zapp.03.histpmag history paleomagnetism

7/27/2019 ZApp.03.HistPmag History Paleomagnetism

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Appendix 3: Introduction to and history of paleomagnetism and rock magnetism.

Introduction

The studies of rock magnetism and paleomagnetism, which cover the magnetic

materials in rocks and the record of the magnetic fields they carry, have a surprisingly

long history, although the burst of activity at the middle of the C20th

and since dominates

the subject. As early as 1691, the chemist Boyle showed that bricks acquired a direction

of magnetization parallel to the ambient field in which they were fired. In the middle of

the C19th

, Melloni in Italy had shown that when a basaltic rock was heated and then

cooled in the geomagnetic field, it acquired a magnetization in the direction of the

ambient field, as had Boyles brick. At the end of the C19th

, Folgheraiter published a

series of papers in the proceedings of the Italian Accademia dei Lincei in which the work

by Melloni was cited and expanded to include results showing that the direction of

magnetization found in young basalts was parallel to the recent geomagnetic field. The

classical method of measuring the magnetism of a sample was to observe the effect of its

field on a suspended magnet in a torsion experiment.

Fortunately, rocks contain fine particles of various iron oxides such as magnetite,

titanomagnetite, maghemite, hematitie and sulphides such as pyrrhotite and greigite.

These fine particles are excellent magnetic memory elements and indeed just like those

used in magnetic tape. In this discussion we will not delve deeply into the origin of

magnetism. For example we will not worry about the differences between

ferromagnetism and ferrimagnetism, but simply recognize once again that magnets in the

general sense are aligned in magnetic fields. Once magnetized, the fine particle magnets

found in rocks need strong fields, or some other energetic process to reset their

- Mike Fuller


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magnetization. Given this, the reader may well ask how the weak geomagnetic field

manages to set their magnetism in the first place. Once again somewhat fortuitously, the

formation of the rock succeeds in setting this magnetization and recording the weak

geomagnetic field. One such process upon which much of the paleomagnetic record is

based is the magnetization that lavas acquire as they cool.

Neel theory.

The magnetization acquired when rocks cool in a magnetic field is often

misrepresented as the alignment of particles in molten magma that later becomes a rock.

Actually, the magnetization is usually acquired 100s of degrees below the temperature at

which basaltic rock solidifies. The properties of this thermal remanent magnetization, as

it is known in the trade, were established in the 1930s and 1940s by Koenigsberger in

Germany, Thellier in France and Nagata in Japan. Then, in the late 1940s, Neel

published a theory of TRM that has survived to this day and is the cornerstone of rock

magnetism theory.

Neels theory takes as a starting point the well-known fact that a magnetic particle

is in a lower energy state, if its magnetization is parallel to the ambient field, than if its

magnetization is in any other direction. The magnetic of energy a particle in a magnetic

field is the product of the magnetic moment of the particle and the field (-m.H), which in

turn is equal to the product of the saturation magnetization and volume and field (js.v. H)

Immediately below the Curie point when the particle has become magnetic, the

magnetization carried by the particles is in an unstable state subject to thermally driven

fluctuations in direction. Despite these fluctuations, there is a statistical bias at any instant


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in favor of magnetization close to the field direction because that is the lower energy

state. To be anthropomorphic, we may say that particles are happier in the lowest energy

state with their magnetization close to the field and therefore stay longer in this state than

in any other.

As temperature continues to fall, the energy driving the fluctuations decreases and

the bias in the directions of magnetization in the various particles is preserved as a record

of the field present during cooling. As Neels theory explains this change is very sudden,

so that the magnetization of particles changes from being unstable with changes in

direction in nanoseconds to stabilizing, so that the record is held for billions of years, in a

temperature drop of just a few degrees. We call this the blocking temperature of thermal

remanent magnetization for a particular particle.

