Ocean Floor Geologic Changes

Answer choices

1. A

   No Impact2% picked this

   There are types of rock other than basalt that are known to

   The existence of other rocks that have magnetite or some other magnetic component has nothing specific to do with the ocean-spreading theory.

   2%
2. B

   Correct64% picked this

   The ages of the earth's magnetic reversals have been verified by means other than examining

   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.

   64%
3. C

   No Impact13% picked this

   Pieces of basalt similar to the type found on the mid-ocean ridge have been found

   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%
4. D

   No Impact14% picked this

   Along its length, the peak of the mid-ocean ridge varies greatly in height above

   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.

   14%
5. E

   No Impact8% picked this

   Basalt is the only type of volcanic rock found in portions of the ocean floor

   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%