Self-study
Professor Robinson: A large meteorite impact crater in a certain region was thought to be the clue to explaining the mass extinction of plant and animal species that occurred at the end of the Mesozoic era. ████████ ███ ███████████ █████████ ██ █████ █████████ ██ ███ ████ █████████ ████ ███ ██████ ████ ██████ ████ ██████ ███ ███ ███ ████████ ████ ██████ █████ ████████████ ████ ███████ ███ ████████ ██ ███████ ████████ █████ ██ ████ █████ ███ ███ ██████████████ █████ █████████ ██ ███ ████ ███████ ██████ ████████ █████████ ████ ██████ ███████ ████████ █████ ███ ████████ ██ ███ ████ ██ ███ ████ ███████████
Summary
The author concludes that the meteorite that formed a large impact crater was not the cause of the mass extinction of plants/animals that happened at the end of the Mesozoic era.
Why? Because of the following:
There’s a crystalline structure of rocks recovered from the site of the crater.
When molten rocks (melted rocks) crystallize, they display the polarity of the Earth’s magnetic field at the time they crystallize.
The recrystallized rocks found at the site display a polarity that’s different from the polarity that existed at the time of the mass extinction.
Notable Assumptions
The author makes at least the following assumption about timing:
The molten rocks recrystallized around the same time that the mass extinction occurred. (This is why the author thinks the differing polarity shows the meteor didn’t cause the extinction.)
19.
Each of the following is ██ ██████████ ██ █████ █████████ ██████████ ████████ ███████ ███████
a
The crater indicates ██ ██████ ██ ████ ████ ██████████ ████ ██ ████ ██████ ███ ████ ███████████
Not necessary, because the author’s arguing the meteor that made the crater didn’t cause the extinction. So if (A) were negated — if the impact was NOT of enough size to cause the mass extinction — that doesn’t undermine the author’s reasoning.
b
The recovered rocks ██████████████ ███████ █████ ████ ███████
Necessary, because if it were not true — if the recovered rocks recrystallized MORE than shortly after they melted — then that could explain why the polarity of the recrystallized rocks is different from the polarity at time of extinction. The meteor could have hit, caused extinction, and then the rocks recrystallized later.
c
No other event ██████ ███ █████ ██ ████ █████ ███ ██████ ██████ ███ ███████
Necessary, because if it were not true — if some OTHER event DID cause the rocks to melt after the impact formed the crater — then that could explain why the polarity of the recrystallized rocks is different from the polarity at time of extinction. The meteor could have hit, caused extinction, and then after the rocks recrystallized at that time, they could have melted and recrystallized again later, with a different polarity.
d
The recovered rocks ██████ ██ █ ██████ ██ ███ ██████ ████ ██████ ███ ███████
Necessary, because if it were not true — if the rocks did NOT melt as a result of the impact that formed the crater — then the rocks could have recrystallized after the impact and extinction, which explains why they have a different polarity from the polarity at time of extinction.
e
The mass extinction █████ ████ ████████ ████ █████ ███ ██████ ████ ██████████ ██████ ███
Necessary, because if it were not true — if the mass extinction would NOT have occurred soon after the impact — then that could explain why the recrystallized rocks have a different polarity from the polarity at time of extinction. The meteor could have hit, the rocks recrystallized, and later the mass extinction resulting from the meteor occurred.