When two tectonic plates collide/converge, usually a continental and an oceanic plate, the oceanic plate is submerged by the tectonic one because it is denser. The plate dives into the mantle and is consumed. The boundary, therefore, keeps ‘eating’ up the oceanic plate hence reduces it over time. An example is the Mariana Trench (a subduction zone) in the Pacific ocean. The Pacific ocean, therefore, continues to shrink over time.
On the other hand, divergent boundaries (such as the one in the Atlantic Ocean) make new crust.
