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
██ ████ ██ ███ █████ █████ ███ ███████ ███ ████████ ██████████ ████████ ████████████ █████████ ████████████ ██ ███ ████ ██████ ███ █████ █████ ███████ █████████ ██ ███ ██████ ███ ██████ █████████ ██████ ██ ███████ █████████ ████████ █████ ████ █████ ███ ███ ██████ ███ █████ ████ ████ ███ █████ ██ █ █████████ ███████████ ███████ ██ ████ ████ █████████ ██████████ ███ ████ ███ ██ ████ ██ ██████ ████ ██ ███ █████████ ██████ ███ ██████ ████ ██████ ████████ ███ ███ ████ ████ ████████ █████████ ███
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
████ ██████ ███ █████████ ██ ███████ █████ ██ █████████ ██████ ██ ██ ████ ███ █████ ██████ ███ █████ ███ ████ ██████ ███ ████ ██████ █████████████ █████ ████ ████ ███ ██████ ████████ ███ ████████ █████ ███ ████ ██████ █████████ ████████ ███████ █████████████ ███ ███████ ██████████ ███ ████ ██ ███████████ ████████ █████ ████ ██ █████████ ███ ████████ ███████████ ██ █████ ████ ██████ ███ ████████ ████ ██ ███ ███████ ██████ ████████ ██████████ ████ ████ ████ ██ ███████ █████ █████ ████ ██ ████████ █████████ ████ ███ █████ ███████ ████████ ████████ ████████ ████████ ██████████ ██ ████ █████ ██ ██ ██████ ████ ███ █████ █████ █████ ████ ████ ███ █████████ ██████ ██ █ ████ ██ ███████ ███████████ ███ █████ █████ ██ █
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)
16.
Which one of the following ████ ██████████ █████████ ███ ████ ████ ██ ███ ████████
a
In the 1950s, ██████████ ███████ █████ ████████ ██████████ ███ ███████ ██ █████ ███ █████ █████ ███ ██████ ████ ████████ ██ █████ ████
This doesn’t capture the fact that new discoveries altered scientists’ views about the ocean floor. The main point isn’t merely that scientists refined their views; it’s about what caused them to change their views. In addition, the views discussed in the passage aren’t merely about how the ocean floor was formed “many millions of years ago.” The scientists also have beliefs about how the ocean floor is currently affected.
b
The discovery of ████████ ████████ ██████████ ██ ███ █████ ███ ██████████ ██ █████████ █ ███ ██████ ██ ███████ ███ ███ ████████ ████████ ██ ███ █████ ██████
Not supported, because the author states that basalt “was already known to locally distort compass readings on land.” This implies that there was no “discovery” of basalt’s magnetic properties; this was already something known. Since (B) is not supported, it can’t be the main point. (B) is also too narrow, since the main point isn’t focused on a new explanation for magnetic striping on the ocean floor. Rather, it’s about the fact that we’ve discovered magnetic striping patterns and the spreading of the ocean floor, and that these discoveries have changed views about the ocean floor.
c
In the 1950s, ███ ███████████ ███████████ ███ ██ ███ ██████████████ ██ ██████████ █████ █████ ███ ███████ ██ ███ ███████
This best captures the main point, which the author expresses at the beginning of the passage, and supports in the rest of the passage: “But this idea became insupportable as new discoveries were made.” “This idea” is a reference to the belief that the geology of the ocean floor had remained unchanged for millions of years. New discoveries, such as
d
Local distortions to ███████ ████████ ███ ███████ ██████████ ████ ███████████ ██ █████ ████ █████ ███████ ████ █████ ██ ███ █████ █████ ██ ██████ ███ ███████ ██████
Not supported and too narrow. It’s not supported because the author never connects magma to distortions of compass readings. In addition, (D) is too narrow, because it doesn’t capture the idea that new discoveries have shown that the ocean floor has not remained unchanged for millions of years.
e
The discovery of ███ █████ ███████ ████████ ██████████ █████████ ██████████ ██ ███ ████ ██ ███ ███ ██████ █████ ██████ █████ ██ ████ ███ ██ ███ █████████ ██ ███ ██████ █████████ ██████
This is too narrow, because it doesn’t capture the idea that the ocean floor has not remained unchanged for millions of years. The author’s point isn’t merely that we’ve discovered the mid-ocean ridge; it’s that the mid-ocean ridge, as well as a discovery related to magnetic variations, have changed scientists’ beliefs about the ocean floor.