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.

Trench Flipping
If the descending oceanic plate is carrying a continent, the less dense continental material cannot sink, so it dives into the trench behind the leading oceanic crust until it gets stuck. This crumples its leading edge into folded mountains and causes some of the oceanic crust of the overlying plate to be deposited on top of the continent. Pressure steadily builds up until the trench “flips,” and the previously overriding oceanic plate dives under the continental crust. This could explain why most ocean trenches are found along the edges of continents.

If a trench has flipped because of the arrival of a continent, and the newly subducted plate also carries a continent, a collision of land masses is unavoidable. When this happens, subduction terminates along the collision zone, the trench disappears, and the continents collide, resulting in the birth of a new mountain range.

Sometimes an entire plate can disappear if the plates’ leading edge is being consumed in a subduction trench faster than new crust is being added at the ridge on its trailing edge. When this happens, the ridge slowly moves toward the trench and the whole plate is eventually drawn down into the mantle, causing a global rearrangement of other plates and their borders.
   
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