The Earth's early atmosphere: Volcanoes also produced nitrogen which gradually built up in the atmosphere and there may have been small proportions of methane and ammonia.

The idea

Volcanic eruptions released more than water vapour and carbon dioxide. They also produced nitrogen, which gradually accumulated in the atmosphere because it is relatively unreactive and was not removed quickly. Over time, nitrogen became a major component of the atmosphere. There may also have been small proportions of methane and ammonia in the early atmosphere. These gases are thought to have been present in minor amounts. For example, ammonia and methane can be produced by volcanic activity and by reactions in the early environment. This statement explains how nitrogen became abundant and acknowledges uncertainty about methane and ammonia.

What earns the marks

• States that volcanoes produced nitrogen.
• Explains that nitrogen gradually built up in the atmosphere.
• Identifies that there may have been small proportions of methane.
• Identifies that there may have been small proportions of ammonia.
• Uses language of uncertainty, such as ‘may have been’, for methane and ammonia.

Common mistakes and how to put them right

• Claiming nitrogen was the main gas in the early atmosphere.

  Put it right by stating carbon dioxide was main, with nitrogen building up gradually.

• Stating methane and ammonia were major gases.

  Put it right by describing them as small proportions.

• Confusing nitrogen with oxygen.

  Put it right by noting nitrogen is unreactive and built up, while oxygen was little or absent.

Exam tips

• Use ‘gradually built up’ when describing nitrogen to show the change over time.
• Include the word ‘small’ when referring to methane and ammonia proportions.
• Avoid saying methane and ammonia definitely existed; use ‘may have been’ to reflect the theory.