#18 - Must Be True

[Administrator]

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

The correct answer choice is (A)

This question asks us to identify a feature of magnetite grains. Passage organization is once again
key: even though magnetite grains are described mostly in the first paragraph, the third paragraph
provides additional evidence concerning the magnetic variations of the rocks in which magnetite
grains are found. Use both paragraphs to prove the correct answer choice.

Answer choice (A): This is the correct answer choice. When magma cools to form solid basalt, the
alignment of the magnetite grains is “locked in,” recording the earth’s polarity at the time of cooling
(line 20-23). It is likely that the alignment of the magnetite grains in the youngest basalt is “locked
in” to the earth’s current polarity, but what if some of the youngest basalt was formed before the
latest reversal of the earth’s magnetic field? This is a moot point, according to the third paragraph, as
the youngest rocks all have normal polarity (lines 44-45), proving that answer choice (A) is correct.

Answer choice (B): The first paragraph clearly states that in magma, grains of magnetite align
themselves with the earth’s magnetic field at the time of cooling (lines 16-23). There is no evidence
to suggest that this is a behavior of most, but not all, magnetite grains. This answer choice is
incorrect.

Answer choice (C): No mention is made regarding the presence of magnetite grains in other types of
rock besides basalt. This answer choice is incorrect.

Answer choice (D): The author never discussed the relative size of magnetite grains. This answer
choice is incorrect.

Answer choice (E): The exact size of magnetite grains is not presented or alluded to in the passage.
All we know is that they tend to behave like “little compass needles” (line 18). This answer choice is
incorrect.

[mab9178]

Thank you for the explanation.

I arrived at the answer through different reasoning. Please examine my reasoning in case my thought process is erroneous and I arrived at the correct answer-choice by accident.

Paragraph 1, lines 12 through 14 define "normal" polarity as having the same polarity as the earth's present magnetic field. Paragraph 3, lines 44 and 45 state that "the youngest rocks all have the normal polarity."

The word "normal" overlaps between the definition of the term "normal polarity," and the characteristic of the youngest rocks. Furthermore, the word "present," in the definition of the term"normal polarity," is synonymous with the word "current" in answer-choice A.

In retrospect, since the youngest rocks are characterized as having "normal polarity," and a polarity is said to be "normal" when it is the same as the earth's polarity, then answer-choice A must follow: "In the youngest basalt, [magnetite grains] are aligned with the earth's current [or present] polarity."

Why do lines 20 through 23 relevant? I feel that they cast doubt on the certainty. The definition of what constitutes "normal," one the other hand, appears to be sufficient and dispositive of the polarity of the youngest rocks/basalt.

Am I missing anything?

Thank you

[katehos]

Hi mab9178,

Your reasoning using the definition of 'normal polarity' in lines 12-14 and the statement about the youngest basalt having 'normal polarity' in lines 44-45 is largely correct! The important utility that lines 20-23 offers, however, is the clarification of how the 'grains of magnetite' themselves come to permanently have such polarity. These lines specifically differentiate magnetite grains from some other form/structure of magnetite, which allows you to confidently conclude that answer choice (A) is correct! Each of these lines, when taken together, all help fortify the answer.

Hope that helps!
Kate

[mab9178]

Thank You Katehos.