When volcanic lava solidifies, it becomes uniformly magnetized in the

Question

When volcanic lava solidifies, it becomes uniformly magnetized in the direction in which the Earth’s magnetic field points. There are significant differences in the direction of magnetization among solidified lava flows from different volcanoes that erupted at different times over the past several million years. Therefore, it must be that the direction of the Earth’s magnetic field has changed over time. Since lava flows differing by thousands of years in age often have very similar directions of magnetization, the change in the direction of the Earth’s magnetic field must take place very gradually over hundreds of thousands of years.

Which one of the following, if true, most seriously undermines the conclusion that the change in the direction of the Earth’s magnetic field happened very slowly?

(A) The changes in the direction of the Earth’s magnetic field are determined by the chaotic movement of iron-containing liquids in the Earth’s outer core.

(B) There has not been a change in the direction of the Earth’s magnetic field since scientists have begun measuring the direction of magnetization of lava flows.

(C) The direction of the Earth’s magnetic field has undergone a complete reversal several times over the past few million years.

(D) A lava flow has been found in which the direction of magnetization in the center of the flow differs significantly from that on the surface, even though the flow took only two weeks to solidify completely.

(E) Since the rate at which molten lava solidifies depends on the temperature and altitude of the environment, some lava flows from volcanoes in certain areas will take years to solidify completely.

Bunuel · Answer

Same stem, Assumption question: https://gmatclub.com/forum/when-volcani ... 06625.html

nurba92 · Answer

Choice D looks very obvious, but we are not told about different layers, is it surface or center. So I took it as a trap answer. Why can we bring so diverse info?

Posted from my mobile device

PTD1995 · Answer

Official Answer

Answer choice (A): The argument was that lava indicates something about Earth's magnetic field, not that lava is the only or best way to measure the Earth's magnetic field. As long as lava is sufficient, it does not matter whether other methods would work, so the argument does not need to assume that only lava would work.

Answer choice (B): As delivered in this choice, the only legitimate interpretation of "consistency" has to do with thickness and composition. The use of "consistency" in this choice should not be interpreted as having anything to do with the direction of magnetization.

C): This is the correct answer choice. Since scientists take the readings to indicate a very distant cause, scientists have to assume that nothing unpredictable happened in the meantime that would make it virtually impossible to be certain that the readings reflect the distant cause. If something unpredictable did occur in the meantime, the readings might not indicate anything certain about the original state of the lava flow.

Answer choice (D): The argument does not need to assume anything about the frequency of eruptions. As long as some lava exists, the measurements might be possible.

Answer choice (E): The argument did not suggest that magnetized rocks serve to magnetize the lava; the argument clearly implies that Earth's magnetic field causes the magnetization. This choice is wrong, because it weakens the idea that the readings necessarily indicate the direction of Earth's magnetic field. Even if you incorrectly argue that the magnetized rocks would align with the magnetic field, there is no reason to believe that those rocks are necessary.

savwildeye · Answer

Powerscore Complete Question Explanation

Weaken. The correct answer choice is (D)

This stimulus deals with the direction of the magnetization of lava based on its flow. Based on a single premise—often there is little directional change over thousands of years—the author draws the questionable conclusion that these shifts must always take place over long spans of time. To weaken this argument, we should look for the answer choice which disproves this assertion, or somehow increases the weakness of the single piece of evidence.

Answer choice (A): This stimulus regards the speed at which the changes take place, not their cause, so this answer choice is irrelevant to the inquiry, and incorrect.

Answer choice (B): Even if there have been no recent magnetic directional changes, this does not affect the strength of the conclusion that such changes must take place over long time periods.

Answer choice (C): This answer choice is wrong for the same reason as incorrect answer choice (B) above; the number of these changes does not affect the argument that the changes must always take place slowly, over long periods of time.

Answer choice (D): This is the correct answer choice. If this is true, then it is clear that the referenced directional changes can take place over time spans far shorter than presumed by the author.

Answer choice (E): The length of time required for solidification of molten lava is irrelevant to the question of whether magnetic directional changes require long periods of time, so this answer choice is incorrect.

bestreturn · Answer

I arrive at D by eliminating all other answers, but not sure why D is correct

Tucky · Answer

Doesn't option (D) directly contradict the premise?

When volcanic lava solidifies, it becomes ''uniformly'' magnetized in the direction in which the Earth???s magnetic field points.

bb · Answer

Yes, it does, but there is a new piece of evidence or new information. It can contradict, because the question says if true, which means, you must assume, both are true. So you are running a good test but running it on the wrong question so to speak.

Which one of the following,  if true,

chetan2u · Answer

D surely stands out as an answer but I would rate C also as a likely contender albeit second to D.

C tells us that there has been reversal of direction several times, so the evidence of similar magnetic direction over a period of thousand years may be because the magnetic direction has completely reversed once or more in these thousand years. This would undermine the conclusion.

Tucky · Answer

Hey, I don't think (C) adds much to the argument. Rather, it is inferable from the stimulus itself:

Below line form stimulus:

''There are  significant differences in the direction of magnetization  among solidified lava flows from different volcanoes that erupted at different times over the  past several million years ''

chetan2u · Answer

The magnetic directions observed from solidified lava separated by thousand of years are the same. So the magnetic directions has not changed over these thousand years, and thus the conclusion that magnetic directions changes very gradually.

Now C tells us that the direction has reversed many times in few million years. It could be that the direction has changed and taken a full circle in these thousand years. So our conclusion falls apart.

But as I said D is better placed.

When volcanic lava solidifies, it becomes uniformly magnetized in the

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards

GMAT Critical Reasoning (CR) Questions

When volcanic lava solidifies, it becomes uniformly magnetized in the

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards