There is much evidence that the age of Earth is ~~4.6 Ô 109 years. Our knowledge of events during the first half-billion years of Earth’s history is fragmentary at best. No rocks have yet been found that are older than 4.0 Ô 109 years. One important piece of evidence is that the concentration of volatile elements in the atmosphere and Earth’s crust is very low compared to expectation from Solar System abundances (Holland 1984). These elements appear to have been lost because of some catastrophe, or series of catastrophes, that occurred during the later stages of the formation of Earth. The nature of such catastrophes is not known, but there are several candidates. The massive collision that appears to have led to the formation of the Moon could have stripped most of the atmosphere from early Earth. Beyond evidence from the compositions of the atmosphere, the solid Earth, the Moon, and many meteorites, the history of the first half-billion years is largely a matter of speculation, model building, and calculation. After that, increasingly abundant sedimentary remains bear evidence, although often unclear, about conditions when they were deposited. In the following discussion, I have assumed that Earth had reached essentially its present size by 4.0 Ô 109 years ago, and the calculations take this time as a beginning. A few sedimentary rocks not much younger than that have survived, giving us a faint glimmer of the conditions prevalent at that time; those rocks suggest the presence of both liquid water and a sub-aerial source for the sediments (Holland 2006).

Illustrative rates

Some calculations may be helpful to set in perspective the later history of the oceans and to obtain a simple appreciation of the magnitudes of various processes involved.