A long, narrow, but very deep depression in the ocean floor where two adjacent plates converge, and one descends beneath the other. For this reason ocean trenches are also known as convergent plate boundaries. ‘The arc like structure of many ocean trenches can be qualitatively understood by the ping-pong ball analogy. If a ping-pong ball is indented, the indented portion will have the same curvature as the original ball, that is, it will lie on the surface of an imaginary sphere with the same radius as the ball. The lithosphere as it bends downward might also be expected to behave as a flexible but inextensible thin spherical shell’ (D. L. Turcotte and G. Schubert 2002). An alpinotype orogeny is also termed an ocean trench orogeny.