wazoheat

wazoheat t1_jdra4eq wrote

Specifically for Earth's magnetic field, when rock with magnetic materials is formed it retains magnetism from its environment, giving us information about the strength and direction of the geomagnetic field at the time the rock formed. So if you find certain rocks of a certain age you can use them to get information about Earth's magnetic field at the time.

For volcanic rock, this is because molten rock is too hot to be magnetic. As it solidifies and cools below a certain critical temperature (its Curie temperature), any magnetic minerals will retain the magnetic field of their environment, and so these rocks preserve information about the strength and direction of the geomagnetic field when they formed. This is the same principle by which bar magnets are created.

There are also several ways that sedimentary rocks can end up with a "fossil" magnetic field. The study of these phenomena is called paleomagnetism.

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wazoheat t1_j5oxpe7 wrote

The core is not stopping or reversing it's course. That would be impossible due to being a violation of conservation of momentum.

The study that has been making it's rounds in the media suggests that the core has slightly changed it's rotational velocity relative to the surface, so that it is now spinning slightly slower compared to the surface rather than slightly faster as has been previously noted. They also show evidence that this may be a cycle that reverses every few decades. This is unrelated to the magnetic dynamo of the earth and it's roughly 100,000-year cycles.

The media coverage on this study is probably the worst I have ever seen. It's a very simple concept to explain, but if you explain it correctly it's boring, so I have to imagine that the journalists involved are being wilfully mislead, writing willfully misleading articles, or some combination of both.

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