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passage 3
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
Wrong ·
In light of new discoveries
Discovery 1 (Phenomenon) ·
Odd magnetic variations on ocean floor
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.
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
18.
According to the passage, which ███ ██ ███ █████████ ██ ████ ██ █████████ ███████
a
In the youngest ███████ ████ ███ ███████ ████ ███ ███████ ███████ █████████
b
In magma, most ███ ███ ███ ██ ████ █████ ██████████ ████ ███ ███████ ████████ ██████
c
They are not █████ ██ █████ █████ ██ ████ ███████ ███████
d
They are about ███ ████ ██ ███████ ██████ ██ █████
e
They are too █████ ██ ██ ███████ ██ ███ █████ ████