Present-day continental crust, which typically has a thickness of 30–40 km, did not form all at once. While estimates carry large margins of uncertainty, at least 50% is thought to have formed during the Archaean and most of the rest during the subsequent Proterozoic, by upwelling of hot, liquid rock from the mantle. Most of the older rock was later remelted into granite or eroded into sediment, but in a few parts of the world the original rocks are exposed at the surface, mainly as a result of uplift and overlying deposits being stripped away. At the start of the Archaean the whole Earth was under water.
- igneous rocks
- chemically precipitated sediments, such as salts, cherts and limestones (the latter increasingly composed of tiny organic shells)
- clastic sediments, such as clays, sandstones and conglomerates, made up of fragments of older igneous and chemical rocks.
By far the most common igneous rocks in the Earth’s crust are continental granites and oceanic basalts. Clastic, sedimentary rocks constitute a relatively small proportion of the total but are common, after the Archaean, at the top of continental crust. They occur most voluminously in areas of uplift, where weathering, landslides and tectonic dislocations erode the mountains and distribute the product across low-lying plains and basins. The most common sediments are mudrocks (mudstones and shales). When rocks are subjected to intense heat or pressure, they recrystallise in a process known as metamorphism. Among the more familiar types of metamorphic rock are marble (metamorphosed limestone) and slate (metamorphosed mudstone).
How the first cratons came into being – the crustal blocks around which further crust nucleated to produce continents – is not well understood, but certainly most Archaean rocks reflect conditions that were catastrophic. This was a more violent time than any other in Earth history, and dominated by igneous processes.
Around 35 craton fragments have been counted, some of which amalgamated in the Archaean and early Proterozoic to form larger units. The cratons floated on currents of magma, occasionally colliding and buckling into tight folds. As yet the crust was too soft to support high loads. Eventually some areas began to rise above water.
Although some tracts underwent extraordinarily violent and complex folding, on a gross scale the geology of the Archaean is relatively simple. Characteristic rocks include early Archaean gneisses, granite-greenstone belts, and late-Archaean clastic sediments that accumulated around the craton margins.
These include Earth’s oldest rocks, in northwest Canada and west Greenland, as well as making up the basement of cratons elsewhere. The original deposits – igneous and sedimentary – were later metamorphosed by high heat and pressure and strongly deformed, often forming beautiful wiggly bands. Pillow lavas and patchy beds of conglomerates and chemical sediments occur within the sequences, denoting intervals when the rate of deposition slackened.
- catastrophic volcanism gradually abated
- depositional gradients steepened as cratons became increasingly emergent
- erosion and sediment recycling became dominant.
Both during and after deposition, the greenstone successions were intruded by huge granitic domes or batholiths, measuring up to 100 km across. Their chemistry is consistent with partial melting of water-infused basalts. These inflating plumes of low-density magma rose through the still soft greenstones and shouldered them aside. By the mid to late Archaean some may have reached all the way to the surface; others were exhumed in the Proterozoic and later. The ascending granites transferred with them a significant proportion of heat-producing elements, enabling the lower crust to cool and stiffen.
Continental crust continued to be generated and cratons to amalgamate in the late Archaean. As their interiors stabilised, widespread sedimentary platforms and basins began to develop around the margins, showing that continental crust had attained sufficient rigidity to sustain sedimentary piles many kilometres thick.
Canada’s Slave craton is a fairly typical example. After deposition of submarine basalts up to 6 km thick (2.73 Ga, column c), partially emergent volcanic and volcaniclastic sequences accumulated, then turbidites (2.68 Ga), as collisions led to folding and intrusions of granitic domes, and the transient newly formed mountains were unroofed. The turbidites, in turn, were followed by conglomerate-sandstone sequences along regional fault systems (2.60 Ga).
Comparable stratigraphies are observed in other places, such as the Dharwar (southern India), Zimbabwe, Wyoming cratons and parts of the Yilgarn craton (western Australia).
It is difficult to see how 1 billion years (beginning, say, from 3.5 Ga) can credibly be allocated to such processes. Averaged over a thickness of 30 km, 1 billion years translates to a rate of 0.03 millimetres per year. A single granite batholith in the Pilbara Terrane (above) spans a period of 650 Ma – equivalent to the entire length of the animal fossil record, much later in time. Volcanic eruptions do not erupt in slow motion. Before 3.3 Ga, sedimentary rocks such as sandstones, which originate from the erosion of igneous rocks, are rare. Everything about the Archaean indicates large-scale processes taking place rapidly, with occasional interludes. Even if we halved the timespan to allow for intervals of erosion and non-deposition, the average would still be only 0.06 mm per year: less than the thickness of a sheet of paper. Archaean geology is incompatible with the dates produced by radioisotope dating. While it is impossible to say exactly how long the period occupies, it seems more fitting to think in terms of thousands of years than of billions.