The Earth's early atmosphere: 6 billion years.

The idea

This statement gives the age of the Earth as about 4.6 billion years. It is a key timescale for the early atmosphere: the Earth formed about 4.6 billion years ago, and the atmosphere developed over millions of years afterwards. For example, the first atmosphere is thought to have formed from volcanic gases released early in Earth's history, long before the stable composition of the last 200 million years. When answering questions, use 4.6 billion years as the age of the Earth and place events such as formation of the early atmosphere, appearance of life and changes in oxygen levels in order after that date. The value is approximate and is supported by evidence from meteorites and rocks.

What earns the marks

• State that the Earth is about 4.6 billion years old.
• Use the timescale to place the formation of the early atmosphere after the Earth formed.
• Recognise that the value is approximate and based on evidence such as meteorites and rocks.
• Order events correctly, for example Earth's formation before the development of the atmosphere.
• Distinguish 4.6 billion years from the 200 million years of stable atmospheric composition.
• Use the timescale when describing how the atmosphere changed over time.

Common mistakes and how to put them right

• Writing 4.6 million years instead of 4.6 billion years.

  Put it right: Use 4.6 billion years.

• Confusing the age of the Earth with the 200 million years of stable atmosphere.

  Put it right: Keep the two timescales separate.

• Placing the early atmosphere before the Earth formed.

  Put it right: The Earth formed first, about 4.6 billion years ago.

Exam tips

• Write the value as 4.6 billion years and keep the unit clear.
• Use the timescale to sequence events when describing atmospheric change.
• Check whether a question asks about the age of the Earth or the period of stable composition.