Subduction zones are where oceanic crust is destroyed. and returned to the mantle. When cold dense oceanic crust converges with less dense oceanic or continental crust it sinks back down into the mantle at a trench and is reincorporated into the mantle.
Mantle convection currents combine with slab pull to keep the plate moving downwards.This can form a destructive boundary 1000km s long. A pattern of earthquakes with increasing depth follos the descending slab- Benioff zone. As the plate descends it takes sea sediments with it or scrapes them off as an accretionnary prism. The sediments and minerals in the slab release fluids which cause the mantle above to melt and magma to rise to the surface. Where it erupts it forms a volcano.
The Pacific ring of fire is a 40 000km/ 25 000 mile horseshoe shaped subduction zone which bounds the east,north and western sides of the Pacific ocean. The Pacific ocean is being subducted on three sides forming deep ocean trenches and the volcanoes of the Andes,Mexico,Cascades,Alaska,Kamchatka,Japan,Phillipines,Indonesia and New Zealand. 75% pf the worlds volcanoes are here.