Q25 - When volcanic activity solidies

[sumukh09]

Totally lost with this question. Surprised it wasn't brought up by someone in the forum. Helpp

[nbayar1212]

I remember this question; definitely tricky!


The stimulus tells us that there is a relationship between the direction of solidified lava and the earth's magnetic field. At the bottom, the stimulus states that because the directions of magnetization of volcanoes that erupted even thousands of years apart from each other are nonetheless similar, we can conclude that the earth's magnetic field must have changed vert slowly.

Q25 is trying to get us to undermine that conclusion so we are looking for an answer choice that tells that the change DIDN'T HAPPEN SLOWLY.

a) Gives us additional information about the process but nothing about the rate of change.

b) Tells us things haven't changed since scientists started measuring these things but we don't know when scientists started measuring changes in the magnetic field.... if they started 10 years ago it wouldn't be surprising that they haven't found a change...

c) Tells us that the direction of the magnetic field has changed several times over the past few million years but millions of years seems pretty long and slow.... seems more like it strengthens.

d) Is the correct AC because it implies that the change happens pretty quickly. So, if we know that lava becomes magnatized in the direction of earth's magnetic field, AND we know from this AC that within one lava flow there are multiple directions of magnatization, we could conclude that the earth's magnetic field in fact can change relatively quickly since it changed within two weeks!

e) This tells us that for at least one lava flow (i.e. some), it takes a while for it to solidify completely. Even if we know this, however, we still don't know anything about the direction of earth's magnetic field and certainly not anything about its rate of change.

[WaltGrace1983]

Just wanted to add a bit to (E). I think you could also just say its wrong because it is focused on the idea that lava flows will "take years" to solidify completely. This is implying a relatively low number (2 years, 5 years, etc.) yet the conclusion is talking about HUNDREDS OF THOUSANDS OF years.

I thought (C) was a little tricky but it is put in the context of millions of years so it is a bit weak. Plus, even if it said "The direct of Earth's magnetic field has undergone a complete reversal several times over the past few thousand years" then all this would do is just basically negate the conclusion. I guess this could happen but I would think that such a weakening technique would be too "easy."

[513852276]

For D, since lava flow in the direction of magnetization changed, but for rest of part it needs two weeks to change.It suggests a possibility that although lava flow differing with small changes, magnetization may changes rapidly.

I remember this question; definitely tricky!


The stimulus tells us that there is a relationship between the direction of solidified lava and the earth's magnetic field. At the bottom, the stimulus states that because the directions of magnetization of volcanoes that erupted even thousands of years apart from each other are nonetheless similar, we can conclude that the earth's magnetic field must have changed vert slowly.

Q25 is trying to get us to undermine that conclusion so we are looking for an answer choice that tells that the change DIDN'T HAPPEN SLOWLY.

a) Gives us additional information about the process but nothing about the rate of change.

b) Tells us things haven't changed since scientists started measuring these things but we don't know when scientists started measuring changes in the magnetic field.... if they started 10 years ago it wouldn't be surprising that they haven't found a change...

c) Tells us that the direction of the magnetic field has changed several times over the past few million years but millions of years seems pretty long and slow.... seems more like it strengthens.

d) Is the correct AC because it implies that the change happens pretty quickly. So, if we know that lava becomes magnatized in the direction of earth's magnetic field, AND we know from this AC that within one lava flow there are multiple directions of magnatization, we could conclude that the earth's magnetic field in fact can change relatively quickly since it changed within two weeks!

e) This tells us that for at least one lava flow (i.e. some), it takes a while for it to solidify completely. Even if we know this, however, we still don't know anything about the direction of earth's magnetic field and certainly not anything about its rate of change.

[roflcoptersoisoi]

(D) is clearly the answer choice but I eliminated it because I thought it contradicted some of the background info (the first sentence) however I understand why it is correct.

I'm not not quite sure why (E) is incorrect, can something give an additional explanation?

[ohthatpatrick]

On a Weaken question, it’s helpful to keep the Anti-Conclusion in your head as you read through answers. We want an answer that helps us argue that the magnetic field does NOT take hundreds of thousands of years to change direction.

(D) helps us argue that because it describes a lava flow in which the magnetization changed within two weeks.

(E) does nothing to help us argue that. It just says that some lava flows take years to solidify completely. Okay. Who cares? Unless you tell me, like (D) did, that within those two years the magnetization changed, then this answer has nothing to do with the conclusion.

The author had no particular commitment to how long it takes lava flows to solidify. So whether its days, weeks, or months, she wouldn’t care.

The author is only committing herself to how long it takes for the Earth’s magnetic field to shift. And again, this answer choice doesn’t speak about magnetization at all.

[jay.tom01]

Doesn't answer choice D just seem to negate/contradict the premise immediately preceding it which basically says, "lava flows/solidified lava that are a few years apart have SIMILAR directions in magnetization," and D is just saying, "here's one lava flow, which solidifies in two weeks, and has DIFFERENT directions in magnetizations? Is it because this new lava flow is supposedly found LATER on that it doesn't contradict the premise? Or am I interpreting the prompt/answer choice correctly?

[ohthatpatrick]

Yeah, it doesn't contradict anything ... it says lava flows that are thousands of years apart are often very similar in direction.

[jay.tom01]

Hmm. So in that case, the anti-conclusion would be more like, the earth's magnetic field change does NOT ALWAYS take place gradually. Like, sometimes it takes place gradually AND other times rapidly? And had the conclusion used a word like "can" instead of "must," D would NOT weaken?