Reading Comprehension

PT133 · S4 · P3 · Q19 Ocean Floor Geologic Changes

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Until the 1950s, most scientists believed that the geology of the ocean floor had remained essentially unchanged for many millions of years.

Topic

The discovery and explanation of ocean floor spreading and the patterns of magnetic striping on the sea floor.

Framework

Old / New

Main Point

Until the 1950s, scientists believed the ocean floor was geologically unchanged for millions of years, but new discoveries—especially magnetic striping patterns and their alignment with Earth's historic magnetic reversals—revealed that the ocean floor is continually being renewed at mid-ocean ridges through a process called ocean floor spreading. ( – end of first paragraph, is the pivot sentence.)

P1: The Old Belief and the Puzzling New Discovery

Up until the 1950s, the consensus was that the ocean floor didn't really change. But when scientists started seeing weird magnetic differences in ocean floor rocks, they realized this old idea couldn't hold up. The explanation? As molten rock cools, its magnetic minerals lock in the direction of Earth's magnetic field, which has actually flipped many times in the past.

P2: Mapping the New Patterns and Formulating the New Theory

As researchers mapped more of the ocean floor, they saw clear patterns of alternating stripes with normal and reversed magnetic polarity centered on mid-ocean ridges—a network of underwater mountains. Scientists proposed the new idea of "ocean floor spreading": new crust forms at these ridges and slowly moves away, creating the striped pattern and gradually building the ridge.

P3: Evidence Supporting Ocean Floor Spreading

Several pieces of evidence supported ocean floor spreading: rocks at the ridge crest are very young and get older farther away, the youngest rocks all have normal polarity, and the ages of the stripes match documented reversals in Earth’s magnetic field—showing a striking correlation that backs up the spreading theory.

19.

If the time intervals between the earth's magnetic field reversals fluctuate greatly, then, based on the passage, which one of the following is most likely to be true?

  1. Compass readings are most likely

    Opposite, if anything

    Distorted compass reading came from being atop basalt that was out of whack with the Earth's current polarity. There's nothing special about the peak of the mid-ocean ridge connected to being out of whack. To the contrary, since that's where the new magma spew out of, that has the youngest basalt and thus it should usually have cooled in alignment with the current polarity of the Earth's magnetic field. Thus, if you're near the peak of the mid-ocean ridge, you're dealing with modern basalt, it's aligned, so it won't distort.

    15% picked this

  2. It is this fluctuation that

    Unsupported Causal Connection

    This is just a Word Salad (smushing together different terms from the passage). There's nothing in the passage connecting magnetic field to the topographical ridge line of the mid-ocean ridge. Ridge lines of mountains are caused by volcanos and tectonic plates and stuff like that, not by magnetism.

    12% picked this

  3. Correct

    Some of the magnetic stripes

    Why this is right

    This is very easy to support language, because the alternative is that "all the stripes of basalt are roughly the same size". The stripes of basalt are a combination of two factors, current polarity of Earth + how much ocean floor spreads during that period. If the ocean floor spreads 30m while the polarity is positive, then there will be a 30m stripe of positive basalt. We are told that the ocean floor seems to spread at a pretty constant rate (several cm per year), so a shorter interval at positive polarity will mean a thinner stripe of positive basalt. A long interval at negative polarity will create a fat stripe of negative basalt. Thus, if the polarity intervals fluctuate a lot (but the ocean-floor spreading goes at a constant rate), you'll get stripes of different sizes .

    Skill tested: Inference · how this choice captures the passage's function is the move to repeat next time.

    62% picked this

  4. Continental rock is a more

    Unsupported Comparison

    We have no way to relate the magnetic field to a comparison between continental and oceanic rock. As far as we know, rock is rock. If it has magnetite in it, then when it cooled into solid form (whether on land or in sea), then it locked in its compass needle.

    5% picked this

  5. Within any given magnetic stripe

    Too Strong: does not vary

    First of all, this has no connection to the question stem. In fact, the question stem is emphasizing fluctuates greatly, which means much variation. So it's counterintuitive that we would then predict lack of variance. Moreover, if the Earth is at positive polarity for 100,000 years, then all the basalt that came out of the mid-ocean ridge during that period will be in the same stripe on the ocean floor. If we pick up on rock from that stripe, it might be 50,000 years old, and if we pick up another one, it might be 150,000 years old. Can we say "the age does not vary"?

    5% picked this

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