LSAT 133 | Section 4 | Reading Comprehension: Q17

RC Question 17 Explanation

DISCUSSION: Look back at the passage analysis section. I described the whole theory: the mid-ocean ridge forms from magma. This ridge spreads out as the crust is pulled apart. The sections of the ridge have differing polarity, as they formed in different magnetic eras.

We know how old land-based rocks are, and we thus figured out when the Earth reverse polarity. Scientists then looked at the mid ocean ridge, and assumed it moved apart at a constant rate. This would let them judge the age of each section of the ridge.

There was a strong correlation between the predicted age/polarity of each section of the ridge, and the measured polarity of that age derived from land based rocks.

So this “remarkable” correlation is consistent with the theory and helps prove the theory correct.

___________

1. The passage says that the spreading may occur at a rate of several cm per year (line 54). But we don’t know if that is fast or slow: we have no context for such a statement. 
2. We’re not just trying to explain the existence of the ridge. We’re trying to explain the magnetic striping of the rock extending away from the ridge. The correlation is between the age of this rock and the magnetic history of the Earth, so this correlation supports the full theory.
3. Strength of the magnetic field is never mentioned as being a factor for anything. 
4. CORRECT. The theory has multiple parts: why the mid-ocean ridge exists, why there are strips of alternating polarity extending from the ridge, etc. The correlation supports all parts of the theory.
5. The passage never mentions if the reversals happen at set intervals. This isn’t part of the theory.