Question 26 from UPSC CSE 2018 - Geography

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?

IASCODEX · Accepted Answer

Correct Option: C. 1 and 3 only

Strategy: Focus on statements with geological and atmospheric clues. The Earth's magnetic field is widely known to reverse periodically—supporting Statement 1. Exclude Statement 2 since knowledge about primordial atmosphere confirms there was no abundant oxygen or absence of carbon dioxide when the Earth formed. Statement 3 is plausible, as photosynthetic life is historically known to have altered Earth's atmosphere. Therefore, select options containing Statements 1 and 3 only.

Key Info: Earth's Magnetic Field Reversals: The geomagnetic field has undergone numerous polarity reversals over geological time, with the average interval over the last 20 million years being about 200,000–300,000 years, as recorded in seafloor spreading zones and stratigraphy. These geomagnetic reversals are evident from the symmetric magnetic striping on the ocean floor.

Early Earth's Atmosphere: Earth formed around 4.6 billion years ago. The initial atmosphere was hydrogen and helium, which escaped due to solar wind. Secondary atmosphere formed from volcanic outgassing, mainly consisting of water vapor, carbon dioxide (CO2), methane (CH4), ammonia (NH3), sulfur compounds, and negligible free oxygen (O2). There was substantially more CO2 (10–200 times current levels) and virtually no oxygen.

Origin of Life and Atmospheric Modification: Life began at least 3.5 billion years ago with prokaryotic microbes. The appearance of photosynthetic cyanobacteria (about 2.5–2.3 billion years ago) initiated the Great Oxygenation Event, which drastically increased atmospheric O2 and reduced CO2. The evolution of photosynthesis fundamentally changed atmospheric chemistry and enabled aerobic respiration, profoundly altering Earth's environment.

Relevance: Understanding magnetic field reversals aids in plate tectonics chronology. Knowledge of atmospheric transitions reveals the interplay between geology and biology in shaping Earth's habitability.

IASCODEX · Answer

Option A: 1 only

IASCODEX · Answer

Option B: 2 and 3 only

IASCODEX · Answer

Option D: 1, 2, and 3