Q19

[timmydoeslsat]

I saw that this question was discussed somewhat in the passage discussion thread, however, I am not understanding why (C) is correct.

I have a great grasp on this passage, as this stuff interests me, but I cannot understand how (C) is likely to be true at all.

When I saw the question stem telling me that if time intervals between the earth's magnetic field reversals fluctuate greatly...I did not know how to tie that in with my understanding of the passage.


I know from the passage that the earth's magnetic fields reverses its polarity. I do not know if it is at a consistent rate, a fluctuating rate?

So by this question stem telling me about a hypothetical situation of the time interval of these reversals fluctuating greatly...I do not know what to make of it.


I will say this, the last part of the final paragraph states that if we assume that the floor moved away from the spreading center at a rate of several cm's per year...that may indicate a constant rate, but we are assuming that.

I do not know how that constant rate ties in magnetic stripes of basalt being much wider than others.

[noah]

I'm impressed with anyone who can say this stuff interests them - it's complex stuff!

With 19, we're asked what would most likely happen if the time interval between magnetic field reversals were to fluctuate wildly. Let's think about it: We know the ocean floor is spreading (or created at the ridges) at a constant rate of several centimeters per year. And, we know that the new ocean floor material demonstrates the magnetic field present at the time it's created. So, if the reversals happened every 10 years, we'd expect to see bands of regular width (10 x several centimeters = ~30cm?).

But, if the reversals happened like this--1920, 1921, 1940, 1950, 1999, 2011--then the bands would be of different widths. That's what (C) states.

(A) is out of scope - we have no information about compass readings near the peaks.

(B) is unsupported. We aren't told why the seams happen the way they do, and there's no reason to connect them to polarity reversals.

(D) is out of scope. Continental rock?

(E) is unsupported. It actually doesn't really make sense - within a stripe, there might be basalt made at the beginning of a "polarity era" (new term, coined right here, folks!) and some made at the end. If I were to take an educated guess, a polarity era lasts hundreds of thousands of years if not more, so it's actually likely that the rock within a stripe are from vastly different ages.

[jamesquinnw]

It might help to represent this one visually.

First, the stimulus states that stripes are composed of basalt of a single polarity. Therefore, if the polarity of newly formed basalt is determined by the earth's current polarity, we can infer that a reversal in the earth's polarity would cause the basalt stripe being formed at the time of the reversal to end and a new basalt stripe to begin (and, if there were no such reversal in the earth's polarity, we would have no reason to assume that the current band of basalt would not continue to grow until such a reversal occurred).

If the floor is spreading at a rate of 2cm/year, we can represent the resultant basalt band visually- let's say I = 2cm. For every year the earth stays at a single polarity, we just add one "I" to our visual representation. If the earth's polarity stayed constant for 3 years, for example, then the basalt band formed during that period would look like this in our representation:

III (i.e. 6cm).

If the earth's polarity stayed constant for a much longer period, say 10 years, our band would look like this:

IIIIIIIIII (i.e. 20cm)

The visual representation makes it obvious that the width of the basalt bands are directly proportional to the length of the intervals between reversals in the earth's polarity.