The Discovery of Palaeomagnetism and Acceptance of Continental Drift

By the middle of the 20th Century, scientific advances and exploration of the seafloor finally 'opened the doors' (and the minds of the scientists) to the acceptance that continents can, and do, move.
The following discussion describes the major discoveries and key characters responsible for this, and covers:

• Palaeomagnetism
• Palaeomagnetism of the British Isles
• Palaeomagnetism of the World
• Vine and Matthews
• Proof and Acceptance

Palaeomagnetism

As has been known for centuries, the Earth is magnetic, with poles at either end. These poles are close to (but not exactly coincident with) the Earth's axis of rotation, and both its position around the geographic pole, and the strength of the magnetic field, vary.
In the 20th Century, the magnetic North Pole moved over 1000 km towards the geographical North Pole, and in the decade of the 1990's the strength of the magnetic field fell by 1%.

What causes the magnetic field is not known for sure. It is most likely that it is related to movements in the iron-rich, partially-liquid, core. Convection within the core would set up a dynamo effect, which would produce a magnetic field. This field acts through and over the surface of the planet, and beyond.
Magnetic compass needles point toward their opposite pole, as do the magnetic components of iron-rich, molten rock. Within lava, these are magnetite crystals, which become permanently magnetised in the direction of the Earth's magnetic pole when it cools and hardens into rock.
Thus, as lava flows and sets, it 'records' the relative direction of the magnetic pole at the time it solidifies. Furthermore, because the magnetic field acts through the planet (rather then just over its surface), the angle of dip (the inclination) of the magnetite crystals record the distance from the pole, which thus indicates the latitude at which the rock formed.
If either the solid rock, or the pole, moves, the original orientation and inclination of the magnetic pole with respect to the rock is retained within the magnetite of the rock.

In 1849, a French physicist (Achilles Delasses) had made the discovery that rocks were magnetised in parallel with the Earth's magnetic field; and in 1906 another Frenchman (Bernard Brunhes) made the more startling discovery that some rocks are magnetised in the exact opposite orientation to the Earth's field. This led him to propose that the phenomenon was the result of a past reversal in the polarity of the global magnetic field.

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The Palaeomagnetism of the British Isles

Patrick M. S. Blackett

In the early 1950's, this magnetism of old rocks was used to investigate palaeomagnetism - the new study of the history of the Earth's magnetic field. Blackett had long been interested in the concept of continental drift, and believed that magnetic measurements might help determine whether the great land masses had indeed shifted their position.
Studies of British rocks of known age from a variety of locations revealed two surprising factors:

• The magnetic orientation (the direction to which the magnetic component pointed) deviated by as much as 30° from the modern position of the North Pole, depending on the age of the rock.
• The magnetic inclination (the dip, that indicates the latitude at which the rock 'set') was far less than it ought to have been. This also depended on the age of the rock, and could only have occurred of the rock had been formed at a far more southerly latitude.

Together these factors led to an inescapable conclusion: That Britain had rotated through 30° of longitude, and at the same time drifted north from a more southerly latitude, over the past 200 million years.

Polar Wandering

Another British scientist thought differently, and proposed that it was the poles, not the land, that had wandered. Like Blackett, he (S. Keith Runcorn) used magnetised rocks from the British Isles (and Europe), but plotted instead the projected movement of the magnetic (north) pole over time, in what is now termed a 'polar wandering curve'. He presented his findings in 1955, commenting that they did not indicate continental drift, they showed the movement of the magnetic pole.
However - very soon, his measurements were compared with a similar set, plotted for North America. The palaeo-positions given by the magnetised North American rocks was an almost perfect match with those that Runcorn had calculated - except they are all displaced west by 30°. As the Earth has got only one magnetic (north) pole, the only possible explanation was that the two continents, North American and England (within Europe) had once been 30° closer together - or, there had once been no Atlantic Ocean separating them. Runcorn was converted, and concluded that although the poles may have wondered, the continents had also drifted.

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Palaeomagnetism of the World

However; although more evidence was found around the world, especially in the thick layers of the Deccan traps in India, the scientific establishment was still not impressed. Their reasoning was good - the science of palaeomagnetism was (and remains) inexact. Rocks are undependable recorders of magnetism to any great degree of accuracy, and many scientists thought that the palaeomagnetic evidence for continental drift was based on "inadequate sampling, inaccurate measurements, and unjustified assumptions". It was especially easy to distrust the palaeomagnetic arguments when they lead to the improbably speculation that the Earth's magnetic field could reverse itself, transposing its north and south poles.

