Discuss the various stages of formation of the earth's atmosphere.

There are three stages in the evolution of the present atmosphere. 

• The first stage is marked by the loss of primordial atmosphere. 
• In the second stage, the hot interior of the earth contributed to the evolution of the atmosphere.
• During the cooling of the earth, gases and water vapour were released from the interior solid earth. 
• This started the evolution of the present atmosphere. 
• The early atmosphere largely contained water vapour, nitrogen, carbon dioxide, methane, ammonia and very little of free oxygen. 
• The process through which the gases were outpoured from the interior is called degassing.
• Continuous volcanic eruptions contributed water vapour and gases to the atmosphere. 
• As the earth cooled, the water vapour released started getting condensed. 
• The carbon dioxide in the atmosphere got dissolved in rainwater and the temperature further decreased causing more condensation and more rains. 
• The rainwater falling onto the surface got collected in the depressions to give rise to oceans.
• The earth’s oceans were formed within 500 million years from the formation of the earth.  
• Finally, the composition of the atmosphere was modified by the living world through the process of photosynthesis. 
• around 2,500-3,000 million years before the present, the process of photosynthesis got evolved. 
• Life was confined to the oceans for a long time. Oceans began to have the contribution of oxygen through the process of photosynthesis. 
• Eventually, oceans were saturated with oxygen, and 2,000 million years ago, oxygen began to flood the atmosphere.