Professor Robinson: A large meteorite impact crater in a certain region was thought to be the clue to explaining t...

Victoria on May 6, 2021

Hi @yckim2180,

Happy to help!

Let's start by going through the stimulus.

Professor Robinson concludes that the meteor was not the cause of the mass extinction of plant and animal species that occurred at the end of the Mesozoic era.

Why? Because of the crystalline structure of rocks recovered at the site of the impact.

When molten rocks crystallize, they display the polarity of the Earth's magnetic field at the time of crystallization. However, these rocks display normal polarity even though the magnetic field was reversed at the time of the mass extinction.

The professor makes a number of assumptions in reaching this conclusion. Our job is to select the answer choice which does not outline one of these assumptions.

Overall, the professor's argument is assuming that the meteor caused the rocks to melt and then recrystallize. As the rocks recrystallized when the Earth's magnetic field was normal, the professor concludes that the meteor must not have struck when the extinction occurred.

Answer choice (B) is a necessary assumption. What if the meteor struck at the time of the extinction, causing the rocks to melt, and then the rocks stayed molten for a long period before recrystallizing? Then it would be entirely possible that the meteor caused both the melting of the rocks and the extinction and that the Earth's magnetic field had time to reverse before the rocks recrystallized.

Answer choice (C) is also a necessary assumption. If something else caused the rocks to melt after the meteor, then it would be possible that the meteor caused the extinction because the crystalline structure of the rocks would have formed after the extinction event.

Answer choice (D) is incorrect for the same reasons as answer choice (C).

Answer choice (E) is incorrect because, if we negate this assumption, then the extinction would not have occurred soon after the impact. This would make it possible that the meteor struck when the Earth's magnetic field was normal and then the magnetic field reversed and the mass extinction event occurred.

Answer choice (A) is correct because Professor Robinson does not make this assumption. In fact, this assumption supports the opposite of Professor Robinson's conclusion because it suggests that it is possible that the meteor caused the extinction.

Hope this helps! Please let us know if you have any further questions.