https://www.theregister.co.uk/2019/12/03/old_earth_oxygen/

The chemical reaction that produced the outburst of oxygen on Earth was sparked by volcanoes belching carbon dioxide after a major tectonic plate shifted about 2.5 billion years ago, according to the latest research.

A new study published in Nature Geoscience on Monday describes a scenario that could explain two ancient milestones that transformed our planet’s climate into the oxygen-rich habitat we enjoy today: the Great Oxidation Event and the Lomagundi Event.