ATTERNS AND PROCESSES IN THE PHYSICAL ENVIRONMENT (b) Oceanic–oceanic plate collision – trenches and island arcs
The Mariana Trench is the deepest part of the ocean. It is over 11 km deep. The highest mountain on land (Mount Everest) at over 8 km high would fit easily into it.
When two oceanic plates collide, one is usually subducted beneath the other. Where one plate bends beneath the other, deep trenches form. These are the deepest parts of the ocean. The Mariana Trench marks the place where the fast-moving Pacific plate collides with and subducts under the slower-moving Philippines plate. Trenches are also marked by strong deep earthquakes due to the stresses that are released as the plates slide into the asthenosphere, e.g. Kobe 1995. Oceanic-oceanic plate collision zones are also marked by curved lines of volcanic
islands known as island arcs. These volcanoes form when the subducted plate descends and melts, creating rising bubbles (plumes) of magma that break through the crust above. These active volcanoes have built up on the sea floor over millions of years until they appear above the surface of the water. The Aleutian Islands, the Philippine Islands and Japanese islands have all been formed in this way.
Oceanic crust Sea Trench Continental crust
Island arc – a curved line of volcanic islands that mark the subduction zone