Q21

[legalrabbithole]

Double posting on this passage...goes to show how much this passage ruined me.

I got #21 right but after reviewing the question, I realize that I didn't have a strong reason for choosing it. I re-read the passage, but can't find corresponding evidence for this inference. I was able to weaken/eliminate the other four, so if someone could please walk me through the correct answer (A) that would be great!

[demetri.blaisdell]

No way to predict what the answer will be on a general question like this. Let's dive in (no pun intended).

(A) doesn't seem like it's in the passage. But we're looking for an inference that is supported by the passage. If the ocean floor is spreading from the middle (mid-ocean ridge) and the rocks are getting older as you go further away from the ridge, then the oldest rocks would be the furthest away from the middle of the ocean. It's a "baby step" inference to say that the rocks near the continents are the furthest from the mid-ocean ridge.

(B) is a little tempting, but the whole point of using the rocks is that the polarity is set at the time they cool. If it could change, all the rocks would have the same polarity.

(C) doesn't make too much sense. Are SCUBA divers navigating at the ocean floor? In a more technical sense, we have no way of making a comparison between magnetic interference on land and in the sea (or on the water, as it seems to imply).

(D) sounds reasonable, but there is no support for it anywhere in the text. Why should we think the magnetic fields ever have to weaken?

(E) is contradicted by the passage. There are only two polar possibilities (current and reversed). They flip-flop back and forth so about half of the bands should correspond to today's polarity.

I hope this helps clear up your confusion. Let me know if you have any questions.

Demetri

[griffin.811]

A little confused as to why C is so easily ruled out. I understood what the passage was saying about A, and almost went with it, but after wasting too much time looking over the lines that supported both A and C I felt C was better supported.

If the basalt is strongly magnetic, and makes up the majority of the ocean floor, and it is true that it is infact the magetic properties of the basalt that is distorting the compass readings on land, it should hold that when in the sea/ocean, you're compass would be even more distorted.

Also supporting this idea was the fact that compasses were only known to be "locally distorted" which could easily be interpreted as near shore lines, and only areas near the ocean. So the closer to the ocean the more likely to have your compass distorted.

The only way I could see this not being true is if there is, on land, some other mineral that when combined with the affect of the basalt from the sea, tends to reek more havac on compasses than the basalt alone (for instance the basalt, combined with the use of cell phone transmissions, which are non existent at sea, both act on the compass needle while on land). Even still, we have no reason to believe this is the case and there could just as easily be other sea minerals that have the same affect!

Thanks for you help!

[abrenza123]

I agree... I'm not quite sure why A is the stronger choice when they both seem to be making the same "baby step" inferences...

[maryadkins]

I'm not sure how we'd infer (C). We don't know anything about what happens on land from the passage, which is all about the ocean. (All we know about land is that magnetite distorts compasses on land, from line 11.) It's too much of a stretch to compare land to water with regard to how they'd affect compasses.

(A) on the other hand relies on the part of the passage that tells us that the youngest rock is at the mid-ocean ridge, which is in the middle of the ocean.

[renata.gomez]

Hi,

Would it still be effective to eliminate E because of the word "any" rock? The passage only discusses certain volcanic rock so E would be discussing rocks not necessarily covered in the passage.

Thank you!

[bswise2]

Hi,

Would it still be effective to eliminate E because of the word "any" rock? The passage only discusses certain volcanic rock so E would be discussing rocks not necessarily covered in the passage.

Thank you!

I am not a scientist, but based on what I am getting from the passage, magnetic polarity occurs because the grains of magnetite in magma align themselves with the earth's magnetic field while the magma cools, locking in the magnetic polarity (see lines 16-23). So I believe that this explanation strongly suggests that we are only talking about volcanic rock when we are talking about magnetic polarity. However, I guess we cannot know this for sure...there could be some other process X that also effects the magnetic polarity in other rocks that the passage doesn't talk about. Regardless, I would be careful though because the question is asking for a "most strongly supported," and I don't think E is wrong because of "any rock that exhibits present-day magnetic polarity" (note that the argument is not talking about any rock--the argument is talking about any rock that exhibits present-day magnetic polarity). It is arguably supported by the passage that these rocks are necessarily formed from magma, thus allow the described process of establishing polarity to occur--i.e., only volcanic rock. If E was a different, more correct answer, I wouldn't necessarily eliminate it because of "any" in this case, just because of the nature of how magnetic polarity is established (and because it is a "most strong supported" question). That's just my impression...someone can feel free to correct me if they disagree.

I do also have a question for whoever can help me--can someone point out where in the passage it says that the ocean floor spreading process occurs in the middle of the ocean? I got this question wrong and after rereading the passage a few times, I cannot find where I am supposed to infer that this was occurring in the middle of the ocean and not, say, closer to the continents.

[ohthatpatrick]

Sure, the 2nd paragraph continually refers to the mid-ocean ridge.

Where is the mid-ocean ridge? Mid-ocean.

Since ocean floor spreading CREATES the mid-ocean ridge, we could infer that ocean floor spreading it happening mid-ocean.

[BarryM800]

It's hard for me to interpret "mid-" as right in the middle, in a geographic (location) sense. Where is the middle of the ocean, particularly since there are four big oceans? Alternatively, it could be about the depth of the ocean floor, i.e., middle level mountain ridge/range. Then, what about "mid-air"? Where is the middle of air, since "air" and "ocean" are in the same category semantically? So I thought it just means there are underwater mountain ridges, as opposed to relatively flat areas, e.g., plain or plateau, in land terms.