The Earth's magnetic field is the result of a complicated dance of compressed, therefore heated, iron and the "flowing" but not exactly rotating inner core.
It's possible that a planet with a more silicate makeup than Earth could have a strong magnetic field so long as it posesses a similar iron core. But it would seem to me that if it had a significant core of iron, there would be iron in the crust, also, because of volcanic eruptions.
As far as my astronomic studies go, silicate planets have very little iron anywhere in them. Earth is thought to have "gathered" its iron while it was still a molten ball of magma as iron-rich asteroids fell into its gravity. Then iron, being heavier than the rock, fell to the core of the fluid ball and eventually formed our iron core. The reason we have iron deposits near the surface is because after the crust cooled, volcanos carried the material to the upper crust and surface.
Someone feel free to smack me if I'm wrong, I don't swear by this explanation.
Faultline
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