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 ?
- (a) 1 only
- (b) 2 and 3 only
- (c) 1 and 3 only ✓ UPSC's answer
- (d) 1, 2 and 3
Why the answer is (c)
• Statement 1 is correct: geomagnetic reversals, recorded in seafloor rock magnetism, occur irregularly, on average every few hundred thousand years; the last was about 780,000 years ago.
• Statement 3 is correct: photosynthetic cyanobacteria transformed the early atmosphere by releasing oxygen (the Great Oxidation Event, c. 2.4 billion years ago).
• Statement 2 is wrong: the early atmosphere had almost no free oxygen and was rich in carbon dioxide, water vapour, nitrogen and methane.
• Hence 1 and 3 only, option (c).
Why the other options are wrong
- (a) 1 only
- Statement 3 is also correct: life modified the early atmosphere.
- (b) 2 and 3 only
- Statement 2 is wrong: early Earth had no free oxygen and much CO2.
- (d) 1, 2 and 3
- Statement 2 is wrong.
Asked in the GS Paper I of the UPSC Civil Services Preliminary Examination 2018, held on 3 June 2018. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.