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I am trying to find an explanation to a planet in which the gravity is not constant, thus allowing some continents to be situated 2-3 km above surface while others are "grounded" on the surface.

The planet also has 2 kinds of rain because of the gravity:

  1. Normal rain: falls as in Earth from the clouds to the surface.
  2. Unusual rain (relative to Earth): from the surface up to aliment the continents that are above the surface.

I have read the question A planet with changing gravity, however I think it is different to this question.

Edit: The continents float due to gravity being different in certain part of the planet. There are not mere islands. It is like having Asia and Europe floating above ground while Africa, Antarctica, the Arctic and the Americas are grounded on the surface

Edit 2: As stated on the comment section to @Renan's answer, the explanation may be outside of Physics, Astronomy, etc as we know them, but not should not involve magic.

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    $\begingroup$ I'm having a hard time figuring out what you're really asking. Are you asking how you can have "floating islands"? If so, then you need a lot more than just tweaking gravity a little... Could you please Edit to clarify what your goal is? $\endgroup$ – a CVn Dec 19 '18 at 14:16
  • $\begingroup$ @aCVn I have edit the question. Floating continents more accurate than floating islands $\endgroup$ – Reddy Lutonadio Dec 19 '18 at 14:36
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A floating continent without magic would mean the continent is in orbit. That presents different problems like, if it's high enough to be geosynchronous orbit, nobody can breath, and if it's low enough to breath, the continent is traveling at a constant 28,800 km/hour. You can imagine the issues with that.

So you are left with magic.

A possibility, hand waved planetary evolution.

A binary planet with magnetic deposits in certain land masses. The planets are extremely close to each other, and the magnetic field of one planet, pulls up entire landmasses on the other.

You could use this to make a very unique world, but you will need to handwave or magic out how these planets ever managed to exist without colliding into each other or breaking up. But this gives you some loose science up front, that falls apart on closer inspection (like star trek warp drive)

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    $\begingroup$ That would be spectacular :) Sadly, inhabitants of that planet would disagree. $\endgroup$ – Artemijs Danilovs Dec 19 '18 at 17:39
  • $\begingroup$ @ArtemijsDanilovs Whoops! I fixed the numbers in my answer. $\endgroup$ – Trevor D Dec 19 '18 at 17:41
  • $\begingroup$ Add to that: if they can breath, then the friction with the atmosphere will cause the continent to burn like a meteorite and then crash upon the planet's surface. $\endgroup$ – Renan Dec 19 '18 at 18:01
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    $\begingroup$ @ReddyLutonadio It's your choice which answer to accept, and when. We do however generally recommend waiting at least a day before accepting an answer, as questions with accepted answers tend to receive less attention from others in the community. Having an accepted answer could in principle deprive you of even better answers. You can unaccept and reaccept at any time. $\endgroup$ – a CVn Dec 19 '18 at 18:17
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    $\begingroup$ Alternatively magnetic repulsion could be suspending the continents in the air. But again a bit farfetched. $\endgroup$ – Tyler S. Loeper Dec 19 '18 at 19:04
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You can have continents at different elevations without wildly varying gravity, tectonic plates will do the trick for you. Earth is already like that:

Highs and lows

If you meant floating continents, though, you will need for that.

As for river flowing upwards, you have two options. Either have frequent tidal bores:

A tidal bore, often simply given as bore in context, is a tidal phenomenon in which the leading edge of the incoming tide forms a wave (or waves) of water that travels up a river or narrow bay against the direction of the river or bay's current.

...Which do happen on Earth as well, or, you know, use for that too.

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  • $\begingroup$ Floating continents without magic $\endgroup$ – Reddy Lutonadio Dec 19 '18 at 14:52
  • $\begingroup$ @ReddyLutonadio that's a nope. Might as well divide by zero. $\endgroup$ – Renan Dec 19 '18 at 15:06
  • $\begingroup$ Looking for an explanation that may bring answers not based on physics, astronomy as we know them, but more credible than magic ;-) $\endgroup$ – Reddy Lutonadio Dec 19 '18 at 15:11
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    $\begingroup$ Any answer not based on physics would be magic. $\endgroup$ – Abigail Dec 19 '18 at 17:21
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    $\begingroup$ For the elevation difference between continents (2-3 km) we need to go further and propose that there are 2 types of continental plates - "regular" and "lightweight". "Lightweight" would, on average, ride a few kilometers higher from the ocean floor than the regular ones. $\endgroup$ – Alexander Dec 19 '18 at 17:34
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Even if you could come up with a realistic scenario where gravity can be different on one planet (spoilers: it can't), lower gravity wouldn't make a landmass be higher or even float. As @Trevor D said, you are left with magic as the only recourse.

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  • $\begingroup$ Gravity is already different on planets. See mascons. $\endgroup$ – L.Dutch Dec 19 '18 at 17:57
  • $\begingroup$ You're really nitpicking. If you want to get technical about it, gravity is always different by location and the gravity in one point of any planet is not the same as at another point. But the difference is tiny and sure as hell wouldn't lead to floating continents. $\endgroup$ – T Nguyen Dec 19 '18 at 18:01

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