Ocean trenches happens at areas where two plates collide (converge). As convectional
currents in the upper mantle converge and sink, this causes one plate to move towards the
other and two plates collide. Plates collision causes denser oceanic crust
(oceanic plate) to sink or slip beneath lighter continental crust (continental plate).
Hence this is a zone of subduction, compression and destruction. Oceanic crust/plate is
made up of heavy material (baslatic rocks). Along a collision plate boundary, thinner,
denser oceanic crust is squeezed down and subducted along the subduction zone and melts in
the magma of the upper mantle (asthenosphere). It is consumed, hence destroyed in the
mentle beneath the continent (along the Benioff zone). Subduction produces a deep Ocean
Trench on the ocean floor, parallel to the plate boundary.
Examples are:
- The Marians Trench (over 11 km deep) in the west Pacific Ocean is produced by the subduction of the Pacific plate beneath the Philippine Plate. Oceanic crust is pushed into the mantle.
- The Peru-Chile ocean trench (off the west coast of South America) is formed when the
Nazca Plate
and the South American Plate collide. The former slips beneath the latter.