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KRONOS Vol III, No. 4
Geogullibility And Geomagnetic Reversals
RALPH E. JUERGENS
There was a time when the earth's magnetic field was thought to be such a permanent feature that compass needles would always point north. It was a time when geologians could scoff at Velikovsky's citation of evidence that the polarity of the field was reversed in historical times.
But today things are different. Earth science has leapfrogged past Velikovsky to a position from which it can patronize him as a victim of overly conservative thinking.(1) We are now assured that reversals of the earth's magnetic field were so frequent in the past that hundreds of them are recorded in the geological annals of the last 70 or 80 million years (though none shows up as an historical event).
The work that provoked this remarkable about-face has allegedly confirmed the complementary hypotheses of continental drift and sea-floor spreading. It has made the theory of plate tectonics into established dogma, which now rules the minds of earth scientists with more rigidity than the generalities of uniformity ever could. Since these interlocking notions are likely to be the last bastion of defense for geology and geophysics against the barbarous ideas of Velikovsky, one may reasonably wonder: What is all this business about a succession of magnetic polarity cycles?
The great stampede to embrace sea-floor spreading and continental drift had its beginnings in the discovery that ocean bottoms around the world bear scars in the form of distinctive patterns of remanent rock magnetism. The first hint that this was so came in 1952, when Ronald G. Mason of the Scripps Institution of Oceanography towed a magnetometer off the stern of a ship sailing from Samoa to San Diego. The instrument revealed an intriguing series of magnetic anomalies – field intensities higher or lower than theory would predict – along the track of the ship. It appeared that the anomalies were arranged alternately positive and negative – high and low magnetic intensity, respectively – with remarkable consistency.
Over the next decade, Mason and other Scripps scientists, notably Victor Vacquier – co-inventor of a method for stabilizing magnetometers against the motions of airplanes and ships – and Arthur D. Raff, dragged a broad region of the North Pacific Ocean with similar devices. They established that the anomalies were largely arrayed in long, parallel lines, striping the bottom of the sea with alternating bands of excess and deficient field intensity. Their work also suggested that the lineated anomalies might be aligned with, and related to, portions of the North Pacific Rise – the "mid-ocean ridge" in that part of the world.(2)
In 1963, F. J. Vine and D. H. Matthews of Cambridge University proposed that the peculiarly systematic magnetic features, mapped as positive and negative anomalies by all workers up to that time, were actually alternating strips of crustal rock magnetized with normal and reversed polarity. They pointed out that if sea-floor spreading were real, and if the earth's field actually reversed itself from time to time while new crust was being formed and pushed away from its supposed point of origin at a mid-ocean ridge, then each bit of new crust would become magnetized in keeping with field polarity prevailing at its birth. The result would be a symmetric pattern of alternately normal and reversed magnetism centered on each undersea ridge; in sequence, the widths and amplitudes (magnetic intensities) of anomalies on one side of any ridge should closely mirror those on the other side.(3)
Such symmetry had not been observed by the Scripps team working in the eastern Pacific Ocean because the North Pacific Rise is unusual; it wades ashore in western North America and has no underwater eastern flank to be explored by oceanographers. But even as Vine and Matthews were drafting their proposal, their vision of symmetry in anomaly patterns was being justified by explorations in the Atlantic Ocean, whose ridge runs accommodatingly right down its center. By 1965 the symmetry prediction had been substantiated by work in many other parts of the world, and the sea-floor-spreaders were welcoming new riders for their bandwagon every day.(4)
Meantime, geologists working on land had identified several apparent geomagnetic reversals, as recorded in the remanent magnetism of extensive rock formations; and had "dated" them radiometrically. They had worked out the "history" of several periods of normal and reversed magnetism going back about 3.5 million years.
The oceanographers enthusiastically adopted the magnetic calendar of the geologists and quickly extended its reach in time by a factor of 20.
