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 over time have shaped the world that we know today. The concept is fairly new (gaining currency in the late 1960s), but it has revolutionised our understanding of geological processes. For example, it helps us understand why earthquakes and volcanoes occur in some parts of the world and not others, and how mountain ranges formed. Mountain ranges vary in age from relatively young ranges such as the Alps and Himalayas to ancient ones such as the Appalachians, but whatever their age, all formed as a result of plate tectonics, either through the subduction of ocean crust under continental crust (top picture, left) or through the collision of two continental plates (top right).

New ocean crust forms at mid-oceanic ridges, where magma wells up 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. Some 15-20 cubic kilometres of new crust form in this way every year, with a similar amount being subducted under the continents. Although this may sound like a lot (it is enough to bury every building in London) it translates to an average spreading rate of just 2-15 cm per year. Whether present-day rates of spreading are typical of those in the past is open to question.

Listed on the Links page are several good websites on the subject. An immediate overview (for readers who would like some background knowledge) can be obtained at:
http://scign.jpl.nasa.gov/learn/plate.htm

The ocean crust has been completely replaced at least twice!

The ocean basins are relatively young in geological terms. While every continent has portions of land going back to the Archaean, no part of the present ocean crust is older than the Jurassic period, shown as blue in the colour-coded image below.

The Jurassic is the time when the Atlantic began to open from a rift in the supercontinent ‘Pangaea’ (below). In other words, the entire ocean crust that existed before the Jurassic – more than 70% of the Earth’s surface – has been recycled back into the mantle and replaced.

During the immediately preceding Permian and Triassic periods the plate-tectonic system was less active than before the Permian or after the Triassic. We can see this from reconstructions of the geography. Nearly all the continents were joined together into a single landmass, and they maintained that basic arrangement throughout that time. Before the Permian and after the Triassic they were much more mobile.

Such a pattern is also consistent with the kind of marine limestones that formed during this hinge-like 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).

To experience a ‘quicktime’ animation of the Earth’s changing geography from the Permian to the present, see http://pgap.uchicago.edu/global290-0pgeogrev.mov

In the periods before the Permian the Earth looked even less familiar. Click on the link below for a representation of how the landmasses are thought to have been distributed in the Ordovician.
http://jan.ucc.nau.edu/~rcb7/Ord.jpg

When we bear in mind how enormously the distribution of the continents changed in the course of the Palaeozoic, it becomes clear that the Earth’s oceanic crust must have been recycled back into the mantle more than once in its total history. By contrast, continental cratons and their roots have not been consumed to anywhere near the same extent, because they are made up of minerals that are lighter than the ocean floors. Overall, their volume and areal extent have been increasing over time.

Terrestrial crust has been replaced just once

Geologically it is well known that the planet’s terrestrial crust has been replaced only once, and this, not gradually over the whole of Earth history but right at the beginning. One of the great mysteries of geology is that rocks of all kinds are completely missing from the first ‘700 million years’ of Earth history. This period is known as the Hadean, of which the only traces that survive are tiny, very rare crystals containing the element zircon. These indicate that, contrary to earlier notions of what the primordial Earth was like, both continents and oceans existed at that time. But they have all gone, and it seems that they were very quickly gone – before the succeeding Archaean period even started.

One of the key concepts of recolonisation theory is that the world that disappeared at the end of the Hadean was the antediluvian world: the world before the great cataclysm. It was ‘recycled’ back into the mantle to become, quite literally, the Hadean underworld. In its place, new crust formed, and, so far as the continents were concerned, it formed little by little, out of water. Thus modern geology and ancient Hebrew tradition combine to inform each other.

Although the antediluvian world perished suddenly and violently, the process of ocean-crust replacement that continues still today is a sobering reminder of how it is possible for entire slabs of crust to disappear into the mantle and be completely destroyed.

Plate tectonics, radioactivity and recolonisation

Following that cataclysm, the face of the planet changed dramatically, and as a consequence so did its climate and the environments in which plants and animals made a living. They were colonising environments that were continually evolving and quite different from the ones to which they had previously been adapted. In their own way plants and animals also had to evolve if they were to keep pace with these changes. Many of them in fact became extinct – not so much because they failed to adapt, as because every so often sudden catastrophes punctuated Earth history and wiped them out.

Plate tectonics is the driving force behind most geological processes. What drives plate tectonics, however, is less clear. Part of the answer is that the decay of radioactive elements in the mantle releases heat, the heat powers convection currents, and the convection currents carry the ocean plates along with them. However, the convecting mantle is depleted in heat-producing elements, giving rise to a disparity between the heat produced by radioactivity and the heat measured to be coming from the mantle. Present rates of radioactivity and secular cooling are insufficient to account for the heat still given off. They are also insufficient to drive the convection currents themselves. Mantle convection calculations do not exhibit plate tectonic behaviour unless it is imposed by the modeller (Tackley 2000). Consequently, if rates of radioactive decay have been constant over time, there must be another source of energy, usually suggested to be the pull of gravity exerted by the subducting slab.

Recolonisation theory proposes that rates of radioactive decay were much faster in the past: rising in the pre-Cataclysm period, then declining. As a result of the heat generated, pressure built up in the mantle until eventually the Earth’s crust ruptured. Plate tectonics can therefore be understood as a consequence of the heating up of the Earth’s interior. Temperatures in the mantle varied in proportion to the amount of radioactive elements present, and viscosity varied according to the amount of water mixed in. Accordingly some parts of the mantle were lighter and more mobile than other parts, causing convection cells to form, and circulation within the cells accelerated as plumes of magma rose to the surface and drew other mantle in their train.

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 91:160-161 (1998).

Plate tectonics describes the movement of solid crustal plates as they ride on top of hot, mobile mantle. Since this movement is driven by convection cells, the cells had to develop first. The proto-continents that formed in the Archaean formed at the same time as the cells developed, by vertical tectonics (diapirism). Cratons were floating on top of the mantle and constantly on the move, with some converging and colliding. Horizontal (plate) tectonics does not seem to have played a substantial role in continental growth until the Proterozoic, and by the time plate tectonics became the main 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. The large rises and falls in sealevel can be correlated to high/low plate tectonic activity during the Phanerozoic 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.

See also:
The Hadean Cataclysm
New land in the Archaean