Ah. A review of your past posts suggests you're writing a sci-fi story.
Still, a quick wiki of plate tectonics or continental drift would probably be better than having to give us all credit in your book for doing your basic research for you.
Cold is the short answer. Ice cores from Greenland are pretty compelling. Earth was in one of its frequent ice ages back then. Volcanic eruptions, and possibly collisions with asteroids/comets, appear to trigger ices ages.