuated by
extraordinarily rapid field change.”
In fact, Coe and Prevot (1989) were not arguing that the reversal process itself was rapid (indeed the normal and reverse section are separated by quite a number of lava flows (more than 50 flows). What they concluded was that during the transition between stable normal and reverse magnetization, the field would fluctuate in strength and directions (Figure 6) and might reach a near normal position before returning to an intermediate direction (in fact they documented what appears to be 2-3 of these 'rebound effects'). This conclusion is quite different from what Humphreys would have you believe regarding the magnetic field changes. The total length of time needed to complete a full reversal may have been on the order of thousands of years although the transitional field behavior may have fluctuated over much shorter time intervals during the reversal process. The 'impulses' documented in the paper may have involved periods of fluctuation on the order of a few degrees/day although this seems to be at odds with timescales of magnetic field models.
Figure 6: VGP (Virtual geomagnetic poles in the Steens Mountain sequence). Each square
represents an instantaneous record of the magnetic field. Note that the sequence has a
number of intermediate directions.
I should add that the final verdict on what exactly these transitional fields represents is not completely settled. One paper seldom referenced at the creationist sites is the 1999 update by Coe and colleagues (Camps, P., R. S. Coe, and M. Pr�vot, 1999, The hypothesis of transitional geomagnetic impulses: geomagnetic fact or rock-magnetic artifact?, J. Geophys. Res., 104, 17,747-17,758). In this paper, the authors attempt to verify their hypothesis of rapid field fluctuations (again NOT reversals!!). In their abstract they note:
"In order to check the spatial reproducibility of the paleomagnetic signal over distances of up to several kilometers, we have carried out a paleomagnetic investigation of two new sections (B and F) in the Steens summit region which cover the second and third directional gap. The main result is the description of two new directions, which are located between the pre-second and post-second impulse directions. These findings weigh against the hypothesis that the geomagnetic field caused the unusual intraflow fluctuations which now appears to be more ad-hoc as an explanation of the paleomagnetic data."
So, all of this excitement about rapid reversals is based on a misunderstanding of the literature and even the rapid fluctuations that originally led to the excitement now appear to be, at best, an ad hoc explanation. In the conclusions they regard the extremely rapid transition as rather less likely although they also have trouble reconciling the new data with a remagnetization hypothesis.
Problem 3. Technical Analysis
In his most recent discussion on the decay of the magnetic field, Humphreys (2003) takes on the assertion by Brent Dalrymple that the decay of the dipole moment is balanced by the growth in higher order harmonics (so-called quadrupolar and octupolar components of the field). As an aside, Humphreys goes on to say that evolutionists have been using Dalrymple's explanation without reviewing the work of creationists. Is this a valid criticism? Let me state that the scientific work of creationists goes largely unnoticed by mainstream science for good reason. The normal venue for getting science accepted, reviewed and criticized is to publish in the mainstream literature and present the results at National and International meetings. Humphreys has not attempted to publish his works in the mainstream literature and, so far as I know, has never presented his work at conferences such as the Annual Meeting of the American Geophysical Union. Creationists complain of bias, but this is certainly not true of international conferences. Creationists like Steve Austin, Kurt Wise and John Baumgardner are fairly regular staples at the Geological Society of America meetings and AGU. Most mainstream scientists don't even know Humphreys exists, let alone his notions regarding the age of the earth. The few of us who do pay attention to his 'publications' do so mainly to set the record straight and defend against bad science teaching in America and elsewhere. It does not help his case when he refuses to publish data he says led to his conclusions (see archeomagnetic discussion above). Secrecy in science is most often associated with fraudulent science. If Humphreys is really interested in other scientists taking his science seriously, then he must submit his work in the same manner as the rest of us. Otherwise his work will remain invisible and irrelevant.
