PrepTest 133, Section 4, Question 20
Until the 1950s, most scientists believed that the(440 more words not shown)
Which one of the following(12 more words not shown)
- AThere are types(12 more words not shown)
- BThe ages of(16 more words not shown)
- CPieces of basalt(15 more words not shown)
- DAlong its length,(14 more words not shown)
- EBasalt is the(16 more words not shown)
Reading the passage
The spreading theory says the floor pulls apart, magma rises through the weak zones, and new crust forms. My first prediction is fairly direct: evidence of that movement or of magma forming new crust would help. Let's see what support the choices actually offer.
Working the answers
- A
Other rocks distorting compasses doesn't tell us whether the floor spreads. Basalt doesn't have to be uniquely magnetic for the proposed process to work.
- BCorrect
This isn't the direct observation I first had in mind. It's support for the evidence used to test the theory. In the last sentence, scientists compare the ocean's stripes with the dates of Earth's magnetic reversals. If another method verifies those dates, we have more confidence in the timeline behind that comparison—and therefore in the match that supports spreading. Strengthening a theory's supporting evidence can strengthen the theory without directly observing the movement.
- C
Simply finding similar basalt on land doesn't show that oceanic crust forms at the ridge and moves outward. The useful land-rock evidence in the final paragraph involves dates and magnetic orientations, not resemblance alone.
- D
Different ridge heights don't tell us whether spreading produced the ridge. The relevant issue is the process, not whether every part reaches the same height.
- E
The spreading theory doesn't require basalt to be the only volcanic rock near the continents. Excluding other rock types there doesn't establish the proposed movement.
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