Paleomagnetism has many complications and subtleties – as I’ve learnt the hard way in the last six years, it’s not a simple matter of sticking your sample in the machine and getting a direction out. For a start, the direction that the magnetometer initially spits out is often not in a geologically meaningful reference frame. The magnetic intensity of a sample is usually measured along orthogonal x, y and z axes, defined such that the top of a sample is the x-y plane, and the z axis points downward through it.

However, if we examine a paleomagician in the act of drilling for a sample, we see that he is drilling into the outcrop at an angle. Therefore, the x-y plane defined by the top of the sample does not correspond to the real horizontal plane; neither does the z-axis of the core point straight down with respect to the Earth. This skewing of the co-ordinate axes means that the direction of the magnetic vector you measure in the lab is also skewed away from the ‘real’ direction. Plus, because each core is drilled at a different angle, this skew will be different for every sample from an outcrop, so even very similar actual magnetization directions might appear to be very different as measured.

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