Subduction happens when

Subduction, Latin for "carried under," is a term used for a specific type of plate interaction. It happens when one lithospheric plate meets another—that is, in convergent zones —and the denser plate sinks down into the mantle. Continents are made up of rocks that are too buoyant to be carried much farther than about 100 kilometers deep.

Explanation:

answerhydrogenous sediment;