    Sentence 1: What was the most  important discovery that led to the  understanding that Earth's  lithosphere is separated into  tectonic plates?    Sentence 2: There is an intimate  coupling between the movement of the  plates on the surface and the  convection of the mantle: oceanic  plate motions and mantle convection  currents always move in the same  direction, because the oceanic  lithosphere is the rigid upper  thermal boundary layer of the  convecting mantle.