Earth processes cycle so powerfully that little evidence remains of early planetary faces and flows. Critical and intriguing clues lie in the formulation of Earth rocks and minerals when compared to those on the Moon and other planets.
The oldest mineral crystals on Earth, zircons from western Australia, date back 4,300 million years. The oldest Moon rock brought back by Apollo astronauts is 4,200 million years old. Granite rocks found near Canada's Great Slave Lake go back 3,960 million years. Scientists believe these evolved from even older crustal material, melted and remelted by the restless Earth.