Q24 - When volcanic lava solidifies,

[phlee004]

Hey my fellow LSATers!

I have a quick question on this lava question. So I understand that "C" is correct because this is a Necessary Assumption question and thus I am able to negate the answer choice to say that "all solidified lava has changed... magnetization unpredictably" which would definitely weaken the argument... But, I was wondering about choice "A". If the question stem were a STRENGTHEN question, would answer choice "A" be correct?

Thanks in advance!

-Phil

[giladedelman]

Thanks for the question.

I would say, No, (A) would not strengthen this argument. Here's the core:

P: lava magnetizes in direction of Earth's magnetic field + there are differences in magnetization direction among lava from millions of years apart ----> C: Earth's magnetic field must change gradually

(A) says that only lava can be used to measure the direction of magnetization. Fine, but did the Earth's magnetic field change or not? Whether lava is the only kind of evidence or not has no impact on whether we can draw this particular conclusion.

Does that answer your question?

[WaltGrace1983]

What is the conclusion?

The stem says simply that we are looking to analyze "it must be that the direction of the Earth's magnetic field has changed over time."

Yet the stimulus seems to imply that "direction of the Earth's magnetic field must take place very gradually" is the conclusion to be analyzed.

I find (C) to be a much better answer for the stimulus' implied conclusion but not a very good NA for the first conclusion.

[rinagoldfield]

Thanks for your post Walt!

The argument core has two conclusions, one intermediate and one final. That “the direction of the Earth’s magnetic field must have changed” is an intermediate conclusion. Add to that intermediate conclusion that the flows differing by thousands of years often have very similar directions of magnetization, and we get the final conclusion, that the change must take place very gradually.

This final conclusion isn’t implied – it’s directly stated in the stimulus.

HOWEVER, as you noted, Walt, this question does not ask about the whole argument. It asks about the first conclusion, the one that says that “the direction of the Earth’s magnetic field must have changed.”

Do read past the question stem – this is the part of the argument we’re concerned with.

Here’s the part of the argument we’re concerned with:

P: There are significant differences in the direction of magnetization among solidified lava flows

C: the direction of the Earth’s magnetic field must have changed

(C) is a necessary assumption for the conclusion at hand. To conclude that the magnetic field itself changed, we need to know that the solidified lava didn’t randomly change its direction on its own.

Hope that helps. Keep up the great work.

--Rina

[uhdang]

I was confused with two conclusions. I feel like this is NOT a intermediate conclusion leading to final conclusion but a combination of two arguments. First argument sort of sets a background knowledge to the second argument in a way. So, my core looks like:

(Sub-argument)

Volcanic lava magnetizes in the same direction as Earth’s magnetic field points when solidifies + differences in the direction of magnetization among solidified lava fro different volcanoes from different times over the past several million years.

==>

direction of the Earth’s magnetic field has changed over time.

(Main argument)

Lava flows differing by thousands of years in age often have very similar directions of magnetization

==>

change in the direction of the Earth’s magnetic field take place very gradually over hundreds of thousands of years.


@ Assumption analysis

The argument assumes that frequently showing very gradual change in magnetic field is the ONLY explanation for very similar directions of magnetization. What if magnetic field changes too often in various directions and it just happens to show gradual pattern in more frequently while the actual direction change occurs every second? (very unlikely, but who knows?) Since the question is asking for assumption from main argument, I didn't try looking for an assumption in sub-argument.

Now, let's get into the answer choices.

A) “only” here is a very strong language. We don’t even know the reason for concluding gradual change is definite. Narrowing it down to lava as the only reason is a hasty move. To verify it, negating this would read, "there is at least some other way than lava (not only lava) that can be used to measure the direction of the Earth's magnetic field as it existed in the distant past." -- and this has no impact on the conclusion. We are concerned with how fast Earth's magnetic field changes direction. Irrelevant.

B) If this is true, this would rather present a possibility to hurt the conclusion, because it would indicate a possible false dating for solidified lava. Who knows different elements in different lava could cause different dating? Some materials might record direction of magnetization even before solidifies, disrupting the conclusion.

C) While the conclusion advocates for gradual change, eliminating a possible unpredictability looks necessary. Negating this would read, all solidified lava has changed the direction of its magnetization unpredictably. If all of them are unpredictable, then we can’t establish the pattern with certainty, and concluding “gradual change” would be less reliable.

(On a side note, I am a bit hesitant to write a negation of "not all" to "all." is this right?)

D) How often is not our concern and has no influence on the conclusion. Irrelevant.

E) If this is true, solidified lava could show irregularities in magnetization. So, this would hurt a reliability of the data. Weakens.

[YiZ98]

Thanks a lot for the previous posts. However, I'm still having some problem with answer choice C.

I suppose that for necessary assumption questions, the correct answer choice, if negated, should be incompatible with the overall argument/conclusion. Then if we negate C, it becomes "All solidified lava have change the direction of its magnetization unpredictably." But couldn't this still be consistent with the conclusion of the first half of the argument if the Earth's magnetic field also changes unpredictably?

In other words:
All solidified lava have change the direction of its magnetization unpredictably --> Earth's magnetic field changes unpredictably --> Earth's magnetic field changes

rewrite:
~Choice C --> Earth's magnetic field changes unpredictably --> intermediate conclusion

Therefore, even with C negated, it seems that we can still reach the desired intermediate conclusion. I understand that we can only choose the best out of five, but this still puzzles me.

Hope somebody can help!