Since tectonic plates are held together by lateral stress, friction, and gravity, and the Earth is a sphere, they work just like a full-circle arch or full-sphere dome. So can the crust be compared to an arch?
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5$\begingroup$ I have a hard time to project how a full circle arch relate to plates in any way. It would be great to have more context to help us answer better. $\endgroup$– marsisalieOct 16, 2015 at 14:04
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1$\begingroup$ This question is split from the question [Is it possible that the recent droughts are signs of epic crust failure?] (earthscience.stackexchange.com/questions/6725/…) $\endgroup$– Teki HakenOct 16, 2015 at 18:16
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$\begingroup$ I think you're forgetting buoyancy. $\endgroup$– Semidiurnal SimonOct 24, 2015 at 8:37
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1 Answer
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An arch is a poor analogy for tectonic plates. The lithosphere is supported beneath by the mantle, unlike an arch which is unsupported beneath. If the lithospheric plates were only held together only by lateral stress, friction with surrounding plates and gravity, how would they move relative to each other? How would forces at the plate interfaces vary? How would subduction work without the whole thing falling apart? I am not a geologist but I cannot see how this arrangement would work.
The mantle "fluid" that sits underneath the lithosphere convects and drives plate motion. It accepts subducting plates and emits new oceanic crust along the mid-oceanic ridges. The support everywhere underneath the lithosphere by the asthenosphere makes an arch a poor representation for the physics at work.
Yes, you are correct that at the tectonic plate edges there is lateral stress and friction and that the whole plate is subject to gravity. What you are missing is that the crust is everywhere supported underneath. An arch relies on the lateral stress to stay standing while tectonic plates do not.
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$\begingroup$ I hesitate to fully accept your answer because the mantle flow only "carries" (more like pushing up hard and destabilizing) the full weight of oceanic crusts at the ridges. The subduction zones show that the littoral plate is like an arch resting on top of the subducting plate that slides under. Most of the littoral plate's weight should be carried by the oceanic plate at their point of contact (the subduction zone). The mantle below the littoral plate does not look like it is carrying the full weight of the plate. However, I agree full crust=arch is poor comparison. $\endgroup$ Oct 16, 2015 at 22:18
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2$\begingroup$ @TekiHaken what are you basing your claims on? $\endgroup$– GimelistOct 17, 2015 at 10:47
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2$\begingroup$ @TekiHaken The subduction zones don't show that at all. A subduction zone is simply a place where one plate is forced below another. This isn't the only place the crust touches the mantle - you're picturing the plates as rigid when in reality they can flex. The lower plate is deformed and bent downward under the one that's sliding over it, and forced deeper into the mantle beneath. The main force holding the plates up is their buoyancy relative to the mantle. $\endgroup$ Oct 28, 2015 at 15:31