Logical ReasoningDifficulty: Medium

PT15 S3 Q24 Explanation

When volcanic lava solidifies, it becomes

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

TopicsNecessary Assumption

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Stimulus

When volcanic lava solidifies, it becomes uniformly magnetized in the direction in which the Earth's magnetic field points. There are significant differences in the direction of magnetization among solidified lava flows from different volcanoes that erupted at different times over the past several million years. Therefore, it must be that the direction the Earth's magnetic field must take place very gradually over hundreds of thousands of years.

What this question is testing

Necessary Assumption

Your task

Find the assumption the argument requires in order for its conclusion to hold.

Common trap

Answers that would help the argument but aren't strictly required (sufficient, not necessary).

Winning move

Negate each choice — the right one breaks the argument when negated.

Reading along? Open the full official question in LawHub — we show a fragment here and keep the reasoning in our own words.

The question
24.

The argument that the direction of the Earth’s magnetic field has changed over time requires

Answer choices

  1. Trap6% picked this

    only lava can be used to measure the direction of the Earth’s magnetic field as it existed

  2. Trap9% picked this

    a single volcano can produce lava of differing consistencies during

  3. Correct81% picked this

    not all solidified lava has changed the direction of its

    Why this is right

    Answer C is correct.

    Skill tested: Necessary Assumption · how this choice captures the argument's function is the move to repeat next time.

  4. Trap0% picked this

    there are fewer volcanic eruptions now than there were millions of

  5. Trap4% picked this

    as lava flows down the side of a volcano, it picks

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