1. The Archean Earth was hotter (from impacts and more radioactive decay). A more vigorous mantle convection probably kept most plates smaller than today. (left: 3 billion years ago)

2. Earliest atmosphere formed by degassing (gases spewed out by volcanoes) would have been rich in N 2, H 2 O, CO 2. Earliest crust would have been basaltic (close to average mantle composition), Mg- rich and Si-poor.

3. http://www.sciam.com/1999/0799issue/0799bernsteinbox1.html Meteorite impacts may have vapourized the early atmosphere many times, sterilizing the Earth. But comets brought water and possibly many organic chemicals which are the building blocks of life.

4. Where are the Archean rocks (black patches)

5. Acasta gneiss, in Nunavut: a metamorphosed granite. Grains of zircon (ZrSiO 4, upper right), a tough mineral rich in U, Th, were dated in 1998, as 4.03 billion yrs old.

6. Much of the Canadian Shield is Archean (yellow on map), and it hosts rich metallic mineral resources.

7. (Australia) (Nunavut, Canada) (Greenland)

8. The O 18 /O 16 ratio of oldest zircons may indicate that liquid water was present earlier than we thought.