The "dated" field-reversals of the land-based calendar were assumed to correspond to the first few anomaly changes in the magnetic record closest to the axis of any mid-ocean ridge. Then, on the further assumption that the rate of sea-floor spreading away from each point along any ridge must be generally uniform, successive anomalies at increasing distances from each such "spreading center" were assigned ages on the basis of total distance "traveled." Almost overnight the lineated magnetic anomalies were transformed into a geomagnetic calendar spanning nearly 80 million years of earth history.(5)
By 1968 oceanographer J. R. Heirtzler could boast: "We have now identified 171 reversals of the earth's magnetic field over the [last] 76 million years and believe that the dates assigned to each are quite accurate... On the whole the evidence for the correctness of the time scale is so strong that it can now be used in turn to study variations in spreading rates over the ages in certain disturbed areas."(6)
Five years is a remarkably short time for a new suggestion to become established dogma. Even more remarkable, however, is that this particular suggestion, based on a faulty first premise and built largely on circular reasoning, should have seduced a whole generation of earth scientists and have been codified as dogma at all.
We review: One assumes that sea-floor spreading is a reality; one assumes that sea-floor spreading arranges magnetic anomalies in broad bands parallel to individual segments of discontinuous ocean ridges; one assumes that sea-floor spreading proceeds at steady rates; and one concludes that the symmetry of anomaly patterns confirms seafloor spreading and that the spacings of the anomalies confirm uniform spreading rates. Such reasoning must certainly raise some eyebrows. And, as we shall soon see, the assumption, or presumption, that reversed magnetism is responsible for most ocean-bottom negative anomalies is itself untenable.
The tenuity of this entire argument is disturbing enough. Yet when one takes a closer look at the methodology of its proponents, any urge one may have to savor it, much less swallow it, must be choked off.
The ocean-floor magnetic anomalies routinely paraded forth in support of sea-floor spreading are mapped on the advice of magnetometers towed along just beneath the surface of the sea. The instruments trace magnetic profiles whose major features, reflecting high and low values of field intensity, average a few tens of kilometers across and about one-half of one percent of the total magnetic field in amplitude, or profile "height." Each profile is a jagged line with numerous dips and rises at irregular intervals. The deeper dips are interpreted as negative anomalies, and the higher rises are classed as positive anomalies. Repeated voyages along parallel tracks at right angles to mid-ocean ridges yield series of profiles whose major features more or less line up and, when plotted on maps with the help of magnetic "contour lines," show up as broad stripes essentially parallel to ridge axes.
So far, so impressive. But when magnetometers are lowered and towed close to the sea bottom, producing profiles that should be even more reliable than those of near-surface tows, a rather different picture emerges.
The first such deep tow, in 1967, yielded a magnetic profile for a track in the eastern Pacific Ocean in which anomalies, both positive and negative, were ten times more numerous and three to four times more pronounced in intensity than those in a companion profile obtained with a shallow-tow instrument. The investigators contained their astonishment: "Thus what appears to be uniformly magnetized crust at the ocean surface is seen to be contaminated with material of differing magnetization at depth."(7) From the figures and comparisons just cited, this would appear to be something of an understatement.
