- Where two pieces of oceanic crust on different plates collide, one is subducted beneath the other.
- The crust that is subducted may be marginally denser of the two plates or is the one which is moving more quickly.
- These form characteristically curving lines of new volcanic land known as island arcs such as Aleutian Islands with deep ocean trenches, Marianas Trench in the Pacific Ocean which is more than 10km deep.
- Such island chains may develop over millions of years to become major landmasses such as Japan and Indonesia. Subduction produces frequent shallow deep focus earthquakes, some of which are immensely powerful 2004 Asian tsunami 9.0 on Richter scale.
