Imagine a world covered in a shallow sea (usually only 4 metres or less) that has little land, where there is land its in the form of fairly small islands (less than 1sq KM)

This will be populated externally so it doesn't need to be life bearing.

Is such a world geologically plausible ?

What world creation scenarios could explain such a world ?

  • 2
    $\begingroup$ shallow sea: of water? $\endgroup$
    – guido
    Nov 20, 2014 at 16:39
  • 2
    $\begingroup$ oh come on, I mean really - what do you think.... $\endgroup$ Nov 20, 2014 at 16:42
  • 3
    $\begingroup$ You'd be surprised about what will be used for the liquid $\endgroup$ Nov 20, 2014 at 17:06
  • 11
    $\begingroup$ @ChrisCamacho Titan has seas of liquid Methane, so its a completely reasonable question to ask. I do think 4m is extremely shallow at a planetary scale. Such a planet would be neat if it had tides, causing different areas to get exposed. $\endgroup$ Nov 20, 2014 at 17:44
  • 2
    $\begingroup$ I was under the impression that there was a large period in earths history where there was huge shallow seas cover large parts of the planet. $\endgroup$
    – bowlturner
    Nov 20, 2014 at 18:04

4 Answers 4


I'll say plausible (though I question the extremely shallow depth...200m not 4m maybe?), though perhaps not the most likely. Earth is unique (so far at least) within our solar system with tectonic plates, so it's more than feasible to have planets lacking this plate structure.

The problem you will face is how the planet releases it's heat. On planets without plate tectonics, you get very large volcanoes as the means through which it releases its heat (Olympus Mons on mars is a good example of the size these volcanoes reach like this). I've seen a theory that applies to Venus that see's the entire planets crust 'melt' from internal heat in a 200 million year long event...I guess a melted and reformed crust might look like this?, seems like a stretch though. You basically need a planetary body that's cool enough internally that it doesn't have the need to expel this heat. Best if the planet has no moons as they tend to cause gravitational effects that stir up a planets interior causing more heat...and no tide to move the water around, leaving it the consistent depth)

Planetary bombardment might be hard to explain away as well. Though not as true today, the galaxy used to be a messy place with asteroid impacts being nearly commonplace (look at the moons bombardment history if you want an idea of the timeline there). Craters leave deep impacts and large rings (might actually explain the island presence you are going for).

Planets are never really round...they have bulges and the sort (earth has 4 I believe) and the rotation of the planet tends to redistribute water as well.

Those are the issues...lets give this a go:

My preferred explanation for such a bodies existence is an impact event that rips apart a larger planet. You have a larger, cooling body that suffers a massive impact that rips a section of the planet away. This surface section ripped off doesn't contain the same heat and pressure as the larger planet once did, so you are working with a cooler interior to start

This planet fragment (now a planet in it's own right) then undergoes a massive volcanic event releasing a magma flow that basically extends planet wide, filling in the cracks and leaving the planet coated in a volcanic crust. Should be close to round at this point.

No clue how the water got there (perhaps released with the lava), but the planets water undergoes a 'snowball earth' style effect where the planet entirely freezes over (freezing effect is easy as the sun this planet orbits can easily undergo an extended cold/dormant phase). This glacial 'covered in ice' period works to further flatten the planet by covering it in a large mass of heavy ice. Sun goes back to normal and melts everything leaving you with a world that you are looking for. It might not be quite prefect, but a few millions of years of erosion with no forces such as volcanic or tectonic activity to reshape the world should continue to flatten it out. Marine life such as coral tends to help this as well...millions of years worth of shelled life remain can also flatten out the seabed as it settles.

Not sure on how feasible this is though...comments and corrections more than welcome.

