Oldest rocks, earliest life, heaviest impacts, and the Hadean–Archaean transition

Many geologists have their own favourite part of the vast geological time scale, from which there is plenty to choose for all types of pure and applied research, and for detailed speculation. My own favourite is that ancient time range during which the Earth s surface changed from something quite unfamiliar and shaped by largely non-uniformitarian processes, to something increasingly recognisable in uniformitarian terms. That has been referred to as the Hadean–Archaean transition, often defined by arbitrary time boundaries like 4.0, 3.9, and 3.8 Ga, which are not necessarily associated with any specified event horizon. The first and last sentences of the chapter on the Hadean Earth in Jonathan Lunine s well-known textbook Earth – Evolution of a Habitable World (1999) read as follows: