Prelims 2018 · Q57
MediumConsider 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?
Answer & explanation
Answer: (c) 1 and 3 only
Statements 1 and 3 are correct. The Earth's magnetic field has flipped repeatedly in the geological past, and early life released oxygen that changed the atmosphere. Statement 2 is false because the early atmosphere had carbon dioxide and very little free oxygen.
- ✓ 1. Rocks record past polarity flips of the magnetic field. NASA describes a history of flipping every few hundred thousand years, though the flips are irregular and the last was about 780,000 years ago.
- ✗ 2. The early atmosphere was mostly water vapour, nitrogen, carbon dioxide, methane and ammonia, with very little free oxygen. It was not 54% oxygen and it did contain carbon dioxide.
- ✓ 3. Once life evolved in the oceans, photosynthesis added oxygen to the water and later the air. About 2,000 million years ago oxygen began to flood the atmosphere.
Remember · Early atmosphere: water vapour, nitrogen, CO2, methane, ammonia, almost no free oxygen. Oxygen came from photosynthesis by early life.
📘 Read it in NCERT: Class 11 Fundamentals of Physical Geography, Ch 2 (practise this chapter) · Class 11 Fundamentals of Physical Geography, Ch 2 (practise this chapter)
Sources
- NCERT Class 11 · Fundamentals of Physical Geography, Chapter 2 “The early atmosphere largely contained water vapour, nitrogen, carbon dioxide, methane, ammonia and very little of free oxygen.”
- NCERT Class 11 · Fundamentals of Physical Geography, Chapter 2 “Oceans began to have the contribution of oxygen through the process of photosynthesis. Eventually, oceans were saturated with oxygen, and 2,000 million years ago, oxygen began to flood the atmosphere.”
- NASA Science: Earth's Poles & Magnetic Reversal ↗ “Earth's magnetic poles and their history of flipping every few hundred thousand years.”
Question and answer: UPSC's official GS Paper I (2018, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink ·