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We love turtle worlds. We already have a question about turtle planets : How do I prevent my turtle from collapsing under its own gravity? but let's scale it down.

I would like to have a waterball planet inhabited by gigantic oceanic creatures. The biggest (or not ?) creature of them all is a turtle which lives with the top of its shell above water surface. Those turtles have such a long lifespan that their shell is subject to erosion and a deep soil forms on top of it. You eventually get mountains, lakes, forests... A beautiful landscape and a nice background for a fantasy story.

If possible, I would also like the creature to move slowly (swimming or maybe walking on the ocean floor ?) such as once in a few centuries, civilizations from two different turtles can meet and exchange culture, genetic diversity and deadly diseases.

They don't need to be specifically turtles, or even looking like turtles if not possible. They however require to be alive and sentient, because I need them into my magic system (a mother nature sort of thing where you "ask" for magic).

Would such a creature be possible with as few handwavium as possible ? If absolutely necessary, magic is available, with whatever properties you need. The less magic the better.

The maximum turtle size I need is about 1000 - 2000 miles / 1500 - 3000 km. (that's Australia-sized) It's okay if only smaller turtles can move, and the biggest/oldest (dead-est ?) turtles are stuck to one place. If such a size is not possible, how large is the biggest possible turtle ?

How old would a turtle be in order to get nice erosion and landscape ?

What lanscape features would be available, or not, on such structure ? Small mountains, lakes... ?

For those two questions, keep in mind that we are on a waterball planet, with powerful wind and storms as explained in this question : Ocean and air currents on a water planet

If that's two many questions in the same post, or two broad, please let me know. And thank you for your time !

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    $\begingroup$ Related: worldbuilding.stackexchange.com/questions/317/… $\endgroup$ – Tim B Oct 25 '15 at 15:51
  • $\begingroup$ @TimB right, I forgot this one, thanks ! The question here is not exactly the same, as part of my creature needs to be above water, but still strongly related. $\endgroup$ – Petit Lama Oct 25 '15 at 16:37
  • $\begingroup$ Do your turtles have to be living members of Testudines, or can these be metaphorical turtles, like segments of crust moving around on a mantle which exhibit some living properties? $\endgroup$ – Cort Ammon Oct 25 '15 at 16:44
  • $\begingroup$ @CortAmmon I'd like them to be alive, with a conscious mind, but they don't need to be turtles properly. $\endgroup$ – Petit Lama Oct 25 '15 at 16:46
  • $\begingroup$ Another question: you say they move slowly, so that cultures may meet every few centuries. What's your concept of "slow?" If there's a lot of mass to these turtles, they might need to move on the plate techtonic sclae of movements, in which case breaking barriers between humans every few centuries is tortise-and-the-haire haire fast, blazing through the oceans. $\endgroup$ – Cort Ammon Oct 25 '15 at 16:59
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When I read this, I absolutely had to develop something that was turtles all the way down. Hopefully it fits well enough with your magic system.

The time it takes for erosion to occur is really based on the materials involved. Keratin and bone erode much faster than stone. However, a world where the landmasses are nothing but keratin would be boring. Maybe there's a way to make their shells stone. Also, if they were stone, large landmasses would be arbitrarily easy to make. The stone shell would simply fuse with the ocean floor, creating a stationary continent just like we have today.

So let's give these turtles a reason to put on some serious rock-based sunblock. Let's run with the idea that they're part of a magic system, and need to hold onto magic. Perhaps their magic is directional, and you can take control of their magic from above. They develop a hard top shell to prevent magic from being stolen. This works for small turtles, but as they grow, their magic is going to be more valuable. Bone and Keratin are simply not enough to protect them from those who would steal it. They need something stronger, like stone.

Fusing stone is not easy. It takes a lot of heat, and then you have to deal with that ridiculous heat after you're done forming a stone layer. To combat this, the larger turtles may develop chutes up through their body, lined with fireclay or similar refractories. Then all they must do is wander the ocean until they find a volcanic vent, and direct the lava on top of them.

Now this is fine for small turtles. We understand how reasonably sized creatures might move around. Obviously, as we get more and more massive, challenges of buoyancy will arise. The turtle will have to be able to hold up its stone shell while it moves. And this is where the turtles all the way down come in.

Think back to finding Nemo, with Crush the turtle. Fearless, laid back, and not going to be stopped for anything. If Crush were to get too heavy to swim, do you think he'd lie down on the couch with a beer and lament his misfortune? No. He'd find a way to surf anyways!

Enter the lava of the planet's mantle. It's fluid...ish. For a heavy turtle who refuses to give up, it might be the best surfing available to him, so why not! With a lot of care, he might be able to redirect the plate techtonics to slowly shift him around, surfing not in the ocean, but on the planet's inner layers themselves! Never underestimate the willpower for a surfer to find a way to surf!

Now we have the tools needed to work with the incredible size of these turtles, because they are no longer having to support themselves with organic compounds. They have used the power of molten rock to build supports around themselves, so standard geology rules can apply from there on. Of course, this surfing isn't always easy. Its hard to find the right magma flows to surf that wont leave you stranded when they cool. Fortunately, Crush loves to teach the next generation. He moves along the mantle, shaping the volcanic flows below to give the younger turtles a place to realize they can surf the mantle. Some give up, of course and become stationary islands with no turtle under them, which are rapidly eroded away by the forces of time. But some learn to surf the mantle in their own right.

Science based? Maybe. Right now, I'd say it'd be hard to believe that you could surf the mantle, but if you had an entire species of turtles exploring the best ways to make it happen, who knows?

And maybe one day one of them learns to surf the currents nearer to the core. Maybe they can hear whispers of our planet that we cannot, and transmit that upwards, giving us the magic to surf one day.

Hang ten!

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    $\begingroup$ Wow. Now that's metal. Magma surfing turtles certainly do have an appeal ! And the vertical-leaking magic do work : humans living on the turtle are in the right place to make use of magic leaks into the shell. $\endgroup$ – Petit Lama Oct 25 '15 at 17:36
  • $\begingroup$ They'd need to be magic not to get fried by the lava :) $\endgroup$ – Tim B Oct 25 '15 at 17:44
  • $\begingroup$ @TimB That woudl help! My original thought was that they'd be willing to work with the lava the same way we do: through protective clothing. If there's enough rock between their sensitive bits and the lava that they're trying to surf, there's room to manage temperature gradients, just like a glassblower when working with orange-hot glass. It wouldn't be quite the same level of contact with the surf as, say, nothing but a board between you and the ocean, but I like to think Crush would make it work =) $\endgroup$ – Cort Ammon Oct 25 '15 at 17:57

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