PrepTest 107, Section 3, Question 19
Professor Robinson: A large meteorite impact crater in(91 more words not shown)
Each of the following is(9 more words not shown)
- AThe crater indicates(13 more words not shown)
- BThe recovered rocks(5 more words not shown)
- CNo other event(11 more words not shown)
- DThe recovered rocks(11 more words not shown)
- EThe mass extinction(11 more words not shown)
Reading the argument
How do these rocks tell us when the impact happened? That connection matters more than the scientific vocabulary.
Robinson says molten rocks preserve the Earth's magnetic polarity when they crystallize. The recovered rocks show normal polarity, while the field was reversed at the time of the extinction. Robinson takes that mismatch to show that this impact didn't cause the extinction.
But which event are the rocks dating? Did they melt because of the impact and then crystallize promptly? What if they had formed earlier, or crystallized much later? In those cases, today's crystal structure could tell us about a different time.
There is another timing question too: how soon would an impact produce the extinction it caused? If its effects took long enough, the magnetic field could change between the impact and the extinction. A mismatch would then be less useful for ruling out the proposed cause.
What we’re looking for
This is a necessary-assumption EXCEPT question. Four answers protect connections the timing argument relies on. We're looking for the one Robinson does not need. For each choice, ask whether rejecting it would break the connection between the rocks, the impact, and the extinction.
Working the answers
- ACorrect
Robinson doesn't need the impact to have been more than large enough to cause the extinction. The argument tries to rule out that impact on timing grounds. It could also turn out to have been too small, giving another reason to reject it as the cause.
That would not undermine the timing argument. Even an impact just large enough, rather than more than sufficient, would not change the polarity comparison. This size claim is not required for Robinson's conclusion.
- B
The rocks need to crystallize soon after melting for their polarity to be useful evidence of the melting event's time. A long delay could allow them to take on the magnetic polarity of a later period. Robinson could then be comparing the extinction with the wrong date.
- C
A later melting event could reset the evidence. Imagine the impact melted the rocks during the reversed-polarity period, but a volcanic event melted them again during a normal-polarity period. Their present structure could reflect the later event even if the impact occurred at the time relevant to the extinction. Robinson relies on that not having happened.
- D
The melting must be tied to the impact being investigated. Otherwise, why treat these rocks as evidence of that impact's timing? Rocks that melted for an unrelated reason could crystallize at another time and have a polarity that tells us nothing useful about when the crater formed.
- E
The proposed cause and effect must be close enough in time for this comparison to rule out the cause. If the impact could lead to an extinction much later, normal polarity at the impact and reversed polarity at the extinction would not separate unrelated events. They could be different stages of the same causal story.
Robinson relies on the extinction following the supposed impact promptly, just as the argument relies on the rocks preserving useful evidence of the impact's time.
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