Polar Reversals

Within a decade, by the 1960's, the proposition of a polar reversal gained respectability, when a team collected and studied tonnes of rock from around the world. Using radioactive testing methods to date the samples, they identified nine magnetic reversals in the past 3 million years. Later studies have shown that the Earth has switched its magnetic polarity at least 171 times in the past 76 million years - but the changes show no pattern or predictability. The reversals themselves take from 5 000 - 10 000 years to happen, and once established, can last for 10's of thousands, to millions, of years. The most recent was 700 000 years ago; but whether another is due in the near future is unknown. However, some Danish scientists believe that one is due within the "next millennium", citing a recent increased speed of movement of the magnetic pole, and weakening of the magnetic field, as evidence. For more about this, please got to Physics Web

Seafloor magnetism

As the surface geologists investigated the rocks around the world, those working on the ocean floor met with more mysteries. In the mid-1950's, a research ship carried out a detailed investigation of a large patch of the Pacific Ocean floor, tugging behind it a 'mag-fish', which measured the intensity of the Earth's magnetic field. When the results were plotted, a curious pattern of stripes, of alternating strong and weak magnetic fields, was observed. The zebra-like pattern turned out to run roughly parallel with the coast; and a similar pattern of stripes, of alternating magnetic strength, was also seen in adjacent parts of the ocean.
For years, nobody knew what to make of it.

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Vine and Matthews

Back in England in Cambridge University, the presence of Sir Edward Ballard had attracted an enthusiastic team of young scientists, who were adherents of the controversial subject of continental drift. Fredrick J. Vine, a graduate student, and his supervisor Drummond Matthews were among them, and both were firmly convinced by the theory of continental drift.
In 1962, they carried out an ocean survey along part of the Mid-Ocean Ridge, and saw the same evidence of zebra-like patterns of strong and weak magnetic intensity that had been observed in the Pacific years earlier, and other oceans since. Although they varied, some of these magnetised stripes were over 20  miles (30km) wide.

Puzzling over Hess's "geopoetry" of sea-floor spreading and their own results, they suddenly realised that they probably had proof, there in front of them, of its veracity. The magnetic stripes may not, they thought, reflect the intensity of the magnetic field, but its direction. They were both aware of the idea of pole reversal; what if the stripes that were thought to indicate only a weak magnetic field instead held a magnetic imprint of a field in the reverse direction?

Thus, they reasoned:
If hot mantle material was welling up in the Mid-Ocean Ridge, it would be magnetised in the direction of the Earth's magnetic field as it cooled. If the seafloor was spreading, then this band of magnetised rock would be carried slowly away from the ridge. And if the pole reversed from time to time, then the stripes on the seafloor, which were parallel to the ridge, would be magnetised in alternate directions. Furthermore; since the dates of pole reversals had been roughly calculated, then the magnetic pattern on the spreading sea floor would document not only the floor's age, but also the rate at which it was spreading.

Vine and Matthews gathered more data, and in 1963 were able to publish an article, "Magnetic anomalies over Oceanic Ridges" in the scientific magazine Nature.
But they had little response - most scientists thought that polar reversal was as unlikely as the idea that the sea floor might spread - so a theory combining the two was impossible to take seriously.

At about the same time, a Canadian geophysicist (Lawrence W. Morley) had reached exactly the same conclusion - but his paper had been rejected with a short, sharp note, saying that "such speculation makes interesting talk at cocktail parties, but is not the sort of thing that ought to be published under serious scientific aegis".

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Proof and Acceptance

Evidence mounted and more great minds applied themselves to the theories. Although Vine and Matthews did not win many supporters with their paper, they did gain very important adherents. Between them, they produced enough proof to change the views of the global geological community, and most of the population - who were probably more willing to accept the 'evidence of their own eyes' in the form of a world map than had been the scientists who knew that continents could not possibly move, whatever appearances said.

Tuzo Wilson

One who did take seriously the idea of seafloor spreading and polar reversal was a Canadian, Tuzo Wilson. He was converted from the traditional cooling-and-contracting Earth theory by this combination of palaeomagnetic evidence and Hess's 'seafloor spreading geopoetry'. In 1964, he came to Britain to participate in a 'Continental Drift symposium', sponsored by Britain's Royal Society. There, Sir Edward Bullard presented a map that he had created using the latest computer technology to match the coasts of Africa and South America. This, and other presentations at the landmark gathering convinced a lot of sceptics, who had gone to with no intention or interest in conversion.

