age and early evolution of the Earth and Solar System

age and early evolution of the Earth and Solar System One of the questions that geologists are most frequently asked is ‘How do you know the age of the rocks?’ The early geologists were aware of the fact that there was a regular order of superposition in sedimentary strata (for example, Nicholas Steno (1638–87), and to William Smith (1769–1839) is attributed the recognition that one can use the fossil assemblages contained in the rocks to identify individual formations, because fossil assemblages are different in rocks of different ages and do not repeat themselves. The early geologists, however, had no idea of the magnitude of the time-spans represented by rock sequences. In the nineteenth century, the definitions of the eras and systems of the geological column were based on fossils and sequences (see stratigraphy). Such definitions leant heavily on the work of Sedgwick (1785–1873) and Murchison (1792–1873), but there was still no absolute timescale.

The earliest attempt to determine the age of the Earth was by Bishop Ussher, who in 1654, on the basis of the Scriptures and reportedly the misidentification of a crinoid fossil as an ear of corn, dated the creation at 26 October 4004 bc at the ‘sensible hour of 9 a.m.’ Buffon in 1749 estimated that at least 75 000 years were required to produce the known fossil-bearing strata. In the 1860s Lord Kelvin used the cooling rate of the Earth to arrive at an estimate of 98 million years, with lower and upper limits of 20 million and 400 million years. John Joly, in 1899, used the saltiness of the sea to arrive at an age of 80 to 90 million years for the Earth. Although we now know that the methods used were flawed, Kelvin's estimate was hailed as ‘established by the laws of physics’. Nevertheless, T. C. Chamberlin in the United States argued that it was incorrect and so the laws of physics must be wrong.

The discovery of radioactivity in 1896 supplied a means of calibrating Earth history, and in 1907 Boltwood showed that a figure of at least 400 million years (and possibly 2000 million years) was more correct. Various new methods of radiometric dating have since been developed, based on the decay of elements (for example U–Pb, K–Ar, Rb–Sr, Rh–Os, Sm–Nd, radiocarbon, fission tracks). The accepted age of the Earth rapidly increased to 3.5 billion years and we now accept 4.5 billion years as the likely age of the Earth. Radiometric dating requires no more than the application of a simple equation, but the rock must be impermeable, and have lost or gained neither daughter nor parent isotopes. Nowadays radiometric dating can be effected with great precision using sophisticated instruments, such as the ion probe. As a result, progressive metamorphic changes (caused by heating and pressure), diagenetic changes (caused by burial), and stages in igneous differentiation can be dated in rocks. The entire geological column can in fact be calibrated (Fig. 1).

The development of the Solar System

It is almost universally accepted that the Sun, the planets, their satellites, and the asteroids grew from a cloud of gas and dust, contracting under its own gravity. The cloud originally had some degree of rotation, so, as the centre contracted, the conservation of angular momentum forced the rest of the cloud into a flattened disc, the Solar Nebula, rotating in the same plane as the Sun. This process probably occupied about 10⁴ years. Nucleosynthesis had already started by the time this stage was completed and the Sun shone brightly; observations of very young stars support such an evolutionary scenario. Material was then lost to interstellar space and condensation also occurred, producing the solid bodies of the solar system. The density of the nebula decreased because of these processes; heat radiated away more easily and cooling occurred. The planets condensed and aggregated from the nebular material.

The solar abundances of elements (measured spectrometrically) and the abundances in primitive meteorites (measured analytically) enable us to estimate the composition of the original nebula. Most stony meteorites are composed of rounded microscopic lithic particles (chondrules), made up of common minerals such as olivine and pyroxene, and these are widely held to be condensation globules, although only in primitive meteorites such as carbonaceous chondrites have they not suffered secondary modification. The chondritic meteorites can be radiometrically dated like igneous rocks, and they are believed to have been formed over a period of 100 000 years or so by condensation 4.5 billion years ago. This is taken as the age of Solar System (and also the age of the Earth).

The favoured scenario for the Solar System requires that bodies further from the Sun will contain progressively more volatile material, and this appears to be the case; there is evidence of water-ice in the outer planets and their satellites. There are two models for the accretion of the planets: slow/heterogeneous and rapid/homogeneous. The answer probably lies between these extremes.

Astronomical evidence of the inclination of orbits favours condensation for the satellites of planets, not capture; but Triton, Jupiter's largest satellite, has a retrograde orbit and might have been captured from somewhere in the outer Solar System; and there may be other captured satellites.

Age calibration for the Moon, Mercury, Venus, and Mars

We have good calibration based on radiometric dating for the lunar surface rocks; and, surprisingly, radiometric dating puts the main cratering events as between 3.92 and 3.17 billion years, and the oldest Pre-Nectarian rocks are dated at 4.17– 4.54 billion years. After 3.2 billion years, by which time most of the lunar surface had been formed, the calibration is weakly based. The cratering on the Moon is widely held to be due to impacts.

