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. In many parts of the world these rocks are exposed at the surface, either because overlying deposits have since been stripped away or because the primeval surface was never buried. Naturally, to say that there was no continental crust at the beginning of the Archaean is tantamount to saying that the whole Earth was then under water.

Rocks may be analysed into three basic types:

• igneous rocks
• chemically precipitated sediments, such as rock salt, cherts and limestones (later examples of which often contain a high proportion of shells)
• clastic sediments, such as sandstones and conglomerates, made up of fragments of older igneous and chemical rocks.

By far the most common igneous rocks in the Earth’s continental crust are granites and basalts. Sedimentary rocks comprise a relatively small proportion of the total and in places have been subjected to intense heat and pressure, causing them to metamorphose into a crystalline state. Gneiss, for example, is metamorphosed granite (or metamorphosed sediments derived from granite), slate is metamorphosed silt or mud, and marble is metamorphosed limestone.

Clastics are recycled rock and occur most voluminously in areas of uplift, where weathering, tectonic movements, and flows of water erode the mountains and distribute the sediment over lower-lying plains and basins, often of huge extent. Tectonically active regions may have gone through more than one cycle of mountain-building, erosion and redeposition.

The processes that created the first cratons – the crustal blocks around which further crust formed to produce continents – are not well understood, but generally Archaean rocks present evidence of catastrophic conditions. This was a more violent time than any other in Earth history, with most deposits having an igneous origin.

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, while the sea around them seethed and steamed. Here and there towers of semi-molten granite rose above the surface, then collapsed and crashed back into the sea, for the crust was as yet incapable of supporting high topographic loads. In due course some areas began to rise permanently above water.

Although some tracts underwent extraordinarily violent and complex folding, the basic geology of the Archaean is simple on a gross scale.

Characteristic rocks of the period are early Archaean gneisses, granite-greenstone belts, and the sediments that accumulated on late Archaean platforms around the craton margins. The chemistry of the igneous rocks is mostly different from that of post-Archaean eruptions.

Gneisses

These include Earth’s oldest rocks, in northwest Canada and west Greenland, as well as forming the basement of cratons elsewhere. The process by which they formed remains unclear but their chemistry is consistent with partial melting of water-infused basalts under high temperature and pressure. Pillow lavas and patchy interbeds of conglomerates and chemical sediments sometimes occur within them, denoting brief intervals when the rate of deposition slackened. Throughout the Archaean they were strongly affected by episodes of intense metamorphism and deformation, associated with inter-craton collisions and the rise of granite domes.

Granite-greenstone belts

After the basements had cooled and solidified, basalt lavas surged up through dense swarms of dikes in the gneisses to lay down the thick deposits known as greenstones – again, mostly metamorphosed. Greenstone belts, which are unique to the Archaean, occur on every craton, and are so-called because the rocks are greenish. Typically they comprise a lower, dominantly volcanic group and an upper sedimentary group. The sediments are mostly cherts, jaspers and banded iron-formations (all hydrothermal), followed by shales, sandstones, conglomerates and quartzites – eroded, reworked volcanics that were deposited rapidly by turbidity currents and debris flows. This pattern of decreasing volcanics and increasing clastics (rocks broken off from lavas) reflects the progressive phases of a single, continuous sequence, during which:

• catastrophic volcanism gradually abated, and
• depositional gradients steepened as cratons became increasingly emergent.

Both during and after deposition, the greenstone successions were intruded by huge granitic domes or batholiths, measuring up to 100 km across. 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.

Platforms

Continental crust continued to be generated and cratons continued 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 1-6 km thick (2.73 Ga), emergent volcanic sequences culminated in basins filled with turbidite fans (2.68 Ga), as collisions led to folding and more upward intrusions of granitic domes, and the transient mountain belts were unroofed and stripped. The turbidites, in turn, were followed by conglomerate-sandstone sequences (2.60 Ga) along regional fault systems.

Comparable stratigraphies are observed in many cratons, 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, whereas volcanic eruptions do not erupt in slow motion. Everything about the Archaean indicates large-scale processes taking place continuously and rapidly. If we halved the timespan so as to take account of 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 decades.