Earth’s Early History
- Earth & Space
- Ages 14–18 · Grades 9–12
What you’ll find out
- Earth’s oldest rocks are rare because the crust is recycled; zircons, meteorites and Moon rocks fill the gap.
- The young Earth was molten and bombarded, then formed a crust, oceans and an oxygen-poor atmosphere.
- Photosynthetic microbes caused the Great Oxidation Event, enabling the ozone layer and complex life.
No sign-up and no real names. In the lesson you check your understanding as you read, and practise until it sticks.
Clues to a young planet
Earth formed about 4.6 billion years ago. Its surface has been constantly recycled by plate tectonics, erosion and volcanoes, so very little of its earliest rock survives. To reconstruct early history, scientists turn to other evidence. Tiny, tough crystals called zircons can survive being eroded and reburied many times. Some from Western Australia are about 4.4 billion years old. Meteorites are leftover fragments from the formation of the Solar System. They have barely changed and give ages of about 4.6 billion years. Rocks from the Moon, which lacks plate tectonics and weather, also preserve a record of early times.
How the planet changed
The young Earth was a hostile place. At first much of its surface was molten, and it was bombarded by asteroids. As it cooled, a solid crust formed. Water vapour released by volcanoes condensed into the first oceans. The early atmosphere contained carbon dioxide, nitrogen and water vapour, but almost no oxygen. The first life, simple single-celled microbes, appeared in the oceans more than 3.5 billion years ago. Later, bacteria evolved photosynthesis and began releasing oxygen. Around 2.4 billion years ago, oxygen started building up in the atmosphere. This paved the way for more complex life.
Curriculum links: NGSS HS-ESS1-5 · NGSS HS-ESS1-6 · England KS4: Chemistry · NCERT Geography, Class 11