Let us return to the discussion of energy in the main field. As noted by Humphreys, the terminology gets to be somewhat unwieldy; however, the magnetic potential V can be given as:
where V= potential function; P is the Schmidt quasi-normalized form of the associated Legendre polynomial of degree l and order m. The coefficients g and h are called the Gauss coefficients with as the geocentric axial dipole term, as the geocentric axial quadrupole term and as the geocentric axial octupolar term. The International Association of Geomagnetism and Aeronomy publishes the values for these gauss coefficients on a 5-year basis together with estimates of secular variation. The units are given in magnetic induction units (nT=nanoteslas) for each of the gaussian coefficients. So, what does all this have to do with using the decay of the magnetic field to estimate the age of the earth? The answer, quite frankly, is not much at all. Humphreys seems to think it is of some importance. He spends much time calculating the energies of the dipole and off-dipole terms (octupole, quadrupolar and higher order terms) for the past century and concludes that the dipole energy has fallen off more rapidly than the off-dipole terms. I don't dispute this, but do dispute the conclusions reached by Humphreys that this observation has anything whatsoever to do with the age of the earth. Humphreys seems to think that we are merely observing some terminal ohmic dissipation of the main field resulting from the post-flood stabilization in the core. In his 'dynamic decay' scenario, each successive reversal removes energy from the main field and we are now witnessing the 'death' of the main field. However, we have already seen that all of that his ideas regarding reversals, field intensity from archeomagnetic data (and in fact the lack of evidence for a global flood) is based on bad data, or even worse, invented data. Humphreys can calculate energies until the cows come home, but the archeomagnetic data that are available indicate that there were no magnetic reversals in the past 5000 years (they are simply an artificial invention of Humphreys). Furthermore, the archeomagnetic data show that the dipole moment fluctuates with time and if anything, the long term trend from the archeomagnetic data (Figure 4-top) shows the dipole moment increasing with time (even on a compressed scale)! Studies of relative paleointensities from deep sea cores and lake records also show that the virtual dipole moment varies with time and shows no indication of long-term decay (see Merrill et al., 1998)
So, what led to Darymple's assertion regarding the off-dipole terms? Geologists have long wondered if reversals were accompanied by extinctions due to the removal of the Earth's magnetic shielding. No correlation between extinction and reversal has been observed leading most to conclude that the decay of the dipole was compensated for by an increase in higher order fields or that the incoming solar radiation at the time of a reversal was not significant enough to cause extinction (Jacobs, 1994). There is no need for these higher order fields to exactly balance the dipole decay, nor is there any reason to assume that this must occur in a 1:1 fashion or that they must be temporally correlated. Certainly, none of the field equations argue that there must be a balance between the decay of the dipole and a corresponding increase in off-dipole terms (in fact the toroidal and poloidal fields decay on separate time scales) . Humphreys is extrapolating 100 years of data (some of which are only estimates) to reach his conclusions. However, we can see from figure 4 above, that the variations in dipole moment through time is simply part of the expected behavior for a stochastic geodynamo. Measuring the strength of the dipole during transitional period is fraught with difficulty. Early thoughts were that the dipole was reduced to zero and then grew back in the opposite direction. The few observational data collected (see Merrill et al., 1998 for complete discussion) suggest that the virtual dipole moment during a transition falls about to about 25% of its 'usual' value. Therefore, the decay in the energy of the dipole field compared to the first two harmonics given by Humphreys is nothing more than an observation of the most recent 30 years of geomagnetic data. In no way can it be reliably extrapolated to yield an age limit for the earth. Lastly Humphreys argues that some of the energy is unlikely to be transferred into toroidal fields. As evidence, he cites an article by Lanzerotti et al. (1985). This is not the last word on the magnitude or longevity of toroidal fields in the core (Figure 7). In fact, it amounts to nothing more than a handwave to get rid of one of the two options for explaining the observed decrease in energy. What Humphreys needs to show is conclusive evidence that the longevity of the toroidal fields are <<100 years and that their contribution is <<< 1.4% decrease documented in his study. For example, a subsequent comment by Lanzerotti (1994) on a paper by Zhang and Fearn (1993) suggested that this 'invisible' component of the magnetic field may be on the order of a few gauss at the Core-mantle boundary (this is approximately equivalent to the dipole component of the poloidal field at the surface) to as much as 50 gauss within the core (more than 20x the dipole component). These are not trivial numbers as Humphreys would have you believe and therefore cast further doubt on his conclusions regarding the preservation of the field energy during a reversal.
Figure 7: Schematic of poloidal versus toroidal fields in the core. The
purple shading represents the extent of the outer core. Poloidal field
lines exit the core and are sensed on the surface. These fields are
described via spherical harmonic expansion given above. Toroidal fields
are confined to the core and hence not detected. Toroidal fields may;
however, be converted to poloidal fields and vice versa given the proper
motion in the outer core.
Humphreys has argued in the creationist literature that the Earth's magnetic field is in terminal decay and that its maximum age can be no more than 10,000 years. As shown above, his conclusions are based on undocumented reversals in the archeomagnetic record, a mistaken conclusion regarding the time it takes for the magnetic field to reverse and an extrapolation based on the last 30 years of magnetic observation. Furthermore, Humphreys argues that the magnetic field of the earth at creation was much higher than the present-day value. This conjecture is totally at odds with observational data and thus is mere speculation. Humphreys does accept reversals, and if they all happened in the year of the flood, then they would occur roughly at the rate of 1 per day. There is no observational evidence to support this frequency of reversals. However, if Humphreys is correct that all the reversal occurred in the year of the flood, then the strata corresponding to the flood must extend from the Archean to the most recent sedimentation since reversals are well-documented in that interval. Humphreys refuses to publish his work in mainstream literature or to present his ideas to mainstream science via annual conferences.
