What may happen if the entire crust of Earth was one plate?

Just a guess: If the entire crust of Earth was one plate, then Earth MAY be flat, and revolve in place either horizontally or vertically. Like I said, I’m guessing, because I have no clue how gravity would act upon this theoretical flat Earth, what it would cause.

Many volcanoes breaking through the earths crust. The plate wouldn’t have room to move with the conduction of lava underneath. It would be like bending your knee with a big scab. It would crack and flex up or down. In short, it wouldn’t work out very well