Fascinating. I'd always assumed that the magnetic field was a combination of the chemistry of the particulates recombining, and the density of the mass accumulated. I had no idea that it fluctuated.

I think what you are looking for is the theory of geodynamo, which is referred to in the article you shared:

"Earth’s magnetic field is generated in its outer core, where swirling liquid iron causes electric currents, driving a phenomenon called the geodynamo that produces the magnetic field.
"Because of the magnetic field’s relationship to Earth’s core, scientists have been trying for decades to determine how Earth’s magnetic field and core have changed throughout our planet’s history. They cannot directly measure the magnetic field due to the location and extreme temperatures of materials in the core. Fortunately, minerals that rise to Earth’s surface contain tiny magnetic particles that lock in the direction and intensity of the magnetic field at the time the minerals cool from their molten state."

https://websites.pmc.ucsc.edu/~glatz/geodynamo.html