old oceanic crust is consumed where
Crust is consumed in subduction zones. When two plate move together, one of them must slide under the other one. This is called subduction. When an oceanic plate and a continental plate meet, the denser oceanic plate subducts under the less dense continental plate. The subducting plate slips down into the mantle where it is heated, melts and is recycled. Thus spreading centers and subduction zones act like a conveyor belt, manufacturing crust at one end and consuming it at the other. The deepest parts of the ocean occur in trenches along subduction zones.