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In my sci-fi universe, I have an earth-sized planet orbiting a K-type star. This planet (which has life) has about 0.6 G of surface gravity, an atmospheric pressure of 0.8 bars, and no continents. Water covers 60% of the surface.

Now, here’s the tricky part: I would like this planet to be girdled by a giant mountain range or plateau, about 10 km above the average altitude or sea level. This plateau should be continuous, separating the two hemispheres and preventing their indigenous life forms from mixing to any great extent.

What natural/cosmic phenomenon could explain this geographical oddity?

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    $\begingroup$ Oceans are easier to get and not any worse than mountains at preventing life forms from mixing across them... And I have never heard of a pass-less continuous mountain range. Ever the Pamirs and the Himalayas have passes; people have been crossing from India to Tibet and vice-versa since always. $\endgroup$
    – AlexP
    Apr 3 at 19:31
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    $\begingroup$ The definition of what constitutes a continent is defined by convention, an is based more on history than actual geography, so you can call anything you want a continent. $\endgroup$
    – Mathaddict
    Apr 3 at 20:17
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    $\begingroup$ An Earth-sized planet with only 0.6g is going to have a lot less iron in it's core. It might not even be geologically active. Definitely won't have a magnetosphere, so scientifically it will be a dead planet. I write all that to say... Write your darned story. Utterly and completely ignore the "natural/cosmic phenomenon (which) could explain this geographical oddity". $\endgroup$
    – RonJohn
    Apr 4 at 6:09
  • $\begingroup$ @AlexP The specified height is about 10% greater than Everest though, and the Tibetan plateau is high enough to have been a significant barrier to migration, and to thus leave evolutionary impacts. See eos.org/articles/… While OP's mountain range does seem unlikely to have arisen by purely natural processes (thus the Q), they didn't use the science-based or hard-science tags, and didn't specify that the belt be completely passless - only that it's a significant barrier to migration. $\endgroup$
    – Sarah Messer
    Apr 4 at 17:49

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Well, let's look at the options proposed for the origins of the equatorial ridge on Iapetus:

  1. Your world used to spin a lot faster, but tidal evolution has slowed it down, and the crust was rigid enough that it left this ridge around the equator while relaxing into a more spherical shape.
  2. It could be the result of volcanism / convective activity, like continents on Earth, but with lighter continental rock compared to the mantle, or a hotter, more fluid mantle, or faster spin, or all three, so that Coriolis forces push all of the light continental stuff towards the equator where it accumulates into a ring.
  3. Your planet used to have a ring system (or at least used to have a bigger ring system--maybe it still has rings), and ring material has been constantly raining down on the equator for a few million years, piling up several kilometers worth of dust (which compresses into rock).
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  • $\begingroup$ I'd like to point out that it's difficult to have Earth-like life on a planet with rings. See "If Earth had rings" by Artifexian. $\endgroup$
    – nearsighted
    Apr 3 at 22:11
  • $\begingroup$ @nearsighted Meh. The planet already has lower gravity and less atmosphere, and rings won't prevent liquid water or sunlight reaching the surface $\endgroup$
    – Logan R. Kearsley
    Apr 3 at 23:53
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It could be explained using normal tectonic plates if you're okay with some islands dotted around the oceans. If you had a relatively small number of plates, mostly moving towards the equator, you could conceivably get a mountain range spanning around the equator. It wouldn't be perfect ─ it might look something like this (not a great model, just a quick Inkarnate sketch) ─ but it'd be close.example of what it could look like

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