Professor Robinson: A large meteorite impact crater in a certain regio

Fact : Meteorite impact crater was the clue to explain the mass extinction of plant and animal.
Conclusion : The impact that formed this crater was not the culprit(for mass Extension)
Supporting Fact : Earth’s magnetic field was reversed at the time of the mass extinction. The recrystallized rocks recovered at the site display normal magnetic polarity(Not reversed Magnetic field) Hence ~ Conclusion.

The Author is trying to tell Ideally if the meteorite impact caused the extinction Those Rocks that are Crystallized(after impact) should have the same reversed Magnetic Field(as that of Earth's). Just because they do not have the author is concluding that meteorite impact did not cause Extension.


Assumption Question should always be approached using Negation Technique: In this Question those Choices that Weaken the Argument when negated should be eliminated

(B) The recovered rocks recrystallized shortly after they melted.
Negation: The recovered rocks did not recrystallized shortly after they melted: This Weakens the Argument as it provides a possibility that: "Impact resulted in Extension and resulted in reversal in Earth’s Magnetic Field , but at the same time explains why the Rocks did not have reversal in Magnetic Field(as they did not Crystallize immediately)" [Alternate Cause]

(C) No other event caused the rocks to melt after the impact formed the crater.
Negation:Other event caused the rocks to melt after the impact formed the crater : This also weakens the Argument as it creates the new possibility that after the impact occurred the Rocks are melted again Crystallized again [Data Error]

(D) The recovered rocks melted as a result of the impact that formed the crater.
Negation: The recovered rocks did not melt as a result of the impact that formed the crater. This also weakens the Argument(It is actually similar to Option B) [Alternate Cause]

(E) The mass extinction would have occurred soon after the impact that supposedly caused it.
Negation: The mass extinction did not occurred soon after the impact that supposedly caused it: This Weakens the argument Ex: The Creates the possibility that meteorite impact further caused Chain of actions that caused the Extension

Hence A is the answer.

Though not sure.. i guess the wording in A makes it wrong..

The crater indicates an impact of more than sufficient size to have caused the mass extinction

no need to be more than suff size.. can create mass extinction even if the crater is of suff size..

Experts please check in..

This is an arduous question as we are looking for 4 assumptions among non-assumptions, so it might take more time to answer. However, it's clear why A is the answer as the size of the impact that caused the crater is irrelevant to whether the crater can explain the extinction. The other options somehow connected the premise to the conclusion.

@chetanu : please help me to understand this question.

This is a confusing passage, so let's start with the conclusion: "the crystalline structure of rocks recovered at the [impact] site indicates that the impact that formed this crater was not the culprit" responsible for the mass extinction of plant and animal species that occurred at the end of the Mesozoic era.

What is the point of this passage?

The first sentence describes an explanation that had previously been accepted: "A large meteorite impact crater in a certain region was thought to be the clue to explaining the mass extinction of plant and animal species that occurred at the end of the Mesozoic era."

However, evidence from the "the crystalline structure of rocks recovered at the site" goes against that explanation. So, there was a commonly held view, and then new evidence suggested that the prevailing view was not accurate.

What was that evidence?

• The "Earth’s magnetic field was reversed at the time of the mass extinction."
• "The recrystallized rocks recovered at the [impact] site display normal magnetic polarity."
• "When molten rocks crystallize, they display the polarity of Earth’s magnetic field at that time."
• Thus, we can infer that the rocks recovered at the site--if they were molten--did not crystallize at the time of the mass extinction.
• In other words, if molten rock was in fact created by the meteorite impact and if that impact occurred at the time of the extinction, then we would expect the recrystallized rocks recovered at the site to display reversed magnetic polarity. Thus, according to the author, the fact that the rocks display normal magnetic polarity is evidence that the impact that formed the crater was not the cause of the mass extinction.

Each of the following is an assumption on which Professor Robinson’s argument depends EXCEPT:

(A) The author argues that the evidence from the rocks suggests that the impact that formed the crater was NOT the cause of the mass extinction. If (A) were not true, then it would be further evidence to SUPPORT the author's argument. Thus, (A) is not a required assumption and might be our answer.

(B) What if the meteorite did cause the extinction at a time when the magnetic field was reversed, but then it took thousands of years for the molten rocks to recrystallize? By that time, the Earth's magnetic field may have returned to normal, explaining why the recrystallized rocks recovered at the impact site display normal magnetic polarity. This would disrupt the author's logic, so (B) is a required assumption.

(C) What if the molten rocks crystallized at the time of the extinction (giving them reversed magnetic polarity), but then, years later, when the magnetic field was normal, the rocks melted again and recrystallized? That would explain why the recrystallized rocks recovered at the impact site display normal magnetic polarity, disrupting the author's logic. (C) is a required assumption.

(D) If the rocks did not melt as a result of the impact that formed the crater, then their polarity would not be an indication of the Earth's polarity at the time of the impact. Thus, the crystalline structure of the rocks recovered at the site would be irrelevant, and the author's argument would fall apart. (D) is a required assumption.

(E) Perhaps the impact occurred at a time when the Earth's polarity was normal, but it took hundreds or thousands of years for the effects of that impact to finally cause the mass extinction. For example, maybe the impact set off a chain reaction that would, after hundreds of years, significantly alter the temperature of the planet, thus causing a mass extinction. By the time the extinction took place, the polarity of the Earth may have become reversed. This would explain why the polarity of the recrystallized rocks recovered at the impact site does not match the polarity of the Earth at the time of the extinction. Thus, (E) is a required assumption.

