LR Question 19 Explanation, by LSATHacks
QUESTION TEXT: Researchers working in Western Australia have…
QUESTION TYPE: Must be True
FACTS:
- Workers found microdiamonds that are 4.2 billion years old.
- They are the earliest known fragment of Earth’s crust.
- The Earth was formed 4.5 billion years ago.
ANALYSIS: On some “must be true” questions, there’s nothing to draw. Instead, you must clarify the facts and see how they combine.
Here, we know that microdiamonds formed 300 millions years after the earth did, and that microdiamonds are part of the earth’s crust.
Therefore, we can say that the crust formed, at latest, 300 million years after the earth did.
Two wrong answers make the error of assuming that because we have evidence, it’s the only possible evidence. It’s possible that the crust formed elsewhere, or earlier, but we don’t have evidence yet.
___________
- CORRECT. See the analysis above. Microdiamonds were part of the earth’s crust, so they are the latest point the crust could have formed.
Sidenote: In history, this is known as a “terminus ante quem”, meaning that “this thing happened at the latest at this date”. - Not so. Our earliest evidence for the crust is from Western Australia. But the crust could have formed elsewhere – perhaps the evidence didn’t stick around, or we just haven’t found it yet.
- This contradicts the passage. The microdiamonds, part of the crust, were there 300 millions years after the earth formed, at the latest.
- No, microdiamonds are the earliest components we know of. There could be earlier ones that we haven’t found yet.
- Nonsense. We know that the microdiamonds we found were part of the crust. But it’s possible that other microdiamonds formed that weren’t part of the crust.
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The terminology is rather confusing in this one to me. It seems that they use the terms “early crust” interchangeably with “crust”, but how is one to know that those two are the same thing… it seems that there have been other questions in different logical reasoning sections that have been very particular about such subtle differences in terminology – why would we assume that both “early crust” and “crust” are identical? What if there was a “later crust” or a “somewhere in between time period crust” that we do not know / are told about here?
Good point/tough question. It’s very subtle, but the argument treats the two as the same in the sense that the early crust is the crust, just earlier. If the two were not the same, how would this discovery have any bearing on figuring out when the crust started to form? I think you make a good point about that being ambiguous, though. However, if the early crust and crust weren’t the same then no answer would work.
-Reply from LSAT Tutor, Morgan Barrett