Q18

[nnn2108]

Where in the passage does it say that "in the youngest basalt, [magnetite grains] are aligned with the earth's current polarity"?

All it says, is that when the magma cools and forms solid basalt, the alignment of the magnetite grains is "locked in," recording the earth's polarity AT THE TIME OF COOLING.


Couldn't the earth's polarity have changed, the second the most recent basalt had cooled?


Or is it saying that the magma is basalt, just not in its solid form, and thus the magnetite grains are aligned with the earth's current polarity?

[rinagoldfield]

Thanks for your post, nnn2108.

This is definitely a tricky question. We have to pull together a couple of pieces of evidence to answer it. (A) is supported by the lines you noted (17-24): “grains of magnetite…align themselves with the earth’s magnetic field…when the magma cools to form solid basalt, the alignment of the magnetite grains is ‘locked in.’”

As you note, we still need more support to fully prove (A), which we get in lines 44-45: “the youngest rocks all have normal polarity.”

These lines tell us that the youngest rocks are aligned with the earth’s current polarity; as a result, the magnetite grains there must also align with the earth’s current polarity.

Best,
Rina

[Vivi]

Why C is not right?

Line 17-18 says " 'mg' align with ...", C says "most of them align with"...

can "all" safely infer to "most"? ( if all X, then most of all must be X... Could anyone give an example that fails it? )

And similarly, if "all" is given, can 'some' be inferred?

Thank you in avance!