Earth is the only one of the rocky Inner Planets that has substantial continental crust, the rest being largely basaltic worlds. That explains a lot. For a start, it means that almost 30 percent of its surface area stands well above the average level of the basaltic ocean basins – more than 5 km – because of the difference in density between continental and oceanic lithosphere. Without continents and the inability of subduction to draw them back into the mantle Earth would remain a water-world as it is thought to have been during the Hadean and early Archaean Eons. The complex processes involved in geochemical differentiation and the repeated reworking of the continents through continual tectonic and sedimentary processes has further enriched parts of them in all manner of useful elements and chemical compounds. And, of course, the land has had a huge biosphere since the Devonian period that subsequently helped to draw down CO2 well as evolving us.
It has been estimated that during the Archaean (4.0 to 2.5 Ga) around 75% of continental crust formed. Much of this Archaean crust is made up of sodium-rich granitoids: grey tonalite-trondhjemite-granodiorite (TTG) gneisses in the main. Their patterns of trace elements strongly suggest that their parent magmas formed by partial melting at shallow depths (25 to 50 km). Their source was probably basalts altered by hydrothermal fluids to amphibolites, unlike the post-Archaean dominance of melting associated with subducted slabs of lithosphere. Yet most of the discourse on early continents has centred on when plate tectonics began and when they became strong enough to avoid disruption into subductible ‘chunks’. Yet 10 years ago geochemists at the University of St Andrews in Scotland used hafnium and oxygen isotopes in Archaean zircons to suggest that the first continents grew very quickly in the Hadean and early Archaean at around 3.0 km3 yr-1, slowing to an average of 0.8 km3 yr-1 after 3.5 Ga. In 2017 Geochemists working on one of the oldest cratons in the Pilbara region of Western Australia developed a new, multistage model for early crust formation that did not have a subduction component. They proposed that high degrees of mantle melting first produced a mafic-ultramafic crust of komatiites, which became the source for a 3.5 Ga mafic magma with a geochemistry similar to those of modern island-arc basalts. If a crust of that composition attained a thickness greater than 25 km and was itself partially melted at its base, theoretically it could have generated TTG magma and Archaean continental crust. Three members of that team from Curtin University, Western Australia, and others have now contributed to formulating a new possibility for early continent formation (Johnson, T.E. et al. 2022. Giant impacts and the origin and evolution of continents. Nature, v. 608, p. 330–335; DOI: 10.1038/s41586-022-04956-y).
Tim Johnson and colleagues base their views on oxygen isotopes in Archaean zircon grains from the Pilbara. The zircons’ O-isotopes fall into three kinds of cluster: low 18O that indicate a hydrothermally altered source; intermediate 18O suggesting a mantle source; high 18O signifying contamination by metasedimentary and volcanic rocks. The first two alternate in the 3.6 to 3.4 Ga period; 4 clusters with mantle connotations occupy the 3.4 to 3.0 Ga range; a cluster with supracrustal contamination follows 3.0 Ga. This record can be reconciled agreeably with the geological and broad geochemical history of the Pilbara craton. But there is another connection: the Late Heavy Bombardment (LHB) recognised on most rocky bodies in the Solar System.
Bodies with much more sluggish internal processes than the Earth have preserved much of their earliest surfaces and the damage they have suffered since the Hadean. The Moon is the best example. Its earliest rocks in the lunar Highlands record a vast number of impact craters. Their relative ages, deduced from older ones being affected by later ones, backed up by radiometric ages of materials produced by impacts, such as melt spherules and basaltic magmas that flooded the lunar maria, revealed the time span of the LHB. The maria formed between 4.2 and 3.2 billion years ago and the damage done then is shown starkly by the dark maria that make up the ‘face’ of the Man in the Moon. The lunar bombardment was at a maximum between 4.1 and 3.8 Ga but continued until 3.5 Ga, dropping off sharply from its maximum effects. Earth preserves no tangible sign of the LHB, but because it is larger and more massive than the Moon, and both have always been in much the same orbit around the Sun, it must have been subject to impacts on a far grander scale. Projectiles carry kinetic energy that enables them to do geological work when they impact: 1/2 x mass x speed2. The minimum speed of an impact is the same as the target’s escape velocity – 2.4 km s-1 for the Moon and 11.2 km s-1 for the Earth. So the energy of an object hitting the Earth would be 20 times more than if it struck the lunar surface. Taking into account the Earth’s larger cross sectional area, the amount of geological work done here by the LHB would have been as much as 300 times greater than that on Earth’s battered satellite.
The Earth’s early geological history was rarely seen in that context before the 21st century, but that is the framework plausibly adopted by Johnson and colleagues. Archaean sediments in South Africa contain several beds of impact spherules older than 3.2 Ga, as do those of the Pilbara. The LHB also left a geochemical imprint on Earth in the form of anomalous isotope proportions of tungsten in 3.8 Ga gneisses from West Greenland (See: Tungsten and Archaean heavy bombardment and Evidence builds for major impacts in Early Archaean; respectively, July and August 2002). Johnson et al. suggest a 3-stage process for the evolution of the Pilbara craton: First a giant impact akin to the lunar Maria that formed a nucleus of mafic-ultramafic crust from shallow melting of the mantle; its chemical fractionation to produce low-magnesium basalts; and in turn their melting to form TTG magmas and thus a continental nucleus. They conclude:
‘The search for evidence of the Late Heavy Bombardment on Earth has been a long one. However, all along it seems that the evidence was right beneath our feet.’
I agree wholeheartedly, but would add that, until quite recently, many scientists who referred to extraterrestrial influences over Earth history were either pilloried or lampooned by their peers as purveyors of ‘whizz-bang’ science. So, many ‘kept their powder dry’. The weight of evidence and a reversal of wider opinion over the last couple of decades has made such hypotheses acceptable. But it has also opened the door to less plausible notions, such as an impact cause for sudden climate change and even for mythological catastrophes such as the destruction of Sodom and Gomorrah!
See also: Timmer, J. 2022. Did giant impacts start plate tectonics? arsTechnica 11 August 2022.
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