Shouldn’t there have been a magnetic field for as long as there has been a spinning iron core? That would be pretty much as long as there has been an Earth.
But when did the earth start spinning?
Yeah, it’s the motion of the currents in the mantel from the core to the crust and back again as the heated material cools the generates the field. It’s hard to see this heat exchange without a crust.
Exactly, that’s all it takes. But, not being an expert, I was sitting here speculating why geologists would think the early Earth’s core wasn’t spinning? The Earth had just formed. Were the interior dynamics of the planet too chaotic at that point or something? I’m glad you asked the question.