EARTH STRUCTURE AND ORIGINS 429 geology’s dark ages We know something of this period from the Moon – the lunar rocks brought back by Apollo and Lunik date from this period – and it is reasonable to invoke a bombardment of the Earth, like the Moon, during this period However, there is no trace of this in the geological record of the Earth The lack of evidence of ejecta from the Moon in Space means that it is difficult to attribute all the heavy early cratering of the Moon to bombardment by impacting objects from Space (see Solar System: Meteorites) Nevertheless, the early Earth must have swept up asteroids, comets, and any other debris in its path for a few million years after it attained its present size The Earth, after accretion, must have heated up internally and melted in order to separate, gravitationally, the core (part liquid and part solid), mantle, and crust This heating would have been caused by the radioactive decay of elements and by gravitational energy – the latter as the dust, or dust and planetesmals, condensed and accreted into a ball It has also been suggested that heat could have been produced by impacts on the surface, though much of this heat would have dissipated into Space The heavy elements, mainly iron, would have gravitated to the core, and the lighter silicate material would have ended up in the mantle and crust A primitive and highly mobile surface layer, perhaps a magma ocean, would have initiated the crust, and from this small-scale proto-continents separated More ‘way-out’ models for the formation of the crust invoke early impacts, the subsidence of large impact-generated basins in the basaltic proto-crust, and partial melting of the basalt to form the proto-continents The early protocontinents were certainly small, and the extent of the continents increased until around 3200 Ma, when the Kaapvaal (South Africa) and Pilbara (Western Australia) cratons were formed The very lightest volatile elements would have separated at the same time to form the proto-ocean and proto-atmosphere This volatile component would have had a bulk composition of 10–20% water and contents of other volatiles in line with those of carbonaceous chondrite meteorites, according to the chondritic Earth model The first atmosphere was almost certainly reducing rather than oxygenating Our present oxygen-containing atmosphere was derived later, though how much later is debatable, and relates, in its origin, to the onset of biological activity How and when life originated is still a mystery – there are models that suggest that it originated within the early developing Earth, but there are also models that suggest that it arrived on the planet from comets or primitive meteorites (see Origin of Life) See Also Earth: Mantle; Crust; Orbital Variation (Including Milankovitch Cycles) Igneous Rocks: Kimberlite Magnetostratigraphy Mantle Plumes and Hot Spots Minerals: Zircons Origin of Life Plate Tectonics Solar System: Meteorites; Mercury; Moon; Venus Volcanoes Further Reading Anderson DL (1999) A theory of the Earth: Hutton, Humpty Dumpty and Holmes In: Craig GY and Hull JH (eds.) James Hutton: Present and Future, pp 13 35 Special Publication 150 London: Geological Society Edwards K and Rosen B (2000) From the Beginning London: Natural History Museum Hamilton WB (2002) The closed upper mantle circulation in plate tectonics In: Plate Boundary Zones, pp 359 410 Geodynamics Series 30 American Geophysical Union Hancock PL and Skinner BJ (2000) Oxford Companion to the Earth Oxford: Oxford University Press Harland WB, Armstrong RL, Cox AV, et al (1990) A Geologic Timescale 1989 Cambridge: Cambridge University Press Holmes A (1965) Principles of Geology, revised edn London and Edinburgh: Nelson McCall GJH (2000) Age and early evolution of the Earth and Solar System In: Hancock PL and Skinner BJ (eds.) Oxford Companion to the Earth, pp 11 Oxford: Oxford University Press McCall GJH (2003) Pole up the pole Geoscientist 13: Price NJ (2001) Major Impacts and Plate Tectonics London and New York: Routledge Rothery DA (1992) Satellites of the Outer Planets Oxford: Clarendon Van Andel TJ (1994) New Views on an Old Planet, 2nd edn Cambridge: Cambridge University Press Vita Finzi C (2002) Monitoring the Earth Harpenden: Terra Wilde SA, Valley JW, Peck WH, and Graham CM (2001) Evidence for the early growth of continents and oceans from