Reading Comprehension

PT133 · S4 · P3 · Q20 Ocean Floor Geologic Changes

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Until the 1950s, most scientists believed that the geology of the ocean floor had remained essentially unchanged for many millions of years.

Topic

The discovery and explanation of ocean floor spreading and the patterns of magnetic striping on the sea floor.

Framework

Old / New

Main Point

Until the 1950s, scientists believed the ocean floor was geologically unchanged for millions of years, but new discoveries—especially magnetic striping patterns and their alignment with Earth's historic magnetic reversals—revealed that the ocean floor is continually being renewed at mid-ocean ridges through a process called ocean floor spreading. ( – end of first paragraph, is the pivot sentence.)

P1: The Old Belief and the Puzzling New Discovery

Up until the 1950s, the consensus was that the ocean floor didn't really change. But when scientists started seeing weird magnetic differences in ocean floor rocks, they realized this old idea couldn't hold up. The explanation? As molten rock cools, its magnetic minerals lock in the direction of Earth's magnetic field, which has actually flipped many times in the past.

P2: Mapping the New Patterns and Formulating the New Theory

As researchers mapped more of the ocean floor, they saw clear patterns of alternating stripes with normal and reversed magnetic polarity centered on mid-ocean ridges—a network of underwater mountains. Scientists proposed the new idea of "ocean floor spreading": new crust forms at these ridges and slowly moves away, creating the striped pattern and gradually building the ridge.

P3: Evidence Supporting Ocean Floor Spreading

Several pieces of evidence supported ocean floor spreading: rocks at the ridge crest are very young and get older farther away, the youngest rocks all have normal polarity, and the ages of the stripes match documented reversals in Earth’s magnetic field—showing a striking correlation that backs up the spreading theory.

20.

Which one of the following would, if true, most help to support the ocean floor spreading theory?

There are types of rock
No Impact
The existence of other rocks that have magnetite or some other magnetic component has nothing specific to do with the ocean-spreading theory.
1% picked this
Correct
The ages of the earth's
Why this is right
Almost none of us would be attracted to this answer on the first pass. It seems to just be signing off the on the truth of a "Premise". One of the strongest pieces of evidence for ocean-floor spreading was the "remarkable correlation" between the ages of the earth's magnetic reversals and the striping pattern found near the mid-ocean ridge. We were told that the ages of the earth's magnetic reversals "had been assigned by measuring the magnetic orientation of continental volcanic rocks, whose ages we had already determined". In other words, we had already decided how old a bunch of rocks were near a volcano on land, and those rocks have the compass needle thing that tells us whether the Earth was in normal polarity (positive North Pole / negative South Pole) or reversed polarity. So if they had a 300 million year old volcanic rock with reversed polarity and a 250 million year old volcanic rock with normal polarity, they would decide that "the earth's polarity switched between 250-300 million years ago". All this answer is saying is that cross-referenced that timeline we built with some other means to verify that our timeline of magnetic reversals was correct. By verifying our timeline, we make it a more trustworthy metric of when polarity changed, and so the "remarkable correlation" between mid-ocean stripes and the polarity timeline feels even more real than before. But this answer does nothing new. None of us were worried that the polarity timeline derived from the volcanic rocks was messed up. We had never been told it was definitely right, and now this answer is telling us it's definitely right. It's a very, very bizarre answer. It's has very little strengthening effect, but it's more than nothing, and more than every other answer, so that makes it the winner.
Skill tested: Strengthen · how this choice captures the passage's function is the move to repeat next time.
65% picked this
Pieces of basalt similar to
No Impact
The ocean-spreading theory doesn't really care if there is basalt on land as well or only in the ocean. The only relevance of basalt is that it gives off magnetic patterns, and the magnetic patterns near the mid-ocean ridge seem to line up with reversals in earth's magnetic polarity.
13% picked this
Along its length, the peak
No Impact
The ocean-floor spreading theory doesn't really make any predictions about whether ridge height would vary greatly or only slightly. Every mountain range on earth varies greatly in terms of height above sea level, so we'd expect an underwater mountain range to vary greatly as well (whether that mountain range was still currently changing or pretty much settled in place). You know mountains -- they love their peaks and valleys, which vary greatly in height.
13% picked this
Basalt is the only type
No Impact
The ocean-spreading theory doesn't really care where on Earth there is or isn't basalt. It doesn't care whether near the continents there are portions of only-basalt, or whether basalt is always mixed in rather evenly. The only relevance of basalt to this theory is that it gives off magnetic patterns, and the magnetic patterns it gives off near the mid-ocean ridge seem to line up with reversals in earth's magnetic polarity. Whatever basalt is doing on the continents, in (C), or near the continents, in this answer, doesn't really have anything to do with analyzing the mid-ocean ridge and interpreting whether it's fixed or spreading.
8% picked this

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