5 upvotes, 3 direct replies (showing 3)
View submission: Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science
Why are all subduction zones underwater?
EDIT: Thanks for the awesome information!
Comment by SaiphSDC at 20/07/2022 at 15:43 UTC
18 upvotes, 0 direct replies
Subduction zones require a more dense rock to impact a less dense rock, and this causes the dense rock to descend.
The dense rock will sit lower, and is descending, creative a low region for water to flow into.
Rocks of the same density colliding push up into mountain ranges, which of course rises above the water and causes any water to flow away.
Comment by I_likeIceSheets at 20/07/2022 at 18:43 UTC
9 upvotes, 1 direct replies
Finally an Earth Science question!! For subduction to occur, there needs to be two plates of Earth's crust with different densities. In other words, one plate has to be denser than the other. The denser plate is the one that sinks and the more buoyant plate is the one that prevails. The newer oceanic crust is simply made of denser rock than the older continental crust, so the oceanic crust sinks.
Comment by turtley_different at 20/07/2022 at 23:22 UTC*
3 upvotes, 1 direct replies
They aren't. India is subducting under Asia albeit inefficiently due to the similar density of continental crust (which, ultimately, forms the Himalayas and the complex duplexing under them).
However it is true that most subduction zones are underwater.
Firstly, this is because oceanic crust is a denser than Continental crust, therefore, in resolving the forces at work continental plates, compression is best accommodated by subducting an oceanic plate somewhere in the system (and the oceanic plate will be underwater)
Secondly, over long periods of time continental subduction is not favourable as you are trying to subduct a buoyant material. However, a subducting oceanic plate undergoes metamorphic transformation and becomes denser (more dense than the surrounding mantle) making subduction of oceanic plates self-reinforcing and thus more likely to persist for long periods of time.
Overall, oceanic plates are more likely to start subducting, and will stay subducting for longer. Therefore the overall earth system has far more subducting faults underwater than between continents.