Subduction Zones
When two oceanic plates collide head-on at a convergent boundary, one is subducted beneath the other and
returns to the mantle, completing the tectonic cycle begun when the oceanic crust initially erupted at a mid-
ocean ridge.
Anatomy of an island arc-trench system.
Arc trench pap
Volcanic island arc
Fore-arc
Accretionary
Trench
Oceanaic Sichesphere
Melting occurs in the mantle above the subducted plate when the plate reaches depths of 100 to 150 km. As a
result, the volcanic arc associated with a subduction zone forms at some distance from the deep-sea trench.
The area between the volcanic arc and the trench, called the arc-trench gap.
1. Based on island arc-trench geometry (see above) and the fact that melting typically occurs at about
the same depth in all subduction zones, what seems to be the major factor that controls the arc-trench
gap distance?
