The Earth’s crust is divided into mobile plates: with that realisation, many long-standing mysteries were immediately solved. Cartographers mapping the outlines of Africa and South America had speculated as early as the 16th century that these distant continents might once have been joined. In the 20th century geologists took up the speculation, pre-eminently Alfred Wegener, who in 1912 pointed out that the stratigraphy and fossils of these continents were also similar and the puzzle of ancient glacial tillites occurring in the southern hemisphere at low latitudes would be solved if South America and Africa had later moved both apart and northwards. The biggest problem was how to account for the solid basalt crust of the oceans between the continents. A conceptual breakthrough came in the 1960s. Assimilating remotely-sensed information about the world under the water, geologists realised that the Atlantic floor itself was expanding. To compensate, slabs of ocean crust elsewhere were descending into the bowels of the Earth like escalators in the London underground. It was time for geology to experience its own Copernican revolution. Just as the Earth was not stationary in space, so the arrangement of its continents and oceans was not stationary in time.

Plate tectonics explains many things. Volcanoes occur where an oceanic plate plunges beneath a continental plate (diagram top left). With increasing pressure, the pore water in the subducting plate gets squeezed out, rises into the lithosphere above and reduces its melting point, creating pockets of magma. Mountain ranges such as the Andes and the Zagros-Taurus mountains (including Mount Ararat) consist entirely of volcanoes erupted along the margins of subducting oceanic crust. Other mountain ranges – from relatively young ones such as the Alps and Himalayas to ancient ones such as the more subdued Appalachians – are the result of collisions between continental plates (diagram top right). Such collision zones are also the places where earthquakes occur, as the strain of two adjacent plates grinding in opposite directions suddenly gets released.

At the mid-ocean ridges new crust forms. Magma rises to the surface, spreads out on either side of the ridge axis and solidifies, with the result that the newest rock lies next to the spreading centre and progressively older rock further out. 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. A corresponding volume is subducted under the continents. Although this sounds like a lot, the average spreading rate in each direction is around 3 cm per year, and volcanism is very spasmodic.

The ocean crust has been completely replaced at least twice!

Away from the ridges, as the crust gets older, an ever greater thickness of carbonate ooze overlies the volcanic crust, much of it consisting of microscopic fossils. Where the ooze is accessible to core drilling, the deepest of these fossils can be used to date the underlying crust, showing that even the oldest parts are relatively young. In contrast to the continents, which have basements going back to the Archaean, the oceans are no older than the Jurassic (marked blue in the colour-coded image).

So this is why the mutually facing coastlines of the Africa and South America seem to correspond. At the beginning of the Jurassic the supercontinent ‘Pangaea’ rifted apart, dense basaltic magma erupted within the gash and as water poured in, the widening seaway gradually became the Atlantic Ocean. Since then, the entire ocean crust – 70% of the Earth’s surface – has been recycled back into the mantle and replaced by new material from the mid-ocean ridges. Like human skin, the crust is continually being renewed.

The geography of the world used to look very different

When erupting lava cools, the orientation of the Earth’s magnetic field becomes locked in the orientation of its iron minerals. In ancient rocks this imprint can be compared with the magnetic field where the rocks are now located to determine how far, north or south, a continent has shifted. Similar rock and fossil sequences can also show how distinct continents were once joined together. Techniques such as these demonstrate that during the Permian and Triassic, immediately before the Jurassic, South America, Africa, India, Madagascar, Antarctica and Australia were all parts of one super continent.

During the Permo-Triassic, the plate-tectonic system was less active than before or after those periods. While vigorous subduction may still have been going on between ocean plates, the land was virtually immobile.

Such deductions are consistent with the kind of marine limestones produced then. In times of low plate tectonic activity, as at present, the carbonate dissolved in seawater precipitates in the form called aragonite. When plate tectonic activity is high and submarine volcanism releases huge amounts of chemicals into the oceans, the carbonate takes another form, calcite. Permo-Triassic limestones are predominantly of the aragonite kind and therefore suggest low hydrothermal flux into the oceans (Steuber & Veizer 2003).

In the periods before the Permian the Earth looked even less familiar (see the series of Palaeozoic snapshots above). Britain, for example, was not a distinct entity at all: Scotland in the early Palaeozoic lay near the equator and was separated from southern-hemisphere England and Wales by an entire ocean. The Scottish highlands were once part of North America – pushed up as the once separate continents collided – whereas the English lowlands lay thousands of miles away to the south. The suture line running NE-SW through Scotland, the Isle of Man and Ireland can still be traced.

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 change was going on, it is clear that the entire Earth’s oceanic crust must have been replaced several times. By contrast, cratons – the cold, old nuclei of continents – 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. And gravity is the force behind plate tectonics. The ridge, being more elevated than the rest of the plate, exerts a pushing force against the continent, and the plate, being denser, tends to sink. The pull of the sinking slab is by far the stronger of the two forces. Convection currents created by the cooling of the oceanic crust as it moves away from the spreading centre also play a role (though the cells are smaller than implied in the diagram). Hot convecting material rises towards the surface, softens the crust, thins it, melts and erupts. Other convecting material circulates away from the ridge and draws the plate with it.

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 given off at the surface. Only around 20% of the heat flux seems to originate from radioactive 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 ‘first-order 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. The mantle is (now) solid, and 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 today’s 1-15 cm per year. 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. 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 – which provides the clock for measuring geological time – is assumed to have been constant because if it had been faster, spreading rates would have been faster. Any fundamental investigation of past spreading rates is precluded.

Spreading rates today 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. With ultra-slow spreading, magma supply is intermittent and shut off most of the time; with ultra-fast spreading it is more continuous, though still episodic. Thus magma supply has a marked effect on ridge morphology, rock chemistry, and on how faults are segmented, and 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 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. Because the Earth is much younger than the age which assumes constant rates of decay, ‘thermal catastrophe’ is not implied.

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 – geologically known as the Hadean era. The only traces of rock to have survived from that time are a few detrital crystals of zirconium silicate. Otherwise that former world has gone.

AAccording to recolonisation theory, the world that disappeared was the antediluvian world. Rates of radioactive decay were then much faster, causing the temperature and pressure of the Earth’s interior to rise. However, a subterranean ocean isolated the continent from the increasing pressure, and since there were no surface oceans, nor were there cycles of seafloor spreading and subduction. Eventually, mantle pressures reached catastrophic levels, the pillars sustaining the continent collapsed and the continent foundered. In a brief onslaught known as the Late Heavy Bombardment, asteroid impacts completed the demolition, the most powerful of them creating lava-filled ‘cratons’ that subsequently became the nuclei of continents. Meanwhile the subterranean water flooded the sinking continent, which thereby became the floor of a world ocean, and cracks in the floor became the spreading centres for new ocean crust. Thus was initiated the new regime of plate tectonics. Sliding under the buoyant margins of the cratons, the Earth’s primeval crust was cycled back into the mantle, and the antediluvian world became, quite literally, the Hadean underworld.

The chemically once homogeneous mantle was by this time partly molten, having separated into a core composed mostly of iron (still partly liquid) and a mantle rich in magnesium silicates. Mantle temperatures reached a maximum in the late Archaean (Herzberg et al 2010). Plumes of lighter magma rose and 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.

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

Cratons formed by vertical tectonics (diapirism), riding on top of hot, mobile mantle, constantly on the move. Towards the end of the Archaean many began to collide and amalgamate. Only then did horizontal (plate) tectonics begin playing a major role in continental growth and by the time it became the chief engine of growth, in the Palaeozoic, magmatism was on an altogether smaller scale.

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 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