Iron-rich minerals inn some rocks hold a record of the Earth's magnetic field at the time of their formation
A large number of rocks are collected then dated, and the direction of palaeomagnetism measured
This data is then plotted as an apparent polar wandering curve
The curves for South America and Africa suggest that, before 160 Ma, one north pole was in two positions at the same time
In fact, the magnetic pole cannot significantly change position
Instead, if we assume the north pole remained fixed, it must be the continents that have moved
If the two continents are re-positioned next to each other, the two curves match up, and there is then one position for the pole
The curves diverge only after the continents started to drift apart
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