Because the statistical bias was acquired at high temperature, the magnetic field

needed to achieve the bias was relatively small and the weak geomagnetic field was

sufficiently strong to be recorded. However, at room temperature the magnetization is

blocked and given the insignificant thermal fluctuations at this temperature they cannot

disturb the blocked magnetization. The weak geomagnetic field certainly cannot affect it

any more. For this reason, the particle retains its magnetization and is not affected by the

geomagnetic field any further. We can collect samples whose magnetization was acquired

millions, or even billions of years ago, measure it, and build up the history of the

geomagnetic field, Similarly the paleomagnetism of lunar samples and meteorites records

the fields in which they acquired their magnetization and allow us to glimpse of these

fields. As always there are complications and difficulties, but thermal remanent


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magnetization can demonstrably carry a high fidelity record of the field in which it is

acquired.

The discovery of Geomagnetic Field reversals

In 1903, Brunhes reported one of the most important discoveries that would ever

come from this paleomagnetic record of the ancient geomagnetic field carried by rocks.

He discovered the reversal of the geomagnetic field. He had heard of the work with

pottery by the Italians and knew that these fired clays gave excellent records of the field

in which they were fired. He decided that natural baked clays, such as those baked by

lava flows should also give a good record of the field. As related to me by Carlo Laj,

Brunhes had a friend in the department in charge of road construction in France and so he

asked this friend to let him know if any road cuts through lava flows with baked

sediments turned up. When his friend did come across one such site, he told Brunhes,

who collected from the fresh road cut. To Brunhes surprise, when he measured the

direction of magnetization of the baked clay he found that it was magnetized not in the

direction of the present geomagnetic field, but in exactly the opposite direction. Instead

of being magnetized to the north and down, as was appropriate for the region of France

where the road cut was, the clay was magnetized to the south and up. Brunhes also

chiseled out some of the basalt and found that this too was reversely magnetized. He

interpreted his results as evidence of a reversed geomagnetic field at the time the lava

initially cooled. The description of his work in his papers takes us back to a wonderful

era in science.


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Additional evidence for reversals accumulated during the first half of the C20th

.

Work by Matuyama in Japan demonstrated that all of the youngest lavas he investigated

were magnetized in a direction similar to the present geomagnetic field, but about half of

the clearly older flows were magnetized in the reversed direction. With the advent of the

new paleomagnetic efforts in the early 1950s, Roche working in the Massif Central of

France and Jan Hospers in Iceland produced convincing evidence of the geomagnetic

field reversal, as did members of Blacketts group at Imperial College. Hospers had even

suggested that in recent geological times the field had reversed roughly once every half

million years and took less than 10,000 years to do so. Both estimates were on the

money. Nevertheless, the topic of reversals was still controversial at mid-century. Much

of the reason was the discovery by Dr. Seiya Uyeda and Professor Nagata of Tokyo

University that some rocks when cooled in a magnetic field, acquired a magnetization in

exactly the opposite direction to that of the field. For some people, it was easier to accept

that the record in rocks was faulty than that the field could actually reverse, so field

reversals remained controversial.

The record of secular variation.

Meanwhile accurate records of the recent history of the geomagnetic field were

being built up. These studies established the history of what is termed secular variation.

Just one year after Boyle had shown that bricks could record the magnetic field in which

they were heated and allowed to cool, Halley had shown that this secular variation

involved a westward drift of the field. It was nearly another 200 years before Chevallier


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developed a secular curve using a series of flows from Mt. Etna that were historically

dated and could be used to show that Halleys observation persisted from earlier time.

In France, Emile Thellier and his wife had taken up the challenge of the recent

record of the geomagnetic field and laid the modern foundations of what is now called

archaeomagnetism. So far in this account we have only discussed the direction of

magnetization, but magnetization is a vector having direction and magnitude, as is the

geomagnetic field. The Thelliers were interested in the strength of the field, as well as its

direction and they found, like the Italians before them, that pottery was an ideal material

for these studies. That pottery contained very fine magnetic material, which was

magnetized during the firing of the clay and retained a faithful record of the field was

well known when they started work.

Koenigsberger, Thellier and Nagata had established that the intensity of thermal

remanent magnetization is proportional to the field in which it is acquired. One might

therefore think that it would be very easy to get the intensity of the field in which the

pottery was fired and acquired its magnetization. All that was needed would be to

measure the intensity of the magnetization of the pottery, give it a thermal remanent

magnetization in a known laboratory field and the ratios of the strength of the

magnetizations would give the ratio of the strengths of the fields. This did not turn out to

be quite so simple because the heating required to give the thermal remanent

magnetization in the laboratory sometimes destroyed the magnetic material in the pottery,

so that there was less material to acquire the laboratory thermal remanent magnetization

and the ratio no longer gave the correct ratio of fields.


