#20 - Weaken

[nmgee]

I understand why (A) could be correct, but I’m having trouble seeing why (C) wouldn’t undermine the continental drift theory evidence.

(C) describes a situation that is the opposite of the evidence used in the passage (about N. America and European rock formations). So I’m unsure of why this would not undermine the evidence for the theory the passage describes. Can someone please help? Thanks!

[James Finch]

Hi nmgee,

The argument about basalt formation and magnetism showing evidence of continental drift requires an inference that if the continents hadn't moved in relation to each other "over the past few hundred million years," the basalt magnetic fields in North America and Europe would still be the same, meaning that because their magnetic fields are different now, they must have physically moved in relation to each other.

Answer choice (C) doesn't undermine this argument because of the timescale: basalt formed relatively recently (ie in the last century) wouldn't have time to drift, meaning these new pieces would still be very close to the same relative location they were when they formed and were magnetized. It would take much longer than a century for the continents to drift enough for the magnetism to be noticeably different for basalt in North America and Europe.

(A), however, effectively undermines the theory of drift by undermining seafloor spreading, and thus plate tectonic theory, and ultimately continental drift.

Hope this clears things up!

[ashpine17]

so i didnt' make the whole connection between basins and continent age and I wanted to know how I was suposed to spot that because it was only mentioned once or twice and i thought it was a detail

[Robert Carroll]

ashpine,

Details can certainly be relevant to questions. The last paragraph is rife with words indicating evidence: "proof", "confirmed", "explained", "evidence". If we're trying to undermine evidence, this paragraph looks like a key place to start. It's not as if we have to understand the significance of that when reading the passage originally - the nature of question #20 invites us to look back at this paragraph that is discussing evidence all over.

Robert Carroll