Reading Comprehension

PT133 · S4 · P3 · Q17 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.

17.

The author characterizes the correlation mentioned in the last sentence of the passage as "remarkable" in order to suggest that the correlation

indicates that ocean floor spreading
Out of Scope: extremely slow
It's implied that scientists now think that the ocean floor spreads apart from this central ridge at a rate of several cm per year (because that then makes the striping pattern line up with polarity reversals). We're never told whether this is a fast, moderate, slow, or extremely slow rate of spreading. And calling the correlation "remarkable" has nothing to do with the speed of ocean floor spreading. It has to do with amazement and how good a fit the striping pattern is for the cycle of magnetic reversals.
1% picked this
explains the existence of the
Too Strong: explains existence
The remarkable correlation between stripes and reversals doesn't explain the existence of the mid-ocean ridge. It is one of several lines of evidence that support the idea that the ocean floor is spreading apart along the mid-ocean ridge.
3% picked this
demonstrates that the earth's magnetic
Out of Scope: stronger magnetic field
Nothing in the passage is ever talking about the magnetic field getting stronger or weaker. We're only talking about it flipping polarity from positive at the North Pole to positive at the South Pole.
2% picked this
Correct
provides strong confirmation of the
Why this is right
As almost all correct answers on Local Purpose do, this answer connects the detail we're being asked about back to the Framing Idea under which it exists. If we picture an outline with a heading supported by bullet points, they ask us about the purpose of a bullet point and we need to answer by saying it's there to flesh out that heading. As we discussed, the heading of this discussion is the first sentence of the last paragraph: "This [ocean floor spreading] theory is supported by several lines of evidence: First ... Further ... Finally, there is a remarkable correlation between stripes and polarity reversals." For some, "strong confirmation" might seem like it's worded too strongly, but by saying there is a remarkable correlation, the author is saying, "It's an incredibly tight fit between prediction and data. This is a pretty compelling sign that we're on the right track."
Skill tested: Local Purpose · how this choice captures the passage's function is the move to repeat next time.
88% picked this
reveals that the earth's magnetic
Out of Scope: very regular intervals
At no point in the passage are we told that the polarity reversals occur at very regular intervals (i.e. "once every 100,000 years"). We're only told that scientists have assigned ages to the reversals, but those periods between reversals might vary a lot in length. Maybe it reverses 1 million years ago, reverses back 200,000 years later, reverses back 50,000 years later, reverses back 350,000 years later, etc. We're never told that the striping pattern consists of same-sized stripes. There could be a thick stripe followed by a thin stripe followed by a medium stripe, etc.
6% picked this

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