The term plate tectonics refers to the fact that the Earth’s crust is divided into mobile segments called plates, the formation and destruction of which have shaped the world as it is today. The concept is relatively new. Although first proposed back in 1915, it was long regarded as implausible and gained currency only in the 1960s, when geologists began to assimilate a wealth of new information about ocean floors. Once accepted, however, it quickly revolutionised the science. Plate tectonics has turned out to be the system that drives all large-scale geological processes. As a concept, it is the closest thing geology has to a unifying theory.

Among the most spectacular phenomena affecting the Earth’s crust are earthquakes, volcanoes and the growth of mountain belts. Earthquakes occur as the strain of two adjacent plates grinding in opposite directions is suddenly released. They therefore help us locate the boundaries between plates, whether on land or under the sea. Volcanoes mostly occur where an oceanic plate plunges beneath a continental plate (diagram top left). Pressure increases with depth, and as it does so, the pore water in the subducting plate is squeezed out, rises into the lithosphere and reduces the melting point of the rocks above. Mountain ranges such as the Andes and the Zagros-Taurus mountains (including Mount Ararat) are primarily volcanic. Others – from relatively young ranges such as the Alps and Himalayas to ancient ones such as the more subdued Appalachians – are the direct result of plate collisions, either as ocean crust and continental crust converged or as two continental plates converged (diagram top right).

Most new crust forms at the mid-ocean ridges, where magma rises to the surface, spreads out on either side of the ridge axis and solidifies. The youngest rock therefore lies next to the spreading centre, while further out the rock becomes progressively older. With a length of over 65,000 kilometres, the ridge system is the longest and volcanically most active mountain chain on the planet, each year generating over 20 cubic kilometres of new crust, with a corresponding volume being subducted under the continents. Although this sounds like a lot, the average spreading rate, adding up both directions, is a mere 6 cm per year.

The ocean crust has been completely replaced at least twice!

Away from the ridges, an ever greater thickness of carbonate ooze overlies the volcanic crust. Where it is accessible to drill-core expeditions, the crust can be determined by radioisotope dating. This has established that the ocean basins that even the oldest parts are relatively young in geological terms. In contrast to the continents, all of which have basal rocks going back to the Archaean, no part of the ocean basins is older than the Jurassic period, shown as blue in the colour-coded image (above).

The Jurassic – outcrops of which are well exposed along England’s southern coast – was the period when the Atlantic began to open from a rift in the supercontinent ‘Pangaea’ (shown below). Since then, the entire ocean crust that existed before the Jurassic – 70% of the Earth’s surface – has been recycled back into the mantle and replaced. Like the human skin, the crust is being continually renewed.

The geography of the world used to look very different

The world’s past geography can be reconstructed by a variety of means. For instance, a continent’s changing position relative to latitude can be inferred from fossil records of the Earth’s magnetic field. When erupting lava cools, the orientation of the field becomes locked in the orientation of its iron minerals, and in ancient rocks this imprint can be compared with the magnetic field at their present location to determine just how far, latitudinally, the continent has shifted. Similarly, the joins between continents that were once parts of supercontinents can be inferred by matching similar rock sequences and fossil zones. By these means it can be shown that during the Permian and Triassic, immediately before the Jurassic, the present continents of South America and Africa were contiguous. India – revealed by its magnetic data to lie much further south – adjoined Madagascar and Antarctica. Antarctica continued into Australia.

During the Permo-Triassic, the plate-tectonic system was less active than before or after those periods, at least so far as its interaction with continents was concerned. This can be seen from reconstructions of the geography. Nearly all the continents were joined together into the single landmass Pangaea, and they maintained that basic arrangement throughout that time. While it is possible that vigorous subduction was still going on between ocean plates, the land was almost immobile.

Such a pattern is also consistent with the kind of marine limestones that formed during that interval. Limestones consist mostly of calcium carbonate, and when plate tectonic activity is low, the calcium carbonate dissolved in seawater takes the form known as aragonite – as at the present time. When plate tectonic activity is high and submarine volcanism releases huge amounts of chemicals into the oceans, calcium carbonate takes its other form, known as calcite. Permo-Triassic limestones are predominantly of the aragonite kind and therefore indicate low hydrothermal flux into the oceans (Steuber & Veizer 2003).

In the periods before the Permian the Earth looked even less familiar (as depicted above in a series of Palaeozoic snapshots). Here we are back in a world of multiple continents, but with shapes and arrangements that looked totally different. Even an island as small as Britain would have been unrecognisable. In the early Palaeozoic Scotland lay near the equator and was separated from southern-hemisphere England and Wales by an ocean. An ancient fault running NE-SW through Scotland, the Isle of Man and Ireland marks the closure of the ocean. While Scots have always considered themselves different from the English, it was a surprise to everyone to learn that the highlands were once part of North America – pushed up as the once separate continents collided. The lowlands belonged with England.

The map of the world changed enormously in the course of the Palaeozoic, and since there was also, before then, an Archaean and a Proterozoic when even more revolutionary shiftings were going on, it is clear that the Earth’s oceanic crust must have been totally replaced several times. By contrast, continental cratons have hardly been consumed at all, because they consist of minerals that are lighter than the ocean floors. Overall, their volume and areal extent have been increasing over time.

Plate tectonics and heat

Plate tectonics is the driving force behind most geological change. But what drives plate tectonics? Given the existence of zones of extension (the mid-ocean ridges) and other zones of compression (the trenches where plates subduct), the answer must be the pull of gravity. The trench is a zone of weakness arising from the difference between continental crust and oceanic crust. The ridge, being more elevated than the rest of the plate, exerts a pushing force towards the trench, and at the trench itself the plate, being denser than the continental crust and mantle, tends to sink. Convection currents that result from the cooling of the oceanic crust also play a role. Hot convecting material rises towards the surface, softens the crust, thins it, and erupts.

