Merely what the title states. My knowledge of geography/geology/*logy is limited to high-school, and some snippets, and snatches of conversation.
As I understand Earth's magnetic field is attributed to the bi-layered nature of it's core. Proto-Earth probably did not have a magnetic field. Apparently much later Earth's core cooled enough to separate into two layers - causing the field to appear
How long did it take for Earth's magnetic field to first appear? As a corollary, did water appear first on Earth, OR did the magnetic field appear first?