The Archaean is the oldest time division of the extant crustal record. Radioisotope dating gives it an age of 4.0–2.5 billion years (Ga), spanning just over a third of Earth’s preserved history, and 4.0 Ga is more or less the age of the oldest rocks. The only remains of the surface older than that are crystals of the mineral zircon (ZrSiO4). They are very rare and the vast majority come from just one locality, the Jack Hills in Western Australia. With cooling ages up to 4.37 Ga, the zircons crystallised as magma before the Cataclysm solidified not far from the surface. The rocks were subsequently destroyed, leaving these highly durable remnants to be incorporated into younger, post-Cataclysm sedimentary rocks.
Conventional geology struggles to know why the Hadean aeon is almost completely absent from the primary rock record, but one possibility is simply that the original land was destroyed. If we suspend belief in the inflated timescale, the Hadean/Archaean boundary equates to the time of the Flood-Cataclysm, and the Archaean itself to the first few thousand years immediately after the Cataclysm, at the start of which the planet was under water and new land began to form in place of the old.
Rocks from the earliest Archaean are nearly all igneous: that is, they originate from magma directly rather than indirectly through the break-up and redeposition of pre-existing rock. Many of the lavas contracted into pillow shapes, indicating that the eruptions were submarine, and and the paucity of clastic rocks such as sandstones and conglomerates, products of weathering, erosion and tectonic upheaval, indicates that there was little land above sea-level. The general picture is of a world submerged, with here and there volcanic piles protruding like ancient versions of Surtsey Island, rapidly shedding material from their unstable slopes. For a very brief period there may have been no dry land at all, just like the situation described in the Hebrew tradition: ‘The waters prevailed so mightily upon the earth that all the high mountains under the whole heaven were covered.’ With the old creation destroyed, terrestrial crust had to form anew, by massive underwater extrusions.
Although Genesis describes the emergence of land in one locality, there is no way of knowing whether that locality was representative. Thickening Archaean crust in eastern India is thought to have emerged around 3.3 Ga ago. Marine barite deposits from different parts of the world suggest that weathering of emergent crust was influencing the chemistry of seawater as early as 3.7 Ga (Roerdink et al. 2022). The early Eoarchaean was the only time when Earth was wholly under water and in that respect provides a perfect match for the Hebrew record.
- gravitational energy, converted into heat as the planet grew in size and density and as it differentiated into silicate mantle and metallic core
- kinetic energy released by the collision with some other planet-sized body that gave birth to the Moon (there are also other, less popular theories of how the Moon originated)
- kinetic energy from ongoing bombardment by large asteroids and comets
- much higher heat production from radioactivity, since the ratio of parent to daughter elements was then much greater.
In such conditions water could not have existed on Earth at all, and the nascent Earth was a long way inside the ‘snow line’ (between Mars and Jupiter) beyond which water in the protoplanetary disc could have existed anyway as a liquid. Until quite recently, the absence of Hadean rocks was seen as evidence that Earth was even too hot for solid crust to form (Valley 2005). The Earth should have been engulfed in a great ocean of magma.
In contrast to the longstanding paradigm of a hellish early Earth devoid of oceans, continents and life, the Hadean zircon record, and the micro-rocks that they encapsulate, largely grew under a range of conditions far more similar to the present than once imagined.
For all we know, even people and animals could have lived on it.
Since the water had to have come from somewhere, the most common suggestion is that it came from comets hitting the Earth after the proto-crust had solidified. This is to build supposition upon supposition, since we don’t have any evidence of a primeval crust as such (that is, of a top layer that solidified from magma), we therefore don’t know how early it might have solidified if it existed, and within the standard narrative we don’t know where the cometary water itself came from. The suggestion only displaces the problem.
Here again the Moon provides an instructive contrast. Seismological data indicate that, today, it has a solid iron-rich inner core, a molten outer core wrapped in a layer that is only partially molten, and a solid mantle and crust. The mantle consists of iron- and magnesium-rich silicate and the crust comprises two types of terrain, highlands consisting mostly of grey-white anorthosite, which is a plagioclase-rich rock, and low-lying maria consisting of dark basalt similar in composition to the mantle. Early on, the initially solid, homogenous interior must have melted, at least in part, allowing iron to sink to the centre and the least dense plagioclase to rise to the surface. The dark patches are solidified extrusions of mantle that welled to the surface in response to the most violent of the asteroid impacts. The impacts melted both the crust beneath them and the mantle immediately below the crust (if the mantle was not already molten). Since the smaller asteroids did not have sufficient force to puncture the crust, the smaller craters were not flooded with magma. The oldest rock sample is an anorthosite whose crystallisation dates to 4.36 Ga, by which time the interior must already have separated into core, mantle and crust. The interior of the Earth began differentiating no later than 4.42 Ga (Morino et al. 2017). The crucial point is: if the Moon was once covered in a magma ocean, how is it that the Earth never was? And relatedly, why are the crystallisation temperatures of lunar zircons typically 300 °C higher than terrestrial zircons (Harrison et al. 2017)?
