Subduction Zones

  Where two tectonic plates converge, if one or both of the plates is oceanic lithosphere, a subduction zone will form.  An oceanic plate will sink back into the mantle.  Remember, oceanic plates are formed from mantle material at midocean ridges.  Young oceanic lithosphere is hot and buoyant (low density) when it forms at a midocean ridge.  But as it spreads away from the ridge and cools and contracts (becomes denser) it is able to sink into the hotter underlying mantle.  Three key features are associated with subduction zones: a deep ocean trench, a volcanic arc on the overriding plate parallel to the trench, and a plane of earthquakes, shallow near the trench and descending beneath and beyond the volcanic arc.