1. A Brief History of a Unifying Theory
Plate Tectonics
A Brief History of a Unifying Theory
Image at
San Andreas fault zone, Carrizo Plains, central California. Photo by Ian Kluft.

2. Accumulation of Observations - Evidence
Plate Tectonics as the Unifying Concept of Earth Science
Accumulation of Observations - Evidence
Patterns of continents
Paleontology
Geology
Patterns of sea floor ages
Patterns of seafloor depth
Patterns of seafloor sediments
Patterns of magnetism
Patterns of volcanoes
Patterns of earthquakes

3. Earth’s Great Puzzle Pieces
1620 – Sir Francis Bacon observed similarities of coasts of Africa and South America … “no mere accidental occurrence.” A few years later it was suggested that they were once one, but had been separated by the Flood.
1782 – Benjamin Franklin, based on observed oyster shells on mountain tops “The crust of the Earth must be a shell floating on a fluid interior.... Thus the surface of the globe would be capable of being broken and distorted by the violent movements of the fluids on which it rested.”
1799 – Alexander Von Humbolt, German explorer and naturalist, observed the similarities in the geology and features of the west coast of Africa and east coast of South America (separated by a valley filled by the flood)
USGS image
In 1858, geographer Antonio Snider-Pellegrini made these two maps showing his version of how the American and African continents may once have fit together, then later separated. Left: The formerly joined continents before (avant) their separation. Right: The continents after (aprés) the separation. (Reproductions of the original maps courtesy of University of California, Berkeley.)
Left: The formerly joined continents before (avant) their separation
Right: The continents after (aprés) the separation
Geographer Antonio Snider-Pellegrini made these two maps showing his version of how the American and African continents may once have fit together, then later separated

