Explanation:oceanic plates are formed at divergent boundaries mainly at mid ocean ridges. The material that is poured out are basaltic lave in nature. Basalt is denser and heavier than the granite that makes up continental plates.
Oceanic plates are much thinner than the continental plates. When an oceanic plate meets a continental plate at a convergent boundary the oceanic plate is forced under the continental plate and destroyed. The convergent boundaries between ocean and continental plates create subduction zones. The oceanic plate is pushed under the continental plate and melted. This melted material becomes magma that may reach the surface as volcanos, or solidify under the surface as granite.
Continental plates are much thicker that Oceanic plates. At the convergent boundaries the continental plates are pushed upward and gain thickness. The rocks and geological layers are much older on continental plates than in the oceanic plates. The Continental plates are much less dense than the Oceanic plates. The Granite and recycled material are much lighter than the heavy basalt layers of the ocean plates.