Consider the following statements:The Earth's magnetic field has reversed every few hundred thousand

Q: What is UPSC Prelims?

UPSC Prelims is a competitive examination conducted in India. This question is from the 2018 UPSC Prelims Geography paper.

Q: How can I prepare for UPSC Prelims Geography?

To prepare for UPSC Prelims Geography, practice previous year questions (PYQs), understand concepts thoroughly, and solve mock tests regularly. This 2018 question covers Interior of the Earth – Layers, Plate Tectonics & Earthquakes topic.

Q: What is the difficulty level of this question?

This UPSC Prelims 2018 Geography question has a medium difficulty level and is marked as an important question.

Q: Where can I find more UPSC Prelims previous year questions?

You can find more UPSC Prelims previous year questions on AspirantMitraa. Browse by subject, topic, year, and difficulty level to practice effectively.

Question

Consider the following statements:The Earth's magnetic field has reversed every few hundred thousand years.When the Earth was created more than 4000 million years ago, there was 54% oxygen and no carbon dioxide.When living organisms originated, they modified the early atmosphere of the Earth.Which of the statements given above is/are correct?

Accepted Answer

Correct Answer: C — 1 and 3 onlyStep-by-step Solution:Statement 1 — Earth's magnetic field reverses every few hundred thousand years: This is correct. Earth's magnetic field undergoes periodic polarity reversals — a well-documented geological phenomenon. The geological record shows that these reversals (called geomagnetic reversals) occur on average every 200,000 to 300,000 years, though the intervals are irregular. The most recent reversal occurred about 780,000 years ago (the Brunhes-Matuyama reversal). Scientists monitor the current weakening of the magnetic field as a possible precursor to another reversal. ✓Statement 2 — Earth's early atmosphere had 54% oxygen and no carbon dioxide: This is incorrect. When Earth formed approximately 4.6 billion years ago, it had almost no atmosphere initially. The early atmosphere that developed (through outgassing from volcanoes) was dominated by water vapour, carbon dioxide (CO₂), methane, ammonia, and hydrogen sulphide — with little to no free oxygen. The current oxygen-rich atmosphere (about 21% oxygen) developed over billions of years through photosynthesis by early life forms. There was certainly not 54% oxygen in the primordial atmosphere; in fact, the composition stated is essentially the reverse of reality — high CO₂, no oxygen. ✗Statement 3 — Living organisms modified Earth's early atmosphere: This is correct. When photosynthetic organisms evolved (cyanobacteria, about 2.7 billion years ago), they began producing oxygen as a byproduct of photosynthesis. Over time, this oxygen gradually accumulated in the atmosphere — first saturating the oceans, then flooding the atmosphere approximately 2 billion years ago. This Great Oxygenation Event fundamentally transformed Earth's atmosphere, enabling the evolution of aerobic life. Living organisms — through photosynthesis and respiration — are a major driver of atmospheric composition change. ✓Why other options are incorrect:Option A (1 only): Statement 3 is also correct.Option B (2 and 3 only): Statement 2 is factually wrong — early Earth had high CO₂ and no free oxygen.Option D (1, 2 and 3): Statement 2 is incorrect.Key Concept: Evolution of Earth's atmosphere has three stages: (1) Loss of primordial hydrogen/helium atmosphere; (2) Secondary atmosphere from volcanic outgassing — rich in CO₂, water vapour, little/no oxygen; (3) Modification by life through photosynthesis — oxygen increased, CO₂ decreased, creating modern atmosphere. Earth's magnetic field (magnetosphere) is generated by the movement of molten iron in the outer core — it protects Earth from solar wind and cosmic radiation. Magnetic reversals are recorded in ocean floor basalts (paleomagnetic evidence).

Master UPSC Prelims with Our Test Series

More Questions from Interior of the Earth – Layers, Plate Tectonics & Earthquakes

Frequently Asked Questions

Master UPSC Prelims with Our Test Series

More Questions from Interior of the Earth – Layers, Plate Tectonics & Earthquakes

Frequently Asked Questions