4. New land in the Archaean
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. At the start of the Archaean there was no differentiated continental crust and the whole Earth was under water.
Rocks may be analysed by origin into three basic types:
- igneous rocks
- chemically precipitated sediments, such as salts, cherts and limestones (younger cherts and limestones may include a high proportion of organic matter)
- 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 Earthâ€™s crust are continental granites and oceanic basalts. Sedimentary rocks comprise a relatively small proportion of the total, mostly in the upper crust. When rocks are subjected to intense heat or pressure, they recrystallise â€“ a process known as metamorphism. Among the more familiar types of metamorphic rock are marble (metamorphosed limestone) and slate (metamorphosed mudstone).
Clastic sediments occur most voluminously in areas of uplift, where weathering, tectonic dislocations and flowing water erode the mountains and distribute the product across low-lying plains and basins. Tectonically active regions may have gone through more than one cycle of mountain-building, erosion and redeposition. The most common sediments are mudrocks (mudstones and shales).
How the first cratons came into being â€“ the crustal blocks around which further crust nucleated to produce continents â€“ is not well understood, but 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 permanently above water.
Although some tracts underwent extraordinarily violent and complex folding, on a gross scale the basic geology of the Archaean is relatively simple. Characteristic rocks include early Archaean gneisses, granite-greenstone belts, and the sediments that accumulated on late Archaean platforms around the craton margins.
These include Earthâ€™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 beds of conglomerates and chemical sediments sometimes occur within them, denoting brief intervals when the rate of extrusion slackened. Throughout the Archaean they were strongly affected by episodes of intense metamorphism and deformation, associated with the ascent of granite domes and collisions between one craton and another.
After the basements had cooled and solidified, basalt lavas surged up through dense swarms of dikes in the gneisses to lay down the thick underwater deposits known as greenstones â€“ again, now mostly metamorphosed. So named because the rocks are greenish (rich in iron), these belts occur on every craton and are unique to the Archaean. 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 and conglomerates â€“ 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
- 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. 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.
Canadaâ€™s Slave craton is a fairly typical example. After deposition of submarine basalts up to 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 intrusions of granitic domes, and the transient mountain belts were unroofed. 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. Volcanic eruptions do not erupt in slow motion. Before 3.3 Ga, sedimentary rocks such as sandstones, generated from the erosion of volcanics, are virtually non-existent. 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 thousands of years than of billions.