The next year, a deep tow farther south in the Pacific Ocean produced a similar profile for another team of researchers. At one particular point along their "deep magnetics" profile the investigators found that a sharply defined dip corresponded quite nicely to a "reversal" inferred from surface readings. Their report emphasizes that this dip is characterized with "about twice the average anomaly amplitude" of the profile as a whole, the plain implication being that here was unequivocal backup evidence for the "reversal" read into the surface profile. What is not emphasized, however, is that several dips of comparable amplitude, though of lesser width, appear elsewhere along the deep-tow profile, at locations where surface readings give no hint of negative anomalies. The report downgrades these unwanted features as "the ever present deep magnetic anomalies of lower [sic] amplitude and shorter wavelength" and dismisses them: "The possibility that each of these anomalies records a short-lived reversal in polarity can be eliminated because no such short-period reversals have been reported from core data [on ocean-bottom sediments]."(8)
This airy rejection of evidence assumes that reversed magnetism in ocean sediments gives evidence of reversed geomagnetic polarity in the past history of the earth; whether it does or not is another story. But the argument just quoted has a more fundamental weakness, related to a widely recognized inadequacy in research methods. The conventional view is that "deep sea sediments... are formed extremely slowly, the deposition rates probably being less than 1 cm per 1000 years. Thus a sample 2 cm high [the typical size of a specimen taken from a core sample for analysis] will have [a natural remanent magnetization] direction representing an average of the earth's magnetic field over 2000 years or more."(9) Any evidence for a short-lived reversal within such a specimen would be lost in a flood of signs of normal polarity.
The same argument can be dismissed in the context of Velikovskian earth history, where a magnetic reversal would most likely accompany a global catastrophe. Simultaneously, or nearly so, violent waters would be sweeping sea bottoms clean in some places and dumping enormous loads of detritus in others. Elsewhere, stratigraphic records of more peaceful times would be mutilated Here and there the sequence of unconsolidated sediments might be fortuitously preserved, or it might even be duplicated through folding.(10) Chaotic records would be the rule wherever disordered currents held sway, however briefly. The placid waters needed for settling particles to align themselves magnetically might not be restored anywhere on earth before the geomagnetic field was re-established in its normal mode.
But for the proponents of sea-floor spreading it was clearly imperative that the multiplicity of anomalies disclosed by deep tows be explained away; at all costs, the proud magnetic calendar had to be preserved. If an appeal to the sedimentary record was not quite up to the task, the unwanted data could be assigned to "time variation either in the paleofield intensity or in the properties of the injected magmas";(11) no one was upset by the thought that the entire phenomenon under study could be similarly disposed of. The case was quickly closed, the problem was said to be solved, and the treasured time scale was saved embarrassment.
However, the data supporting the notion of free-wheeling crustal plates are not only hand-picked. They are also processed in a most revealing way.
As already noted, magnetic profiles obtained by crossing the world's rifted and unrifted mid-ocean ridges consist of continuous wiggly lines with many ups and downs at irregular intervals. From the tops of the highest peaks to the bottoms of the deepest valleys along these traces, magnetic-intensity variations amount to very small percentages of the total field. Some valleys are relatively deep, others not so deep; some peaks are considerably higher than average, others not so high. How does the geophysicist determine which of these features, other than the most obvious of them, are truly negative or truly positive anomalies?
For the confirmed sea-floor-spreader, the procedure is simplicity itself. The investigator averages all the highs and lows in the readings along a single profile, then subtracts the result from each of them "in order to define positive and negative anomalies" (emphasis added).(12) This manipulation puts "zero" – a datum line separating positives from negatives – squarely through the middle of the profile. He does not ask: What if I were to lower the datum enough to eliminate half the negative anomalies defined by my procedure, or raise it enough to eliminate half the positive anomalies? He is quite satisfied, and he proceeds to interpret his positive anomalies as the time tracks of normal-polarity epochs, his negative anomalies as the tracks of reversed-polarity epochs.
So much for methodology. Even without examining the fundamental assumption of the Vine-Matthews hypothesis that led to all this, one is amazed that faith in sea-floor spreading can be maintained on the basis of such arbitrarily selective rites.
But now let us look to the idea that negative anomalies can be laid to reversed remanent magnetism, positive anomalies to normal magnetism. We shall find that this assumption is so faulty as to raise doubt that there is any evidence at all for reversed magnetism in crustal rocks of the ocean floor.
Perhaps the best approach is to backtrack in the history of this research to the late1950's – to the work of the Scripps scientists who then had the field of oceanic magnetic anomalies pretty much to themselves.