Tim Thompson discusses several other flaws with both the Humphreys and Barnes models at the TalkOrigins website.
1In fact only the first 4000 years are averaged on a 500 year basis.
2including one where I 'rightwardly compressed' the time scale as claimed by Humphreys.
3Of course Humphreys could claim that this definition of a reversal is somewhat ad-hoc and that any time with the field direction located in the opposite sense to the previous should be called a reversal. I would like to note however that some of the records detailing reversal stratigraphy are taken from fine-grained sedimentary rocks. The process of detrital remanent magnetization is fairly complex, but most studies indicate that the particles must settle in a quiet environment in order to align themselves with the field. As the sediment is compacted and de-watered these particles 'lock-in' the field. All of this takes time and a global tempest such as the Noachian flood would not favor quiet settling of these particles through the water column.
4how these lavas managed to be erupted above water and undergo subareal weathering during a global flood is another subject.
creationists have whined that by artificially saying a reversal must be
stable for longer than 2000 years means I can't give Humphreys a fair
shake. That's simply not true. I can, for the sake of
argument, change that statement to 'stable for an hour' and nothing
changes. What Humphreys has not shown is any record of a field
reversal in the archeomagnetic record. He simply invented those
data. In contrast, conventional science notes that magnetic
reversals have been found in rocks
from Archean all the way to 780,000 years ago. According to
Humphreys, nearly all the rocks on earth were formed during the
flood. That creates some additional problems for the 'flud'
Barnes, T. G. "Decay of
magnetic moment and the geochronological implications," Creation
Quarterly 8 (June 1971) 24-29.
Barnes, T., 1973. Origin and destiny of the earth's magnetic field. Creation life publishers, San Diego California.
Bruhnes, B, 1906. Recherches sur le direction d'aimantation des roches volcaniques, J. Phys. 5, 705-724.
Brush, S. G., 1983. "Finding the age of the earth: By physics or by faith?" In Zetterberg Evolution versus Creationism: The Public Education Controversy. Phoenix, AZ: The Oryx Press
Camps, P., R. S. Coe, and M. Pr�vot, 1999, The hypothesis of transitional geomagnetic impulses: geomagnetic fact or rock-magnetic artifact?, J. Geophys. Res., 104, 17,747-17,758.
Clement, B., 2004, Dependence of the duration of geomagnetic polarity reversals on site latitude, Nature, 428, 637-640.
Coe, R. S. and M. Prevot. , 1989, "Evidence suggesting extremely rapid field variation during a geomagnetic reversal," Earth and Planetary Science Letters 93, 292-298.
Coe, R. S., M. Pr�vot, and P. Camps. 1995. New evidence for extraordinarily rapid change of the geomagnetic field during a reversal. Nature 374:687–692.
David, P., 1904. Sur la stabilitie de la direction d'aiminantation dans quelques roches volcaniques, C. R. Acad Sci Paris, 138, 41-42.
Humphreys, D. R., 1988. "Has the earth's magnetic field ever flipped?" Creation Research Society Quarterly 25, 89-94.
Humphreys, D.R., 1989. "The mystery of the Earth's Magnetic field, ICR Impact Series 188, Institute for Creation Research, San Diego, CA.
Humphreys, D.R., 1993. The Earth's magnetic field is young, ICR Impact Series, 242, Institute for Creation Research, San Diego CA.
Humphreys, D.R., 2003. The Earth's Magnetic Field: Closing a Loophole in the Case for its Youth, Creation Digest.com (http://www.creationequation.com/Earths_Magnetic_Field.htm).
Jacobs, J.A., 1994. Reversals of the Earth's magnetic field 2nd Edition, Cambridge University Press, Cambridge, UK 346 pp.
Lanzerotti, L.J., 1994. Comment on "How strong is the invisible component of the magnetic field in the Earth's Core", Geophysical Research Letters, 21, 2339-2340.
Mankinen et al. (1985) The Steens Mtn (Oregon) geomagnetic polarity transition. 1. Directional history, duration of episodes and rock magnetism, J. Geophys. Res., 90, 10.
McElhinny, M.W. and Senanayake, 1982. Variations in the geomagnetic dipole 1. The past 50,000 years, J. Geomag. Geoelectr., 34, 39-51.
Merrill, R.T., McElhinny, M.W. and McFadden, P.L. The magnetic field of the Earth, Academic Press International Geophysics Series, 63, 1998.
Prevot et al., (1985) The Steens Mtn (Oregon) geomagnetic polarity transition 2. Field intensity and discussion of reversal models, J. Geophys Res., 90, 417.
Zhang, K. and Fearn, D.R., 1993. How strong is the invisible component of the magnetic field in the Earth's core?, Geophysical Research Letters, 20, 2083.
*Special thanks to MH and AB for pointing out some corrections/updates.