1. One theory suggests that during the first billion years of the Earth’s existence, there was intense volcanic activity that released gases that formed the early atmosphere and water vapour that condensed to form the oceans.
2. At the start of this period, the Earth’s atmosphere may have been like the atmospheres of Mars and Venus today, consisting of mainly **carbon dioxide** with **little or no oxygen gas**. Volcanoes also produced **nitroge**n which gradually built up in the atmosphere and there may have been small proportions of **methane** and **ammonia**.
3. When the oceans formed carbon dioxide dissolved in the water and carbonates were precipitated producing sediments, and **reducing the amount of carbon dioxide in the atmosphere.**