Mason, Vacquier, and Raff, following long-established procedures, identified positive and negative anomalies by computing values for the local general field – values ranging globally from about 25,000 gammas at the geomagnetic equator to about 65,000 gammas near the geomagnetic poles – and subtracting such theoretical results from their magnetometer readings. They neither insisted that the computed values were necessarily the true values for local elements of the general field nor assumed that subtracting them must neatly balance highs and lows along their profiles. Their maps claim nothing more than that the ocean-bottom areas investigated bear lineated magnetic features that cause magnetometer readings to differ very slightly from theoretical expectations, and that the differences are both positive and negative in character.
This same team certainly did not make the great assumptive leap of equating positive and negative anomalies, respectively, with normal and reversed remanent magnetism in crustal rock formations, or at least not before Raff described their findings in Scientific American for October 1961.(13)
The magnetic maps of the Scripps scientists actually portray features totally inimical to the interpretation that ranks of lineated anomalies record sea-floor spreading and the passage of time. More often than not, two, three, or even more anomalies of the same kind, positive or negative, run parallel only for limited distances, then are merged into a single grid-like or tree-like pattern by cross-grain anomalies, also of the same kind. To conceive of each of the long, parallel branches as the product of an age different from that which produced each of the others, one must imagine the anomaly from which all of them spring to be itself of several ages identical to each of the disparate ages of its appendages. This is a matter of no little difficulty.
Distracting details of this nature are customarily ignored and frequently omitted from maps displayed by the proponents of sea-floor spreading. The gnarled and misshapen limbs and branches of the actual anomaly patterns are caricatured as arrays of simple, parallel lines, neatly numbered in sequence away from mid-ocean ridges and often even confidently named for the individual polarity epochs that supposedly gave rise to them.
To the geologist or geophysicist whose special interests lie elsewhere, yet who is anxious to be counted among the riders of the bandwagon, it may seem entirely reasonable to attribute negative anomalies to reversed magnetism, and to embrace sea-floor spreading as the one and only explanation for the "evidence." But when closer examination of the raw evidence challenges the received wisdom, it should at least be questioned.
The fact that reversed magnetism was not even suspected in connection with the lineated anomalies for more than ten years following their discovery would seem to tell us something.
In Raff's Scientific American article of 1961, one finds no mention of geomagnetic reversals, although the author examines a number of possible explanations for the strikingly (though imperfectly) lineated magnetic anomalies. It cannot be maintained that contemplation of geomagnetic reversals was not yet in vogue. Paleomagnetic studies on the continents and islands of the world had for some years already engaged the attentions of earth scientists. And as early as 1955 we find an opinion-maker like S. K. Runcorn of Cambridge University acknowledging magnetic field-reversal as an almost certain fact of earth history (see Earth in Upheaval, "Magnetic Poles Reversed").
Eight more years were to pass before Vine and Matthews would argue that negative anomalies must give evidence of reversed rock magnetism. In the meantime, the evidence itself, as viewed by those closest to it, must have seemed readily explainable in more prosaic terms.
Indeed, it was well-understood, as predicted on theory and verified again and again in continental magnetic surveys, that normally magnetized geologic formations could account for the entire ocean-bottom picture, imposing negative as well as positive anomalies on profiles. Vine and Matthews themselves conceded this.(3) Each magnetized formation is itself a magnet. Its field lines, like those of the earth, emerge from one pole, loop in all directions all the way around, and re-enter at the other pole. And as the lines make their loops, they reinforce the earth's field here, oppose it there, giving rise to positive and negative anomalies.
