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For example the Northern Africa is getting closer to Europe, where the depth of the sea is lower than in south African coast

Could it be that the water's pressure pushes the continent from where the sea is deeper?

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    $\begingroup$ No. The water pressure from the ocean is minimal in terms of what is needed for moving the plates. Water pressure at the bottom of the ocean is perpendicular to the bottom. What is needed is horizontal tensions. $\endgroup$ – arkaia Aug 31 '17 at 11:54
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    $\begingroup$ If that were the case, how would a supercontinent like Pangaea ever have broken up? $\endgroup$ – Nuclear Wang Aug 31 '17 at 15:07
  • $\begingroup$ @aretxabaleta Could envision where folks could follow the thinking that vertical water weight combined with a slanted coastline could actually cause a net force horizontally. Perhaps the comeback would be that the slope of the coast is minimal on the large scale. But could still see how any such force, even if fairly small, be a driving force if it's the dominant force horizontally... and could yet see the further draw of the idea in thinking that it lines up well to the very small velocities (from our typical perspective). Would be great to see a more complete breakdown for an answer! $\endgroup$ – JeopardyTempest Aug 31 '17 at 15:09
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    $\begingroup$ @GabrieleCitossi Props on the creative ideas, but no. Convection in the mantle is what moves the plates. The mantle convects because it's the most efficient way to transfer heat and the continents float, in a sense and drift on top of the convection This is well established. $\endgroup$ – userLTK Aug 31 '17 at 18:45
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    $\begingroup$ Good. Mine was only an hypotesis $\endgroup$ – Gabriele Citossi Aug 31 '17 at 22:33
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Water pressure on both sides of the continent makes sure that all horizontal forces cancel out. So water pressure will not move a continent to the left or right because the pressure on the other side exerts the same force just in the opposite direction (assuming equal water depth). But the water pressure puts the continent under compression and will push the two sides of an ocean apart (putting the oceanic plate underneath under tension). In total, however, these forces are far too small to have any impact on plate tectonics.

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    $\begingroup$ So when you say "will push the two sides of an ocean apart", you're not saying that it actually results in that, just that it has a small force that would do that if not outweighed by other factors? $\endgroup$ – JeopardyTempest Sep 1 '17 at 2:16
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    $\begingroup$ Yes, correct: there is a force that pushes the two sides of the ocean apart, but the forces are far too small to actually cause some motion because the oceanic plate below is strong enough to counter the force. It's like if you put water in a bowl: the water pressure pushes the sides of the bowl outward, but the bowl is in general strong enough not to actually let the sides move. $\endgroup$ – Wolfgang Bangerth Sep 12 '17 at 1:53
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This would not explain many observed movements. For instance, the part of North America west of the San Andreas fault is moving northwards WRT the land east of the fault. Similarly, the Indian subcontinent was once an isolated land mass. Water pressure would thus have acted equally on all sides, and it wouldn't have moved to collide with Asia and so raise the Himalayas.

Further, the value of a theory is in its explanatory power. Conventional plate tectonics explains a lot of things: the pattern of sea floor spreading seen around mid-ocean ridges, the geology of rocks at subduction zones, the location of volcanos, and more. The water pressure theory addresses nothing except the movement.

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  • $\begingroup$ Well, but plate tectonics also doesn't exactly a lot. It says that plates move, and we can explain surface features this way. But plate tectonics itself doesn't explain why plates move. Thinking about water pressure as one way to make plates move is a legitimate -- though ultimately incorrect -- attempt at explaining the missing cause of the motion. $\endgroup$ – Wolfgang Bangerth Sep 12 '17 at 1:55
  • $\begingroup$ @Wolfgang Bangerth: In fact the plate tectonic theory DOES say a lot more than just that the plates move, including why they move. That's why Wegner's original continental drift theory (early 20th century?) didn't get much support: no one could explain how the continents moved, despite evidence that they had. It wasn't until half a century or so later, and the discovery of seafloor spreading from the mid-Atlantic ridge, that the mechanism began to be worked out. Really, this should be explained in any introductory geology text! $\endgroup$ – jamesqf Sep 13 '17 at 2:57

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