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#25 - When volcanic lava solidifies, it becomes uniformly

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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.
lbayliyeva@unm.edu
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1. Why is answer choice B incorrect?

It seems that answer choice B stands directly in contradiction to second sentence in the stimulus.

Answer choice B says:
"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."

Second sentence in the stimulus says:
"There are significant differences in the direction of magnetization among solidified lava flows from different volcanoes that erupted at different types over the past several million years."


2. The explanation online argues that "if there has no been recent magnetic directional changes, this does not affect the strength of the conclusion that such changes must take place over long time periods." This makes sense. I am not certain why the online explanation assumes that answer choice B is only referring to "recent magnetic directional changes," when I do not see the word "recent" or its synonym being used anywhere in the sentence.

Thank you so much for your help in advance.
Luke Haqq
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I'll start with part of your second question. I think the online explantation derives "recent" from the answer choice is because it stipulates that it's talking about "since scientists have begun measuring the direction of magnetization of lava flows."

Concerning the first question, note the distinction in the stimulus and answer choice between the direction of magnetization in the direction of Earth's magnetic field. So answer choice (B) could be assumed to be true--the magnetic field hasn't shifted since they've been studying it, but that neither weakens nor contradicts that there are differences in directions in the magnetization of lava flows.

There are two conclusions in this passage. The first is "therefore, it must be that the direction of the Earth's magnetic field has changed over time." However, the question is about the second conclusion, that the change took place over hundreds of thousands of years. The passage states that when lava solidifies, it uniformly is magnetized in the direction of Earth's magnetic field. But if it were the case that lava showed a change in direction within a small period of time (as in answer (D)), then the conclusion that Earth's magnetic field changed slowly would be weakened.
KaterynaM
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I'm not exactly sure as to why answer choice E is incorrect.
I picked it because since the rate at which lava solidifies varies, "some lava flows will take years to solidify completely", which in turn would not accurately reflect the direction of Earth's magnetic field points. By that logic, the direction of Earth's magnetic field could've changed direction drastically while the lava was in the process of being solidified.
Jonathan Evans
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Hi, Kateryna,

Thanks for the question! Let's step through this argument to make sure we're on the same page.

  • Solidified lava points uniformly in the direction of the magnetic field.
  • Solidified lava from differing time periods points in different directions.
  • The direction of the magnetic field must have changed over time.
  • Solidified lava from times periods thousands of years apart often points in similar directions.
  • Conclusion: The change in direction of the magnetic field must be gradual.

The question task instructs us to weaken this conclusion. How could it be possible that even though solidified lava from times periods thousands of years apart often points in similar directions the change in direction of the magnetic field might not be gradual?

Let's consider answer choice (E). What if lava from different flows in different areas takes different amounts of time to solidify? Will this information by itself call into question the idea that the change in direction of the magnetic field must be gradual?

We do not in fact know whether "this longer cooling process would not accurately reflect the direction of Earth's magnetic field points." The author of the argument would argue that this longer cooling process (measured in years) is insignificant compared to the variations in magnetic field direction over thousands of years, which the author would contend are still minor.

To make answer choice (E) weaken the argument we would have to introduce new assumptions not contained within the stimulus, assumptions that this longer cooling process would exhibit variations in magnetism that would call into question the gradual change hypothesis.

Given the information in answer choice (E), we do not know anything about the observed variations in magnetic orientation within the longer-solidifying lava; to make this answer choice work, we have to bring in outside information and/or unwarranted assumptions.

What we're looking for here is a rock solid (sorry :-D ) case that shifts in magnetism might have occurred very quickly. This is what we have in answer choice (D). (D) gives us direct information that states that magnetism shifted significantly while certain lava was cooling. Compared to the epochal time periods discussed in the stimulus, such rapid change completely upends the author's hypothesis, and we don't have to bring in any outside information to make it work. That's why (D) is a stronger answer compared to (E).

I hope this helps!
hassan66
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I answered D correctly because I ruled out A-C and between D and E, the "take years" in E could mean 5 years or 100,000 years so it wasn't specific enough to weaken the argument for me. But I still don't fully understand how D is correct. I thought that D attacked the premise which I thought ACs weren't supposed to do. The stimulus says that when the volcanic lava solidifies, it is uniformly magnetized. But in D, it says that the direction of the lava differs from the center to the surface so it indicates a change. Also, isn't the second half of D irrelevant? It doesn't matter how quickly or slowly the lava solidifies because what we care about is the change in the magnetic field? Or is it that the direction of the magnetization from the center to the surface was able to change in 2 weeks which is proof that it doesn't need to take thousands of years? Also, in D, it is just "a lava flow." But all it takes is one example to the contrary to weaken an argument right? Thank you!
Jonathan Evans
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Hi, Hassan,

Good reasoning! What we know from the premises is that when lava solidifies, it is magnetized uniformly in the direction of the magnetic field at the moment at which that magma solidifies. However, we cannot assume that all the lava in a lava flow solidifies simultaneously. It could be possible that part of the lava flow solidifies at a different time than another part of the same lava flow.

In fact, the second half of (D) seals the deal for us. We might come into this answer choice thinking, "Well sure, different parts of the lava flow might have different magnetic directions, but what if the lava flow took hundreds of thousands of years to finish?" The second part of answer choice (D) tells us, "No. This lava flow only took two weeks to solidify, but different parts of it have different magnetic directions. Wow, the direction of the magnetic field must have shifted within only two weeks!"

The response to your last question is yes! If we have one example to the contrary, then this example would weaken our conclusion. We're looking for the best answer available, and since this answer does cast doubt on the conclusion, it's a contender. Since there's no better answer available, it is the credited response.

I hope this helps!