Continental crust is much lighter in weight than oceanic basaltic crust. So when oceanic crust collides with continental crust, the oceanic plate subducts downwards. At a certain depth the oceanic crust heats up and melts the continental crust above it. If this melted continental crust reaches the surface, you get volcanoes spewing lava that further increases the height of the continental plate.

When the earth first began to solidify the lighter weight minerals would have floated to the top, so there may have been a short period of time when the earth was covered with continental crust, but rising basaltic magma and volcanic hotspots would have quickly broken the plates up and plate tectonics would have begun.