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)
17.
The author characterizes the correlation █████████ ██ ███ ████ ████████ ██ ███ ███████ ██
a
indicates that ocean █████ █████████ ██████ ██ ██ █████████ ████ ████
Although the author mentions that the correlation in the last sentence is based on the assumption that the ocean floor spreads only several centimeters per year, this doesn’t prove that the correlation is used to indicate or prove the speed of ocean floor spreading.
b
explains the existence ██ ███ ██████ █████████ █████
The reference to correlation isn’t part of an attempt to explain the existence of the mid-ocean ridge. The attempt to explain the mid-ocean ridge
c
demonstrates that the ███████ ████████ █████ ██ ████████████ ████████ ████ ██████████ ████████
The author never suggests the correlation mentioned in the last sentence proves anything about the strength of the magnetic field. The correlation relates to the direction of magnetic orientation, not the strength of magnetic force.
d
provides strong confirmation ██ ███ █████ █████ █████████ ██████
This best captures the purpose, which is to
e
reveals that the ███████ ████████ █████████ ████ ████████ ██ ████ ███████ █████████
There’s no evidence that the earth’s reversals have occurred at “very regular intervals.” We don’t know that the time between each reversal is about the same. Although we do know that there’s a correlation between the duration of a particular orientation before the field reverses and the striping pattern in the ocean, this doesn’t imply regularity to the reversals. Maybe some reversals take a thousand years, others take five hundred thousand years.