The Hadean Eon (~4.6 to 4.0 Ga) is short on rocks that represent it. In fact geologists only know of a single 20 km2 outcrop within that age span (~4.3 Ga): the Nuvvuagittuq Greenstone Belt (~4.3 Ga)on the eastern shore of Hudson Bay. Even that age remains disputed. But a few, tiny detrital zircon grains extracted from much younger sandstones yield an age range up to 4.4 Ga: barely enough to refute James Hutton’s ‘No vestige of a beginning’. So, the Hadean is long on speculation, most based on less than 3% of all the detrital zircon grains that have been dated. What zircons can tell us is based on their isotopic geochemistry, their trace-element content and even tinier granules of a few other minerals that they encapsulate. The data from them suggest the presence of some kind of felsic magma production that crystallised at low temperatures (~700° C) and was exposed to watery fluids. All very vague compared with what can be gleaned with confidence from post-4.0 Ga rock exposures. But there is a sound astronomical context and a theory based on geophysical and geochemical processes known from experiment and observation of later geology, that can shed a little light.
The planetary system began to form by gravitational accretion of material in a protoplanetary disc of pre-solar gas and dust. The first step would have been gravitational sticking together of dust particles. Fast when this cloud was dense but slowing as the available starting material was depleted by growing planetesimals. This early accretion would easily have radiated away the heat generated by the gravitational potential energy that was released. But that became less effective as the accreting bodies grew to sizes of tens to hundreds of kilometres. Studies of meteorites, formed by collisions of larger planetesimals, show that they became hot enough to melt their contents and even to undergo internal, geochemical differentiation. The current view of the next step is that gravitational perturbations associated with Jupiter drove bodies ranging from asteroidal to Mars size into chaotic motion through the Solar System. Assembly of protoplanets thereafter was dominated by collisions. In the case of the proto-Earth this involved its collision with another, Mars-sized body, to result in the formation of the Moon and the early Earth, each initially enveloped by magma oceans. This event can be considered to be the starting point for all subsequent geological processes on both bodies. But that did not ‘calm down’ planetary bombardment. Plenty of large asteroids were still around: their size range can be judged roughly from those that remain in the Asteroid Belt, that are up to 940 km across in the case of the dwarf planet Ceres. This repository of Hadean objects is what motivated Tim Johnson of Curtin University, Western Australia and three Australian colleagues to ponder on the influence on the Hadean Earth of far more bodies, large and small, hurtling around the early Solar System (Johnson, T.E. et al. 2026. Impact heating and the hidden Hadean. Science, v. 392, p. 1408-1412; DOI: 10.1126/science.aeb5402. PDF requests to tim.johnson@curtin.edu.au).
The impact history of the Earth has largely been expunged by tectonics, erosion and sedimentary burial. Johnson et al. assumed an early impact flux from the almost pristine ‘stratigraphy’ of lunar cratering scaled up to the roughly 13 times greater gravitational pull of the Earth. They calculated that energy being released by impacts and partly incorporated into the Earth during the Hadean outweighed that being generated by internal radioactive decay by several orders of magnitude. Hadean tectonics was thus thermally dominated by impact energy, whose supply probably fluctuated wildly because of different sizes of impacting bodies. By far the largest crater on the Moon – the South Pole-Aitken basin – is 2500 km across. It formed about 4.3 Ga ago when a body 200 km wide struck the lunar surface. Being larger and having a greater gravitational pull, Earth would have suffered up to ten collisions of this magnitude.
In Archaean and later times tectonics became the main means of shedding ‘smoothly’ generated internal radiogenic heating. Dated lunar rock samples strongly suggest that such awesome bombardment lasted until the early Archaean, around 3.8 Ga ago. Traces of this Late Heavy Bombardment are anomalous tungsten isotopes in gneisses of that age from West Greenland (see: Tungsten and Archaean heavy bombardment; July 2002). Internal heating now governs the physical behaviour of rock: whether it is ductile or brittle. Modern-style lithosphere is brittle, hence plate tectonics. The mantle beneath, in the long term, behaves in a ductile fashion, hence convection. As thermal energy built up with each massive impact neither thermal conduction nor bulk convection in the deeper mantle – i.e. the general state of Earth’s present thermal balance – would have been sufficient to check its effects. Rock would need to melt and magma move rapidly in vast amounts to the surface to dissipate energy by radiation into space: by far the most efficient planet-cooling process. The authors also modelled the geotherm – the variation of temperature with depth – established by conductive heat loss and radiation from the surface under Hadean conditions. This is shown in the figure and explained below.
The thick white line is the modelled conductive geotherm for the ‘coolest’ impact-heating scenario; a usually safe scientific approach. The thin white line shows beginning of melting of hydrous basaltic crust: the ‘mafic solidus’ – the blue area to its left remains solid. The dark to light green shading towards the right marks increasing percentages of basalt melting in 10% steps (dashed white lines). The palest area at right represents a completely molten crust, beyond the ‘mafic liquidus’. The dashed purple line is the liquidus of mantle peridotite. Moving leftwards, the solid purple, pink and orange lines represent the beginning of melting (solidus) for peridotite, anhydrous basalt and sodium-rich granite respectively
The modelled Hadean geotherm shows very rapid temperature increase down to about 7.5 km. It passes across the solidi of granite, hydrous basalt, anhydrous basalt and mantle peridotite: everything begins to melt. Clearly, whatever its composition, the uppermost Hadean crust would have been in a partially molten condition below about 3.5 km. At depths of 10 km or more, between 40 to 70 % of basalt would be molten. The distinction between brittle and ductile becomes meaningless in the light of Johnson et al.’s analysis of Hadean impact heating. Not only does the modelling rule out any rigid lithosphere and plate tectonics during the Hadean, it also explains the almost complete absence today of tangible Hadean rock. In particular, continental crust dominated by granitic rocks was probably recycled continually and literally into the Hadean ‘melting pot’. Convection would have dominated Hadean tectonics, but rather than taking the modern form of isolated plumes it would have been chaotic.
Suddenly, beginning about 3.9 Ga a rich record of albeit disputed tectonics emerges during the Palaeoarchaean and then evolves onwards to modern planetary behaviour. The heavy bombardment had stopped.
See also: Asteroid assault made ancient Earth too hot and chaotic for continents to form. EurekAlert; 25 June 2026. Why Earth Could Not Hold On to Its First Continents Until the Asteroids Stopped Falling. Science Blog; 25 June 2026.
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