When two plates converge, the type of plate boundary that forms is largely determined by the densities of the two plates involved. Oceanic crust is denser than continental crust, so when an oceanic plate converges with a continental plate, the oceanic plate is typically subducted beneath the continental plate. This type of plate convergence is called subduction.

When two continental plates converge, the plates are often of similar densities, so neither one is subducted. Instead, the plates collide and deform, creating mountains and other geological features. This type of plate convergence is called continental collision.

The density of a plate is determined by its composition. Oceanic crust is composed primarily of basalt, a dense volcanic rock. Continental crust is composed of a variety of rocks, including granite, which is less dense than basalt. The presence of water in the pores of continental crust can also make it less dense than oceanic crust.

The difference in density between oceanic and continental crust is a major factor in determining the type of plate boundary that forms when two plates converge. Subduction is the most common type of plate convergence, and it occurs when an oceanic plate converges with a continental plate. Continental collision is less common, and it occurs when two continental plates converge.