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The ocean has some of the deepest locations on earth, typically caused by subduction zones. Why are there no subduction trenches on land?

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Continental crust is thick and relatively buoyant. Ocean crust is much thinner, and relatively dense.

Hence ocean crust "goes down" more easily, and the density can even help pull the ocean slab due to metamorphism.

In contrast, the continents are almost like floating scum which won't go down! When they collide, you get faulting and folding, and the building up of very thick continental crust. Tibet is the best example today. Yes the Tibetan Plateau is high, but the crust is over 100km thick! (think of an iceberg, most of it is below)

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  • $\begingroup$ Also, trenches on land tend to get filled by erosion. $\endgroup$ – Erik Feb 3 at 10:12
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As mentioned in another answer:

Continental crust is thick and relatively buoyant. Ocean crust is much thinner, and relatively dense.

Let's look at a simulation of what might happen. Here's a figure from "Continental collision and slab break-off: A comparison of 3-D numerical models with observations":

enter image description here

Let's look at this in detail.

You start by having a lithospheric plate consisting of oceanic and continental crust, going to the right, and forming a subduction zone under another lithosphere plate carrying continental crust:

enter image description here

Then the oceanic crust keeps going under into the mantle, eventually reaching a stage where both continental crusts are now in contact:

enter image description here

Finally, the oceanic crust tears or breaks off the continental crust part, and sinks into the mantle, with the surrounding asthenospheric mantle filling up the space. There's no dense oceanic crust to pull everything down to form a trench now, so instead you have mountain ranges and the two plates are amalgamated.

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  • $\begingroup$ also known as the Himalaya $\endgroup$ – John Feb 4 at 4:56

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