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In the 1930s Koenigsberger in Germany and independently Emile Thellier in

France developed an incremental heating method that took advantage of the way in which

thermal remanent magnetization is discretely blocked in certain temperature intervals.

The method was based on the three laws of partial TRMs (pTRM) that had been

demonstrated by Thellier. By partial TRM we mean TRM acquired in some restricted

temperature range below the Curie point. The first law was the law of reciprocity which

states that the pTRM acquired in a temperature range of T(1) to T(2) is demagnetized in

the same temperature range. The second law which is the law of independence states that

the pTRM acquired in a particular temperature range is independent of pTRM acquired in

any other temperature range. The third law, the law of additivity states that pTRM are

additive in the sense that the pTRM acquired between

pTRM T(1) to T(2) + pTRM (2) to T(3) ..+ pTRM T(n-1) to T(n)

= pTRM T(1) T(n)

Taking advantage of these relationships one can compare the ratio of thermal remanent

magnetization to the natural remanent magnetization that the rock, or pottery carried

incrementally. In this way one can detect the destruction of the magnetic phases and

hence has an idea of the fidelity of the intensity estimate. Without going further into the

details of how the method worked, it is worth noting that despite many attempts to

improve the method, including some by the author, the method is still the gold standard

of intensity determinations. It is usually referred to as the Thellier-Thellier method.

Following Nagatas example, lets call it the Koenigsberger-Thellier-Thellier method to


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acknowledge the contribution of Koenigsberger, without any disrespect for the work of

the Thelliers. As early as 1938, Koenigtsberger had worked out the basis of the method

and presented results in the form we use to day. This was the same year as Thellier

presented his famous paper on the baked clay.

The results obtained by the Thelliers after the war began the process of building

up the record of the intensity of the magnetic field. It soon became clear that the

remarkable decrease in intensity that had been established by observatory records of the

field since Gausss determination in 1837 continued back through the two millennia since

Christ. In 1958, only the first few intensity determinations were available.

The paleomagnetic test of continental drift.

In addition to the use of these techniques of modern paleomagnetism to

investigate the past history of the field, paleomagnetism had plunged into a second more

controversial area - Wegeners theory of continental drift. The theory had for a long time

had its advocates, but also its disparagers, among whom were many of the more

distinguished theoretically inclined geophysicists of the time, who quite reasonably did

not see how the relatively thin crust of the earth could move around independently of the

material below. Remember that on the grand scale of our planet, the crust, mantle and

core are in about he same ratio as the skin, main fruit that we eat, and core of an apple.

Some of the strongest advocates were southern hemisphere geologists, who were

familiar for example with the detailed similarity of the geological sections in parts of

South America and Africa. These suggested the two had been contiguous at some stage

and moved apart subsequently. This strengthened the notion of drift based upon a simple


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comparison of the west coast of Africa with the east coast of South America. Another

advocate was Arthur Holmes, who wrote a classic text entitled The Principles of

Physical Geology, much read by physicists, needing to brush up their geological

knowledge. Nevertheless for most people in 1950s, continental drift was not

satisfactorily demonstrated - given a room full of geologists, roughly half would argue

vehemently for continental drift, whereas the other half would argue equally strongly and

convincingly against it. Clearly, some new data was required to settle the question.

Probably, Mercanton, who worked between the World Wars in France, was one of

the first to see that magnetic record of the geomagnetic field in rocks might be used to

test the idea of continental drift. Later Blackett1 became interested in this possibility, after

reading the 1948 paper of Johnson, Torreson and Murphy entitled the Pre-History of the

Earths magnetic field. He too saw the possibility of using the paleomagnetic record to

test continental drift. The recorded ancient inclination of the site could be compared with

present inclination. Then, assuming that the earths magnetic field had the same simple

form, when the rocks formed as it does at present, the difference recorded a change in

latitude between the rock present location and where it was when it acquired its

magnetization Any departure of the magnetization from north recorded a change in

orientation of the site by rotation. The situation is analogous to a sailor navigating by

_____________

1

Blackett had been a graduate student of Rutherford in the 1920s and had won a Nobel

Prize for his beautiful work with cloud chambers that had revealed collision products of

atomic nuclei hit by alpha particles.


