Ocean Floor Geologic Changes

Answer choices

1. A

   Out of Scope: extremely slow1% picked this

   indicates that ocean floor spreading occurs at an extremely

   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%
2. B

   Too Strong: explains existence3% picked this

   explains the existence of the global

   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%
3. C

   Out of Scope: stronger magnetic field2% picked this

   demonstrates that the earth's magnetic field is considerably stronger than

   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%
4. D

   Correct88% picked this

   provides strong confirmation of the ocean floor

   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%
5. E

   Out of Scope: very regular intervals6% picked this

   reveals that the earth's magnetic reversals have occurred at very

   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%