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Plate Tectonics (in detail)

 

Several developments took place between Wegener's death and the appearance of plate tectonic theory. First, studies of the magnetic properties of continental rocks revealed some peculiar features that were best explained if continents had, indeed, drifted in the past.

 

Second, after the Second World War, improved technology allowed the ocean floors to be examined in a greater depth than ever before. This came about partly as a result of increased military awareness that it was important to understand the oceans, especially to allow submarines carrying nuclear bombs to travel at great depths in the oceans. The new knowledge of the ocean floors revealed some interesting features that could be explained best if they were incorporated into a theory involving moving continents.

 

The magnetic data that helped to revive the idea of continental drift was based on the principle that many rocks retain a 'memory' of their past orientation with respect to the Earth's magnetic poles. This is especially true for volcanic rocks. The atoms of iron and other magnetic substances in molten lava align themselves in a magnetic north-south orientation just as compass needles point in the same direction.

 

 

When such lava solidifies after it has cooled down, the atoms of magnetic substances in it remain locked in the same orientation that they had when they were molten. Therefore, the direction in which such magnetic components of today's solid rocks are oriented reveals the direction of the Earth's magnetic poles at the time those rocks solidified.

 

The extent to which such 'fossilised compass needles' dip also reveals the distance of the rocks from the Earth's magnetic poles at the time the rocks were molten. At the poles, the magnetic particles point downwards vertically; as they approach the equator they dip less, at the equator they do not dip at all.

 

In other words, from measurements of the magnetic properties of rocks, one can deduce the direction of the poles as well as the latitude that the rocks occupied at the time they solidified.

 

If continents did drift, magnetic studies might reveal this motion, since the continents would presumably have changed their position of latitude during their migration.

 

In the 1950s, the British geologist, Stanley Runcorn (b. 1922) and his colleagues obtained some peculiar results when they examined the magnetic features of rocks in Europe. When magnetic properties were investigated in rocks of different ages from the same geographical area, the Earth's magnetic poles appeared to have changed position at different times during the Earth's past.

 

There were two major possible interpretations of this research: either the poles had drifted and moved around the globe during the Earth's geological history, or Europe had drifted and the poles had remained in the same place. Most scientists remained glued to the static model of the continents and either rejected the data as being unreliable or accepted the idea that the magnetic poles, but not the continents, had migrated.

 

When similar magnetic properties of rocks were examined in other continents, the poles there were also found to have had apparently different positions at different times in the Earth's past.

 

When the predicted paths of migration of the poles as seen from different continents were plotted on the surface of the Earth, however, they did not coincide. If there was one pole, it should obviously have occurred at the same position at the same time for all continents. If the continents did not move, the only other explanation now was that there were several magnetic poles in the Earth's past, which was thought extremely unlikely.

 

However, if the continents were considered to have been juxtaposed at some time in the past, in a manner similar to that proposed for Pangaea by Wegener, then the plotted paths of polar migration did coincide for different continents. This agreed excellently with the idea that the continents had, indeed, been joined together and had later drifted apart. The magnetic poles were static, according to this interpretation, and the continents moved.

 

More scientists were now convinced that continental drift was a possibility, although the idea was still difficult for many to accept.

 

When detailed knowledge about the ocean floors became available, continental drift was finally accepted, and plate tectonics was born. Much of this knowledge of the ocean floor came from research carried out in the USA by Bruce Heezen (1924-1977), Henry Menard (1920-1986) and their co-workers.

 

The ocean floor was revealed as a realm where mountains and trenches were more magnificent and awesome than they were on land. One prominent feature of the ocean floor is the mid-ocean ridge system, which meanders around the globe. This ridge system consists of rugged structures that extend for a distance of 60 000 kilometres (37 500 miles); they are several hundred kilometres/ miles wide and more than 4500 metres (15 000 feet) high.

 

The mid-ocean ridge system is the site of frequent earthquakes and volcanic activity. Most of the volcanic activity occurs on the ocean floor: a rift valley containing molten lava runs along the ridge's length. In some places, for example Iceland, the volcanoes occur above sea level.

