Sometimes, we as professorial-types, come up with some goofy things to teach some fundamental lessons. Thus was born the continent of Cupcakeia, and its accreted terrane of Frosteringia.
I invented Cupcakeia to illustrate the idea of “apparent polar wander,” the phenomenon wherein it would appear that over time, the north magnetic pole wasn’t always near the north rotational axis of the Earth.
I assure you, the north magnetic pole has always been close to the north rotational axis (that or close to the southern rotational axis – but that’s another story). The magnetic poles don’t wander all over the surface of the Earth. But they look like they do because the continents themselves move.
Cupcakeia and Frosteringia were once separate. Cupcakeia was a substantial continent, whereas Frosteringia was a volcanic island chain that formed due to subduction of the plate that carries Cupcakeia below that which carries Frosteringia.
These two fragments of crust eventually moved together and collided to make the present-day continent of Cupcakeia shown at the top of this post.
We will call the moment in time when the continents were in that ancient, separated position ‘Time 1.’ Modern times (as in the upper image) is ‘Time 3.’ A time in between will be called ‘Time 2.’
During each of these times, the rocks on Frosteringia and Cupcakeia that are forming record the direction toward the magnetic north pole, as shown by the little arrow on each continental fragment.
As time goes on, and the plates move, new rocks still record the positions of the magnetic north pole. But the continents have moved.
We can go in and measure the direction toward the magnetic north pole by studying the rocks on both Frosteringia and Cupcakeia. We can measure the recorded magnetic field in rocks and we can assign an age to rocks. From that we can determine the pattern of change of position of the magnetic pole.
If we assume that the continents haven’t moved, then it looks like the magnetic poles have moved around, sometimes substantially. Not only that, but it also looks like the poles have moved differently for Frosteringia and Cupcakeia. Every continent has its own pattern.
This pattern of motion of the magnetic pole is what we call ‘apparent polar wander.’ What we know, of course, is that the poles don’t move. So if we fix the magnetic pole, then we can know where the continents once were. Since the amount of wander is different for Frosteringia and Cupcakeia, we know they weren’t always connected.
Because we know that Frosteringia was once separate, but is now stuck to Cupcakeia, we call it an ‘accreted-‘ or ‘suspect terrane’ (and yes, that’s how we spell terrane in geology). Much of the western part of North America was built in this way.
The magnetic field as recorded in rocks tells us the direction to the magnetic pole, but also where the rock was in latitude. Was it in the northern or southern hemisphere? Was it close to the equator or far away. By studying magnetism stored in rock we can determine with some confidence the past positions of continents and reconstruct ancient supercontinents like Pangaea and Rodinia. (This post says a little about these two supercontinents.)
Apparent polar wander is one of those concepts in geology that is important, yet very easy to misunderstand. Hopefully, this will help clarify it a bit.