The magnetic latitude of a magnetized formation determines how steeply its axis will be inclined with respect to the ground surface (assumed level). And this, in turn, influences the form of the anomaly curve it will impose on a profile. Reinforcement (positive-anomaly buildup) will be more pronounced at higher magnetic latitudes, and opposition (negative-anomaly buildup) at lower magnetic latitudes. But anywhere on the globe most magnetic profiles will display both positive and negative anomalies in close association and traceable entirely to normally magnetized rocks.(14)
At middle and high magnetic latitudes, if two normally magnetized bodies of rock are fairly close together, their fields combine to oppose the main field in the space between them. A profile along the line joining their positive anomalies will show a deep negative anomaly between them. And this is precisely the picture that appears, repeated over and over, in profiles of ocean-bottom anomalies.
This is the picture, that is, in middle latitudes. Near the magnetic equator it changes.
We have already noted that normally magnetized rocks near the magnetic equator produce anomalies in which negative values are markedly pronounced and positives considerably subdued. This is because buried magnets here are oriented horizontally, parallel to the local lines of the main field. In this position, the longer, outermost lines of the looping field from the buried magnet oppose the geomagnetic field, while only the shorter, sharply curving lines near the poles of the buried magnet reinforce the main field. The lengthwise (north-south) signature of such a magnet is a profile with a stronger negative anomaly terminating at each end in a weaker positive anomaly. And its crosswise (east-west) signature is simply a smooth dip (negative anomaly) and recovery without positive-anomaly boundaries.
On the other hand, reversed magnets buried near the magnetic equator have quite the opposite effects. They produce pronounced positive anomalies as far as they extend in a north-south direction.
It follows that, were the ocean floor near the magnetic equator striped with alternating bands of normal and reversed magnetism, one would expect to find lineated anomaly patterns there as well defined as anywhere, though reversed in sign from the general pattern at middle latitudes. But what do we actually find? "Ridge-anomaly patterns tend to break down near the magnetic equator if the ridge trend is north-south because the amplitudes are apparently comparable to the 'magnetic noise.' "(15)
Lineated, normally magnetized formations would yield profiles exactly like those apparently found: Opposition to the main field would be stronger along the axis of each magnetic body, weaker away from the axis; but opposition would prevail as well in the spaces between magnetic bodies. A magnetic profile across such a region would show very little relief. And if we did not attempt to balance "highs" and "lows" by drawing an arbitrary datum line too low, we might well recognize the profile as that of a series of parallel, normally magnetized rock formations.
Quite clearly, then, the total picture fails to support the hypothesis of sea-floor spreading. Interpreting the lineated anomalies that extend for great distances parallel to mid-ocean ridges requires only a presumption that magnetized rocks of normal polarity behave at sea much as they do on land. This simple truth undoubtedly delayed the arrival of the anomalous idea that the sea floor may be read as a magnetic calendar, or diary, of earth history.
Writing in 1961, Arthur Raff favored an explanation according to which "the strongly positive anomalies are produced by ribbons of highly magnetic, volcanic basalt that flowed into previously formed channels and solidified. [Of all the models considered,] the strips of solidified lava seem to us the most likely." Earlier in his article he had noted that "the right pattern of stress," even without a flow of molten rock, might explain the lineated anomalies, "since the magnetization of rocks varies under stress." But he rejected this piezo-magnetic explanation, characterizing the necessary forces as "highly unusual and unlikely." Nevertheless, he reasoned that the lineations must reflect "some sort of ridged or striated pattern in the crustal rock, obscured by overlying sediment." He added that the magnetic anomaly maps bear "a striking resemblance to the patterns that appear in Bakelite and Lucite when they are placed under stress. This suggests that the present structure is the fossil record of ancient stresses."
Thus, having rejected enormous, global stresses as a direct cause of the oceanic anomaly system through Raff looked again to stresses to produce crustal deformations and cracks through which lava could flow to fill elongated depressions. Presumably the forces implied by the latter interpretation are of magnitudes more readily conceivable.
Yet the scale of the phenomenon remains global regardless of the magnitudes of the specific forces involved. It is interesting to observe that Raff chanced to follow Velikovsky in associating tectonic forces global in scale with disturbances in the rotational motion of the earth. But he searched for mechanisms with uniformitarian vision and found only weak effects arising from tidal actions in a peaceful planetary system.
