This boulder is volcanic in origin and yet the rest of the rock in

Question

This boulder is volcanic in origin and yet the rest of the rock in this area is sedimentary. Since this area was covered by southward-moving glaciers during the last ice age, this boulder was probably deposited here, hundreds of miles from its geological birthplace, by a glacier.

Which one of the following, if true, most seriously undermines the conclusion drawn in the argument above?

(A) Most boulders that have been moved by glaciers have not been moved more than 100 miles.
(B) The closest geological source of volcanic rock is 50 miles south of this boulder.
(C) The closest geological source of volcanic rock is 50 miles north of this boulder.
(D) There are no geological sources of volcanic rock north of this boulder.
(E) No other boulders of volcanic origin exist within 50 miles of this boulder.

yashikaaggarwal · Answer

Inference: The area of boulder was covered by southward-moving glaciers so, the boulder was probably deposited here, hundreds of miles from its geological birthplace, by a glacier.

Conclusion: The volcanic rock must have surely travelled from north for this particular boulder phenomenon to take place.

The following, which is, most seriously undermining the conclusion drawn in the argument above is,

(A) Most boulders that have been moved by glaciers have not been moved more than 100 miles.(the geographical location or measurement is not stated in passage, hence we can't conclude this) (Incorrect)

(B) The closest geological source of volcanic rock is 50 miles south of this boulder. (We want to know whether the volcanic rock is at north or not, it's south location is of no use) (Irrelevant)

(C) The closest geological source of volcanic rock is 50 miles north of this boulder. (The statement here is strengthening the passage as the volcanic rock are present in north, testifying the theory) (Incorrect)

(D) There are no geological sources of volcanic rock north of this boulder. (This statement is undermining the passage as it's justifying that there is no volcanic rock present in north, and if it's not there, that means the inference and conclusion drawn from passage is wrong) (Correct)

(E) No other boulders of volcanic origin exist within 50 miles of this boulder.(No such parameter given to testify the statement with conviction) (Incorrect)

Answer is D

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ajaygaur319 · Answer

(A) Most boulders that have been moved by glaciers have not been moved more than 100 miles. -  Even if there exists one boulder which has been moved 100 miles then it can be the boulder in discussion 
(B) The closest geological source of volcanic rock is 50 miles south of this boulder. -  The boulder came from North and the second closest boulder can be 55 miles North 
(C) The closest geological source of volcanic rock is 50 miles north of this boulder. -  Strengthens the argument 
(D) There are no geological sources of volcanic rock north of this boulder. - If there are no geological sources of volcanic rock North of this boulder then there is no way that the Southward moving glaciers would've moved it. Weakens the argument 
(E) No other boulders of volcanic origin exist within 50 miles of this boulder. -  There might exist boulders 55 miles North.

(D)  is the correct answer.

Pranithare · Answer

it is saying southward moving, so why do weaken that it didn't come from north.Please explain

Abhishek009 · Answer

Movement of a glacier will look something like this -

https://alanhamr.weebly.com/uploads/1/8/6/0/18607614/8348474_orig.jpg

Now for the Volcanic rock to have been moved from somewhere else it must have been brought down from above (North)!!!

Hope this helps

PragmaticShaw · Answer

The glacier is moving southward doesn't necessarily mean it's coming from the North. What if it's coming from North-East or somewhere else and has changed its trajectory later? Possible, right?

hero_with_1000_faces · Answer

Well, the movement of boulder would be south-west and not southward. Try drawing a two points on a paper.

bv8562 · Answer

GMATNinja   MartyTargetTestPrep  Why is B incorrect?

GMATNinja · Answer

The argument concludes that the boulder was probably carried hundreds of miles from its birthplace by a "southward-moving" glacier. Why? Because the boulder is volcanic in origin, but the "rest of the rock in this area is sedimentary."

Let's now consider (B):

If (B) is true, would it weaken the conclusion above? Not really. The argument claims that the rock originated to the north (i.e. it was carried by a southward-moving glacier). So the fact that a source of volcanic rock exists to the SOUTH isn't relevant to that argument.

But what about the fact that a source of volcanic rock is located only 50 miles away? Perhaps the rock was carried from that source? If so, wouldn't that weaken the argument?

The problem with that line of reasoning is that it would require too many leaps. Presumably, you'd need a northward-moving glacier to move volcanic rock from south to north. But we don't know if there was such a glacier. All we know is the "area was covered by a southward moving glacier during the last ice age."

For both of those reasons, (B) is incorrect.

I hope that helps!

VerbalBot · Answer

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This boulder is volcanic in origin and yet the rest of the rock in

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