The ozone layer forms
An invisible skin in the stratosphere makes land possible
See it on the timeline →In depth
Ozone forms when UV radiation splits O₂ and the fragments attach to other oxygen molecules. The layer is strikingly thin: compressed to sea level it would be about three millimetres. Yet it absorbs practically all UV-C and most UV-B. The consequence is that the surface of the continents became habitable — though it took another billion and a half years before anything took advantage of it. That humans managed to thin that same layer with CFCs within decades, then repair the damage through the 1987 Montreal Protocol, remains the most successful global environmental agreement ever made.
For children
Once there was enough oxygen in the air, sunlight created a thin layer of ozone high above. Ozone blocks the most dangerous ultraviolet radiation. Without that layer all life would have had to hide underwater — and the land would still be bare.
Connects to
Part of Terra Saga — the story of Earth on a timeline. terrasaga.org