earth's early atmosphere
Main gases came from volcano : gases such as :carbon dioxide,water vapour small gases such as ammonia and nitrogen.
Rocks contained a record of the gases that were present in the atmosphere at the time
scientist analyse rocks that formed at different times to provide evidence for the way the atmosphere has changed.
Recent rocks contained iron oxide. older rocks the iron had not been oxidised. this provides evidence that oxygen levels in the atmosphere have increased.
Nitrogen has increased and carbon dioxide has dramatically decreased.