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Institute for Geophysics and Department of Geological Sciences, University of Texas at Austin, 4412 Spicewood Springs Road, Building 600, Austin, TX 78759-8500, USA
‘No vestige of a beginning, ...’ In one sense the first half of James Hutton’s famous statement of 1788 holds true today — the oldest parts of Earth’s crust identified in two centuries of ‘modern’ study look not unlike those being formed at present. Yet the age of the planet can now be determined as close to 4.5 billion years, and radiometric ages of vestigial rocks and minerals approach 90 per cent of that value. The history of the planet is becoming clearer. We can now accurately reconstruct the geography of Pangea in early Mesozoic times. Remnants of older rifted continental margins suggest the existence of previous supercontinents for which hypothetical reconstructions have been proposed — a global-scale Huttonian process of tectonic renewal. Their mode of fragmentation and comparison with the other terrestrial planets suggest a functional connection between surficial plates and deep-seated mantle plumes, which may extend back to the Archaean. The fossil record, geochemistry, and molecular biology provide a timeline for life on Earth which does extend to the Archaean, almost as far as the oldest rocks and minerals. The history of endogenic tectonism, environmental change and biological evolution was punctuated, and to some extent at least influenced, by bolide impacts.
Like Hutton, in the rocks we can still read ‘no prospect of an end’, though astrophysical evidence indicates the demise of the Earth, or at least of its biosphere, will inevitably come 5000 million years into the future when our Sun will become a red giant, enveloping all its planets in a gaseous holocaust. Detection of complex organic molecules in the comas of comets that recently traversed the inner Solar System has shown, however, that the basic chemical building blocks of life are present in interstellar space. Evidence is steadily building up for the existence of planets around other stars of the Milky Way. Hence life need not be limited in either space or time by the existence of the Earth itself. Indeed, contemporary cosmology entertains possibilities that the entire universe that we recognize may be boundless, permanently expanding, or perhaps even part of a continuum of evolving universes. In this ultimate sense, therefore, the concluding sentence of the Theory of the Earth by the ‘founder of modern geology’ can be considered as valid today as it was 200 years ago.
* Also affiliated with the Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia.
