#19 - Must Be True

[Administrator]

Complete Question Explanation
(See the complete passage discussion here: lsat/viewtopic.php?t=14347)

The correct answer choice is (C)

This question asks us to determine the likely outcome of having the time intervals between the
earth’s magnetic field reversals fluctuate greatly. The most efficient approach would be to prephrase
a possible answer by relying on the information in the second and third paragraphs.

Recall that there is a remarkable correlation between the ages of the earth’s magnetic reversals and
the striping pattern of the ocean floor (lines 54-56). If these time intervals fluctuate greatly, some of
them will lead to greater accumulation of basalt than others. Consequently, the width of the magnetic
stripes that form the ocean crust will also vary: the longer the time interval, the wider the stripe. This
prephrase proves that answer choice (C) is correct.

Answer choice (A): At or near the peaks of the mid-ocean ridge, the rocks are young and all have
normal polarity (lines 41-45). No fluctuation in the intervals between the earth’s magnetic field
reversals would affect the compass readings near those peaks. The readings might even be more
accurate near the peaks, because the peaks all have normal polarity. This answer choice is incorrect.

Answer choice (B): It is unclear how a fluctuation in the time intervals between the earth’s magnetic
field reversals could have caused the mid-ocean ridge to wind around the earth. The author presents
no explanation as to why the mid-ocean ridge winds its way around the earth like the seams of a
baseball. This answer choice is incorrect.

Answer choice (C): This is the correct answer choice, as it agrees with our prephrase described
above.

Answer choice (D): Any fluctuation in the time intervals between the earth’s magnetic field reversals
would have a comparable effect on the magnetic orientation of the continental and the oceanic rock
alike. There is no reason to suspect that continental rock would be a more reliable indicator of the
earth’s magnetic field reversals than would be the oceanic rock. As long as we assume a constant rate
at which the ocean floor moved away from the spreading center, we can easily estimate the duration
of the earth’s magnetic field reversals by measuring the width of each magnetic stripe. This answer
choice is incorrect.

Answer choice (E): This is the Opposite answer choice. If the time intervals between the earth’s
magnetic field reversals fluctuate greatly, it is reasonable to assume that some intervals would be
longer than others. Longer intervals, however, would allow for the accumulation of more basalt and
wider stripes with the same polarity, suggesting that the age of the basalt within any given stripe will
vary more, not less.

[Legally Blonde]

Where is their evidence in the passage to support the correct answer (C)? I have been searching the passage to see what information in it supports that answer C is most likely to be true.

[Jonathan Evans]

Hi, Legally,

Kudos to you. You're asking the right kind of question. How can we find support to prove an answer choice?

This question is of particular interest because it introduces a new criterion (or premise) into the question stem itself. Based on both the passage and the fact that "the time intervals between the earth's magnetic field reversals fluctuate greatly," what is likely to be true?

The new fact itself gives you the indicators you need to find the relevant information in the passage. From your knowledge of the structure of the passage, you should realize that the discussion of the reversal of earth's magnetic polarity occurs primarily in the second and third paragraphs. Now you have to anticipate the manner in which this new information in the question stem will help you to draw a significant inference when combined with information from the passage.

Consider the information at the end of the first paragraph: "[after magnetic polarity reversals], when magma cools to form solid basalt, the alignment of the magnetic grains is locked in."

This information combined with the question stem information "the time intervals between the earth's magnetic field reversals fluctuate greatly" is in itself insufficient to lead you to a strong prephrase. However, you should now be prompted to ask the question: "What missing information would be helpful to answer this question?"

If you knew that the basalt were deposited at a certain rate, you could make a valid inference about the size of the stripes of the sea floor deposits. If in a time crunch, you could immediately check the answers and notice that (C) is the only plausible candidate to match this likely prephrase.

You find explicit evidence to support this inference on line 52: "if we assume that the ocean floor moved away from the spreading center at a rate of several centimeters per year"

This information combined with the information in the question stem gives you definitive proof for answer (C).