#21 - Global Reference, 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 for a response that is most strongly supported by the passage, so we must find
an answer choice that is consistent with the author’s reasoning. Because the question stem does
not contain a location indicator, the most efficient approach would be to review the answer choices
and quickly eliminate any that are inconsistent with the passage. When examining the remaining
responses more closely, consider passage organization first—this will help you locate the specific
paragraph(s) that validate your answer.

Answer choice (A): This is the correct answer choice. Recall that the rocks at or near the ridge
crests are very young, becoming progressively older away from the crest (line 42-44). This is
because ridges are structurally weak zones through which magma erupts to create new ocean crust
(lines 34-38). Assuming that the theory of ocean floor spreading is correct, the submarine rocks
near the continents will be the farthest away from the mid-ocean ridge and therefore older than most
other rocks on the ocean floor. This was a difficult inference to make and explains the unusually low
accuracy rate for this question.

Answer choice (B): This answer choice is tempting, because the earth’s magnetic field has reversed
at various times throughout history. This does not mean, however, that the older a sample of basalt
is, the more times it has reversed its polarity. Recall that when basalt is formed, the alignment of
magnetite grains is “locked in,” recording the earth’s polarity at the time of cooling (line 21-23). This
polarity never changes after the initial formation of the rock.

Answer choice (C): In the first paragraph, the author mentions that magnetite is known to locally
distort compass readings on land. Whether the distortion is more likely to occur at sea than on land is
unclear.

Answer choice (D): There is no evidence suggesting that the magnetic fields surrounding magnetite
grains would gradually weaken.

Answer choice (E): This answer choice contains a Mistaken Reversal. Indeed, any rock that was
formed after the latest reversal of the earth’s magnetic field would exhibit present-day magnetic
polarity. This does not mean, however, that any rock with present-day polarity was formed after the
latest reversal of the earth’s magnetic field. Since the earth’s magnetic field has reversed multiple
times throughout history, the rock exhibiting “normal” polarity could have been formed during any
number of periods other than the present. The magnetite in it would have the same “normal” polarity
as the earth’s present-day magnetic field, even though the rock was not formed after the latest
reversal of that field.

[alee]

Hi,

Re. Question 21 of Preptest 63, Section 4, I'm can't find the relevant section in the text to confirm that option A is correct:

"Submarine basalt found near the continents is likely to be some of the oldest rock on the ocean floor"

From the passage, it seems to be that oldest rock should be that close to the surface of the sea- is that correct?

Thanks, confused by this one.

Cheers! thanks for the awesome work!

[Adam Tyson]

Hey alee - good question, and I think I might have a bone to pick with the authors on this one, although A is the best answer of the bunch.

Take a look at the beginning of the third paragraph of that passage, where the author tells us that the rocks at or near the ridge crest are young, and they get older as they move away from the crest. What's missing, in my view, is that the mid-ocean ridge is in fact in the middle of the ocean, and that the continents are at the edges of the ocean. Makes sense, right? My only beef is that they assumed we knew that, and we generally aren't supposed to make those sorts of assumptions.

Still, the passage does tell us that the mid-ocean ridge is under the ocean, not under the continents. If the youngest rock is at the crest, and it gets older as you move away from the crest, then where else could the oldest rock be but at the edges of the continents? There's no place to go that could be further from the ridge crest and still be on the ocean floor. That's why A works.

I hope that helped!

Adam

[LSATls]

Answer choice (A) is just plain wrong. That is, it might be the "correct" answer, but nothing in the passage suggests it should be true, and, moreover, this answer wouldn't be true in real life. I understand the reasoning provided here: if the "mid-ocean ridge" was truly in the middle of the ocean, and if the ridge crest were to be equidistant from every continent, then based on the information given in the passage, it would be true that the oldest rocks were the ones nearest the continents.

But nothing says the ridge crest is equidistant from every continent. I was imagining the mid-ocean ridge to be like it is in real life, as illustrated here: https://en.wikipedia.org/wiki/File:Worl ... Ridges.gif. Plainly we can see that there could be a case where some rocks very very far from ridge -- and therefore older -- are in the middle of the ocean, while other (younger) rocks near the ridge crest are right next to a continent. Absolutely nothing in the article suggests otherwise. This is just irresponsible on the part of the test creators.

[KelseyWoods]

Hi LSATIs!

The LSAT can definitely be frustrating! But remember that as tough as it can be, our job when studying for the LSAT is to try to get inside the minds of the test makers and figure out what it is they want us to choose, even if we think we could have written a better answer choice! It's also really important to remember that the LSAT is not a test of your real world knowledge, and in fact, you shouldn't be bringing in that outside knowledge! Your job is to answer the questions using only the information they've given you. That information might be totally incorrect in the real world, but in LSAT world, it's not our job to determine real-world truths. It's to accept what the test makers have told us and understand what we can and cannot do with that information.

I know that's especially tough to do with a question like this which is very difficult and inherently frustrating. Sometimes we just have to make the best with what they've given us and find the best answer, even if we don't like it. The good news is that the scoring scale allows them to compensate for tricky questions like this that more people than usual may have gotten incorrect. Always gotta look for those silver linings!

Best,
Kelsey

[JulesC]

Hi, I'm still a bit confused on answer A. I understand that basalt found near the continents would be among the oldest basalt formed from that mid-ocean ridge. Nevertheless how can we assume that it is likely to be some of the oldest on the ocean floor? We don't know what rocks lie on the ocean floor. Isn't that one of those assumptions that we are not supposed to make on the lsat?

[Adam Tyson]

If the ocean floor spreading theory is correct, Jules, that would suggest that the ocean floor is constantly moving away from the mid-ocean ridge, where new rocks are being "born." Alternating polarity in bands of rock are found marching away from that birthplace, like children leaving home and moving farther and farther away from their place of birth, never to turn around and go back. (Suddenly I miss my son!)

If they move away at a steady pace, never retreating, then the oldest rocks would be the ones that have gotten the greatest distance away. Thus, the oldest ones that are still on the ocean floor would probably be at the edges of land masses like continents and islands! If they go any further, they will no longer be on the ocean floor! So, it seems likely that the rocks nearest the continents would be some of the oldest rocks down under the sea. How could anything older be closer to the mid-ocean ridge? If some other rock, not basalt, was older than the basalt beneath it, did it fall there from the sky? (Possible - meteors! But those would be a minority, most likely.) Shouldn't any rocks other than basalt be carried along with the moving floor of the ocean, like riding on a conveyor belt?

That's how I justified selecting answer A here. While there MAY be rocks in the middle of the ocean, far from continents, that are older than the basalt near the continents, it still seems likely that the basalt near the continents would be among the oldest rocks down there. Picture that moving conveyor belt, and try to imagine how older rocks might stay out in the ocean, far from land. I love to use visualization like that to help me make sense of questions like this, and perhaps it will help you, too.