The Earth, to me, is like looking into a cauldron in a steel works. At the centre, it is extremely hot liquid. However on the top, which is cooler, nearer the outside air, there are chunks and rafts of slag floating about on top of the liquid. These pieces either say separated or float together to form bigger areas. As the lower liquid cools, a skin of slag forms on the top.
The above is a simple explanation of plate techtonics, where the plates of the Earth bump into each other and sometimes break off bits. At other times, one plate will go over or under the plate which it has bumped into.