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sextant, he can tell the latitude he is at and he can keep a record of the heading of the

boat, but he does not know his longitude without a clock. Blackett also had a sensitive

magnetometer capable of measuring the weak magnetism of sedimentary rocks. Although

the lavas had played a central role in the discovery of reversals, the record from

sediments was needed to build up the paleomagnetic record to test continental drift.

Unlike the lavas and baked clays that acquired a thermo-remanent magnetization (TRM)

during cooling, sediments acquired their magnetization during formation at essentially

room temperature.

The process whereby sediments acquired their magnetization is in principle

simpler than TRM, but is still not completely understood in detail. Certainly, particles

falling through water in the oceans, or in lakes experience a torque due to the

geomagnetic field and tend to align like compass needles. Eventually they reach the

bottom of the water column and become a part of the accumulating sediment. Initially,

they are still free to rotate in the water logged sediments and so in their new environment

they show a statistical bias towards the direction of the geomagnetic field.

However, the process of sedimentation, including, such indignities as bioturbation

produce a good deal of randomization. As the rock loses water and solidifies, the particles

are less and less free to rotate and eventually they are locked in place and their

magnetization is locked in with them. A magnetic record of the field is then retained.

become a part of the accumulating sediment. Initially, they are still free to rotate in the

water logged sediments and so in their new environment they show a statistical bias

towards the direction of the geomagnetic field. However, the process of sedimentation,


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including, such indignities as bioturbation produce a good deal of randomization. As the

rock loses water and solidifies, the particles are less and less free to rotate and eventually

they are locked in place and their magnetization is locked in with them. A magnetic

record of the field is then retained.

This depositional, or detrital remanent magnetization (DRM) tends to be about a

thousand times weaker than the saturation remanent magnetization that the rock carries

after being exposed to a very strong magnetic field, say from an electromagnet. This is

about a factor of ten less efficient than the TRM process. Because sediments often have

less magnetic material in them than do basaltic flows, their magnetization may be very

weak indeed and require very sensitive magnetometers to read the record.

The reason that Blackett had a sensitive magnetometer was because he had

wanted to test a theory of his about magnetic fields and the rotation of massive bodies.

The idea was that the origin of the magnetization of stars was a fundamental property of

their rotation. To check this idea, he built an astatic magnetometer underneath which the

magnetism of a gold cube was to be measured, so that the field predicted by the theory

could be detected. The work developing this system and the negative result were

described in a famous paper by Blackett entitled A negative experiment relating to

magnetism and the earths rotation and published in the Transactions of the Royal

Society.

Paleomagnetic techniques.

This is perhaps a good place to introduce the nuts and bolts of paleomagnetic

studies from sample collection to measurement and analysis.


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Fieldwork and collecting samples

Initially of course the sample has to be oriented and this was done by the simple

geological technique of finding and marking a dip and strike on a convenient flat surface.

The strike was found with what is essentially a spirit level in a Brunton compass. The

magnitude of the dip was found by aligning an inclinometer (also in a Brunton compass)

perpendicular to the strike, and measuring the angle by which the surface dips down from

the horizontal. The samples were then cut, or chiseled out of the face, avoiding

destroying the orientation marking. The large (about 6 x 6 x 6 inches) sample was then

lugged back to the vehicle used for collecting and transported it back to the laboratory for

preparation and measurements. The next stage was messy and slow. The samples were

set in plaster of Paris with the original dip and strike directions now horizontal. Finally,

small inch sized cylinders were drilled and cut for measurement.

It was soon found that taking a portable drill into the field was a much better

method and so we all converted chain saws to rock drills and lugged water and drills

around. The orientation was done after the core had been drilled but before it was broken

out. The great advantages are not lugging the samples back and the fact that the

cylindrical drill samples only have to be trimmed for measurement on returning in to the

laboratory.

