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 and move further away from the Mid-Ocean Ridge]. (Image: Keith-Wiess Geological Laboratories; Rice University)
The older, heavier plate bends and plunges steeply through the athenosphere, and descending into the earth, it forms a trench that can be as much as 70 miles wide, more than a thousand miles long, and several miles deep. The Marianas Trench, where the enormous Pacific Plate is descending under the leading edge of the Eurasian Plate, is the deepest sea floor in the world. It curves northward from near the island of Guam and its bottom lies close to 36,000 feet below the surface of the Pacific Ocean.
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When these two plates collide, one plate goes under, becoming hot liquid, and the other plate gets compressed on top, becoming the "mountain" we call volcanos.
The oceanic crust is less dense than the contential crust so the oceanic crust goes underneaththe conteinetal crust that is called subduction
when these two plates colleped another one plate goes under another plate whenever that plate melt. due to high temperture that plate convert into lava and erupt on the oceanic floor or it may be cause of trenches .
When a place carrying oceanic crust collides with a plate carrying continental crust, one plate will subduct or plunge beneath the other one. Because the oceanic crust is less dense then the continental crust, the oceanic plate will slide beneath the continental one, creating a trench. The heat of the earth's core will cause the oceanic plate to melt creating a line of volcanoes that mirror the same shape and length of the oceanic trench.