 

The ridge is not completely continuous: at various intervals along its length ridges and troughs called fracture zones, occur at right angles to it, creating what appears to be a stitch-like pattern. The San Andreas fault in California is another example of a fracture zone.

 

Another outstanding feature of ocean floors is the presence of trenches, which are above five kilometres (three miles) deep, and, like the ridge system, are sites of earthquakes and volcanoes. The deepest parts of the oceans occur where trenches exist. Trenches are often associated with chains of islands; for example those associated with the Aleutian Islands in the Pacific Ocean.

 

Several theories were proposed to explain these characteristics of the ocean floor's geophysics. An early idea was that the Earth was expanding, causing the sea floor to crack; but there was no evidence for this theory.

 

Another theory, which was widely accepted, was proposed by the US scientist, Harry H. Hess (1906-1969), and was called the 'sea floor spreading' hypothesis. The sea floor spreading hypothesis was an extension of an earlier idea that the mid-ocean ridge formed as a result of heating of the Earth's crust, which, in turn, was said to be due to the underlying mantle being heated by the high temperature of the Earth's core.

 

According to Hess, the Earth's crust 'floats' on the flowing mantle. As the mantle material heats up, it rises and heats up the crust; this causes the crust to expand and crack in vulnerable regions. The mantle material rises through the crack and oozes out onto the ocean floor as molten lava. As it flows upwards through the crack, the lava pushes the crust on either side of the crack outwards.

 

The lava flows out of each side of the crack and cools down as it moves away from the centre of the crack. During this cooling, it solidifies and forms a new layer on top of the existing ocean crust.

 

According to Hess, the Earth was not expanding, and this meant that the new crust formed at the cracks, which correspond with the crests of the mid-ocean ridge, had to be compensated for by a loss of existing crust elsewhere. Hess proposed that the trenches that occurred on the sea floor were the sites of destruction of sea floor crust: the expansion of the sea floor at ocean ridges caused sea floor crust to be forced into the mantle at trenches.

 

This theory explained why it was that rocks obtained from the sea floor were always found to be less than a quarter of a million years old, whereas continental rock had been found that was more than three and a half billion years old.

 

The ocean floor was made of young rocks, since it was continually being replaced by newly solidified lava from the underlying mantle; and old ocean crust was, at the same time, being forced downwards into the mantle at trenches, there to become molten.

 

Hess's theory also required that the rocks of the ocean floor should be older the further away they are from the centre of the mid-ocean ridge, since new lava starts in this region and, over millions of years, slowly spreads outwards. In other words, the newest rocks should occur at the ridge crests and the oldest at the trenches.

 

Hess's theory also provided the first acceptable mechanism for continental drift: as ocean floors were renewed at ridges and destroyed at trenches they would carry the continents with them, rather like objects being carried on a conveyor belt.

 

Evidence for sea-floor spreading was forthcoming, particularly when the age of sea floor rocks was determined from their magnetic properties. This allowed the age of rocks on either side of ridges and trenches to be obtained, and the data was in excellent agreement with the ideas proposed by Hess.

 

Indeed, when these rates of renewal and spread of the sea floor were measured, they were very close to those expected from Wegener's idea of continental drift that was proposed decades earlier.

 

This was a clear indication that continental drift did occur and that it was due at least partly to the cycles of renewal and destruction of the ocean floor attached to the adjoining continents.

 

In the late 1960s the British geophysicists, Dan McKenzie (b. 1942) and Robert Parker (b. 1942), and (independently) the US geophysicist, Jason Morgan (b. 1935), brought together the ideas of Wegener, Hess and other geophysicists who had contributed to our understanding of continental drift and knowledge of the continents and ocean floor.

 

They suggested that the Earth contains a number of plates (there are about a dozen large plates and several smaller ones) that consist of the continental and oceanic crusts as well as the upper regions of the Earth's mantle.

 

In other words, these plates are more than just crust: they also contain mantle material. The name given to the layer of the Earth containing the plates is the lithosphere.

 

Plates are about 100 kilometres (60 miles) thick and have, as their edges, the mid-ocean ridges, the ocean floor trenches and fracture zones. They float on an underlying layer of the mantle called the asthenosphere and may contain either continental crust or ocean floor crust or both.

