(Before I write this post, I think it's about time that I appologise for my horrible pun in the user icon attached to this entry. :P )
In my biology class, my teacher recently was talking about Pangaea. He explained continental drift by simply saying that it was 'because the continents all look like a big jigsaw puzzle'. My class seemed satisfied with the answer, but personally, I really don't think that's an explination. Obviously, it's because the continents look like a jigsaw puzzle that the theory of continental drift came about. But that's not proof, really.
Evidence for plate tectonics comes from many places. One proof is how, for example, Southeast Brazil and Southwest Africa have the exact same layers of rocks underneath them once you pass the rocks that are older than about 70 million years. Other proofs involve fossils and the like. And for this post, I'd like to ramble about my personal favourite set of evidence:
Any rock formed with ferromagnetic minerals (in simpler words, iron) in it is generally magnetic. Such rocks are usually either igneous or sedimentary. The said igneous rocks are formed from once molten rocks (at about 1000ºC); lava is non-magnetic as it is too hot, but once it cools to below 600ºC, it has solidified and the iron particles in them become magnetised towards the direction of the at-the-time magnetic north pole. This magnetisation is very stable and usually gets 'frozen' in lava, so this is quite useful for geologists. As for sedimentary rocks, it's more complicated, as they're formed from particles of already-formed metamorphic/igneous rocks. The iron particles in them are already magnetic; however, as they are weathered they lose their magnetism and when they clump together in the sediments it realigns them in the new direction of the Earth's magnetic field.
All three of the Earth's poles* vary and wobble. By looking at the ancient magnetic rocks, geoscientists can observe the old directions of the poles, and so can trace the path of the ancient magnetic north poles. Using now some 35-year-old diagrams from when plate tectonics was a hotly debated theory to illustrate this:
Here are the polar wandering paths found by looking at old rocks on Asia and North America. The times are marked in millions of years (which is the standard in geology). Estimated positions are in broken lines, certain positions are in continuous line. I'm sure that, 35 years later, a more detailed diagram is available, but I have this here with me and I find it convenient. It also illustrates how this is not a new theory. *nods*
Anyway, as you can see the two paths are more or less parallel, and for this to be explained, the Earth would have to have two axes of rotation or something. Obviously it's easier to explain it by putting two continents together, like so:
And there you have it. *nods*
1. The geographical north pole is at the Earth's axis of rotation
2. The magnetic north pole is the northermost point where geomagnetic field dips at an angle of 90 degrees (vertical). (diagram
3. The geomagnetic north pole is the northern pole of the geomagnetic field's dipole moment.