There are a couple of answers to this:
A: A lot of people back then were heavily Catholic, the Catholic church claimed that the Earth was flat, while others believed the Earth was round.
B: Back then, obviously they didn't have airplanes or satellites, thus people believed the Earth was flat. The reasoning of this is because, if you were to look outside right now, the Earth does look like it could be flat. (that is because you have to be at a certain level of elevation to see the curvature of the Earth). Eventually though, people started reasoning and came to the conclusion the Earth was round. (They were proven right when aerospace technology came around.)
The molten material that comes out volcanoes is magma.
I believe one minerals that cleaves into flat sheets are called Mica. there are others too, though.
