LSAT Explanation PT 22, S2, Q19: Professor Robinson: A large meteorite impact

LSAT Question Stem

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

Logical Reasoning Question Type

This is a Necessary Assumption question. 

Correct Answer

The correct answer to this question is A. 

LSAT Question Complete Explanation

The question type for this problem is Necessary Assumption, which means that we need to identify the assumption on which the author's conclusion does not rely. In this passage, Professor Robinson argues that a meteorite impact crater was not responsible for the mass extinction of plant and animal species at the end of the Mesozoic era. The conclusion is based on the fact that the recrystallized rocks recovered at the site display normal magnetic polarity, while Earth's magnetic field was reversed at the time of the mass extinction.

To better understand this argument, let's use a simple example. Imagine you are trying to find out who ate the last cookie in the jar. You have a clue that the person who ate the cookie left a fingerprint on the jar. However, upon examining the fingerprint, you realize that it doesn't match any of the people who were present. This leads you to conclude that none of those people ate the last cookie. In this case, the fingerprint is analogous to the magnetic polarity of the rocks in the passage.

Now, let's evaluate the argument with a question: "Is there any other event that could have caused the rocks to display normal magnetic polarity?"

Let's discuss each answer choice in detail:

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

This is the correct answer choice, as Professor Robinson's argument does not rely on the assumption that the crater was more than sufficient in size to cause the mass extinction. The argument is based on the inconsistency between the magnetic polarity of the rocks and the Earth's magnetic field at the time of the mass extinction. Even if the crater was just sufficient in size, it would not affect the argument.

b) The recovered rocks recrystallized shortly after they melted.

This assumption is necessary for the argument, as the conclusion is based on the inconsistent polarity between the crater site and the Earth at the time of the extinction. If there was a significant delay in crystallization, it would weaken the argument.

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

This assumption is also necessary for the argument, as the conclusion relies on the assumption that the rocks around the crater reflect the polarity at the time of the impact. If another event melted the rocks, it would weaken the argument.

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

This assumption is necessary because the argument depends on linking the melted rocks to the timing of the impact (and the mass extinction). If something else caused the rocks to melt, it would weaken the argument.

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

This assumption is necessary for the argument, as the conclusion is based on the inconsistent polarities. If we negate this assumption, it would weaken the author's argument, which relies on the idea that if the meteor had indeed caused the mass extinction, the two events would have happened around the same time.

In conclusion, answer choice (a) is the correct answer, as Professor Robinson's argument does not rely on the assumption that the crater was more than sufficient in size to cause the mass extinction.