Previous Year 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?
Correct Answer: C
Detailed Solution
Statement 1 is correct. Paleomagnetic records show that the polarity of Earth’s magnetic field has reversed many times during geological history. These reversals do not occur regularly or with a fixed periodicity; the intervals between successive reversals have varied considerably. Nevertheless, an interval of a few hundred thousand years is commonly used as a broad geological average.
Statement 2 is incorrect. Earth’s early atmosphere was not like the present atmosphere. It largely contained water vapour, nitrogen, carbon dioxide, methane and ammonia, while free oxygen was present only in extremely small quantities. Thus, both the claim of 54% oxygen and the complete absence of carbon dioxide are incorrect.
Statement 3 is correct. Photosynthetic organisms, particularly early cyanobacteria, used carbon dioxide and released oxygen. This changed the chemical composition of the atmosphere and produced a major increase in free oxygen during the Great Oxygenation Event approximately 2.4 billion years ago.