#19 - Professor Robinson: A large meteorite impact crater in

[supjeremyklein]

(A) is about the size of the impact - the impact's degree of sufficiency in terms of size as a causal factor to the mass extinction. negating (A) doesn't do anything to the argument

since the argument relies on chronometric variables/relationships between crystallization and extinction, whether the impact's size would suffice in causing the extinction has nothing to do with whether the meteor is responsible.

in short, (A) is like: impact size indicates responsibility
while the argument is like: time stamp indicates responsibility

[Adam Tyson]

Well put, very succinct, supjeremyklein! Timing is the issue about which the author makes a number of crucial assumptions. Size is not, especially since the author is arguing that the meteor did NOT cause the extinctions. In that case, who cares how big it was?

[elite097]

For choice C why do care if another event caused the melting or not after the impact? What does melting as a result of the impact have to do with the conclusion at all? How will that be a weakener ? We are only concerned about crystallisation at the time of the impact so that it shows the polarity. Whether or not the rock melted long back has nothing to do with this as long as it crystallised during the impact

Moreover, how does it matter if another event caused it to melt or not AFTER the impact. As long as it crystallised at the time of the impact , we don’t need to bother about what happened afterwards



Similarly for D, why are we concerned about timing of melting of rock. We only care about timing of crystallising.

[Jeff Wren]

Hi elite097,

This topic of melting rocks, crystallization, and polarity can be really confusing.

You stated that "We are only concerned about crystallisation at the time of the impact so that it shows the polarity."

This is true, but the problem is that we don't know that the crystallized rocks that were found (and have normal magnetic polarity) were crystallized at the same time as the impact. The professor assumes this, and therefore assumes that the impact crater happened during a time of normal magnetic polarity.

You also asked "why are we concerned about timing of melting of rock. We only care about timing of crystallising."

The melting and the crystallizing go hand-in-hand. The rocks melt (i.e. become molten), and then they recrystallize. I'm not a scientist, but I believe that it is possible for the rocks to melt and recrystallize more than once, and the polarity would correspond to whatever the polarity was during the last recrystallization. (That knowledge isn't necessary to solve the question, but may help shed light on what is going on here.)

In the stimulus, we are told "when molten rocks crystallize, they display the polarity of Earth's magnetic field at that time." Professor Robinson uses the fact that these crystallized rocks have a normal magnetic polarity (combined with the fact that mass extinction took place during a time or reversed polarity) to conclude that the impact crater did not cause the mass extinction.

The professor is assuming here that the impact crater caused the rocks to melt (i.e. become molten) and then those molten rocks crystallized, which is why the professor is using the normal magnetic polarity of the rocks to basically "date" the impact crater. In other words, if the impact crater took place during a time of normal magnetic polarity and the mass extinction took place during a time of reverse magnetic polarity, the two events happened at completely different times (possibly thousands or even millions of years apart) and therefore are unrelated.

One of the problems with this argument is that some other event (completely unrelated to the impact crater) could have caused the rocks to melt and then recrystallize at a much later time, in which case the polarity of these rocks would not tell us anything about the polarity at the time of impact crater. If the rocks remelted at a later time, then they would have recrystallized at that later time with whatever polarity was happening then. Answer C, a defender assumption, rules out this possibility.

Answer D is also a necessary assumption of the argument. If the rocks had not melted due to the impact crater, then they would not have recrystallized at that time, which means the polarity "dating" of the rocks does not necessarily correspond to the time of the impact crater in any way.