These move with respect to one another. The process that allows this is sea-floor spreading. Once formed it moves away. As a consequence, an ocean grows wider with time.
Process is called subduction.
When the plate sinks, it heats up and releases moisture. Addition of this moisture to the asthenosphere causes melting. At transform boundaries, one plate slides past another. As ocean basins grow or are subducted, continents are rafted around. In other words, continents drift around the Earth's surface, and the map of the surface constantly changes.