So, I’m sure I’m neglecting a lot of stuff, but if you had a moon of, say, a Jovian planet, and this moon contains saltwater oceans or some such conducting material encompassing (or within) most of it, and the magnetic field of the planet is not aligned with its axis, so it’s all wobbly, you would have a magnetic field induced in the moon.
That’s what happens in Europa. It’s awesome, but my point is, suppose the moon was set up such that it had both active volcanism (caused, presumably, by tidal forces from the planet on the moon) and this global saltwater ocean (or some similar conducting material) existing concurrently, you would have magma and/or lava cooling within a time-varying magnetic field.
This would be a phenomenal thing to observe, due to the wealth of information it could yield (assuming, of course, that the setup is actually possible); I’ll look at the case of lava cooling rapidly at the surface:
Here on Earth, basalt “freezes in” the magnetic field at the time it cools, so looking at records of basaltic flows can tell you about the latitude at which the basalt erupted/cooled (given by the inclination of the magnetic field in the samples, since this inclination will be different at different latitudes), and the magnetic field strength/polarity at the time it cooled.
With a time-varying magnetic field, with a period of hours (in the case of Europa, I think it’s around 10 or 11 hours - the rotation of Jupiter), you wouldn’t have this same, long-term record (unless the moon also has its own geodynamo, in addition to the induced field), but you would have a short-term record of the magnetic field at the time period in which the lava cooled, and this would allow geologists to determine the order and rate at which the lava cooled (this is for places where it cools real fast - it would start to become more complicated the slower the lava cools, methinks).
And you could look at the history of the moon’s induced field by looking at older extrusive igneous rocks. If there was a point at which there was no induced field, you’d be able to estimate it based on ancient lava floe samples. This could in turn tell you stuff about the history of the moon’s saltwater oceans (assuming that’s your conductor).
Also you can gain some very specific knowledge of the geological history of a region by looking at magnetic field that was frozen in, if you were able to find a way around the problems posed by having a magnetic field that varies over such short time periods.