Here the Hebrew tradition gives us important additional information. The original Earth was two-layered, with a body of water lying underneath the land rather than encircling it – as indeed has been geologically inferred (Kamber et al. 2005). When the interior heated up, the water acted as a buffer. The zircons formed in the upper mantle immediately beneath that buffer. They have the characteristics of continental zircons because when chondritic magma interacts with water its mineralogy changes and the result is a less dense, less iron-rich magma, similar to molten granite.
Differentiation began 4.5–4.4 Ga from the centre outwards, somewhat before the age of the oldest zircons near the periphery. The interior was already molten, and was heating up, not cooling down. Hadean zircons correspondingly become more abundant over time, and peak in frequency 4.13–4.0 Ga. As the heat of the interior increased, pressure also built up. Eventually, the springs exploded, the pillars holding up the land collapsed and the lithosphere disintegrated. Under the onslaught of the asteroids every mountain and hill was laid low. Within 40 days even the highest mountains were submerged, hot magma rising over their underwater ruins. The ocean that was once underneath the land ended up on top of it.
The rims of some zircons can be up to a billion years younger the cores. After the crystals formed and after the enclosing rocks were destroyed, the zircons were entrained back into molten material, which cooled in the course of a second phase of rock formation. The cores represent pre-Cataclysm magmatism below the subterranean deep. The rims represent magmatism after the Cataclysm, primarily in the wake of the huge energy released by the asteroid impacts.
- Earth would not have been sterilised even if the impacts had all occurred simultaneously.
- The surface would have cooled within days, at which point the rate of heat loss would have been limited by conduction from the subsurface.
- The upper 350 metres of the crust could have cooled down to habitable temperatures in under 1,000 years.
- If water had saturated the subsurface, heat would have been lost up to ten times faster and habitable conditions re-established up to an order of magnitude more rapidly.
The findings also help to inform the question in relation to a 40-day bombardment. The impact energy would have been colossal: the equivalent of billions of hydrogen bombs. Most of it would have been absorbed by the thick layer of land, not the water beneath. In contrast to today’s continents, which are 1300? C at their base because of the heat conducted and convected directly from the mantle, this primeval layer would have been relatively cool. Undoubtedly the upper few kilometres would have melted, but as water engulfed the land, rapid quenching of the melt would have formed an insulating crust, just as, on a much smaller scale, an insulating crust forms around pillow basalts. Further cooling would have taken place more slowly.
Thus, although the oceans would certainly have been hotter than now, some of the aquatic life that filled the lakes and rivers might have survived – animal life as well as microbes.
Just how warm the oceans were is difficult to say. One indicator is the oxygen isotope ratio of marine phosphates, since in chemical sediments such as phosphate and calcium carbonate the ratio varies with the temperature of the water from which they precipitated. The oldest such evidence comes from the 3.5–3.2 Ga-old Barberton greenstone belt of South Africa (Blake et al. 2010). When we take into account the isotope ratio of the seawater itself (Johnson & Wing 2020), near-surface ocean temperatures in the range 38–47? C are suggested. Ocean-floor cherts indicate that deeper water was warmer and therefore denser, which makes sense in view of the large volumes of dissolved iron and other chemicals oozing from the mantle. The abundance of these chemicals bears witness to the intensity of seafloor spreading. Devoid of oxygen, saturated in metals, hot and acidic, subsurface waters were uninhabitable. The submerged, formerly terrestrial crust, melted and broken up by asteroid impacts, was in hydrothermal ferment.
Evidence of near-surface life from this time is preserved in the Strelley Pool Chert of Western Australia. Here we have a snapshot of environments all the way from the rocky shore to the volcaniclastic deposits of deep water. During ‘a rare pause in igneous and hydrothermal activity’ stromatolites – sedimentary mats and mounds produced by microbes – formed in the shallows, decreasing in abundance where the water deepened and also higher in the succession where increasing evaporation-induced salinity rendered conditions intolerable. The hydrothermal activity reflected the ascent of a molten blob of granite through the craton margin, causing the shelf to dome up and fracture. Soon after the pause, lava up to 8 km thick flooded the region.
The rocks of Akilia provide no evidence that life existed at or before c. 3.82 Ga, or indeed before 3.67 Ga, or even that traces of life occur at all. The complexity of the rocks (mafic, ultramafic and tonalitic gneisses), multiple episodes of intrusion, intense deformation and high-grade metamorphism, and the absence of any unequivocal evidence that any of the rocks were deposited on the Earth’s surface, suggest that these rocks are wholly unsuitable for the discovery of primordial traces of life.
M J Whitehouse et al, Journal of the Geological Society 166:335-48 (2009)
However, evidence of life earlier than 3.5 Ga continues to be adduced. Primary producers – that is, organisms that feed on inorganic matter rather than other organisms – prefer 12C to 13C when building their tissues, causing organic carbon to be measurably depleted in 13C, typically by around 2.5%. This means that laboratory instruments can detect the presence of life even in the absence of fossils. Organic carbon, along with stromatolites, has been found in Greenland dating to 3.7 Ga. Putative microfossils within iron-rich sedimentary rocks in Quebec date to at least 3.77 Ga. Most startling of all, one zircon from Western Australia preserves an inclusion of organic carbon going back to 4.1 Ga (Bell et al. 2015), immediately before the Cataclysm. Microbes, it seems, were in possession of complex biological machinery from the very beginning.