GEOLOGY

Detailed explanation-1: -The air in Earth’s atmosphere is made up of approximately 78 percent nitrogen and 21 percent oxygen.

Detailed explanation-2: -So how did Earth end up with an atmosphere made up of roughly 21 percent of the stuff? The answer is tiny organisms known as cyanobacteria, or blue-green algae. These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen.

Detailed explanation-3: -In fact, the troposphere contains three-quarters of the mass of the entire atmosphere. The air here is 78% nitrogen and 21% oxygen. The last 1% is made of argon, water vapor, and carbon dioxide.

Detailed explanation-4: -Oxygen is found to be 21% in the earth’s atmosphere which supports life, helps in combustion and gets balanced in an environment through the process of photosynthesis.

Detailed explanation-5: -By around 600 million years ago, concentrations of molecular oxygen in our atmosphere finally reached around 21 percent.