UPSC Prelims 2018 · Question 57 of 99
UPSC Prelims 2018 question on Earths Magnetic Field Reversal
- ExamUPSC CSE
- Year2018
- PaperGeneral Studies Paper I
- SubjectGeography
- TopicGeomorphology
- DifficultyMedium
- TypeStatement
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?
Show answer
Answer: C. 1 and 3 only
Verdict
Statements 1 and 3 are correct, so the answer is (c) 1 and 3 only.
Statement by statement
Statement 1 is CORRECT. The Earth's magnetic field has flipped polarity many times, and over the last 20 million years the planet has settled into a pattern of reversal roughly every 200,000 to 300,000 years, so reversals are the rule rather than the exception.
Statement 2 is INCORRECT. When Earth formed about 4.6 billion years ago it had almost no atmosphere. As the planet cooled, an atmosphere formed mainly from volcanic gases, including hydrogen sulphide, methane and 10 to 200 times as much carbon dioxide as today. There was neither 54 percent oxygen nor an absence of carbon dioxide.
Statement 3 is CORRECT. Once photosynthetic organisms evolved, life began to have a major impact, reducing atmospheric carbon dioxide while producing oxygen, so living organisms did modify the early atmosphere.
Source
NCERT Class 11, Fundamentals of Physical Geography.
How to crack it
Statement 2 collapses under statement 3. If organisms modified the atmosphere by producing oxygen, the early atmosphere cannot already have been oxygen rich. Reading the statements against each other exposes the contradiction without any recall of early Earth chemistry, and the oddly precise figure of 54 percent is itself a warning sign.