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When two oceanic plates collide, the younger of the two plates (because it is less dense)* will ride over the edge of the older plate. *(Oceanic plates grow more dense as they cool or move further away from the Mid-Ocean Ridge). The older, heavier plate bends and plunges steeply through the athenosphere, and descending into the earth, ir forms a trench that can be as much as 70 miles wide, more than a thousands miles long, and several miles deep.