Logical Reasoning

PT15 · S3 · Q25 When volcanic lava solidifies, it becomes

A free, expert breakdown of this official LSAT Logical Reasoning question.

Save & drill this skill — build targeted practice sets from questions like this one
Video walkthroughs — watch every question solved step by step
81 official LSATs — as questions, timed sections & full-length tests

When volcanic lava solidifies, it becomes uniformly magnetized in the direction in which the Earth's magnetic field points.

Conclusion

The change in the direction of the Earth's magnetic field must take place very gradually over hundreds of thousands of years.

Evidence (since)

When volcanic lava solidifies, it becomes uniformly magnetized in the direction in which the Earth's magnetic field points.

Lava flows differing by thousands of years in age often have very similar directions of magnetization.

Evaluate

This argument fits the Explain Curious Fact pattern. The author is trying to explain why we're not seeing more variations in the direction of these lava flows from different eras, given that we know the Earth’s magnetic field changes direction over time.

Curious Fact: Given that we know that the direction of the Earth's magnetic field changes over time, why do lava flows that are thousands of years apart have very similar directions of magnetization?

Author's Explanation: The Earth's magnetic field direction changes gradually over hundreds of thousands of years.

The author is essentially thinking that these lava flows that point in the same direction are part of the same era of Earth's magnetic field. Like maybe in 200,000 B.C. the lava fields point towards the south pole and then by the year 1,000 B.C. they point towards the north pole (a new era). .

The reason fields 100,000 years ago point in a similar way to fields from 80,000 years ago is because it very slowly went from pointing to the south to pointing to the north, and these fields would have essentially been pointing to a similar spot in the slow transition from to the other.

Goal

To weaken, we have to argue that the shifts aren't gradual. They're rapid. Maybe the fields 100k years ago aren't from the same era as 80k years ago, but they match because the magnetic field has already cycled back to the same spot.

i.e. maybe 100k ago the field pointed south. Then 90k years ago, it suddenly switched to the north. Then 80k years ago, it suddenly switched back to the south. This would also allow for the 100k and 80k lava fields to point in the same direction, even though they're from different "eras" (the magnetic field has already rapidly switched back and forth).

25.

Which one of the following, if true, most seriously undermines the conclusion that the change in the direction of the Earth’s magnetic field happened very slowly?

The changes in the direction
Unclear Impact
Without knowing whether the chaotic movement of iron-containing liquids in the Earth's outer core is a sudden thing that changes or something that shifts gradually over time, it's hard to use this to say that the shifts in magnetic field are quick.
8% picked this
There has not been a
No Impact
The time scale of when scientists started measuring this is probably a couple hundred years, which is essentially an instant, on the timescales that we're talking about (hundreds of thousands of years). And if anything, the fact that the direction hasn't changed since we started measuring helps the author argue that it doesn't change quickly.
7% picked this
The direction of the Earth’s
No Impact
If we were learning that the magnetic field has reversed several times within the last hundred thousand years, that would ruin the author's conclusion. But since the author thinks it changes every few hundred thousand years, she'd be expecting it to reverse several times (or more) over the past few million years.
11% picked this
Correct
A lava flow has been
Why this is right
This answer suggests that the magnetic field reversed over the course of two weeks. Lava that cooled at the start of that period is pointing in a different direction than lava that cooled a couple weeks earlier. And lava always matches the direction of the magnetic field, implying that the direction of the field shifted over the course of two weeks.
Skill tested: Weaken · how this choice captures the argument's function is the move to repeat next time.
70% picked this
Since the rate at which
No Impact
The fact that it takes years to solidify doesn't really give us any way to say that the magnetic field changes direction quickly. We'd have to also know that over the course of these years of cooling, the direction that the cooled lava is pointing has shifted.
4% picked this

Continue the review in LSAT Lab

Save this question, watch the video walkthrough, and drill similar questions in your LSAT Lab account.

Save and practice this skill

LSAT Lab

Turn this review into a targeted study plan.

Save this question, drill more like it, watch the video walkthrough, and track your progress in your LSAT Lab account.

Start practicing free