Subduction – When one tectonic plate is partly forced beneath the other as the two collide

The submarine ducked down under waves after the captain shouted, "Action (subduction) Stations!" She was forced beneath the ice cap.

Subduction occurs when a less dense tectonic plate collides with a denser one. The denser and heavier tectonic plate is forced underneath and is melted by the magma in the earth's mantle.

When two thin oceanic crusts converge (called destructive plate margin) the denser crust will subduct.

When destructive plate margins in the ocean occur, trenches form and so too do volcanic islands, all along the plate margin.

When a thin oceanic plate converges with a denser continental (land) plate  (again, called a destructive plate margin) the oceanic crust will subduct.

Again, subduction is when one crust is forced beneath the other as the two collide.