From: DV (Avatar) 30/11/2001 21:51:09 Subject: re: ROCK LEGEND NOTES post id: 518838 If the whole earth formed at the same time, why isn't the majority of rock the same age? It is true that most of the material which formed the earth coalesced over a period of less than a billion years, although there have been minor contributions from meteors since then. On the other hand, the actual rocks that make up the crust are in a state of flux. New igneous rocks form at rifts, and are consumed at subduction zones. Bubbles of magma well up through the crust and cool to form plutons of granite, and are then eroded away, the debris perhaps being deposited and lithified into sedimentary rock. This sedimentary rock might be subjected to hydrothermal activity (for instance), and be converted into any of various kinds of metamorphic rock. For this reason, although the earth as a whole is about 4.5 billion years old, most of the rock you are likely to find lying about on the surface is less than a billion years old. Greg L. ® It is true that most of the material which formed the earth coalesced over a period of less than a billion years, although there have been minor contributions from meteors since then. The Earth probably actually formed from the original pre-solar nebula in about 100 million years or so. A collision with a body the size of Mars around the end of the first major stages of accretion (about 4.5 GY ago) is thought to have formed the Moon. The extremely heavy bombardment of the Earth and Moon continued at roughly the same rate for the next 500 MY before tailing off rapidly (like a decaying exponential function). The Earth and Moon have continued to have been hit by asteroids and comets since, with some impacts having a dramatic influence on the biosphere and the course of life.