4. A Man and His Model Alfred Wegener Current: Contracting Earth
1912: Continental Drift
Observations
Fit of Continents
Geology
Paleontology
Climate belts
Pangea (“all lands”) Ma
Breakup 180 Ma
Rigid bodies moving through yielding seafloor
Image from USGS site: Alfred Lothar Wegener ( ), the originator of the theory of continental drift. (Photograph courtesy of the Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany.)
More information at
But that in itself was not enough to support his idea. Another observation favoring continental drift was the presence of evidence for continental glaciation in the Pensylvanian period. Striae left by the scraping of glaciers over the land surface indicated that Africa and South America had been close together at the time of this ancient ice age. The same scraping patterns can be found along the coasts of South America and South Africa.
Wegener's drift hypothesis also provided an alternate explanation for the formation of mountains (orogenesis). The theory being discussed during his time was the "Contraction theory" which suggested that the planet was once a molten ball and in the process of cooling the surface cracked and folded up on itself.  The big problem with this idea was that all mountain ranges should be approximately the same age, and this was known not to be true.  Wegener's explanation was that as the continents moved, the leading edge of the continent would encounter resistance and thus compress and fold upwards forming mountains near the leading edges of the drifting continents.  The Sierra Nevada mountains on the Pacific coast of North America and the Andes on the coast of South America were cited.  Wegener also suggested that India drifted northward into the asian continent thus forming the Himalayas
Wegener eventually proposed a mechanism for continental drift that focused on his assertion that the rotation of the earth created a centrifugal force towards the equator.  He believed that Pangaea originated near the south pole and that the centrifugal force of the planet caused the protocontinent to break apart and the resultant continents to drift towards the equator.  He called this the "pole-fleeing force".  This idea was quickly rejected by the scientific community primarily because the actual forces generated by the rotation of the earth were calculated to be insufficient to move continents.  Wegener also tried to explain the westward drift of the Americas by invoking the gravitational forces of the sun and the moon, this idea was also quickly rejected.  Wegener's inability to provide an adequate explanation of the forces responsible for continental drift and the prevailing belief that the earth was solid and immovable resulted in the scientific dismissal of his theories.
Problems with the Contracting Earth Theory
At the time of Wegener's accidental reading of the fossil proof of sunken land bridges, the 'scientific consensus' was that of sunken land and continents, now covered in oceans. This land had once provided a migratory path for the former flora and fauna, now found as fossils in diverse continents. Land, of course, was a permanent and unmovable feature of the earth's surface. Although it might sink, land could neither move nor be created afresh.
The sunken land had, it was supposed, suffered from the effects of a 'cooling and contracting earth'. As the core of the earth cooled and contracted, its outer crust collapsed inwards. Mountains had thus arisen, and oceans formed in the depressions, covering the earlier land bridges.
Like many scientists of the time, Wegener noted serious flaws in this theory, even though it was held dear. There were 3 principle flaws, although not the only ones. These were:
The pattern of mountain ranges that occur around the earth. They occur in narrow, curvilinear belts, and are often located along continental edges. Were the earth truly contracting, no such belts, or typical location, would be expected. Instead, mountains would occur randomly scattered all over the Earth's surface.
The age of the mountains. Were the contraction theory true, all mountains ought to be of roughly the same age. However, it was already then known that mountains varied enormously in age, from both fossil evidence, and even radioactive dating techniques. The Caledonians of Scotland had already been recognised as being far far older than the European Alpine system.
The gravity problems. Gravity studies had illustrated the lesser density of continental crust compared to oceanic rock. This caused insurmountable problems for the contraction theory, with its sinking land bridges - which ought 'bob up' again to the surface, if the theory was true.
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"The Origin of Continents and Oceans"
Only months after first reading of the fossils in his library, Wegener formulated his extraordinary thesis. In January 6th 1912, he delivered an address to the Geological Association in Frankfurt, entitled "The Formation of the Major Features of the Earth's Crust (Continents and Oceans)". However; his challenge to the theories of sunken land bridges, and proposal that continents had once been united but had since drifted apart to their present positions, was not well received, and was considered a ludicrous notion. Three years later, Wegener used several lines of evidence from several different disciplines to produce his now-famous "The Origin of Continents and Oceans", which unashamedly trespassed into other sciences. Three more volumes followed, in 1920, 1922, and Each adding further elaboration; but his over-stepping of scientific boundaries, his rejection of long-held beliefs, and even his German nationality (around the time of the Great War and its aftermath, , all Germans were treated with at best suspicion by the rest of Europe), did not win Wegener many converts.
The theory reasons quite simply that if the fossil evidence proves the existence of a land link between now-separate continents, and the concept of land bridges is inconsistent with the concept of isostasy, then the only possible conclusion to be drawn is that the continents were once one, and had somehow separated.
Horizontal movement
A key part of the theory suggested that the oceanic crust of the Earth could be likened to pitch. This is solid, and shatters under the sudden pressure of a hammer-blow. However, under continued pressure and over time, it can flow in a plastic, or ductile, manner. If continents are sitting in a similar substance, and can move vertically - as already suggested under Dutton's theory of isostasy, which had been accepted at that time - then equally, they could move horizontally over and through it.
Continental 'fit'
He further proposed that in attempting to make a continental 'fit', one ought not take notice of the shoreline, but of the edge of the continental shelves instead. This is where the shallow sea dips sharply downwards to the deep ocean floor, and is a far more 'real' edge of a continent. It ignores the fluctuations of sea level, and the majority of coastal erosion sediments. Using this demarcation, the geographical fit of African and South America was even better than had previously been seen. Furthermore, the geological rock strata matched perfectly.
Climatic Evidence
In the early 1920's, Wegener added climatic evidence to his theory. He plotted the worldwide distribution of rock and fossil types that indicated the former locations of tropical climates, deserts and icecaps. The continents showed evidence of a variety of different climates, such as remains of temperate species of trees including beech, maple and oak below the ice of polar Spitsbergen. Rearranging, moving and reuniting the continents into a different configuration is the only possible explanation of the history of these climate changes.
Pangea
By 1922, Wegener had joined all of the present-day continents into one huge landmass. He used both fossil evidence, and eight different geological indicators, including
the coal fields shared by Britain, Belgium and the Appalachian Mountains of the USA,
the red sandstone band that passes through Norway, Britain, Greenland and Canada and
diamond fields of South Africa and Brazil.
This continent he called Pangea - from the Greek meaning "all land". Mountains, he claimed, were the result of continents colliding and crumpling, and he cited the example of India colliding into Asia to uplift the Himalayas. At this stage, he used his earlier measurements of the longitudinal change of Greenland to show that the land was still moving. His estimate was somewhat exaggerated, as he claimed a rate of 118 feet (36 metres) per year, while it was alter shown that the ocean is widening at only about 2 cm per year (so the land on either side only half that). Unfortunately, this mistake (due to imprecise measurements) did nothing to convince the already disbelieving sceptics, who disbelieved that continents could move at all, let alone at that speed.
The Lack of a Process
The biggest problem Wegener had, however, was not this error, but that of being unable to describe the process of movement, or why the continents would move in the first place. Although he made some suggestions, he was aware that they were weak (and were in fact wrong.) This inability to explain the "why" and the "how" of continental movement was a primary reason that his theory was rejected. The weaknesses of his suggested processes were torn apart, and their failure to hold up to simple maths, was used to discredit the whole theory in the international forum.
Alfred Wegener

5. Eduard Seuss 1926 Based on glossopteris fern
The same land plant and animal fossils are found on separate continents!
1926
Based on glossopteris fern
Image from

6. Fossils Match!
USGS image from
noted by Snider-Pellegrini and Wegener, the locations of certain fossil plants and animals on present-day, widely separated continents would form definite patterns (shown by the bands of colors), if the continents are rejoined.

7. Geology Matches, Too!
Wegener used an Alexander duToit graphic to demonstrate the uncanny match of geology between eastern South America and western Africa. Image from 1937.

8. Scientific Community says:
USGS image from

9. No mechanism to make continental drift happen
Scientific Community says:
USGS image from
No mechanism to make continental drift happen

10. Mechanism for Plate Movement!
Arthur Holmes (Late 1920’s)
Interior of Earth has sluggish convection (transport of heat from core); hot stuff rises, cool stuff sinks
New ocean crust injected into ocean floor
But from where?
Photo of Arthur Holmes from
Arthur Holmes (Late 1920’s)
Interior of Earth has sluggish convection (transport of heat from core)
Replaced expanding / contracting Earth theories (fit of continents / mountain belts)
Basaltic layer of ocean = plate that carries continent.
New ocean crust injected into ocean floor (where?)
In 1929, about the time Wegener's ideas began to be dismissed, Arthur Holmes elaborated on one of Wegener's many hypotheses; the idea that the mantle undergoes thermal convection.  This idea is based on the fact that as a substance is heated its density decreases and rises to the surface until it is cooled and sinks again. This repeated heating and cooling results in a current which may be enough to cause continents to move.  Arthur Holmes suggested that this thermal convection was like a conveyor belt and that the upwelling pressure could break apart a continent and then force the broken continent in opposite directions carried by the convection currents.  This idea received very little attention at the time.
Meinesz suggested trenches = downward arm

11. Maurice Ewing Mapping the seafloor 1947-1959 Lockney Texas
Rice University Trained
UTMB - Division of Earth and Planetary Sciences of the Marine Biomedical Institute
Photo of Maurice Ewing from
NOAA image of seafloor at
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.
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?
return to top
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 1/40th of that expected, so could represent the sediment of only 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.
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.
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.
"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.

12. Maurice Ewing Mapping the seafloor 1947-1959 Surprises: Thin sediment
Basalt crust – glasses
Age less than 150 Ma (hadn’t identified a pattern yet)
Ridges – later shown to circle globe
Valley within ridge (Tharp)
Earthquakes along ridges
High heat flow (Bullard)
Photo of Maurice Ewing from
Image of 3D ridge at USGS site:
Computer-generated detailed topographic map of a segment of the Mid-Oceanic Ridge. "Warm" colors (yellow to red) indicate the ridge rising above the seafloor, and the "cool" colors (green to blue) represent lower elevations. This image (at latitude 9° north) is of a small part of the East Pacific Rise. (Imagery courtesy of Stacey Tighe, University of Rhode Island.)
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.
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?
return to top
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 1/40th of that expected, so could represent the sediment of only 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.
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.
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.
"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.