The mantle is hot because with increasing pressure temperature increases. The decay of radioactive elements in the lower mantle also produces heat. However, present rates of radioactivity are insufficient to account for the heat still given off at the surface. Only around 20% of the heat flux seems to originate from radiogenic elements – the rest appears to represent secular cooling, which, when extrapolated backwards over billions of years, ends up with ‘thermal catastrophe’. This is a well-known ‘firstorder paradox in reconstructing Earth’s cooling history, because mantle temperature is believed to have been lower than ~1800°C even in the Archean’ (Korenaga 2006). Present rates of radioactivity also appear insufficient to drive convection currents. Mantle convection calculations do not exhibit plate tectonic behaviour unless it is imposed by the modeller (Tackley 2000).

Spreading rates

Since seafloor spreading rates affect the rate of all large-scale geological processes, an important question is whether rates of spreading have always been as slow as 1-15 cm per year, and there have been various studies to investigate this. Generally, the conclusion has been that rates have been much the same as now, at least during the Phanerozoic – i.e. the last 540 million years as measured by radioisotope dating. However, such exercises have been reasoning in a circle, for spreading rates depend on mantle viscosity, which depends on heat generation, which depends on the rate of radioactive decay, and the rate of radioactive decay is assumed to have been constant. Any fundamental investigation of past spreading rates is precluded.

Past spreading rates can only be quantified through radioisotope dating, and even today rates vary from ultra-slow (less than 1 cm per year) to ‘ultra-fast’ (12-15 cm per year). Although in absolute terms these speeds are all small, the different rates reflect significant differences in magma supply. Magma supply, in turn, has a marked effect on ridge morphology, rock chemistry, and on how faults are segmented. Thus we can determine whether rates have always been in the current range by investigating whether ocean floors of all ages have features characteristic of slow spreading rates. The evidence suggests that in the past rates were consistently ‘ultra-fast’, and thus at least 12-15 cm per year. Perhaps the most obvious evidence is the increasing smoothness of ocean floor away from the ridges. While slow-spreading centres are characterised by deep, fault-bounded valleys, fast-spreading centres are characterised by elevated and much more regular topography.

The height to which ancient mountain belts were thrust up into the air tells a similar story. The ‘Trans-Hudson’ mountain belt that formed across North America in the Palaeoproterozoic, for example, or the mid Palaeozoic highlands of northern Scotland and Scandinavia are thought to have been of Himalayan proportions.

The evidence that spreading rates were faster in the past suggests that mantle convection in the past was more vigorous, and rates of radioactive decay higher. This would then explain why the Earth is radiating more heat than can be explained on the basis of present rates of decay. The excess heat from higher rates in the past is still feeding through.

Terrestrial crust has been replaced just once

Another of geology’s great mysteries is that rocks of all kinds are completely missing from the first ‘600 million years’ of Earth history. This period is known as the Hadean. The only traces of Hadean rock to have survived are a few detrital crystals known as zircons. Otherwise that former world has gone, and its going seems to have been very rapid, before the succeeding Archaean period started.

According to recolonisation theory, the world that disappeared was the antediluvian world. Rates of radioactive decay were then much faster than they are today, the temperature of the Earth’s interior rose rapidly, and consequently so did the pressure. However, a subterranean ocean isolated the continent from the increasing pressure, and since there were no surface oceans, there were no cycles of seafloor spreading and subduction. Eventually, mantle pressures reached catastrophic levels, the pillars that sustained the continent collapsed and the continent foundered. Simultaneously the heat energy released by asteroid impacts – in a brief onslaught known as the Late Heavy Bombardment – turned the sinking continent into a molten jelly. The Earth’s crust was ‘recycled’ back into the mantle, and the antediluvian world became, quite literally, the Hadean underworld. In its place, new crust formed as magma welled up from below and solidified under water.

By this time the chemically once homogeneous mantle was partly liquid, having separated into a core composed mostly of iron (still partly liquid) and a mantle rich in magnesium silicates. Plumes of lighter magma rose and the denser minerals sank, generating vigorous convection cells.

Diapirism effected an enormous transfer of radioactive isotopes, and of contained heat, from the lower to the upper crust within a short period of time in each region, and greatly increased the petrologic stratification of the crust. This transfer allowed the lower crust and subjacent mantle to cool markedly below prior temperatures and thus stabilized cratons and stiffened lithosphere (Ridley 1992).

W. B. Hamilton, Precambrian Research 91:160-61 (1998).

The proto-continents of the Archaean formed by vertical tectonics (diapirism), solid crustal plates riding on top of hot, mobile mantle, constantly on the move. Towards the end of the Archaean many were beginning to collide, to form larger continents. Horizontal (plate) tectonics, however, does not seem to have played a major role in continental growth until the Proterozoic, and by the time plate tectonics became the chief engine of continental growth, in the Palaeozoic, continental growth had, by comparison with earlier rates, virtually ceased.

There is therefore a distinction to be made between the generation of continental crust and the generation of oceanic crust over time. While the formation of continental crust began to tail off after the Proterozoic, ocean crust continued to form at high rates. Large rises and falls in Palaeozoic sealevel can be correlated to high/low plate tectonic activity and imply much faster rates of ocean crust generation than in the present. Today the ocean floor is cooler and less buoyant, and sealevel is therefore at a historical low.

The planet’s terrestrial crust has been replaced only once, and this, not gradually over the whole of history but near the beginning, when the antediluvian world perished suddenly and violently. The still continuing process of ocean-crust replacement is a revelation of how it is possible for entire slabs of crust to disappear into the mantle and be completely destroyed.

See also:
The Hadean Cataclysm
New land in the Archaean
Effects of a higher speed of light