sentences:  The associated trenches are formed as the oldest (most western) part of the Pacific plate crust increases in density with age, and because of this process finally reaches its lowest point just as it subducts under the crust to the west of it.The IBM arc system is an excellent example of an intra-oceanic convergent margin (IOCM).IOCMs are built on oceanic crust and contrast fundamentally with island arcs built on continental crust, such as Japan or the Andes.