The Earth began to accrete some 4.5 billion years ago by the accumulation of impacting planetesimals of various sizes. The impacting bodies converted their gravitational and kinetic energies into heat which probably produced initially a large-scale melting condition of the planet. Whether or not the primordial Earth consisted of a gigantic magma ocean, the combined effects of conduction, convection, and radiation heat transfer processes would have rapidly dissipated the Earth’s energy. Based on Fourier’s theory of heat conduction, Kelvin (1864) determined that the Earth had formed some 200 million years ago. His model was, of course, flawed, for he neglected to consider heat generation by the principal and long-lived radioactive isotopes of U, Th, and K. The energy release from these isotopes still keeps the planet in a rigorous internal motion. The discovery of radioactivity early in the twentieth century and subsequent 50 years of debates have established geochronological dating (Section3.7) and that the oceanic crust is less than 200 million years old. In contrast, the age of the oldest rock on the Earth suggests that the continental crust is older than 3.8 billion years (Wetherill, 1990). Measurements of meteorites place the age of the primal Earth at 4.5 billion years (Badash, 1989).
KeywordsPermeability Entropy Porosity Migration Silicate
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