  • 1
    $\begingroup$ climate on such a planet would be interesting as well...without deep seas to redistribute heat, nor a thick icecap to melt/form with heat changes, this world would be pretty dynamic in it's temperature range. Heat redistribution would be isolated to the air currents, meaning there would be heavy and constant trade winds. No hurricanes though, water would be too shallow to provide the energy to support their formation $\endgroup$
    – Twelfth
    Nov 20, 2014 at 19:03
  • $\begingroup$ I was going to put an answer in here about how the planet would need to be moon-less, but you got it in there. +1 for that. $\endgroup$
    – IchabodE
    Nov 21, 2014 at 5:11
  • $\begingroup$ Quibble: Earth is the only rocky planet in the solar system with a molten core. So, 1 out of 4. Talk to me about Io, Europa, and other bodies that have atmospheres, seas, and cores. $\endgroup$
    – user3082
    Nov 23, 2014 at 6:38
  • $\begingroup$ Any idea how big this planet would have to be and what the gravity might be like? $\endgroup$
    – Len
    Apr 21, 2021 at 18:54

Essentially what you are asking for is a world with a very smooth surface. So how do we get it? Gravity and erosion will both generally act to gradually smooth a planet's surface. Wind and water will chip away at exposed rock and deposit the sand in crevices, while gravity causes things to move downwards and flatten out in general. So what is it that creates mountains and valleys in the first place? Plate tectonics! On earth the continents move around on top of a liquid mantle and bump into each other causing mountain ranges and trenches to form. Volcanoes sometimes peak through the surface and create mountains as well. Geologists have a lot of terms for all of the different mechanisms that cause different formations, but all we really need for our planet to be smooth is for it to have cooled down to the point that its interior is no longer liquid. Without a molten mantle none of the turbulent processes that generate mountains and valleys will happen! As far as how to get your solid mantle planet I think the simplest answer is that the planet is very, very, very old. Time will cause the planet to gradually lose heat to the crust and space, and allow wind and water to erode all of the continents.

  • $\begingroup$ I like where you are going, but issue with a very very very old planet is it would have formed in periods of heavy bombardment and show the scars of a very active impact history. Erosion could account for some of the smaller impact zones dissapearing, but large impacts and the craters left behind would be difficult to explain away. $\endgroup$
    – Twelfth
    Nov 20, 2014 at 18:55
  • $\begingroup$ @Twelfth I don't understand you concern. The earth had periods of heavy bombardment, but no longer shows the craters from that time due to tectonics and erosion. $\endgroup$ Nov 20, 2014 at 19:08
  • $\begingroup$ universetoday.com/19616/… There are still several visible ones on earth. Anything that hits the Canadian shield basically remains there. $\endgroup$
    – Twelfth
    Nov 20, 2014 at 19:40
  • $\begingroup$ @Twelfth The majority of those craters are quite recent in geological time scales, within the last few hundred million years. Give it another few hundred million years of wind and rain and they will be gone too. The earth has continuously been struck by meteors throughout its history and we do not see the vast majority of those craters because their evidence has been eroded over time. As a planet ages the craters on is surface will be eroded, and the rate of new impacts will decrease as the system is gradually cleaned by the planets. I still think a very old planet would have very few craters. $\endgroup$ Nov 20, 2014 at 23:19
  • 1
    $\begingroup$ @Twelfth it sounds like you both agree already - calling it a far future planet rather than an old planet might remove the potential ambiguity. It could be Earth in the distant future. $\endgroup$ Nov 20, 2014 at 23:44

You'd be hard-pressed to get four-meter seas through any natural process -- the simple variation in planetary radius between the equator and the poles will exceed that.

On the other hand, if you don't mind your seas being a hundred meters deep or so, there's an easy way to get them: ice ages. Your planet has had a long (multi-million-year) ice age with a stable sea level. Rivers have produced enormous alluvial plains (think the Atlantic/Gulf coast of North America, only larger). Then, the ice melts.

You've now got three distinct geologic areas: the ocean deeps (areas that were ocean during the ice age), micro-continents (mountain ranges and plateaus during the ice age), and between them, enormous shallow seas ranging in depth from a few tens of meters to a few hundred.


It is believed that one of the major formative events in the Earth's history is the collision between Theia and proto-Earth. This had a massive effect on the planet's formation, injecting a vast amount of energy into the system and leaving us with the hot core and relatively volatile planet that we inhabit.

If that had not happened, we might have ended up on a colder and considerably flatter planet with far less geothermal activity meaning no plate tectonics to speak of and less by way of greenhouse gasses in the atmosphere. This might be a viable starting point for the type of world you are looking for.

If I were creating this world, I might then leave the planet covered in a ball of ice for most of its existence, to smooth it out a little further and then perhaps have changes to it's star heating it up enough to melt the glaciers and render it habitable.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .