PT133.S4.P3.Q19

PrepTest 133 - Section 4 - Passage 3 - Question 19

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P1

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

Conventional Theory · Geology of ocean floor unchanged
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Wrong · In light of new discoveries
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Discovery 1 (Phenomenon) · Odd magnetic variations on ocean floor
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Explanation · When basalt cools, it records the earth's polarity at that time
Basalt begins as magma; it contains magnetic material that freely aligns with the earth's magnetic field at the time; when it cools down to form solid basalt, the alignment is locked in.
P2

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Discovery 2 (Phenomenon) · Global mid-ocean ridge
Global mid-ocean ridge contains stripes of basalt with alternating polarities. Imagine a zebra's stripes where the white stripes point north and the black stripes point south.
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New theory · Ocean floor spreading
New explanation is that the ocean floor is not static. It has been changing as new magma erupts from the ridge. The magma spreads out away from the ridge and cools to lock in the polarity.
P3

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Evidence Supporting New Theory · 3 pieces of evidence
Rocks near ridge crest are young; the youngest rocks have normal polarity; the bands of magnetic orientation as recorded by rocks are corroborated by known age of magnetic reversals.
Passage Style
Critique or debate
Phenomenon-hypothesis (RC)
Show answer
19.

If the time intervals between ███ ███████ ████████ █████ █████████ █████████ ████████ █████ █████ ██ ███ ████████ █████ ███ ██ ███ █████████ ██ ████ ██████ ██ ██ █████

a

Compass readings are ████ ██████ ██ ██ █████████ ████ ███ █████ ██ ███ █████████ ██████

Not supported, because we don’t have any reason to think the polarity of magnetite near peaks of the mid-ocean ridge are any different from earth’s current polarity. The author suggests that compass distortion occurs because some magnetite has reversed polarity. But we don’t know that magnetite found in peaks of the mid-ocean ridge are the most likely to have reversed polarity compared to magnetite found elsewhere.

11%
b

It is this ███████████ ████ ██████ ███ █████ ██ ████ ██████ ███ █████ ████ ███ █████ ██ █ █████████

Not supported, because the author never suggests polarity reversals have anything to do with the cause of the baseball seam shape of the mid-ocean ridge. We don’t have any reason to think the baseball seam shape results from the magnetic field or the direction of it. It may result simply from the shape of various weak points in the earth’s crust.

11%
c

Some of the ████████ ███████ ██ ██████ ██ ███ █████ █████ ███ ████ █████ ████ ███████

Supported. We are told that there’s evidence of a strong correlation between the ages of the earth’s magnetic reversals and the alternating magnetic striping pattern in the ocean’s floor. Remember, these stripes have different polarities based on the polarity of the earth as magma cooled. If the time intervals between the earth’s magnetic reversals change greatly, then, given the strong correlation between the reversals and the striping pattern, we can infer that different stripes are likely to have varying widths. If the time between two reversals is short, then the associated stripe between those reversals will be narrower than the stripe made during a period when two reversals are much farther apart in time.

68%
d

Continental rock is █ ████ ████████ █████████ ██ ███ ███████ ████████ █████ █████████ ████ ██ ███████ █████

Not supported, because the author never provides a basis to compare continental rock to oceanic rock on the reliability of their indications of magnetic field reversals. Although the author mentions that we can determine the polarity of continental rocks, we don’t know that these rocks are any more accurate as indicators of polarity compared to oceanic rock.

5%
e

Within any given ████████ ██████ ██ ███ █████ ██████ ███ ███ ██ ███ ██████ ████ ███ █████

Not supported, because we have no reason to think the basalt within a stripe is exactly the same age. Actually, we have reason to think the basalt would involve different ages, because the basalt forms from magma rising from the earth. It would be strange if the magma within a stripe rose all at the exact same time and cooled at the exact same time.

4%

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