An oceanic-oceanic collision is when two tectonic oceanic plates collide. This convergence creates a subduction zone, which is where one plate is submerged below the other.
When the two plates collide, the more dense of the two is forced below the less dense. As this plate is submerged an area is formed, called a benioff zone. This is when the plate is forced into the mantle where it experiences high-intensity heat. This in turn causes the various elements that make up the plate to melt. As the plate makes its descent into the mantle, earthquakes are triggered at varying depths.
While this is happening, magma pockets are created that range in size depending on the intensity of the earthquakes happening in their vicinity. When magma pockets are large enough, they erupt through to the surface and form volcanic cones. Once these cones are large enough they emerge at the ocean surface, creating island chains and land masses. Famous areas such as Hawaii, Japan and the Caribbean were created by these forces. In addition to earthquakes and volcanic eruptions, these oceanic-plate collisions create trenches and also cause destruction to the ocean's lithosphere, one of the top layers of the ocean floor.