Q20

[pkhosraviani]

So I realize that this is an old, old test, but can anyone help me out with the part about the rocks and the magnetic forces in paragraph 2? I'm just not sure what inference should be drawn from it, and this comes into play in Q20. Any help would be really appreciated, thanks!

[ohthatpatrick]

I'll be honest, I'm stumped.

When I pulled up your post, I initially saw what test/passage it was and just started reading the passage (having not yet seen your question).

And just like you, I was completely befuddled by the explanation of the basalts. I re-read the 2nd half of the 3rd paragraph like 3 times, because I couldn't figure out HOW they were trying to make their point.

For the sake of getting through the answers, we at least know that "showing different alignment = evidence for drifting", but why it does is unclear to me.

It seemed to me like they were saying, "Since basalt takes a permanent snap-shot of the magnetic field when the basalt is formed, basalt formed at the same time on the same continent should show the same picture of the magnetic field."

Okay, makes sense.

"But .. basalt from North America formed at the same time as basalt from Europe shows DIFFERENT pictures of the magnetic field."

Okay, groovy. So we can infer that those basalts were NOT on the same continent.

That much makes sense to me. But how does "not on the same continent" provide evidence for continental drift?

It seems like if you DIDN'T believe in Pangaea or continental drift, you would just say, "Of course their basalts look different -- they're different continents. That doesn't mean they started out as the same continent and then drifted apart. It could just mean that they were always apart."

Since these RC passages are edited down from their original texts into appropriately short excerpts, I'm wondering if some crucial sentences were left out ... or if it was just sloppy writing ... or if we're really just missing something.

I actually thought I had heard this story before, told differently, as evidence for Pangaea.

If you say "basalt formed at the same time on different continents should NOT look the same" and then say "basalt from an early epoch on North America looks the same as basalt from the same epoch on Europe" then you can infer "Whoa, so during that epoch, North America and Europe were the same continent!"

This, to me, is strong evidence for continental drift. By establishing that these two, now separate, continents were at one point on the same continent, you can infer that they've drifted apart.

Anyway, sorry I can't be more illuminating.

[yeun2]

So why is C wrong? Wouldn't it weaken the theory of magnetic field if some basalts in different continents are shown to have the same field alignments? Is it because "some" is not enough to weaken the theory?

[ohthatpatrick]

Yeah, some is very weak and worthy of concern.

But it's also the fact that these were formed "during the 20th century" that would make this unhelpful in judging the deep past.

[andreperez7]

I just did this RC, and I think I might be able to help.

I think the reasoning for answer A was along these lines for weakening continental drift evidence:

New lava is coming in at the ocean basin and pushing continents apart. So, the ocean basins are younger material; continents are older; so it looks like things are getting pushed away from the ridges, and that explains drifting.

But, if you reverse that so now basins are younger and continents, older, it looks like, if anything, continents are drifting towards one another, contrary to and weakening our evidence for continental drift.

I hope I'm somewhat on the right track and not misleading anyone. I was between A and D, and D had "some" and sounded like overly convoluted language so I went with A.

[AyakiK696]

I understand the explanation proposed in the above post, but I just didn't think that A could be the correct answer because it seems like it's contradicted? The passage says that "seafloor spreading" helped explained the "long-standing puzzle of why the ocean basins are so much younger than the continents." So, are we allowed to interpret this as speculation rather than fact?

[ohthatpatrick]

Yeah, it's kinda silly how the correct answer contradicts something that was presented as an observed fact.

But the answer throws in "actually" as way of acknowledging that. It's saying, "if we turned out to be wrong about that ... "

Mainly, I wouldn't worry about it.

Even though the question stem isn't worded exactly this way, it still reads like a "Which one of the following, if true, would most weaken?"

It's not our job to worry about whether the answer is true. The question just asks, "Would THIS finding undermine continental drift?"