UPSC Prelims 2018 Question:

Consider the following statements

1. The Earth's magnetic field has reversed every few hundred thousand years.
2. When the Earth was created more than 4000 million years ago, there was 54% oxygen and no carbon dioxide.
3. When living organisms originated, they modified the early atmosphere of the Earth.
   
   

Which of the statements given above is/are correct?

Explanation:

• The Earth has a magnetic field, as can be seen by using a magnetic compass. It is mainly generated in the very hot molten core of the planet and has probably existed throughout most of the Earth's lifetime.
  
  
• Magnetic reversal, or 'flip' is the process in which the North pole is transformed into a South pole and the South pole becomes a North pole. Earth's magnetic field has flipped its polarity many times over the millennia. So, statement 1 is correct. 
  
  
• The early atmosphere, with hydrogen and helium, is supposed to have been stripped off as a result of the solar winds. 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 free oxygen. So, statement 2 is not correct. 
  
  
• Around 2,500-3,000 million years before the present, the process of photosynthesis got evolved. When photosynthetic organisms first appeared, life began to have a significant impact on the environment. Life was confined to the oceans for a long time. Oceans began to have the contribution of oxygen through the process of photosynthesis through the exchange of gases. Eventually, oceans were saturated with oxygen, and 2,000 million years ago, oxygen began to flood the atmosphere. So, statement 3 is correct. 
  
  

Therefore, option (3) is the correct answer.

Subject: Geography | Fundamentals of Geography