Plates move apart. Rising hot rock pushes crust upwards. Crust stretches, warps, and cracks. Slabs of broken crust collapse to form rift valleys . Rift valleys lengthen and deepen, and eventually meet the ocean.
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 .