The History of Continental Drift - Theories after Alfred Wegener

With Wegener's death, his theory seemed to die. However, in the decades following new discoveries were made, and new and compelling evidence was found. Some of these are described below.

This page discusses:

• First Seafloor Explorations
• 'The History of the Ocean Basins'

First Seafloor Explorations

On Wegener's death in 1930, the consensus was still that of a contracting Earth, and the permanence of the position of continents. This view survived for more than for three decades further, it was so well entrenched. Despite major problems already identified with this theory, and the increasing body of evidence against it, each scientific discipline clung to its own beliefs, and continued to accept the familiar but flawed.
Wegener had some supporters, but he was the instigator of the theory and its main proponent; with his death, his ideas languished for 30 years. Any who were impressed by it kept quiet, having seen the way in which he and his revolutionary ideas had been treated. The theory of continental drift gained new life only when further discoveries, made with other targets in mind, produced increasing amounts of compelling evidence that could no longer be ignored.
While the match of Africa into South America had been observed for many centuries even before Wegener, a key point of Wegener's theory was to use the edge of the continual shelves to match continents, rather than the visible seashores, claiming this was more reliable as the limits of continents. (The continental shelf is covered by shallow sea, then dips suddenly and steeply down the continental slope into the 'abyss', the deep sea below.)
In 1935, attempts - for unrelated reasons - were made to investigate this continental shelf. Was the shelf edge a true permanent geological feature, or was it simply the furthest reach of sediment laid down from the land?

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Maurice Ewing

To investigate the continental shelf, those interested approached Maurice Ewing. He was a physics lecturer, and enthusiastically agreed to investigate the composition of the shelf using explosion seismology. Although Ewing was recruited for his interest and skill in seismology, he soon became a leading light in the exploration of the sea floor, which became his life-long passion.

His investigations showed that the continental shelf is not a permanent feature, but is composed of sedimentary deposits, piled high (up to 12 000 feet - about 3 500 m) above the sloping basement of the ocean floor. This knowledge was interesting and passed on to those who had commissioned it; while Ewing continued in his new-long interest of seafloor investigation. What he and his followers discovered over the following decades added vast amounts to our knowledge of the Earth, and re-awoke the dormant discussion of continental drift. Although interrupted by World War II, Ewing's interest initiated an enthusiastic interest in the deep sea, which continues today as the major discipline of the science of oceanography.

Seafloor surprises

In 1947, The National Geographic Society commissioned Ewing to explore the mid-Atlantic Ridge and the sea floor around it, of which very little was known at the time. Already in the early 1850's an American cartographer, Matthew Maury, had made the first chart of the ocean floor where he had noted an mapped an elevation in its centre which he called "Dolphin Rise"; but little was known, or thought, of it.
When Ewing began his investigations, he - like most earth scientists - expected the ocean sediment to offer fossil clues to the entire history of the Earth, which they knew to be billions of years old. However, the first core samples, take from almost a mile (slightly more than 1 km) down, were perplexing. They contained a layer of recent sediment lying directly on top of another layer what was more than 20 million years old. Inexplicably, there was no trace of the material from the period in between.
Also, the thickness of the sediment proved a surprise - or rather, its lack of thickness. Some predications claimed that the accumulation of 3 billion years of sediments would have built up a layer up to 12 miles (20 km) thick; but beyond the continental shelves, the thickest layers were only a few 1000 feet thick (perhaps as little as 500m). This was just 1/40^th of that expected, so could represent the sediment of only 100 - 200 million years, not the expected billions.
On reaching the Mid Atlantic Ridge itself, Ewing faced still more surprises. Not only was the sediment extremely thin there - but he began dredging up 'glassy' rocks, that had apparently been subject to great heat and pressure. The very thin sediment layer and those glassy lavas that he hauled up indicated that the ocean floor was very young, and of volcanic origin - both suggestions causing great astonishment, and some disbelief.
Two more trips in 1948 revealed yet more surprises. Seismic data indicated that the sea floor was composed of dense basalt, and was only about 3 miles (5km) thick. Prior to this investigation, it had been assumed to be composed of the sunken continental material of the contracting Earth - which was also very much thicker.

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The first sea floor map

At the beginning of the 1950's, Ewing decided to translate the available echo-sounding profiles of the North Atlantic sea bottom into a topographical map. Thus far, six studies had been performed, so the combination of all the existant data seemed to be an obvious move.
The cartographer Marie Tharp was enlisted to help. As her mapping progressed, she drew a deep canyon down the centre of the Mid-Atlantic Ridge, which startled everyone, herself especially.
The actual existence of this valley was doubted, it seemed so unlikely. Then, in 1953, a study for potential earthquake damage (for proposed underwater transatlantic telephone cables) used her map as a base. As the known earthquakes were mapped, they all fell within the valley that Maria had drawn along and between the ridge.

