Logical Reasoning

PT107 · S3 · Q19 Professor Robinson: A large meteorite

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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.

Conclusion (indicates that)

The meteorite impact was not the cause of the mass extinction of plant and animal species at the end of the Mesozoic era.

Evidence

The rocks recovered at the site had a crystalline structure. When molten rocks crystallize, they show the polarity of the Earth's magnetic field at that time. And these rocks display normal polarity, whereas the magnetic field had reversed polarity at the time of the mass extinction.

Evaluate

Since this is very real-world and Causal (not abstract and Conditional), I'd be thinking more through the lens of Objections, less through the lens of Linking ideas.

Given that ... the rocks recovered at the impact site have a different polarity from the polarity at the time of the mass extinction,

How could we still argue that ... the meteorite impact was a causal factor in the mass extinction?

Maybe the rocks recovered from the site were reverse polarity when the meteorite hit but got recrystallized again later (once polarity had become normal).

Maybe the meteorite hit during a time of normal polarity, but the mass extinction it ultimately resulted in took so long to develop that Earth's polarity had flipped in the meantime.

Goal

It seems pretty hard to anticipate answers here, especially given the dense science.

We should be able to find four answers that either seem to safely replicate the author's thinking and/or seem like, that would allow us to argue that the meteorite impact was part of the mass extinction.

19.

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

Correct
The crater indicates an impact
Why this is right
The author doesn't need to assume that the crater was big enough to have caused the mass extinction. He's arguing that the meteorite didn't cause the mass extinction. This answer would actually weaken the author's argument, so he wouldn't be assuming it.
Skill tested: Necessary Assumption · how this choice captures the argument's function is the move to repeat next time.
61% picked this
The recovered rocks recrystallized shortly
Assumed
If the recovered rocks didn't recrystallize for a long time after they melted, then that creates a possible timeline like this: Meteorite hits during a time of reversed polarity and causes the mass extinction. The rocks recrystallize much later, by which point Earth has already switched back to normal polarity. That's why they show mismatching polarity.
9% picked this
No other event caused the
Assumed
If some other event caused the rocks to melt, after the meteorite impact, then that creates a possible timeline like this: Meteorite hits during a time of reversed polarity and causes the mass extinction. At that point, the rocks would have reversed polarity. But then, some other event later causes the rocks to melt. By that point, Earth has already switched back to normal polarity. So when the rocks recrystallize after that later melting, they end up with normal polarity.
10% picked this
The recovered rocks melted as
Assumed
If the recovered rocks didn't melt as a result of the meteorite impact, then looking at the rocks' polarity as a timestamp for when the meteorite hit would no longer make any sense. This negation would allow for the rocks to have melted and recrystallized during some period of normal polarity (could be before the impact or after, doesn't really matter). This means the meteorite could have impacted during reverse polarity, and caused the mass extinction. And the rocks don't have that reverse polarity "fingerprint", because they didn't actually melt and re-crystallize from that event.
7% picked this
The mass extinction would have
Assumed
If the mass extinction didn't occur for long after the impact, then that creates a possible timeline like this: Meteorite hits during a time of normal polarity and causes the rocks to have that normal polarity "fingerprint". The mass extinction caused by the meteorite impact takes a long time to develop, and by that point Earth has switched back to reverse polarity. That's why the rocks have different polarity from the polarity that occurred during the extinction.
13% picked this

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