LSAT 133 | Section 4 | Reading Comprehension: Q19

RC Question 19 Explanation

DISCUSSION: Lines 53-55 show that scientists assume the ocean floor is spreading at a constant rate.

Thus, if time periods between reversals fluctuate, some magnetic strips will be wider than others. More spreading will occur if an interval is 500,000 years, compared to an interval of 100,000 years.

___________

1. The peaks of the ridge formed during the current polarity. So the basalt matches our polarity and won’t distort anything. This answer is just here to confuse you by throwing in a term mentioned in the passage, in an irrelevant context.
2. We don’t know why the ridge goes all around the Earth. Scientists speculate it is because the ocean floor is spreading apart….but we don’t know why the spreading occurs. This is another nonsense answer intended to confuse you.
3. CORRECT. Suppose the ocean floor spreads 2cm per year. Then suppose during one period, polarity doesn’t reverse for 2,000,000 years. That strip will be 4,000,000 cm wide. If during another period polarity reverses after 20,000 years, then that strip will be just 40,000 cm long.
   Lines 53-55 say we’re assuming the spreading is at a constant rate. 
4. Gibberish. I don’t even know how to explain this is wrong. This translates as:
    
   “If some magnetic periods are longer than others, then land-based rock is more reliable than ocean-based rock.”
    
   Nothing in the passage suggests this is true.
5. If some magnetic ages last a long time, then some magnetic strips will be very wide. The basalt at the start of the strip will be older than the end of the strip.