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For my fantasy world, I have what I think is a really cool idea. The world is flat, and elliptical in shape. If you touch the silvery edge, well, let's just say bad things will happen. (Not relevant...) This edge is an impenetrable, indestructible wall.

This world is like a coin, with two sides. Each side is the same (for the purposes of the question), and the giant island slopes downward until it meets itself. This leaves most of the ocean without a solid floor.

Assume that gravity is magically inclined to drag everything down to the equator of the "coin". with 1g of force, and this gravity is the same all around.

The world has a high "ceiling" of sorts, above each surface, uniformly heating and lighting each side of the coin. ~72°F

That is the full extent of the world, no sun, moon, or stars for you to be concerned about.

If an object sunk into the sea, it would sink until it reached the bottom of the oceans, a mile from each surface. A fish happily swimming on one side would appear upside down from the other.

My question is, how would aforementioned changes affect the ocean?

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    $\begingroup$ Could you elaborate on the mirroring? Like is the continent double sided and the ocean is just a layer of water sandwiched by gas , etc ..? $\endgroup$ – Zxyrra Dec 21 '16 at 4:52
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    $\begingroup$ @Aify Of course they can do what they want but it's still possible to answer "given this, you'll get that" as opposed to "you used magic and made it all up so you might as well ignore the question and keep making it up"; a question is a question, and that's kind of what we're here for. $\endgroup$ – Zxyrra Dec 21 '16 at 6:16
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    $\begingroup$ Does it spin, rotate or otherwise or is it entirely static? $\endgroup$ – Separatrix Dec 21 '16 at 9:15
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    $\begingroup$ Also, does it have a moon? $\endgroup$ – Separatrix Dec 21 '16 at 9:16
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    $\begingroup$ If the ocean is just a sheet of water, are all the deep sea fishes just shallow water fishes swimming upside down on the other side? If a ship sinks does it pop up to the surface on the other side? $\endgroup$ – kingledion Dec 21 '16 at 13:21
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This will change both the continent and the sea in unusual ways.

1) The ocean floor will create itself anyways.

Bear with me on this one.
If your world has light (solar radiation), it will have a water cycle, because water molecules will be warmed to the point of vaporization. The water cycle drives weathering and erosion, which wears down the land over time.

As sediment slides off the continent on either side of the "coin", it will sink to the lowest point - where both faces of the world meet at the bottom of the sea. Congruent particles will meet each other and remain at the boundary; because the worlds are exact, mirrored copies, matter will run into its counterpart when it reaches the bottom.

Most sediment that reaches the bottom will remain there, because there are no tectonic forces at the boundary that would lift sections of the seafloor back up. Sedimentary rock will form over millions of years, then become metamorphic due to pressure; a muddy floor with rock underneath will inevitably exist.

2) This means the continent will be flatten and shrink over time.

Tectonic and volcanic forces are primarily driven by the convection of rock deep within a planet. Since this world is only a mile thick, it will not have tectonic plate movement, nor will it be able to support any large volcanoes. Therefore, no new landmasses can form - and the continent will continue to erode from the outside-in until the entire world is a shallow sea.

3) The center of the continent will be an inhospitable desert.

Similarly to how super-continents in the past (on Earth) harbored central deserts, your continent will likely be the same. Water is too distant, and most precipitation falls closer to the coast. It's worth noting that this desert will neither be hot nor cold - it should be at about the same temperature as the rest of the world, since there aren't seasons in a world with no axial tilt - but it will be very, very dry.

4) Many sea creatures will be fine.

Plankton and other microorganisms will still be able to thrive, because the light and nutrients they rely on will still be present in this world. Krill and other creatures that feed on these will in turn not be affected, and so on - so you can still maintain a healthy food chain in this world if it's based on those in the shallows on Earth.

5) Say goodbye to deep-sea ecosystems.

This seems most obvious. Creatures will not evolve to be suited for an extreme lack of light, nor will they evolve to feed on the heat and nutrients found in deep-sea vents. As a result, many large Earth creatures that dive deeper may not be able to survive on this world.

6) Two-world creatures may evolve.

Before the seafloor is sufficiently thick, some creatures may take advantage of protection on one side - the world boundary - and develop so that they're attached to their other-side counterparts! While matter cannot be exchanged, and both halves will require energy, decreasing the efficiency of such a system, they may be able to conserve heat energy.

7) There probably won't be any tides.

On Earth, the pull of our moon is responsible for moving large amounts of water, producing tides. It seems like this world will not have a moon, so you may expect to see somewhat smooth sailing.

8) Wind may be wonky.

One of the important wind-driving pressures we have on Earth is the Coriolis Effect. It's caused by the rotation of the Earth, and results in somewhat spiral-like wind patterns.

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  • $\begingroup$ Just one thing, before sea-floor solidifies, atomic random drift should be able to cause diffusion of both oceans into each other, any very early organisms should also be able to cross the boundary with some effort. Furthermore, our mines reach just below 1 km, so with a bit more advanced technology, it would be possible to create passage to the other side. $\endgroup$ – M i ech Dec 21 '16 at 9:22
  • $\begingroup$ @Miech They described two sides being "exact" copies, mirrored over the boundary; I took this literally and used it to say that all objects will meet their counterparts if they attempt to cross. I'm sure there are other interpretations and if the OP did not say "exact" I would agree that animals and particles would cross. $\endgroup$ – Zxyrra Dec 21 '16 at 12:38
  • $\begingroup$ @Zxyrra if the world orbits a sun, there will be noticeable tides - weaker than tides coming from moon as big as earths, but still definitely not "any". Well, as long as the sun is a star, and not magical lantern. On a world that gets flatter every day, they may reach deep into shore, even if they are not too violent. $\endgroup$ – Borsunho Dec 21 '16 at 15:11
  • $\begingroup$ @Borsunho I considered that, and if the world orbited a star that would be true. However, in order to have both sides stay exactly the same, they need to receive equal light and gravity. This means orbiting a star - even a binary - will not work. I assumed magic lanterns as a result. $\endgroup$ – Zxyrra Dec 21 '16 at 16:56
  • $\begingroup$ @Zxyrra I see. I have taken "exact" copies to more of a initial condition statement than an actual constant relation, since that would require some weird interaction, if both sides were supposed to always exactly mirror each other. $\endgroup$ – M i ech Dec 21 '16 at 23:11

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