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I'm building a world that has a feature that is meant to separate the world in half so civilizations on each half won't contact one another. The feature is magical in origin, but does not seem magical at first glance and the barrier is itself something physical not magical. I want to make it as realistic as possible.

Over the continents the feature could take the form of a high mountain range surrounded by harsh desert but I am open to other ideas. I am not sure what the feature could be over the sea, perhaps a permanent storm?

So the question is twofold:

  • What type of feature would keep a civilization with 18th century technology (advanced age of sail but pre-industrial) to be unable to cross it?

  • What effects would such a feature have on the climate? Keep in mind that the feature splits the otherwise Earth-like world in two so it will be on the scale of the whole planet.

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    $\begingroup$ Can it be along the equator? $\endgroup$ – Patricia Shanahan Nov 9 '19 at 19:02
  • $\begingroup$ Political features work best for separating civilizations, and they have no influence on climate. There was very little interaction in the actual for real 18th century France of Louis XV and the contemporary Qing empire of China, or the Tokugawa shogunate of Japan. $\endgroup$ – AlexP Nov 9 '19 at 19:08
  • $\begingroup$ Europe or Africa sticking with the American plates, perhaps? That way you have the world's landmasses roughly evenly distributed on two megacontinents separated by two oceans $\endgroup$ – nzaman Nov 9 '19 at 19:22
  • $\begingroup$ @nzaman If memory serves ocean-crossing expeditions were launched in 1492 (Columbus), well before the 18th century technology level of the OP was reached (even before the scientific revolution). $\endgroup$ – 11684 Nov 10 '19 at 21:43
  • $\begingroup$ This has been discussed here: worldbuilding.stackexchange.com/questions/151820/… $\endgroup$ – A. I. Breveleri Nov 10 '19 at 21:51
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Iapetus, a moon of Saturn, has a strange equatorial ridge along part of its equator.

The geology of Iapetus is very different from that of an Earth like planet, so I don't know if the forces that produced the equatorial ridge on Iapetus could produce one on an Earth like planet.

It is not clear how the ridge formed. One difficulty is to explain why it follows the equator almost perfectly. There are at least four current hypotheses, but none of them explains why the ridge is confined to Cassini Regio.

A team of scientists associated with the Cassini mission have argued that the ridge could be a remnant of the oblate shape of the young Iapetus, when it was rotating more rapidly than it does today.[4] The height of the ridge suggests a maximum rotational period of 17 hours. If Iapetus cooled fast enough to preserve the ridge but remained plastic long enough for the tides raised by Saturn to have slowed the rotation to its current tidally locked 79 days, Iapetus must have been heated by the radioactive decay of aluminium-26. This isotope appears to have been abundant in the solar nebula from which Saturn formed, but has since all decayed. The quantities of aluminium-26 needed to heat Iapetus to the required temperature give a tentative date to its formation relative to the rest of the Solar System: Iapetus must have come together earlier than expected, only two million years after the asteroids started to form.

The ridge could be icy material that welled up from beneath the surface and then solidified. If it had formed away from the position of the equator at the time, this hypothesis requires that the rotational axis would have been driven to its current position by the ridge.[citation needed]

Iapetus may have had a ring system during its formation due to its large Hill sphere, and the equatorial ridge could have then been produced by collisional accretion of this ring.[5]

The ridge and the bulge could be the result of ancient convective overturn. This hypothesis states that the bulge is in isostatic equilibrium typical for terrestrial mountains. It means that under the bulge there is material of low density (roots). The weight of the bulge is compensated by buoyancy forces acting on the roots. The ridge is also built of less dense matter. Its position along the equator is probably a result of the Coriolis force acting on a liquid interior of Iapetus.[6][7]

https://en.wikipedia.org/wiki/Equatorial_ridge_on_Iapetus1

Some of those methods might be more plausible on an Earth like planet than others.

Since it would take billions of years for life and intelligent life to develop on a planet, either the equatorial ridge would have been produced by gradual geological processes instead of by a terrible disaster, or else those terrible disasters would have to be billions of years in the past by the time of the story.

If an Earth like planet had an equatorial ridge without any passes low enough to have air thick enough to breathe easily, but instead was everywhere so high that oxygen tanks would be needed to cross it, the two hemispheres will be out of contact until and unless high flying aircraft are invented.

Depending on the age of the ridge, life in the two hemispheres could have a common origin or else have developed totally separately and have radically different biochemistry.

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I suggest raising the mid-Atlantic ridge up to just below the surface of the ocean and expanding it to a 100 mile wide band of treacherous shallows and reefs running from pole to pole with another similar band running through the centre of the Pacific.

This band would consist of a series of impenetrable reefs each a few miles wide running from pole to pole each separated from the next by 10 miles of open ocean.

The whole area would regularly be swept by violent storms. There would be no fresh water available other than rain water, no islands to land on and no way for a sailing ship to cross the reefs or for any form of base to be built. In the unlikely event that a small vessel did manage to cross one of the bands, by sailing further they would only encounter another reef.

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I had a similar idea for a setting of my own. I settled on twin granite walls/ridges running alongside each other, 1 kilometre high, too steep to climb and too hard to easily drill through. So it would be a tremendous effort to get over, requiring at least medieval technology to build scaffolding to reach the top.

And in the space between the walls, a narrow, islandless sea with water poisonous to man and fish alike. So anyone who got over the wall somehow would not find anything useful on the other side - even the other wall would be just over the horizon.

So this wall would be mountable since perhaps the 1500s, but nobody would have great reason to get to the second wall and across that until the invention of powered flight. It also should not be high enough to hinder climate or migratory birds that much. Plus the poisonous water in the inner sea would evaporate and turn into poisonous rain alongside the walls, meaning that people would not want to live next to it either.

I am interested in the other potential solutions though.

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In the 18th century there were no airplanes, so the o ly way across the old world and the new world was by ships.

If the oceans were overtaken by some bacteria that secreted an acid gas, such as HCl, nobody would be able to cross. There was no technology to make an hermetically sealed ship, nor to drive one, nor to store breathable air for long term travel. This would also kill all marine life that dwells by the surface, unfortunately. Also coastal cities and would all become ghost cities.

The effect on the weather would probably be minimal on a scale of decades or centuries. But hurricanes would be deadlier than the plague wherever they hit.

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A radioactive zone of death.

Your people are separated by 'death zones' - deserts liberally salted with highly radioactive materials. Anyone who tries to cross them dies, and they don't know why.

The magic part could be that if people try to cross by balloon, dust storms kick up which poison everyone. Those dust storms could also be created when people try to cross by land, to ensure maximum contamination.

18th century people would have no idea what's happening. It's just a cursed land that no one can cross without dying shortly thereafter. And worse, when people do come back, they mysteriously cause other people around them to get sick and die. so there would be a strong cultural bias against even trying.

Since the dust storms would be magic and local, none of this would have any effect on the global climate.

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Does it have to be magical? A world somewhat hotter than earth where the equatorial regions are lethally hot for the native intelligent species would work.

Another possibility is two habitable polar continents with a continuous equatorial ocean. The ocean is warm enough for continuous hurricanes to circle the globe. That would make it very difficult for pre-industrial cultures to cross between continents.

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This reminds me of a tidally locked world. Granted, in tidally locked worlds, the climate is very different on the two halves of a planet, and said halves would likely be uninhabitable, due to their unstable weather. The planet would also be likely to orbit a red dwarf star.

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