PT133.S4.P3.Q18 > Analytics

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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. ███

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.

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.

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.

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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)

18.

According to the passage, which ███ ██ ███ █████████ ██ ████ ██ █████████ ███████

In the youngest ███████ ████ ███ ███████ ████ ███ ███████ ███████ █████████

Supported, but we have to look to different parts of the passage. First, we have to recognize that basalt makes up much of the ocean floor. Next, we have to understand that basalt contains magnetite grains, which “lock in” their polarity when magma cools to form basalt. Finally, we need to understand that the reference to “youngest rocks” in the last paragraph refers to the youngest parts of the ocean floor (basalt). Put it all together, and we can support the claim that in the youngest basalt, magnetite grains are aligned with the earth’s current polarity (also called “normal” polarity).

80%

In magma, most ███ ███ ███ ██ ████ █████ ██████████ ████ ███ ███████ ████████ ██████

Not supported, because we’re not told that “most, but not all” are aligned with the earth’s current magnetic field. We don’t know what proportion of grains are aligned with the magnetic field and what proportion are not.

17%

They are not █████ ██ █████ █████ ██ ████ ███████ ███████

Not supported, because the author never mentions the absence of basalt in other kinds of rock.

They are about ███ ████ ██ ███████ ██████ ██ █████

Not supported, because we don’t know anything about the size of magnetite grains.

They are too █████ ██ ██ ███████ ██ ███ █████ ████

Not supported, because we don’t know anything about the size of magnetite grains.

PrepTest 133 - Section 4 - Passage 3 - Question 18

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