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

[Administrator]

Complete Question Explanation

AssumptionX. The correct answer choice is (A)

A particular crater was thought to have been made by a meteorite that caused mass extinctions in the late Mesozoic era. Professor Robinson does not believe the impact that caused this crater was to blame for the mass extinction. This is because molten rocks reflect the Earth’s polarity at the time they crystallize, and the rocks around the crater are the reverse of the polarity of the Earth at the time of the extinction.

The question is followed by an AssumptionX question, which means that the all of the answers except for one will provide an assumption on which the author’s conclusion is based—that the meteor that caused the crater was also responsible for the mass extinction. The correct answer choice will provide an assumption on which the author’s conclusion does not rely.

Answer choice (A): This is the correct answer choice, because the author did not need to assume that the crater was more than sufficient size to cause the mass extinction—it could have been just sufficient—that, of course, would have been enough. To confirm this as the right answer choice, we can take it away (by logically negating it) and note whether or not the argument is weakened: “The crater was not more than sufficient.” Again, since it could have been just enough, this negated version would not hurt the argument.

Answer choice (B): This is an assumption that the author needs, because the conclusion is based on the inconsistent polarity between the crater site and the earth at the time of the extinction; the argument assumes that the area around the crater reflects the polarity at the time that the crater was formed. A significant delay in crystallization would thus weaken the argument.

Answer choice (C): Since the author’s conclusion is based in part on the assumption that the rocks around the crater reflect the polarity at the time of the impact, if the rocks at the location were melted by some other event, that would weaken the argument. Thus the author’s argument does assume that no other event melted the rocks, so this choice cannot be the answer to this Assumption Except question.

Answer choice (D): The author assumes that the surrounding rocks were melted by the impact—if something else caused the rocks to melt, that would weaken the author’s argument, which depends on linking the melted rocks to the timing of the impact (and the mass extinction).

Answer choice (E): Again, the author’s argument is that the impact happened at some time other than the extinction—this is based on the inconsistent polarities. If we negate, or take away, this assumption, we get:

The extinction would not have occurred soon after the impact.” If that is the case, then that would weaken the author’s argument, which is based on the assumption that if the meteor had indeed caused the mass extinction, the two events would have happened around the same time.

[desmail]

Hi,

For answer choice E, when we negate it, let's say the impact and extinction occurred far apart from eachother. I don't see how this really weakens the conclusion, because if they occurred far apart, it doesn't suggest that the impact that formed the crater WAS the culprit. The rocks would still have eventually crystallized and the polarity would have still been normal and not reversed.

Any additional feedback would be appreciated!
Thanks

[Jon Denning]

Hey desmail - thanks for the question. First let's make sure we understand the argument: the polarity of the crystallized rocks shows that the meteorite impact wasn't responsible for the mass extinction, since the polarity at the time of the extinction doesn't match the polarity of the rocks. The author clearly believes the polarity of the rocks is indicative of when the impact occurred, and that's why he/she feels so confident stating that the impact didn't cause the extinction.

So to weaken that idea we need to show that the polarity of the rocks might be unrelated/unconnected to the extinction (basically disconnect the potential cause from the effect). When we take E and negate it, we get something along the lines of "the extinction did not occur soon after the impact." This attacks the argument by suggesting that the evidence from the impact (rock polarity and the time frame that indicates) is unrelated to the potential effect (extinction). That is, if the impact occurred at time X and captured a certain polarity, but it took a very long time for the impact to cause the extinction, then the impact was still responsible but the lapse in time negates the significance of the polarity. What the rocks looked like at the time of the impact isn't relevant to the time of the extinction because, even though the impact still caused it, the extinction happened much later. So negating E definitely attacks the argument.

Make sense?

[desmail]

But the stimulus says that the polarity was reversed at the time of the extinction If we say that the impact occurred with normal polarity, and then 100 years later it caused the extinction, the polarity at the extinction would still be reversed either way. So I feel like we can't really weaken this because the polarity is proof that one did not cause the other since the polarities are not the same.

So how would the time negate the significance of the polarity? Just because they occur far apart the polarity is still different, assuring us that one didn't cause the other. Are you saying that the polarity would still be reversed but the actual extinction would occur afterwards?

So sorry, but still a little lost on this one. I appreciate your patience.

[Jon Denning]

Yeah, it's a tricky idea. The reason negated E attacks is because it shows that the polarity at the time of impact isn't relevant to the polarity at the time of the extinction, because, even though the impact EVENTUALLY caused the extinction, there was so much time in between that the polarity could have changed.

