LSAT 156 | Section 2 | Logical Reasoning: Q19

LR Question 19 Explanation

QUESTION TEXT: Geologist: As a spinning object’s radius decreases, its rate of rotation …

QUESTION TYPE: Weaken

CONCLUSION: The geologist’s explanation of variations in Earth’s rotation is correct.

REASONING: Earthquakes shift mass closer to Earth’s center, which makes Earth rotate faster.

ANALYSIS: Let’s start with the mechanism. Earthquakes can redistribute Earth’s mass, and that affects rotation speed. That part is fine.

But then it uses that mechanism to explain changes in Earth’s rotation overall. That’s the jump. Just because something can affect rotation doesn’t mean it actually explains the observed changes.

So what’s missing? We need to know that earthquakes actually occur at the right times to explain the changes in rotation. If the rotation changes happen right after earthquakes, that strongly supports the explanation.

Without that, the argument is weak. It is just saying “this could explain it,” not “this actually does explain it.”

So the best answer is the one that connects earthquakes directly to the timing of the rotation changes.

___________

1. This is too broad – it says earthquakes account for most variation, but that’s stronger than needed and not directly tied to the observed changes.
2. CORRECT. If changes in rotation follow earthquakes, that directly supports the geologist’s explanation.
3. Immediate effects aren’t necessary for the explanation.
4. Too general. A high rate of rotation doesn’t connect to changes caused by earthquakes.
5. Wrong direction. It talks about increased earthquakes, not whether they explain the changes.