All of us who have collected samples for paleomagnetism around the world have

had our share of adventures that this sort of behavior in remote parts of the world is heir

to and I have related some of mine in this text.


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Magnetometers

The initial workhorse of the subject was the astatic magnetometer based on

Blacketts approach. The principle of an astatic magnetometer is simple, but hard to

realize experimentally. Two magnets of equal strength and opposite polarity are mounted

on an assembly that also supports a mirror. This assembly is then mounted on a torsion

fiber and its deflection can be observed with a lamp and scale. In theory, such an astatic

system will not respond to a homogenous magnetic field because the torque that is

experienced by the upper magnet is opposed by an equal torque in the opposite sense

from the lower magnet. It therefore will not respond to the main geomagnetic field,

which does not vary significantly on the scale of the distance between the two magnets.

The tricky part is of course to get the two magnets very well balanced and aligned. When

this is achieved a sample can be placed below the bottom magnet and its field is stronger

at the lower magnet than at the upper, so that it unbalances the astatic system. The system

will then rotate and the field from the sample can be sensed. This is a classical torque

experiment used for generations by physicists. With multiple readings the magnetization

direction and intensity can be determined.

These astatic magnetometers are unfortunately tricky instruments to use, as well

as to make. One of the problems is that to make the system more sensitive, the torsion

fiber has made weaker, so that it responds to weaker torques. One type of fiber used was

finely drawn quartz, whose preparation was a very delicate art. As a consequence of

making the fiber weaker, the period of the instrument increases, so measurements take

longer and instrument drift problems become serious. Drift of an instrument may occur

due to any number of small changes in the equipment in response most often to minor


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temperature changes. Many a graduate student of my vintage has cursed these

instruments, as hours were spent watching minute deflections of a spot on a scale to

measure weak samples.

We have already seen what an important role the astatic magnetometer had played

in measuring the weak magnetization of sediments. Initially the Jodrell Bank

magnetometer was used by both Blacketts and Runcorns groups. In a recent talk at an

American Geophysical meeting, Ken Creer acknowledged what an important role

Blackett played, not only in providing the equipment, but in lending his considerable

reputation to the whole endeavor of paleomagnetism.

There were other instruments that measured the remanent magnetization of rocks

by spinning them near to a pick up coil hooked up to sensitive amplifiers. As Michael

Faradays celebrated law of induction formalizes, an electromotive force is induced in the

coil and a current flows proportional to the magnetization perpendicular to the spin axis.

To cut down noise the pick up coil is placed in a -metal shield. By spinning the sample

successively about three mutually perpendicular axes, the total remanent magnetization

of the sample can be found with some statistical redundancy. Eventually with advent of

modern electronic techniques, these became very sensitive and replaced the astatic

magnetometers. Nevertheless most of the classic paleomagnetic studies related to the

movement of landmasses were made using astatic magnetometers

It had also been realized from the early work that rock samples often carried a

magnetic record that consisted of a signal plus noise. The signal could be related to the

field that the rock had initially recorded during its formation. The later magnetizations

acquired by the rock could be of interest, or could be noise picked up by as mundane a


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problem as being exposed to a magnetic field during collection, or processing. Evidently

a procedure was needed to separate the signal from the noise to improve our

paleomagnetic records of the ancient field.

In our discussion of the Koenigsberger-Thellier-Thellier method of intensity

determination we saw that the original magnetization in the pottery, or rock was

incrementally replaced. Thus for example, the magnetization up to 100C was

demagnetized by heating the sample to 100 C, which unblocks that fraction of the

magnetization. The rock was then cooled down in a laboratory field to give a

thermoremanence in the same temperature range. Now, if instead of cooling the sample

in the laboratory field, it is cooled in zero field, or as near as we can get to zero field, then

the magnetization blocked below 100C will be largely randomized and eliminated. We

would have preferentially eliminated the magnetization blocked at low temperature and

we can then remeasure the sample. We can repeat the process going to progressively

higher temperatures. Often the noise is blocked in a lower temperature range compared

with the signal, so we improve the signal to noise ratio. If we are lucky, we may totally

eliminate the noise leaving our signal untouched.

