Obviously plates cannot continue to expand infinitely, and eventually plates must rejoin to form convergent boundaries. There are two different types of crust, continental crust which is relatively light (2.7) and oceanic crust which is denser (3.0). Because of these two types of crust, there are also two types of lithosphere, denser oceanic lithosphere and less dense continental lithosphere. The oceanic crust material welled up from the mantle; therefore, it can be easily cycled back into the mantle. Continental crust, reduced in density by weathering, differentiation and other chemical processes, is more buoyant and cannot sink back into the mantle. 

The specific features associated with a particular convergent boundary depend largely on the two types of lithosphere which meet. Given that there are two types of lithosphere, there can be three sub-types of convergent boundaries. In the first case, oceanic lithosphere meets oceanic lithosphere. Where two oceanic plates meet, one of them will dive back (be subducted) into the mantle. Where oceanic lithosphere meets continental lithosphere, the continent is too buoyant to reenter the mantle and it is the oceanic plate that will be subducted. Finally, if two continents meet, neither plate will be recycled and the resultant collision will join the two plates into one.