The Cambridge Conclave

In early 1965, Wilson and Hess met with Vine and Matthews in Cambridge, where they discussed, of course, the geological implications of sea-floor spreading. During these discussions, it occurred to them that if they were right - and they were sure they were - then the magnetic patterns on either side of the ridge ought to be symmetrical. Careful studies of the maps they had there showed the expected mirror image symmetry. Furthermore, a correlation between the widths of the stripes, estimated rates of seafloor spreading and the (independently calculated) time-scale of polar reversals over the past 4 million years also matched.

Equivalent research by others and in other oceans gave compatible results, and finally, the theory gained growing respectability.
While considering displacements in the ridge where it appeared to take staggered 'steps' and cracks that ran at right-angles from it, it occurred to Wilson that they could not be the normal 'transcurrent' faults that are familiar features of earthquakes and land movement. The faults "faded away" into nothing, and where there would have been compressed or stretched rock at either ends of a normal fault due to land-movement, there were no signs of stress. Further considering the geometry of a spreading ridge, he realised that the cracks could simple be 'tears' in the new rock, where there had been weaknesses in the ridge as it tore apart. Were this the case, the movements along two adjacent faces of the fault would only be in opposition to each other in the section between offsets of the ridge; but beyond that 'overlap', movement would be in the same direction. This would be different to the situation seen in the only faults thus far known, in which the movement along the whole face of the fault is in opposing directions.
He named this new form of fault a "transform fault", and predicted that examination of the cracks running out from the spreading ridge would indeed be a new form of fault.

Sykes and Transform Faults

Wilson claimed that this would explain strange seismic records taken along sections of he East pacific Rise ridge by a young seismologist, Lynn Sykes. He was initially unimpressed, as he doubted the seafloor spreading concept. However, studying the his results in the light of Wilson's new faults, he became a convert, and set out to prove the theory. This he managed to do within his laboratory, studying microfilms taken of earthquakes around the world. Within a few days he had found 20 examples; and every one showed slippage in the opposite direction to that expected by traditional geology. Proof, finally, that would convince the vast majority scientific community. In 1966, Sykes presented his findings to the Geological Society in Americas, and by the next year, almost 70 abstracts for sea-floor spreading had been submitted for an upcoming meeting of the American Geophysical Union, a major scientific conclave within the USA. Even those unconverted attended, intent on showing how their own observations were incompatible with sea-floor spreading and thus disprove the latter. And as each re-examined his data in the light of the new theory, they realised just how workable the new hypothesis was.

Fossil Evidence

Even then, although the scientific community had finally, more than 50 years after Wegener published his "The Origin of Continents and Oceans", became convinced, "conclusive evidence" was not had until a few years later.
In 1968, a number of 30-foot (9m) core samples were taken to from the floor of the Atlantic Ocean. As the fossil skeletons in each was analysed, it was found that the age of the oldest sediment in each core - that immediately on top of the basalt basement - was directly related to the distance of the hole fro the ridge.- the further from the hole, the older were the fossil remains. This indicated a spreading rate of ¾ inch (1.9cm) on each side, which exactly agreed with the rate previously calculated by dating pole reversals and matching them to the magnetic stripes on the ocean bed. Finally - proof of the theory of seafloor spreading, and Wegener's theory of continental movements.

Wegener had died 38 years before; had he been alive, he would have been 88, a not inconceivably great age to have witnessed this final triumph, given his fitness - had he not chosen to trek through a Greenland winter.

'Continental Drift' v 'Plate Tectonics'

Wegener's main problem had been that of finding a mechanism that would carry the continents around the world. This has finally been solved. They do not, as he supposed 'drift across the molten interior', but are instead pulled (and pushed) apart. His theory of 'continental drift', although so very close, was thus shown to be poorly-named. In the light of the recognition of the mechanics of its movement, the theory now favoured is termed 'plate tectonics' - after the Greek for "movements"

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This completes the description about the history of Continental Drift.
You can either read more about the theory of Plate Tectonics, return Home, or use the navigation links on your left.

For any comments, suggestions or contributions, please e-mail me at: portsdown@bbm.me.uk