Only (A) is not a required assumption, so (A) is the best answer.

GMATNinja KarishmaB what does the time of melting have to do with magnetic fields? From the pssage we only care about when it was crystallised . Even if the rock was molten from before the impact, then as long as the crystallisation happened during extinction we can safely say the rock would have reverse polarity and if it didn’t crystallise then , then it would have normal polarity. So not sure how D can be an assumption ? Why do we need to know that melting happened during impact or not since that doesn’t have any impact on polarity in the first place avigutman @experts MartyTargetTestPrep

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Good question. It's the time of crystallization that matters, not the time of melting. So, this question could perhaps be better written.

Here's what's going on though.

The question involves the understanding that rocks on the surface of Earth are not normally molten and do not remain molten for extended periods of time.

So, it would be extremely unlikely that the rocks in question would have been molten at that time without the effects of the meteorite impact. Rather, the rocks would have had to have been caused to be molten by something such as the meteorite impact, and then would have quickly recrystallized.

So, basically, choice (D) is indirectly stating the assumption that the rocks were briefly molten at the time of the meteorite impact, an event that would not likely have occurred unless the meteorite impact caused it.

Author's conclusion: 
Recrystallization does not indicate that impact was the culprit of the mass extinction [because it doesn't contain that magnetic polarity information, which was reversed at the time of the extinction].

Except A all the assumptions are necessary to reach the author's conclusion.
Ex:
(B) The recovered rocks recrystallized shortly after they melted. [Otherwise, how would rock record the magnetic polarisation at an instant when mass extinction happened?]

(C) No other event caused the rocks to melt after the impact formed the crater. [Eliminating another cause]

(D) The recovered rocks melted as a result of the impact that formed the crater. [Eliminating another cause]

(E) The mass extinction would have occurred soon after the impact that supposedly caused it. [Otherwise, how would rock record the magnetic polarisation at an instant when mass extinction happened?]

This is what the argument tells us:

At time T in history, mass extinctions happened. At this time, Earth's polarity was reverse, not normal.
It was believed that a meteorite whose impact is found in certain region X caused it.

Conclusion of Professor: the meteorite was not the culprit.
Why?
When rocks crystallise, they display the polarity of Earth at that time.
The rocks from region X show normal polarity, not reversed. This means that they did not melt due to the meteorite and recrystallise during time T in history.
The meteorite must have hit at a time other than T.

The rocks on Earth's surface do not exist in molten state. Some major impact would be needed to melt them. A meteorite would have that impact.

We have to find 4 assumptions here. So we need things that are necessary for the conclusion above to hold.

(A) The crater indicates an impact of more than sufficient size to have caused the mass extinction.

Our conclusion is that the meteorite was not the culprit. This is counter to our conclusion. So certainly not an assumption.

(B) The recovered rocks recrystallized shortly after they melted.

Yes, we did assume that the rocks recrystallized during time T only.

(C) No other event caused the rocks to melt after the impact formed the crater.

Is an assumption. We are assuming that the rocks were not melted and recrystallised yet again sometime after T when the polarity was normal again.

(D) The recovered rocks melted as a result of the impact that formed the crater.

Correct. We are assuming that the rocks melted because of the meteorite's impact because only then will they re-crystallise and display Earth's then polarity. What if rocks of region X did not melt due to the meteorite. They would display old polarity only which could be normal.

(E) The mass extinction would have occurred soon after the impact that supposedly caused it.

Also an assumption. Since Earth's polarity was reverse during extinction, we are assuming that the rock should show reversed polarity too i.e. there wasn't a whole lot of time to let polarity change between the two events of meteorite hitting and extinction

Answer (A)

Professor Robinson: A large meteorite impact crater in a certain region was thought to be the clue to explaining the mass extinction of plant and animal species that occurred at the end of the Mesozoic era. However, the crystalline structure of rocks recovered at the site indicates that the impact that formed this crater was not the culprit. When molten rocks crystallize, they display the polarity of Earth’s magnetic field at that time. But the recrystallized rocks recovered at the site display normal magnetic polarity, even though Earth’s magnetic field was reversed at the time of the mass extinction.

Each of the following is an assumption on which Professor Robinson’s argument depends EXCEPT:

(A) The crater indicates an impact of more than sufficient size to have caused the mass extinction. - CORRECT. Sufficiency of size wrt to mass extinction is subjective and is not a certainty that a specific size leads to one. So, it is not necessary for the passage.

(B) The recovered rocks recrystallized shortly after they melted. - WRONG. It is necessary for the passage. If not polarity is questionable.

(C) No other event caused the rocks to melt after the impact formed the crater. - WRONG. Easily identifiable as an assumption and thus not the right aanswer.

(D) The recovered rocks melted as a result of the impact that formed the crater. - WRONG. More or less like C as it related to it in a sequential manner.

(E) The mass extinction would have occurred soon after the impact that supposedly caused it. - WRONG. If not then some other factor may have lead to it.

The more longer the passage the more time consuming it is to read. And, more important it is to be read carefully so that by the time one reaches the choice he/she isn't exhausted and misjudges a choice that is wrong. However, i must admit this passage is bit odd. Highlighted text is conclusion and there is further reasoning given for that. Prior to conclusion is generally acceptable notion.

Answer A.