13. Harry Hess and Seafloor Spreading
1962 – startling new theory “History of the Oceans”
New ocean crust at mid-ocean ridges
Ocean crust dragged down at trenches; mountains form here
Continental crust too light; remains at surface
Earthquakes occur where crust descends
Photo of Harry Hess from
Geology professor at Princeton (starting 1932)
Served in US Navy in Pacific in WWII (landed at Iwo Jima), ran depth soundings while traveling from battle to battle
All soundings led to first discovery of mid-ocean ridge, he hypothesized it was hot magma
Relationship to Bob Dietz? Who coined the term sea-floor spreading?
“It explains everything….”

14. Rocks and Magnetism - Tools
When magma cools, takes on signature of Earth’s prevailing magnetic field
Three magnetic measurements can be taken from rocks
Inclination - ~ latitude ~distance to the pole
Declination - ~ direction to the pole
Positive (normal) or negative (reversed) - depending on what Earth’s field is doing
Add age = powerful tool
Earth has magnetic field (Image at
Similar to a giant dipole magnet
magnetic poles essentially coincide with the geographic poles
may result from different rotation of outer core and mantle

15. Magnetic Reversals Earth’s present magnetic field is called normal
magnetic north near the north geographic pole
magnetic south near the south geographic pole
At various times in the past, Earth’s magnetic field has completely reversed
magnetic south near the north geographic pole
magnetic north near the south geographic pole
171 times in last 76 million years … takes 5,000 to 10,000 per reversal. Lasts 10’s of thousands to millions of years
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 years to happen, and once established, can last for 10's of thousands, to millions, of years. The most recent was 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.
return to top
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".

16. Vine and Matthews The Final Push – 1962-1963
USGS image from
Text from a university website that no longer exists
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 years to happen, and once established, can last for 10's of thousands, to millions, of years. The most recent was 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.
return to top
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".
Symmetric patterns of magnetism on either side of mid-ocean ridge

17. Magnetic Stripes on Seafloor
Image credited to USGS,
An observed magnetic profile (blue) for the ocean floor across the East Pacific Rise is matched quite well by a calculated profile (red) based on the Earth's magnetic reversals for the past 4 million years and an assumed constant rate of movement of ocean floor away from a hypothetical spreading center (bottom). The remarkable similarity of these two profiles provided one of the clinching arguments in support of the seafloor spreading hypothesis.
Some information at
Seafloor as a magnetic tape recorder

18. magnetic iron-bearing minerals align with Earth’s magnetic field
When magma cools, it takes on signature of Earth’s prevailing magnetic field
USGS image from
magnetic iron-bearing minerals align with Earth’s magnetic field

19. Oceanic Crust Is Young
USGS Image from
Seafloor spreading theory indicates that
oceanic crust is geologically young
forms during spreading
destroyed during subduction
Radiometric dating confirms young age
youngest oceanic crust occurs at mid-ocean ridges
oldest oceanic crust is less than 180 million years old
oldest continental crust is 3.96 billion yeas old

20. that may be copyrighted.
Age of Continents
Original copyrighted image removed; there is an image available at
that may be copyrighted.
Original copyrighted image removed; there is an image available at that may be copyrighted.

