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I've got two landmasses that are each situated on either side of a 20-50 mile wide sea (with the width varying along its length) where the sea runs for several hundred miles before opening out into the ocean proper. Is there any way that the tectonic plates of my planet could move away or toward each other to create this? If possible I would like to avoid too high levels of volcanic activity and earthquakes although having at least some would be preferable. If a formation like this isn't possible what would be the most similar? (i.e. when the Americas moved away from Europe and Africa at what width was there a stable sea between the two?)

In this sea what would the currents and weather systems look like? Would it be a fairly calm and easy crossing or extremely perilous at most times of year?

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  • $\begingroup$ The weather will depend on the location on the planet. $\endgroup$
    – John
    Sep 5, 2020 at 16:15
  • $\begingroup$ The weather I can't answer without knowing where on the planet we our - so see worldbuilding.stackexchange.com/questions/184276/… for an example of how you'd work out weather from location on planet and geography. $\endgroup$
    – Ash
    Sep 5, 2020 at 16:47
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    $\begingroup$ Why do you ask about the tectonic plates? The sea you mention is about the same dimenisons as the English Channel, which is on one tectonic plate instead of the gap between 2 of them. And most of the time it won't matter to any people on the planet how deep or shallow the sea or channel is or where the line between tectonic plates is. You might want the border between 2 tectonic plates for earthquakes and volcanoes in the area, but if so, you should have said so. And if not, a version of the English channel should be fine for your story. $\endgroup$ Sep 5, 2020 at 16:55
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    $\begingroup$ The English Channel itself is several hundred miles long — 350, to be reasonably precise —- and is of course exactly the same width as the English Channel. $\endgroup$
    – Mike Scott
    Sep 5, 2020 at 17:18
  • $\begingroup$ As you build your world, remember tides. Locks exist for more reasons than altitude. As our moon pulls the ocean up on one side of the planet, the sea level drops on the other. I don't think it would turn your channel into a raging river of death - but it shouldn't be ignored. (Of course, it's your world... it could be a raging river of death....) $\endgroup$
    – JBH
    Sep 5, 2020 at 18:19

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Two tectonic plates slowly moving away from each other will create a channel between them.

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The slower and more regularly they're moving, the more stable the geology will be.

For example, look at the Arabian plate vs the African plate, and the red sea:

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  • $\begingroup$ Good example with the Red Sea! $\endgroup$
    – Willk
    Sep 5, 2020 at 19:54
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Yes it is possible, and it actually exists

Consider the Bosporus + Sea of Marmara + Dardanelles: all together they are about 300 km end to end (190 statute miles, 160 nautical miles) and they connect the Black Sea to the Aegean (and from there on to the Mediterranean and the Atlantic). The Bosporus and the Dardanelles are obviously much narrower than 50 miles (they are long winding narrow straits); but even the Sea of Marmara is narrower than 50 miles. (Its maximum width is about 70 km, or 37 nautical miles, or 44 statute miles.)

The Bosporus, the Sea of Marmara and the Dardanelles

North is up. North-East to South-West: Bosporus Strait, Sea of Marmara, Dardanelles Strait. The pink coloration on the shores of the Bosporus is the city of Istanbul. The large body of water to the North-East is the Black Sea; the Aegean is to the South-West. Bounding box: West 26°, South 39.8°, East 30°, North 41.5°. Map from Wikimedia, public domain.

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  • $\begingroup$ There's also the Red Sea. At the current time, it's rather wider than the OP wants, but during an ice age the sea level would be lower. en.wikipedia.org/wiki/Red_Sea Would seem easy to adjust the tectonic plates to make it open at both ends... $\endgroup$
    – jamesqf
    Sep 5, 2020 at 19:00
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If you accept the Gondwanaland hypothesis. https://en.wikipedia.org/wiki/Gondwana

You can choose any width you want from zero to the width of the Atlantic Ocean.

enter image description here

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    $\begingroup$ The question would be whether it would fill entirely with water before it's wider than that, which turns on the vagaries of the local geography. The longer, the more likely that some places will be high enough to be land bridges for some time. $\endgroup$
    – Mary
    Sep 5, 2020 at 17:01
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    $\begingroup$ Put a glacial lake at one end, and let it scour out the channel in a massive flood when the ice dam melts. $\endgroup$ Sep 6, 2020 at 1:45
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To add to existing examples of tectonic plates separated by salt water, at Silfra in Iceland we also have an example of a newly-formed fissure filled with water, except here it is fresh water because it's on land. In time of course this will widen and join the sea. This is a good example to demonstrate that we do see a continuous range of water-filled fissure widths, so you really can make your geography be whatever you need it to be.

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