Nevertheless, what if the early investigators were basically correct in viewing the suggestively patterned ocean-floor anomalies as "the fossil record of ancient stresses?" The relationships between the anomalies and mid-ocean ridges are fully compatible with the idea that the rifted ridges are the scars of violence done to the earth's crustal fabric. The suggestion that contortions of the crust may be completely masked by overlying sediments was substantiated in the early 1960's through echo soundings; the investigators, Maurice Ewing and several colleagues of Columbia University's Lamont Geological Observatory, concluded that "horizontal currents" – a euphemistic term for catastrophic water sloshings of the kind described by Velikovsky (Earth in Upheaval, "A Working Hypothesis," and elsewhere) – must be held responsible for so emplacing sediments as to erase completely all superficial evidence of the "remarkably rugged" basement complex revealed by their studies.(16)
The much-heralded symmetry of the magnetic anomalies flanking opposite sides of mid-ocean rifts finds explanation not in terms of steadily parting crustal plates but in terms of similar stresses and strains on both sides of lines of primary failure. And the strenuously ignored asymmetry of other parts of the anomaly patterns comes to rest quite conformably on the explanation that those patterns reflect stress distributions; there is no residual problem of assigning discrete ages to the several branches of a single, espaliered anomaly. Indeed, on the ocean floors the major asymmetries in anomaly patterns are known to occur where ridges are interrupted and offset by huge cracks, described as transform faults; cracks are precisely where asymmetries develop in structural models made of Bakelite or Lucite, the materials whose stress patterns, or "fringes," were referred to by Raff.
To judge from similar magnetic anomalies on land, we may elect to favor, with Raff, the idea that basaltic lavas solidified in narrow ribbons beneath the oceans and captured normal magnetism from the earth's field to produce the observed anomalies. Yet later stressings might well have brought piezo-magnetism into play; in a context of global catastrophism one need not reject certain forces simply for seeming "highly unusual and unlikely."
In line with the idea of magnetic anomalies portraying stress patterns in the earth's crust, we may note several supportive observations of recent years:
(1) Stanford University geophysicists Sheldon Breiner and Robert L. Kovach reported in 1967 the results of a two-year search for piezomagnetic effects – changes in magnetic susceptibility and remanent magnetism due to elastic deformation – associated with stress buildups along California's San Andreas Fault, a feature now interpreted as a transform fault between two discontinuous sections of the East Pacific Rise. "Local changes in the observed magnetic field were observed on the Hollister differential record in December 1965 and February, June, July, and October 1966. In every case, and at no other times, abrupt creep displacement of the San Andreas fault... has occurred in the vicinity of Hollister some tens of hours after the magnetic event." With more precise instrumentation, the same investigators observed similar correlations between magnetic events and earthquakes on two occasions in April 1967.(17)
(2) Late in 1969 M. J. S. Johnston and F. D. Stacey, of the University of Queensland, Australia, published a report of a "Volcano Magnetic Effect Observed on Mt. Ruapehu, New Zealand." Their magnetometers detected variations "greatly exceeding variations during the preceding months of inactivity and apparently correlated in detail with the volcanic activity" of April 1968. "The favored explanation is based on the piezomagnetic effect."(18)
(3) The Cannikin nuclear explosion on Amchitka Island in 1971 "so squeezed the island's volcanic rock or produced so much stress in the rock that it permanently altered the island's magnetism."(19) – "Either the compressive stress generated by the explosion produced a change in the magnetism of nearby magnetic rocks by squeezing them, or subterranean stresses produced by the test have not been relieved."(20)
Whatever the actual process or processes by which parallel features flanking oceanic ridges became anomalously magnetized, it appears most likely that powerful tectonic forces were involved. The overall picture, divorced from the ill-fitting frame of sea-floor spreading, evokes explanation in terms of Worlds in Collision and Earth in Upheaval.
