Cratons contain some unusual rocks and valuable ore deposits that bear the imprint of conditions that once prevailed on Earth.

• Until about two billion years ago, Earth’s atmosphere contained one percent or less of its present oxygen levels.
• Two-and-one-half billion years ago, Earth produced five times as much heat as today.

Valuable Ore

These layers are part of South Africa’s 2.1- billion-year-old Bushveld Complex. One 5.5-m (18-ft) thick layer in this Complex contains more than half of Earth’s known platinum reserves.