For Mars, a stratigraphy has been based empirically on crater counts. The oldest rocks of Hesperia Planum are believed to be 3.9 billion years old, various ages suggested down to Mare Acidalium at 1.2 billion years, and an age as young as 300 million years has been suggested for Olympus Mons and the other shield volcanoes. There has, however, been no radiometric dating except on meteorites supposed to have come from Mars and these ‘best ages’ derived from crater counts may be widely out. There may be a link between the suggested age of Olympus Mons and the shield volcanoes and the age determined radiometrically for the Shergottite meteorites, believed to come from Mars, but determinations on such meteorites provide little in the way of support, or otherwise, for the suggested ages derived from crater counts.

For Venus, again, an empirical stratigraphy has been proposed which is based on the superposition of radar-detected features, but a major surprise is that the global distribution of craters considered to be due to impact cannot be distinguished from a completely random spatial distribution, and the global crater retention age may be no more than 300–550 million years. It is therefore believed that no geological units from the first 80–90 per cent of the history of this planet, heavily influenced by volcanic activity, remain at the surface.

For Mercury, prolifically cratered, it is likely that the cratering history and age of surfaces are not greatly different form those of the Moon, but the suggestion that the post-Orientale (Moon) and post-Caloris (Mercury) surfaces are of the same age (3.8 billion years) is highly speculative in the absence of any radiometric rock dating on Mercury.

For the outer planets and their satellites, all that can be said is that for some of their satellites superposition sequences have been derived.

The Solar System is only a minute fraction of the Universe. The size and age of the expanding Universe is calculated by astronomers on the basis of winking stars called cepheids, the nearest of which is 1000–2000 light years away. Recent estimates suggest a figure of 11–12 billion years. This, however, takes one back only to the ‘Big Bang’. There is no evidence of what, if anything, preceded the Big Bang.

The early history of the Earth

We can now consider the early history of the Earth after accretion and condensation. There are four strands to this: the early history of the rocks, the origin of the oceans, the origin of that atmosphere, and the origins of life. The Earth evolved into a planet with many of its present properties in a very short time, geologically speaking: about 500 million years. The oldest known mineral grains in rocks are zircons (for example those 4.10–4.27 billion years old from Mt. Narryer, Western Australia) and the oldest known whole rocks exposed on the Earth's surface—familiar gneisses—are the Acasta gneiss in the Slave Province, Canada, which are about 4.0 billion years old.

There are many models for the ‘Hadean’ period between 4.5 and 4.0 billion years, of which we have no significant record, but it is reasonable to invoke an intense early bombardment, as for the Moon, and the evolution of a zoned body by gravitational separation—solid inner and liquid outer core, dense and very hot; a molten mantle derived from chondritic silicate material; and a primitive, highly mobile surface layer, perhaps a magma ocean, with small sialic proto-continents segregating as a result of vigorous mantle convection (there are, however, other models for the separation of the sialic crust, such as the one advanced by Grieve, who invokes subsidence of large impact basins and partial melting of basaltic volcanic rocks). Whatever model one espouses for accretion and the early separation of the Earth shells, the early segregation of a proto-water ocean and atmosphere is consistent with the 10–20 per cent water and volatile content of carbonaceous chondrites, the favoured chondritic parent material, but the first atmosphere was reducing or only very weakly oxygenic. The first sialic proto-continents were certainly in place by 4 billion years ago, but continents probably only made up 5–10 per cent of the Earth's surface up to 3.2billion years ago, when larger areas were cratonized (for example, Kaapvaal, South Africa; Pilbara, Western Australia). The Archaean, up to 2.5 billion years ago, appears to have been characterized by numerous smaller and faster convection cells in the mantle: the larger cells of present-day plate tectonics probably appeared only after that date. The oldest known sediments recorded are metamorphosed sandy sediments (now metaquartzites), iron formations, and clayey sediments (now paragneisses); evaporites (analogous to present-day salt lake and sabkha deposits) 3.484 billion years old are recorded at North Pole in the Pilbara, Western Australia. Banded iron formations are a feature of early sedimentary sequences, the oxygen probably being derived from photosynthetic organisms and being taken up in the oceans at the time when a reducing atmosphere prevailed.

How life originated we do not know, but primitive unicellular cyanobacteria (prokaryotes) are first seen as fossils in rocks 3.5 billion years old (Onverwacht, South Africa; Warrawoona, Western Australia). They had a long innings, for eukaryotic phytoplankton appeared only about 2 billion years ago when the atmosphere became fully oxygenic, and the Metazoa, multicellular animals, appeared about 600 million years ago in the last 100 million years of the Precambrian. The Precambrian has poor fossil preservation, and these established age limits of fossil preservation do not preclude a somewhat earlier existence of these three life forms; possible metazoan fossils are recorded from rocks 1.3 billion years old.

G. J. H. Mccall

Bibliography

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