Deep-sea Earthquakes

The coincidence of the apparent mid-ocean valley and position of the mapped earthquakes was brought to Ewing, who immediately began to gather all available data on mid-ocean earthquakes. These he plotted, and found that they ran not only through the known oceanic ridge (or rather rift), but also in lines through all the world's oceans. From this, he was able to predict the position of seafloor ridges and rifts in all the world's oceans. Until then, the existence of such ridges had not been known, or even suspected. The confirmation of the predictions ought to have gained him much support - but his hypothesis was still to revolutionary.
In 1956, Ewing disclosed some of his conclusions, but they were rejected. They implied, after all, a widening of the ocean, which was against the generally accepted notion of the Earth's contraction.

Heat Flow

At about the same time, the British Geophysicist Sir Edward Bullard had made more astonishing discoveries. It was already known that the Earth's internal heat came from the radioactive decay of materials in the granite. As the seafloor had been shown to be of basalt (which has no radioactive component) it was assumed that far less heat would be produced by the seafloor.
However; investigations showed that as much heat was radiated by the seafloor as by land; and further studies along what he considered to be the crest of the Mid-Atlantic Ridge gave figures showing very high heat flow there - up to eight times that from land.
Combining Bullard's heat flow figures with the understanding that the 'crest of the ridge' was in fact a deep valley, Ewing concluded that that valley was in fact a crack in the Earth's crust, out of which hot material from the mantle was rising to the surface.

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The first description of the sea floor map

In 1959, Ewing and his associates published the first 'physiographic', or 'picture'. map of the sea floor. Prominent was the mountain range of the Mid-Atlantic ridge, with the rift - in places 12 miles (20 km) wide - running between.

Further exploration showed this ridge to be only a section of a 40 000 mile-long (65 000km) mountain range, that curls around all the oceans of the world.
"The discovery that numerous, previously-known, individual ridge systems were all part of the same worldwide system is probably the most exciting major discovery about earth science made in the past 20 years" claimed one. The curving ridge, exactly central and parallel to the coast of Africa and South America, became impossible to treat as 'mere coincidence', and the discovery called into question all the traditional theories about the Earth. None of these had predicted, or could explain, the ridge's existence."
However; its role remained speculation for some years to follow.

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"The History of Ocean Basins"

By now, discoveries were leading to more questions and puzzles. These included:

• The discovery of conical mountains on the sea floor (called 'sea mounts), some of which had flat tops, as if sliced off (called 'guyots'). The further from the ridge they were found, the deeper they became.
• The absence of ocean-bottom rocks over about 150 million
• The 'missing sediment'
• The thinness of the ocean crust
• The high heat flow along the ridge

Harry H. Hess

Struggling with the above problems, a Princeton professor, Harry H. Hess, produced a startling and original hypothesis - that the ocean floors were moving 'like conveyor belts, carrying the continents along with them'. However - mindful of the conservatism of the scientific establishment, and aware of Wegener's fate, Hess was careful when he committed his ideas to print. He wrote a paper in 1960, which was widely circulated before publication in 1962, entitled "The History of Ocean Basins" in which he claimed
The birth of the oceans is a matter of conjecture, the subsequent history is obscure, and the present structure is just beginning to be understood". He went on to warn his readers that he was presenting "an essay in geopoetry . . . that bordered on fantasy".
Hess followed this with his fundamental proposition - that is accepted today as a basic feature of plate tectonics.
The sea floor is not permanent, but is constantly being renewed. The Mid-Ocean Ridge is indeed a crack in the crust. Through it, hot material from the underlying mantle continually wells up and spreads outwards."
This effect was later termed 'sea-floor spreading', and Hess estimated that new crust is generated at the rate of about half an inch (just over 1cm) a year, on each side of the ridge. At this pace, all the ocean floors of the world would have been formed during the last 200 million years - less that 5% of the Earth's geological history. Hess went on to point out that as the Earth is not expanding, old crust must simultaneously be being destroyed - and he correctly suggested that this happens in the deep ocean trenches, which lie near to the edges of continents.
This process, whereby the old oceanic floor is pulled into the deep trenches, was later termed 'subduction'.

All the problems listed above were solved by Hess's theory, and he drew his evidence together to proclaim an entirely new version of the Earth's major features:
The ocean basins are impermanent features and the continents are permanent, although they may be torn apart or welded together and their margins deformed. The continents are carried passively on the mantle with convection and do not plough through the oceanic crust".
In his belief that it was convection in the mantle that carried the continents it now appears that he was wrong, and he suffered as Wegener did in that his theory turned long-held beliefs upside down. Although his paper was widely read, it was not accepted and was treated (as he himself had perceptively warned) as fantasy. Hard proof was required, rather than just a theory that fitted all the evidence. "Circumstantial evidence" was not enough.

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Please continue to read on, about the discovery of Palaeomagnetism and final acceptance of Continental movements.

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