Valley becomes a long, narrow sea. Crust thins and volcanic activity occurs as molten rock breaks through crust. Volcanic activity forms undersea mountain ranges called oceanic ridges . Convergent Plate Boundaries Plates move toward each other. When an oceanic plate and a continental plate converge, the oceanic plate slips under the continental plate in a region called the subduction zone .
Oceanic plate bends downward, producing an oceanic trench .
The deepest known place in the ocean is Challenger Deep in the Mariana Trench . The subducted plate partially melts to form magma. Magma rises to surface of the land, resulting in volcanic activity. Volcanic activity produces mountain ranges called volcanic arcs . As the oceanic plate subducts, sections of rock hang up but eventually shatter.