21. Tuzo Wilson
Transform faults: opposite sense of movement than expected.
Proven correct (Sykes)
Sealed theory of sea-floor spreading and plate tectonics for most scientists
1960s-1970s
Image of Tuzo Wilson from USGS site J. Tuzo Wilson ( ) made major contributions to the development of the plate-tectonics theory in the 1960s and 1970s. He remained a dominant force in the Canadian scientific scene until his death. (Photograph courtesy of the Ontario Science Centre.)
Graphic of the transform fault at
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.
To solve the problem of the origin of the Hawaiian Islands, for example, Tuzo imagined someone lying on his back on the bottom of a shallow stream, blowing bubbles to the surface through a straw.
The bursting bubbles were the Hawaiian Islands, and they lay in a line because they were swept along the surface by the moving stream. Thirty years later, leading geophysical theorists use supercomputers to solve horrendous equations that Tuzo "solved" in the visualizing region of his brain.
Tuzo's great paper describing this, "A Possible Origin of the Hawaiian Islands," was rejected by the leading American geophysical journal in 1963 on the grounds that it was completely at variance with the latest seismic studies of the region. Undeterred, he sent it to the Canadian Journal of Physics, where it was immediately published because, I suspect, the editors didn't know what else to do with anything so devoid of mathematics.
His second great, yet simple, idea was that of transform faults. Again, Tuzo's approach was visual and non-numerical. And yet it was devastatingly definitive in what it predicted. It did not give us an equation, such as E=mc2, or say that the magnetic field near a wire is proportional to the current flowing through it. Tuzo's transform fault concept said to earth scientists that they were living in a looking-glass world. For earthquakes occurring underwater and in the middles of oceans, he predicted that the rocks everybody believed had moved right to left during the earthquake had moved left to right, and vice versa. This was a wonderful geometric test for the existence of continental drift and plate tectonics. If the rocks moved as Tuzo said, continental drift was a racing certainty. If they didn't, Earth was a far more static place. Wilson capped his transform fault paper (1965) with a stunning synthesis of what we now know as plate tectonics.
In 1967, Lynn Sykes of the Lamont Geological Observatory examined the motion of rocks in 10 earthquakes on two mid-ocean ridges and found Tuzo's predictions were correct in every case. His announcement of this went a long way in convincing people that continental drift had not only occurred in the past 200 m.y., but was going on under our feet today, at the rate at which our toenails are growing.

22. Hydrothermal Vents
The result of seawater percolating down through fissures in the ocean crust in the vicinity of spreading centers or subduction zones
The cold seawater is heated by hot magma and reemerges to form the vents. Seawater in hydrothermal vents may reach temperatures of over 340°C (700°F)
Discovered in 1977 while exploring an oceanic spreading ridge near the Galapagos Islands
Images from:

23. Theory of Plate Tectonics
The upper mechanical layer of Earth (lithosphere) is divided into rigid plates that move away from, toward, and along each other
Most deformation of Earth’s crust occurs at plate boundaries 23

24. How can you calculate the rate of plate movement?

25. Calculating Plate Movement
Pick an object and watch it …
Better on glaciers than on slow moving plates …
Use magnetic reversals … long time periods
Date rocks across a mid-ocean ridge really really carefully … tedious

26. Northern Pacific Basin
Aleutian Trench
Emperor
Seamount
Chain
Image from Google Earth
Hawaiian
Ridge
Midway
Hawaii

27. Hot Spots Stationary magma chambers under mobile plates …
USGS image at

28. Prominent Hot Spots
USGS image at

29. Plate Movement Rates using Hot Spots http://www. classzone

30. Tectonics on Other Planets?

31. Do you recognize either of these locations?
A computer-generated image of the Aleutian Trench (in violet); "warm" colors (yellow to red) indicate topographic highs, and "cool" colors (green to blue) represent lower elevations. Below: The topography of Artemis Corona, a trench-like feature on Venus, shown at the same vertical and horizontal scale as the Aleutian Trench. (Imagery courtesy of David T. Sandwell, Scripps Institution of Oceanography.) - magellan spacecraft

32. Mars Topography
NASA Mars MOLA map; available at

33. Mars Magnetic Field
This is a map of the magnetic field of Mars observed by the Mars Global Surveyor. Red and blue stripes represent magnetic fields with opposite directions, with darker hues representing more intensity. The map is superimposed on a topography relief map from the Mars Observer Laser Altimeter instrument. NASA image