LR Question 19 Explanation
QUESTION TEXT: Professor Robinson: A large meteorite impact crater in…
QUESTION TYPE: Necessary Assumption – Exception
CONCLUSION: The meteor that made the crater in question didn’t cause the mass extinctions.
REASONING: The crystallized rocks shows that Earth had positive polarity at the time.
ANALYSIS: The professor is assuming that the rocks crystallized when the meteor struck and that they show the earth’s polarity at that time.
Most of the wrong answers show how the rocks could have crystallized from some other cause or at some other time.
___________
- CORRECT. Professor Robinson claims that the crater did not cause the extinctions. So it doesn’t matter if the crater was large enough to have caused the extinctions. (A crater can be big enough to cause extinctions, yet not cause any.)
- If the rocks didn’t re-crystallize soon after they melted then they might not accurately show the polarity of the earth at the time the meteor struck.
- This could mean the rocks re-crystallized some time after the meteor struck. Their polarity wouldn’t be an accurate indication of the Earth’s polarity when the meteor struck.
- If the rocks didn’t melt because of the meteor then they aren’t a good indicator of the polarity at the time the meteor struck.
- If the mass extinction occurred long after the impact then there may have been enough time for Earth’s polarity to change.
Recap: The question begins with “Professor Robinson: A large meteorite impact crater in”. It is a Necessary Assumption question. Learn how to master LSAT Necessary questions on the LSAT Logical Reasoning question types page.
More Resources for Necessary Assumption Questions
- Negations Article: Learn about negations on the LSAT.
- Conditional Reasoning Article: Learn about conditional statements.
- Negations Drill: Practice your negation skills.
- LR Diagrams Guide: Learn how to draw LR diagrams.
- Intro to Conditional Reasoning: Learn conditional reasoning basics.
- Intro Course lesson: This intro course lesson covers Necessary Assumption questions.
- Mastery Seminar lesson: This LR Mastery seminar lesson covers necessary assumption questions.
B and E I dislike for using “shortly after” and “soon after”. How often does polarity change? What is soon in this scale? The fact that they both used phrasing like this made me feel better about still eliminating them.
A was of course correct. Is A always correct here? What I mean by that is it’s covering an assumption this guy apparently needed to make about something “I believe the impact was big enough to cause the extinction” when his hole argument is that didn’t happen? Does the guy have to believe it was big enough to do anything interesting to make his argument about how the meteor didn’t actually hit the Earth at the right time to have caused the extinction event? I’m not sure I’ve encountered a question like that.
This part doesn’t matter but…
Out of curiousity, Googling it apparently polarity reversal avg is 200-300k years. Sometimes these reversals happened in sub 10k years. Mesozoic period lasted 186mil years. +500x the long end of the avg range, and nearly 20000x a short reversal. So yeah the language “soon after” feels a little hard to pin down. Presumably faster than half an average reversal period length.
A few clarifications should help here. Since this is a Necessary Assumption EXCEPT question, that means four answers must be assumptions the argument depends on, and A is the only one the argument doesn’t need.
Answer A feels odd at first since the professor’s conclusion is that the crater did not cause the extinction. But since that’s his conclusion, he doesn’t need to assume anything about whether the crater was big enough to have caused an extinction. His argument is entirely based on timing and polarity, not on size. So even if the crater was huge, his polarity argument would still show it wasn’t the culprit. He’s essentially saying:
1. If this crater caused the extinction, the rocks would show reversed polarity.
2. They don’t.
3. So this crater didn’t cause the extinction.
That argument operates independent of the size of the crater, so that’s why A is NOT a necessary assumption.
As for B and E, you’re right in your instinct that those phrases aren’t judged against real-world geographical timelines. They are judged only in relation to the argument’s structure.
B is necessary because if the rocks didn’t recrystallize around the time they melted, then their polarity would no longer reflect the polarity that existed at the time they melted. If they recrystallized millions of year later, the polarity evidence becomes meaningless.
E is necessary because if the extinction happened long after the impact, the magnetic field could have flipped in the meantime. So a mismatch in polarity wouldn’t prove the crater wasn’t the cause.
Both assumptions preserve the professor’s logic: the polarity at the site must correspond to the polarity at the time the extinction-causing impact would have happened.
So even though “shortly after” or “soon after” are vague in everyday language, they are the minimum conditions the professor’s reasoning requires.
Hope that helps clarify some things! I’m not sure what you mean by “is A always correct here?” Your discussion of A is getting to the right idea of why it doesn’t matter for his argument though. Let me know if you have further questions!