No matter wherever I read the process of subduction, I always find this one sentence which says "when the two tectonic plates collide the denser plate gets subducted under the lighter plate because it has less buoyancy" but it doesn't make sense at all. I mean yes one plate is more dense but it's still floating and that's what matters for floatation that it's density has to be lower than the liquid underneath it and if this condition is already there why does density have any role in initiating the process of subduction? I am really confused.
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$\begingroup$ The less dense plate wants to fliat on top. Just like the lower-density immiscible fluid floats on top. $\endgroup$– Jon CusterCommented Jun 29 at 17:01
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$\begingroup$ @JonCuster but that doesn't make sense, I am imagining that the two tectonic plates have not collided yet and just floating over mantle , so in the start itself they are just floating so we can't say that "just because one has more density it will sink" because if that's the case why was it floating earlier before the collision happened? $\endgroup$– Virender BhardwajCommented Jun 29 at 17:32
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First velocities are very low and masses are very large. Momentum ($mv$) is large, but kinetic energy ($mv^2$) is small. So when two plates crash into each other, it is a fender bender. The kinetic energy behind two entire continents is enough to raise a few mountain ranges at the collision site. They continue to move as dictated by momentum.
Second on geological time scales, rock is fluid. Two continents float on the mantle. As they collide, the potential energy to float one on top of the other and raise mountains is on par with the kinetic energy of the collision. That is, buoyancy forces at the region of contact are on par with the forces pressing the two plates together. Buoyancy will force the less dense plate up and the more dense plate down.
Once the denser plate is started down, momentum will keep it moving down.
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$\begingroup$ I didn't understand your argument...like how does buoyancy play a role here I am not able to imagine. Yes one plate is dense than other but it was still floating over the mantle before collision so what special happened at the collision that it sank? $\endgroup$ Commented Jun 30 at 8:32
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$\begingroup$ Suppose a layer of oil and a layer of water meet at a vertical interface. Would the interface stay vertical? The pressure is atmospheric at the top. The pressure is greater at the bottom of the water layer than the bottom of the oil layer. Water at depth pushes the oil back. Water winds up under the oil. It is similar for two plates. $\endgroup$ Commented Jun 30 at 13:02
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$\begingroup$ But the plates here. Are solid not liquid right? We can talk about pressure at the bottom when we are talking about fluids and that too when they have the same height but here the plates are solid so how can we talk about pressure here? $\endgroup$ Commented Jun 30 at 13:19
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$\begingroup$ On long geological time scales, rock does bend and flow. Especially at the base of the plate, where temperatures are very high. After all, moving plates are rock floating on rock. $\endgroup$ Commented Jun 30 at 13:27
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1$\begingroup$ So you are trying to say that the fluid below the oceanic crust is more dense than the fluid below continental crust and when they meet them due to pressure redistribution the continental crust will move up and oceanic crust will move down?? Am I right ? I don't know anything about geology in detail. I was just curious so I asked it $\endgroup$ Commented Jun 30 at 13:48