Evolution of the atmosphere
Topic: Origin and Evolution of the Earth and the Solar System · NCERT: Class 11, Ch 2 "The Origin and Evolution of the Earth"
Meaning
The Earth's atmosphere developed in three stages:
- The first hydrogen-helium atmosphere was stripped away by the solar wind, a stream of charged particles flowing out from the Sun.
- Degassing: gases and water vapour escaped from the hot interior, helped by volcanoes. This formed a second atmosphere with very little free oxygen.
- Photosynthesis by living things changed the mix of gases and added free oxygen.
The Earth's magnetic field helped the planet hold on to this second atmosphere.
Example
At Jhamarkotra, near Udaipur in Rajasthan, rocks contain stromatolites. These are layered mats built by blue-green algae, which made oxygen by photosynthesis. They are a record of the third stage.
Don't confuse with
- Differentiation: this is the layering of the Earth's interior into crust, mantle and core. It is not one of the three stages of the atmosphere's evolution.