Consider maybe an analogous idea: let's say I have a line of dominoes ten miles long, and I tip the first domino. Now we can agree that that action caused the dominoes to all fall, but the time between the initial tip (impact) and the final domino falling (extinction) is so great that whatever was true about the world when I started the chain reaction may not be applicable to the world when the final piece fell. That is, maybe it's sunny out when I knock that first piece over. There's so much time passing as those ten miles of dominoes fall that it could be dark when the last one topples. Does that difference day vs night mean I didn't cause the last one to fall? No. Because the time between allows for things to change. Same idea with polarity: the impact could still be the cause even with the different polarities, because the gap in time could be long enough for the polarity to reverse.

Does that help?

JD

[desmail]

Okay, I think I'm starting to get it now. I think what I didn't understand before was that the polarity is just an observation. The polarity is sort of a coincidence, right? It could have been reversed or normal but it could have been a coincidence that it was reversed at the time of the extinction.

So the author has to assume that the impact and extinction were close together because there is less time for change--and we would be able to observe the changes in polarity much closer. So I guess the author assumes that if the extinction happened right away (or soon enough) the polarity wouldn't have time to change and the two polarities would have been the same (both normal).

What do you think?

[Jon Denning]

Not exactly. The point the author is trying to make with the polarity being different is that, if the impact caused the extinction then the polarity for the two should have been the same, right? But that's only true if you're assuming the extinction came right after the impact (same time more or less, so polarity would be consistent). But if it took 10 million years for the impact's full effects to cause the extinction, then the polarity could easily have switched, maybe even multiple times. The impact would still be the cause, and polarity would be irrelevant because so much time passed. So that's the assumption here: the two events--impact and extinction--happened close together time-wise. That's what's required for the polarity at the time of the impact to mean anything.

Again, another, perhaps easier to process, example. I eat a sandwich that's been sitting around for a while, and later I get really ill. Now a lot of people might conclude the sandwich made me sick. But I come back and say "no, I ate it on Tuesday, and wasn't sick until Saturday" (essentially using the different days for the two events to conclude the first couldn't have caused the second). What's my assumption? That the sandwich would have made me sick soon after eating it. Negate that idea and watch what happens: it can take several days for the effects of food poisoning to be felt (or a lot of time can pass between when you eat something and when it makes you sick). Ouch. If that's the case then me trying to kill the causal relationship by showing the two events happened under different parameters (days, polarity, etc) won't work! The cause and effect can have really different circumstances if it's possible for them to be separated by a long period of time.

Does that crystallize everything for you?

JD

[NeverMissing]

I interpreted answer choice A to mean "At the very least, the crater's size crosses the threshold sufficient to have caused mass extinction." I understand now that the answer choice refers to being greater than the sufficient threshold to cause the extinction. I guess I read "more than sufficient" to mean "crosses the threshold or barrier of sufficiency;" in other words, "AT THE VERY LEAST sufficient, possibly greater."

Am I correct in understanding that had answer choice A said "The crater indicated an impact AT THE VERY LEAST sufficient, possibly greater, to have caused the extinction" then that would be an assumption of the argument?

[Kristina Moen]

Hi NeverMissing,

Here, we have the classic "Some people believe X... but they're wrong! Here's why" argument structure. The conclusion of this stimulus is that "the impact that formed this crater was not the culprit." Answer choice (A) is possibly an assumption of the people in the FIRST sentence - that the meteorite crater is a clue to explaining the mass extinction (which the author of this stimulus refutes). But keep reading....

If you see a sentence like "A is more than B," you would negate that to "A is not more than B," which means A and B could be the same. So this isn't even an assumption of the people in the first sentence, because if the crater indicated an impact of sufficient size to have caused the mass extinction, their argument would still hold.

But again, it's not an assumption of the author. The author is concluding that it's NOT the impact of the crater that caused the extinction.

[coffeehouse]

Can I ask for a clarification on that last response? If answer choice A had read "The crater indicates an impact of sufficient size to have cause the mass extinction," (ie, if one overlooks - like I did - the terms "More Than") I would take that to be an assumption of Professor Robinson's (not merely whomever he cites in the first sentence), because it eliminates an alternate cause for why the commonly held belief (by those in the first sentence) is wrong. His argument is, "Meteor impact not the clue" --because--> "crystalline structure has opposite polarity"; if there was another cause (maybe because it was discovered that the crater impact was smaller than expected), then surely that would undermine the argument? Or am I applying Cause-Effect logic improperly here?