Earth’s rocks can be analysed into successive time zones, from the Quaternary down to the Archaean. The lowest division, the Hadean, is not represented by whole rocks, though there is zircon evidence (ultra-hard crystals within younger sandstones) that land and water existed then. According to recoloni- sation theory, a cataclysm at the end of the Hadean initiated plate tectonics and subducted the pre- Archaean world into the mantle. The chart gives some idea of how finely it is possible to subdivide geological time. Click on image for a pdf.