There are other methods of demagnetization, such as alternating field

demagnetization and chemical demagnetization. All of them depend on somehow

separating the signal from the noise and eliminating the latter. The most commonly used

is alternating field demagnetization. Figure A3.5 (A) illustrates a situation in which the

signal and noise are carried by particles whose magnetization can be reset, or switched,

by different fields. In alternating field demagnetization, we take advantage of these

distributed switching fields of the particles carrying the magnetization. Figure A3.5(A)


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could also be plotted against temperature to illustrate the principle of thermal

demagnetization, in which case it would be the distribution of blocking temperatures that

would be used to separate signal and noise as described In the previous paragraph.

If an alternating field is applied in the absence of any ambient field, for example

when the sample is being shielded from the geomagnetic field. All particles whose

magnetization can be switched by the field are in equal numbers in each direction of the

alternating field. This field is then decreased to zero leaving the particles equally

magnetized up and down as shown in figure A3.5 (B). Those particles not switched by

the field are untouched. If we apply a field in between the switching fields of the signal

and noise, as shown in figure A3.5(A), then we will eliminate the noise and leave the

signal untouched. The application of the technique is rarely quite as simple as the

textbook example, but it is a very powerful technique universally used. It was pioneered

by the Dutch workers and in particular Hans Zijderveld, who also invented a clever

diagram to help interpret the results. In this technique, magnetization, which can be

randomized by a small alternating field is first eliminated and then the sample is

remeasured. The procedure is repeated with the field strength is progressively increased

thereby increasing the signal to noise ratio and again if we are lucky we may isolate the

signal from the field the rock was formed in.

The new results bearing on Wegeners continental drift.

One of the earliest papers to provide convincing evidence that the magnetization

of sediments required movement of at least parts of the crust relative to the magnetic field

was by Clegg, Almond and Stubbs in 1954. They recorded their results in the form of


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radial frequency plots (fig.A3.6). These clearly showed that the declination was rotated

clockwise from north, which was interpreted as indicating that the site had been rotated

by this amount since the rock had been magnetized. The inclination results were a little

more complicated in that they were not antipodal. Presumably, they had not been entirely

successful in cleaning out all of the noise. However, it was clear that the rocks must have

become magnetized at lower latitudes than those at which they were now found.

The authors were able to apply two tests of stability on a geological time scale

that had been published by John Graham in America in 1949. The first of John Grahams

tests is called the fold test. For this, the magnetization is observed across fold, which are

the rumples one sees in sedimentary sections, like those in a carpet that has been buckled

up. If the magnetization is constant when observed with the sedimentary layers folded, it

must have been acquired after the folding. On the other hand, if the magnetization is

constant when the fold is flattened out and the bedding planes returned to their original

configuration, then themagnetization must have been acquired before the folding. The

results from the New Red Sandstone passed this test.

A second test that is not quite so directly useful, but still can be a helpful indicator

is the conglomerate test. Conglomerates are sediments made up of fragments of earlier

sediments including coarse material that can reach boulder size. If the magnetization of

boulders of the sediment of interest are found to be randomly magnetized in the

conglomerate, this is good evidence that their earlier magnetization has survived and not

been reset by the processes of transportation and formation of the conglomerate. On this

count too, the remanent magnetization of the New Red Sandstone passed muster as a


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stable magnetization, probably acquired when the sediment formed some 150 million

years ago.

Another remarkable aspect of this study was that the rocks gave two sets of results

one group was magnetized with an easterly declination and a downward dip shallower

than the present field. The other was magnetized to the south an upward with again a

smaller dip than the present field and a direction rotated clockwise from due south. If one

assumed that the Triassic geomagnetic field was dominantly a dipolar field aligned close

to the rotation axis as is the present field, then it appeared that these sediments must have

been nearer to the equator when they formed and had subsequently rotated clockwise.

Moreover, the two directions were nearly exactly the reverse of each other, so that the

results were also consistent with the field reversing during the Triassic.

