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)
20.
Which one of the following ██████ ██ █████ ████ ████ ██ ███████ ███ █████ █████ █████████ ███████
a
There are types ██ ████ █████ ████ ██████ ████ ███ █████ ██ ███████ ███████ █████████
We have no basis to think other kinds of rock are relevant to the ocean floor spreading theory. We know that basalt makes up most of the ocean floor, but we don’t know other kinds of rock found in the ocean floor and whether those other kinds of rock are among the ones known to distort compass readings.
b
The ages of ███ ███████ ████████ █████████ ████ ████ ████████ ██ █████ █████ ████ █████████ █████████ ██████ ██ █████
This strengthens by providing independent evidence of the timeline of earth’s magnetic reversals. We know that scientists determined the ages of earth’s reversals by examining the magnetite in continental rocks. And we observed a correlation between these reversals and the magnetic striping pattern in the ocean floor. But the pattern in the ocean floor is also based on magnetite — we look at the polarity of magnetite in various strips of the floor. It’s possible that the magnetite we examined in the continental rock and ocean floor do not reliably indicate earth’s actual polarity at different times. By providing independent (non-magnetite) verification of the ages of magnetic reversals, we have more reason to believe in the strong correlation between earth’s magnetic reversals and the striping pattern.
c
Pieces of basalt ███████ ██ ███ ████ █████ ██ ███ █████████ █████ ████ ████ █████ ██ ███ ███████████
We have no reason to think that the fact some basalt is found on land affects the ocean floor spreading theory or the evidence supporting the theory.
d
Along its length, ███ ████ ██ ███ █████████ █████ ██████ ███████ ██ ██████ █████ ███ █████ ██████
We have no reason to think variations in height could affect the ocean floor spreading theory or the evidence supporting the theory.
e
Basalt is the ████ ████ ██ ████████ ████ █████ ██ ████████ ██ ███ █████ █████ ███████ ██ ███ ███████████
We already know basalt makes up “much of the ocean floor.” It’s not clear why learning that some parts of the ocean floor not near the mid-ocean ridge are made up only of basalt would change our opinion about ocean floor spreading or the evidence supporting it.