Somehow, today's well-schooled earth scientist finds it within himself to accept the startling notion that the geomagnetic field has flipped over hundreds of times since the Cretaceous Period, which immediately preceded the present Cenozoic Era in Geologic time. He subscribes to this, apparently, on the assurances of a few specialists claiming to know how an unelectrified earth can generate a magnetic field and, even more, how such a field can be reversed. Yet when Velikovsky argues, on the basis of historical testimony, archaeological evidence, and sound physical principles, that the geomagnetic field has been suddenly reversed on more than one occasion in historical times, the earth scientist looks the other way.
We have seen that a close look turns up little, if any, evidence from the sea bottoms pointing to geomagnetic reversals, and none at all for sea-floor spreading. To the contrary, patterns of rock magnetism beneath the oceans point to global upheaval and turmoil. The earth's crust indeed appears broken into numerous plates, but the cause of all this fragmentation is nowhere to be found in conventional teachings.
Is it too early to predict that the magnetic anomalies of the ocean floor will one day be the final undoing of sea-floor spreading and related concepts which deny the earth an eventful history? One suspects that, were such a revolution to come about, the spirit of Velikovsky's late good friend, Harry Hess – an early advocate of sea-floor spreading – might be the first to applaud.
NOTES AND REFERENCES1. E.g., see S. J. Gould, "Velikovsky in Collision," Natural History, March 1975, pp. 20-26.
2. This work is described by A. D. Raff in "The Magnetism of the Ocean Floor," Scientific American, October 1961, pp. 146-156.
3. Cf., F. J. Vine and D. H. Matthews, Nature 199, 947 (1963).
4. Cf., J. R. Heirtzler, "Sea-Floor Spreading," Scientific American, December 1968, pp. 60-70.
5. Ibid .; see also W. Sullivan, The New York Times, December 10, 1966.
6. Ibid .
7. B. P. Luyendyk, J. D. Maudie, and C. G. A. Harrison, Journal of Geophysical Research 73, 5951 (September 15, 1968).
8. R. L. Larson and F. N. Spiess, Science 163, 68 (3 January 1969).
9. Cf., C. G. A. Harrison, "The Paleomagnetism of Deep Sea Sediments," Journal of Geophysical Research 71, 3033 (June 15, 1966).
10. In this connection K. L. Verosub cautions (Science 190, 48: 3 October 1975): "If a deformation in the sediments is obscured because the sediment is not laminated or if the sediment is sampled by coring and the deformed structures are not recognized, then an anomalous remanent magnetic direction will be observed."
11. B. P. Luyendyk, Journal of Geophysical Research 74, 4869 (September 15, 1969).
12. Ibid . Luyendyk comments: "This value [the arithmetic average that he subtracted from the observed field] is probably within 100 gammas of the regional strength at depth..." This may or may not be true. In any case, however, a shift of his datum upward or downward by 100 gammas would significantly redefine many of the anomalies observed.
13. It should be noted that Raff later, apparently, joined the ranks of the sea-floor spreaders; cf., Journal of Geophysical Research 73, 3699 (June 15, 1968).
14. Cf. H. Jensen, "The Airborne Magnetometer," Scientific American June 1961, pp. 151-162.
15. M. Talwani, X. Le Pichon, and J. R. Heirtzler, Science 150, 1109 (26 November 1965).
16. Reported by W. Sullivan, The New York Times, May 12, 1963.
17. Cf. S. Breiner and R. L. Kovach, Science 158, 116 (6 October 1967).
18. Cf. M. J. S. Johnston and F. D. Stacey, Journal of Geophysical Research 74, 6541 (December 15, 1969).
19. "Atom test blast alters earth's magnetic field," The Arizona Republic (Phoenix), September 5, 1972 (via Washington Post Service).
20. Science News 102, 164 (September 9, 1972).