RC Question 21 Explanation, by LSATHacks
DISCUSSION: The right answer is a bit weak. But it’s a “most strongly supported” question, so the answer doesn’t need absolute support.
Further, none of the wrong answers have any support from the passage. In fact, some contradict the passage.
___________
- CORRECT. The mid-ocean ridge is in the middle of the ocean. Rock gets older the farther it is from the ridge. So rock near the continents is likely to be oldest, especially if the whole ocean floor formed from this ridge.
This answer is easiest to support because it only says “some of the oldest rock”. There could be older rocks elsewhere in the ocean. Further, the question is just “most strongly supported”.
We mainly know that the youngest rock is in the middle of the ridge. It’s a bit of a stretch to say for sure that the rock by the continents is among the oldest. But given the wishy-washiness of the question type, and the vagueness of this answer, we can say that it is “most strongly supported”. - Lines 20-23 say that basalt doesn’t reverse polarity once it forms.
- You may have found this tempting, because basalt distorts compasses. But the basalt is all the way at the bottom of the ocean, presumably too far to distort compasses.
Also, use some common sense: for centuries, sailors navigated with compasses. They couldn’t have done this if basalt on the ocean floor was constantly interfering with compass readings.
If the passage doesn’t support an answer AND the answer defies common sense, it’s definitely wrong. - Why? The passage never mentions that magnetite grains grow weaker.
- The passage contradicts this answer. The magnetic field of the earth has reversed many times. So many pieces of old basalt have current polarity because they formed in a past age when polarity was the same. Polarity reverses. That means it only has two states.
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Hi,
Could you please help me understand how A is the correct answer for this question while for question 20 answer choice C is incorrect. My understanding is that both answers are similar and supported by the passage.
Thanks,
Sammie
“But the basalt is all the way at the bottom of the ocean, presumably too far to distort compasses.”
“Also, use some common sense: for centuries, sailors navigated with compasses. They couldn’t have done this if basalt on the ocean floor was constantly interfering with compass readings”
I thought we were supposed to not introduce our outside knowledge and “common sense” for the LSAT? Also, I didn’t know that compass distortion was a function of distance, and the passage doesn’t mention that. It even says that scientists noticed and measured these magnetic distortions caused by the ocean floor.
Your initial step should be to look for answer choices that are directly linked to statements explicit in the passage (either to select them as correct or to rule them out). However, sometimes you won’t find that; some questions and answer choices call on you to use common sense to make inferences as opposed to having direct evidence in the passage. LSAC does expect you to be able to make common sense inferences that an average college student would reasonably be expected to make.
If magnet interference wasn’t significantly impacted by the distance between the magnet and the compass, then compasses would be extremely inaccurate. There are magnets everywhere, they’d constantly be interfering with compass use. One of the things that prevents that interference is distance from magnets.