Meanwhile the group led by Professor Runcorn, first at Cambridge and later at

Newcastle-upon-Tyne, had made major progress. Ted Irving, who was to play a pivotal

role in the vindication of continental drift by paleomagnetic methods, carried out a study

of the Pre-Cambrian Torridonian sandstone from Scotland and published it as a Masters

thesis in the same year as the Clegg, Almond, Stubbs paper came out in 1954. He too had

found that sites at which he had collected from the Torridonian had moved far from their

location when the sediment acquired its magnetization. The results also fell into what

appeared to be normal and reversed groups, so Irvings results also argued for reversals.

Indeed Irving had plotted his results in a way that suggested the use of the patterns of the

sequence of reversals to correlate different geological sections within the Torridonian.


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Controversies in paleomagnetism.

With the benefit of hindsight, it is clear that by 1958 there were plenty of data

from paleomagnetism consistent with reversals of the earths magnetic field and

continental drift. Why then were these ideas not accepted?

The controversy over reversals continued for another decade. The principal

reason, as we noted above, was the discovery of self-reversal - the process of acquiring

magnetism in a direction to the ambient field. If rocks could acquire thermo-remanent

magnetization in the laboratory opposite to the field in the laboratory, why shouldnt this

happen in nature. There was also a suggestion that there was a correlation between the

more highly oxidized lavas and those that were reversely magnetized. This strengthened

the case for self-reversal because self-reversing magnetization had initially been found in

more oxidized lavas. Yet these ideas neglected the good work of so many people

demonstrating the reality of reversals that it was relatively easy for me to accept the

reversal of the geomagnetic field. Perhaps this was also because of my early experience

of the fieldwork in Arran. John Leng had demonstrated that where the dykes were

reversely magnetized, so was the magnetization of the Old Red Sandstone that had been

heated and reset when the dykes were intruded. Conversely where the dykes were

normally magnetized, so were the baked sediments. This was a convincing argument that

the field had sometimes been reversed and sometimes normal during the period when the

dykes were intruded. Both the dyke and its baked contact had acquired the magnetization

of the field when they cooled.

The demonstration of field reversals was eventually to come much later when it

was found that rocks younger than about 780,000 years were all normally magnetized,


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that before that there was period of some 200,000 when the rocks were reversed. This

pattern was repeated back through geological time with rocks of the same age having the

same polarity. Clearly it was very unlikely that all rocks of a particular age were self-

reversing and much more likely that the geomagnetic field spends time in the normal

state and some in the reversed state. However, the general acceptance of field reversal

came still later when reversals played a key role in establishing the grand unifying theory

of the earth sciences.

The continental drift controversy continued on for years. The possibility of

explaining the results by polar wander was suggested. For some it was easier to accept

the motion of the pole of rotation with respect to the mantle. Tommy Gold had pointed

out in a characteristically amusing way that if a beetle moved on the surface of the earth,

the distribution of the earths mass changes. The moment of inertia of the earth changes

and the rotation pole will adjust or tilt to give rotation about the axis of the maximum

moment of inertia.

In 1957, Runcorns group published work that took the view that polar wander

could explain the observations. They described their results in a paper authored by Creer

Irving and Runcorn in the form of Apparent Polar Wander curves. This was possible

because one could always calculate the magnetic pole that accounted for the

magnetization direction observed providing one made the assumption that the field of the

earth always had the same simple form as the field of a sphere. The horizontal component

through the angle of declination from the meridian gave the direction to the pole. The

inclination, the angle by which the field direction deviates from the horizontal, gave the

distance to the pole through a simple relationship. The curves for the succession of poles


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for Europe and North America are shown.While they were aware that continental driftcould explain the results they preferred the recognized possibility of polar wander. This

hypothesis required fewer assumptions to be made and did not involve the difficulty of

continental crust plowing through the mantle, which was highly implausible.

Within Runcorns group the recognition of continental drift occurred at different

times for the various members. Ted Irving says that he had always been sympathetic to

the idea of continental drift, having been taught it in school. Ken Creer, who took the

crucial step of plotting the first polar wander curves based upon the paleomagnetic

results, must have recognized drift by the late 1950s. Neil Opdyke claims to have been at

Keith Runcorns Damascus Road like enlightenment. He tells of a lunch in the spring of

1956 with Westoll, a distinguished Professor of Geology at Durham University, who was

a strong supporter of drift. At that lunch Runcorn accepted continental drift. Although

there are always difficulties in tracing the development of ideas and temptations for

revisionist history, it is clear that although Runcorn eventually argued very strongly and

correctly for continental drift, he took a lot of persuading initially.

