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?