 

The continents and ocean floors are carried by the floating plates like objects sitting on rafts. When tectonic plates break up, old continents may give rise to several new ones, and these eventually may drift apart, as was the case when Pangaea broke up.

 

When two plates collide with each other, the outcome depends on the particular features of the plates. If the edges of two plates containing oceanic crust collide, either one of them will descend under the other.

 

When the continental edge of one plate collides with the ocean crust edge of another, the oceanic plate always descends under the continental one. The reason for this is that continental crust is more buoyant than ocean floor crust, and so continental crust always rises above the ocean floor.

 

When the edges of two colliding plates both consist of continental crust, neither will sink because of their high buoyancy; instead, the boundary may become crumpled and produce mountain ranges. The Himalayas were formed as a result of such an encounter.

 

Collisions between two continents may unite them together into a single, larger continent. When two plates drift apart, new molten lava from the underlying mantle oozes up to fill the gap: this is the situation that occurs in the mid-ocean ridge.

 

At the trenches, ocean floor crust from the descending side of a plate is destroyed and enters the asthenosphere, there to become molten and eventually to be recycled as new ocean crust. The ocean floor is therefore constantly renewing itself as a result of plates colliding and moving apart.

 

When continents crack, molten lava from the asthenosphere rises to fill the crack. Since this lava consists of basalt, it forms ocean crust when it cools down, and this means that new ocean floor is formed where the crack occurred. This allows a continent to split into two new continents separated by an ocean.

 

The net rate of increase of ocean floor material is essentially zero, because whenever new ocean floor is produced, an equal amount of old ocean floor plunges into the mantle and is destroyed elsewhere. However, this does not mean to say that new ocean floor formed at a ridge is compensated for by loss of ocean floor from the same plates whose borders form this ridge.

 

As long as old ocean floor is destroyed at the same rate that new ocean floor is made, the recycling processes can occur at borders of other plates. For example, the Mid-Atlantic Ridge system, which borders plates carrying North and South America, does not have its own trenches.

 

When new ocean crust is made at this ridge, it is compensated for by a loss of crust from the Pacific plates, which border the west coast of the Americas. In this way, the plates containing the Americas are growing larger in area at the expense of the Pacific plates.

 

Plates may also slide past each other instead of colliding with one another or moving apart; when this occurs, earthquakes frequently result. Fracture zones occur where plates slide past one another. Earthquakes also occur in other places where movement of the Earth's lithospheric plates is intense; they are particularly common at mid-ocean ridges, where two plates move apart, and at trenches, where one plate descends under another.

 

If the major earthquakes that have been recorded are plotted on a map of the Earth, they correspond strikingly to the edges of plates. Similarly, volcanoes map very closely with plate boundaries.

 

Volcanoes do not occur where two plates collide (to produce mountains); nor do they occur where continents slide past each other. They do occur where plates move apart or where one plate descends under another following a collision.

 

When an oceanic edge of a plate descends under another plate, some of the ocean crust and ocean sediment melts in the mantle and the molten substances have a tendency to rise to the surface and create active volcanoes.

 

Japan and the Aleutian Islands contain numerous volcanoes that occur along this kind of plate boundary. Volcanoes also form where plates move apart and molten lava rises to the surface; examples include those along the eastern side of the Pacific plates and those in Iceland.

 

There is a good deal of evidence for the existence of tectonic plates and the consequences that arise from their collisions, separations and sliding can be seen clearly in many places on Earth.

 

Tectonic plates are undergoing these processes today, and they will continue to do so in the future. Continents will continue to drift. Indeed, it has been suggested that all of the continents will, in several hundred million years' time, once more join together: the supercontinent so formed has already been given a name - Neopangaea - by some geophysicists.

 

If Neopangaea does form it will undoubtedly break up again and the resulting fragments will drift apart to produce a geography that will be very different from the one we have today.

 

Human beings have created artificial, political and physical boundaries between their nations, but the natural processes that cause continents to break up and their fragments to drift apart and perhaps to rejoin over hundreds of millions of years are the real creators of the Earth's boundaries.