Self-study
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 (RC)
21.
Which one of the following ██ ████ ████████ █████████ ██ ███ ████████
a
Submarine basalt found ████ ███ ██████████ ██ ██████ ██ ██ ████ ██ ███ ██████ ████ ██ ███ █████ ██████
b
The older a ██████ ██ ██████ ███ ███ ████ █████ ██ ███ ████████ ███ █████████
Not supported, because a given sample of basalt does not change polarity. The polarity of basalt is determined based on the
c
Compass readings are ████ ██████ ██ ██████ █████████ ██ ███ ████ ██ █████
The author never provides a basis to compare the likelihood of compass distortion on sea and on land. We know that there are rocks with reversed polarity both on sea and on
d
The magnetic fields ███████████ █████████ ██████ █████████ ██████ ████ ████████ ██ █████ ██ ███ █████ ██████
We have no basis to think that magnetic fields weaken over millions of years. Although we know that polarity of the earth changes from time to time, this doesn’t indicate that magnetic fields grow weaker.
e
Any rock that ████████ ███████████ ████████ ████████ ███ ██████ █████ ███ ██████ ████████ ██ ███ ███████ ████████ ██████