In 1958, the London group favored continental drift and in 1960, they published a

landmark paper An analysis of rock magnetic data by Blackett, Clegg, and Stubbs. In

this paper they presented the data by reducing the results to a standard city for each

continent and recorded its ancient latitude and rotation given through geological time by

the rocks to give figure A3.9. As we can see the different landmasses have moved with

different velocities, with Indian being the fastest. The landmasses also had different

histories of rotation. In the initial tabulation of the results they used the difference in

inclination between the ancient and present latitude, and the rotation, which they defined


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as the difference between the observed declination transformed to a positive or negative

quantity depending upon whether it was clockwise or anticlockwise. Here they

anticipated methods to be used decades later for determining the rotational history of

small regions with respect to the continental mass, of which they were a part. Without

wishing to reopen old battles, this paper has not, in my opinion, really received its due.

Of course, I may be prejudiced by my relationship with the London group through my

family. Be that as it may, I will quote in detail from the abstract of the paper as follows.

The results of this analysis strongly support the supposition that the

observed wide divergence between the directions of the remanent magnetic vectors of

older rocks and that of the present field is systematic and not a result of purely random

processes occurring through geological time. The most reasonable explanations of the

phenomenon appear to be that (a) the directions of earlier rocks have been changed by

some widespread physical or geological process since the time of their formation, (b) the

earths magnetic field has had strong multipolar components in past geological ages, (c)

a relative drift of the continents across the earths mantle has occurred. Of these

hypotheses, (c) appears to be the most plausible. On the tentative assumption that the

rock magnetic results can be explained by continental drift, it is possible to estimate the

ancient latitude and the orientation relative to the earths rotational axis of each

continent although by paleomagnetic measurement alone changes in it relative longitude

cannot be revealed.

By 1962 it was clear that the polar wander paths for America and Europe,

deviated systematically from each other when the paths were plotted in the configuration

of the landmasses today. However, when the masses were moved into the hypothesized


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drift positions the paths agreed. This was pretty convincing evidence of continental drift.

Yet, continental drift was still not generally accepted until like reversals of the field, it

came along as part of plate tectonics - the grand unifying theory of the earth sciences that

we keep anticipating.

This then gives some idea of the state of development of paleomagnetism that had

taken place up to about the end of the 1950s when I started my research.

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Figure A3.1 Illustration of energy in a magnetic field H

- Mike Fuller


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Normal magnetization in France Reverse magnetization in France

Towards the North and Down. Towards the south and Up.

N Present Field S

Figure A3.2 Cartoon illustrating the discovery of reversely magnetized

rocks in France by Brunhes.


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Figure A3.3 Cartoon of depositional remanent magnetization process.


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(a) torsion head

(b) remote configuration to cut down noise.

Figure A3.4 Astatic magnetometer and (b) remote configuration to cut

down noise.

Figure A3.5 Spinner magnetometer.


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Fig. A3.4 Equipment for thermal demagnetization (a) Use of field

Helmholtz coils within a shielded room to minimize ambient field by

Bob Dunn and (b) Use of nested mu-metal magnetic shields to minimizeambient field by Schonstedt Co.


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(A) (B)

Figure A3.5 Alternating field demagnetization. (A) distribution of

switching fields of particles carrying noise (JA) and signal (JB), (B)

process of demagnetization.


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Figure A3.6. The remanent magnetization in the New Red Sandstone

from various sites in England presented as frequency polygons. Upper

plot declination, Lower plote inclination.

Figure A3.7 Fold, Conglomerate and baked contact tests.

Figure A3.7 Stability tests (a) the fold test, (b) the conglomerate test and

(c ) the baked contact test.


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Figure A3.8 Apparent polar wander curves for Europe and North

America.

Figure A3.9 Paleomagnetic results compiled for different continents.

zapp.03.histpmag history paleomagnetism

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