UPSC Prelims 2018 Q57: Geography – Astronomy & Earth - Sun System
- (a) 1 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer: (c) 1 and 3 only
Explanation
Earth's Atmosphere, Magnetic Field and Life - Geological History
| Statement | Claim | Verdict | Detail |
|---|---|---|---|
| 1 | Earth's magnetic field has reversed every few hundred thousand years | TRUE | Well-established from paleomagnetic studies of mid-ocean ridge basalts. Geomagnetic reversals occur irregularly, with average intervals of a few hundred thousand years (last reversal: Brunhes-Matuyama, ~780,000 years ago). |
| 2 | Early Earth atmosphere (~4000 million years ago) had 54% oxygen and no CO2 | FALSE | Completely reversed. Early Earth atmosphere was oxygen-free (anoxic) and rich in CO2, methane, water vapour, nitrogen, ammonia. Oxygen rose only after the Great Oxidation Event (~2.4 billion years ago) via cyanobacterial photosynthesis. |
| 3 | Living organisms modified the early atmosphere | TRUE | Photosynthetic bacteria (cyanobacteria) began releasing O2 ~3 billion years ago, triggering the Great Oxidation Event and permanently transforming Earth's atmosphere. |
Answer: (c) 1 and 3 only.
Why each option is right or wrong
- (a) Incorrect. Statement 3 is also correct - life dramatically modified the atmosphere through the Great Oxidation Event.
- (b) Incorrect. Statement 2 is factually reversed - early Earth had NO oxygen, not 54%.
- (c) CORRECT. Statements 1 (magnetic reversals) and 3 (life modifying atmosphere) are established science.
- (d) Incorrect. Statement 2 (54% oxygen initially) is completely wrong.
Key point
Earth's atmospheric history: Early atmosphere = anoxic (CO2, CH4, N2, H2O, NH3). Great Oxidation Event ~2.4 bya via cyanobacteria. Modern atmosphere: 78% N2, 21% O2, 0.9% Ar, 0.04% CO2. Magnetic reversals: every ~200,000-1,000,000 years, last at Brunhes-Matuyama boundary.
Remember
The Earth's magnetic field has reversed multiple times throughout geological history, with intervals of a few hundred thousand years between reversals - the last being the Brunhes-Matuyama reversal about 780,000 years ago. Living organisms, particularly photosynthetic cyanobacteria, dramatically modified the early atmosphere through the Great Oxidation Event about 2.4 billion years ago. The claim that early Earth had 54% oxygen is factually wrong - early Earth was oxygen-free.
Common mistakes
- Assuming early Earth had oxygen - it did not; oxygen is a biogenic gas produced by photosynthesis over billions of years.
- Confusing Earth's magnetic reversal (natural, periodic) with magnetic field weakening (currently occurring but not a reversal yet).
- Missing the Great Oxidation Event - one of the most transformative geological events ever, driven by microbial life.
Exam tip
Atmospheric evolution: Pre-2.4 bya: anoxic (CO2, CH4). Post-2.4 bya: Great Oxidation Event, O2 rises. Cambrian explosion at ~540 mya. Modern composition stable for last ~400 my.
Sources
- NCERT Class 11 Geography - Fundamentals of Physical Geography (Chapter on Atmosphere)
- IPCC AR6 Physical Science Basis - Earth's Climate History