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Consider an Earth-like planet. I want a large (Europe/Africa) continent to split in half, with the halves separating in a reasonably short time (by geological standards). The halves need to separate by several hundred miles within a century or so. Ideally it would happen as a single event (maybe taking hours, or days).

It is okay if this is achieved by a strip of land sinking, rather than separating.

I want the event to be not too violent. So a K-T extinction-like event is not a solution.

This event needs to be survivable by most species, although many individuals might die due to flooding or whatever.

A human civilization which once spanned the separated lands needs to be able to rebuild afterwards.

Are there any plausible geological processes (speculative, but not stretching the laws of nature too much) which could cause such a change?

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  • $\begingroup$ I can't remember enough information to justify an answer, but I remember reading about a large lake (in North America) bursting at some point in the recent past, causing a lot of flooding. I still don't know if it would be quick enough to meet your timescale. $\endgroup$
    – dplane
    Feb 27 at 18:29
  • $\begingroup$ @dplane this? $\endgroup$
    – alkahest
    Feb 27 at 18:44
  • $\begingroup$ @alkahest that's what I had in mind, yeah $\endgroup$
    – dplane
    Feb 27 at 19:02
  • $\begingroup$ just be clear even a dinosaur killer level event is not moving continents any faster than they move normally. its like using a single firecracker to move 2 trains. $\endgroup$
    – John
    Feb 29 at 1:33
  • $\begingroup$ You don't need much to get this to be belivable. World of Warcraft pulled off this 20 years ago with the Sundering. $\endgroup$
    – Mermaker
    Feb 29 at 14:57

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Somewhere in the range of a hundred thousand years

Your numbers don't add up. For normal plate tectonics, the fastest recorded speed is 8.5cm/year. For a hundred mile gap, that would take 1.8 million years.

And, for those of us living on the surface of a planet, it's good that it takes that long. The energies involved in moving even that quickly are colossal. Speeding it up tenfold would result in daily earthquakes and probably volcanism, but is likely survivable.

So what about a different mechanism?

Here, let's look at the Black Sea Deluge Hypothesis.

Before that date, glacial meltwater had turned the Black and Caspian seas into vast freshwater lakes draining into the Aegean Sea. As glaciers retreated, some of the rivers emptying into the Black Sea declined in volume and changed course to drain into the North Sea. The levels of the lakes dropped through evaporation, while changes in worldwide hydrology caused global sea levels to rise.

The rising Mediterranean finally spilled over a rocky sill at the Bosporus. The event flooded 100,000 km2 (39,000 sq mi) of land and significantly expanded the Black Sea shoreline to the north and west. According to these researchers, 50 km3 (10 cu mi) of water poured through each day, two hundred times the flow of Niagara Falls. The Bosporus valley roared and surged at full spate for at least 300 days. They argued that the catastrophic inflow of seawater resulted from an abrupt sea-level jump that accompanied the Laurentide Ice Sheet collapse and the ensuing breach of a bedrock barrier in the Bosporus strait.

So, imagine a vast, prehistoric riverbed extending across your fictional continent. A combination of debris and ice formed a dam across the mouth of its source in antiquity, and it eventually dried up, with its endpoint forming a gulf with the sea.

Sudden volcanism dumps an enormous amount of carbon dioxide into the air, but none of the accompanying particulates (somehow), and the planet experiences rapid global warming. Land ice melts in enormous quantities feeding into the source sea, and eventually the ice dam gives way, dumping cataclysmic amounts of water into the riverbed in a vast land-based tidal wave. Millions die.

It won't be a hundred miles wide, and it won't be permanent, but you'll have your dotted line.

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    $\begingroup$ I should add that all of this requires very specific things to happen in a very specific order in a deeply contrived fashion, but fiction is fine with contrivances. $\endgroup$
    – jdunlop
    Feb 27 at 19:14
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Yes. I can see one way. Assume an underground sea. 100 mile wide underground cavern filled with sea water. And then the ceiling collapses. Maybe a meteor punctures the roof. Maybe it just finally wears through. Whatever the case, now the ceiling is unstable and over the next years, chunks keep falling into the sea. Eventually you have a 100-mile gap within a century.

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  • $\begingroup$ Could also be volcanic gas escaping $\endgroup$
    – Pica
    Feb 29 at 17:38
  • $\begingroup$ Is such a cavern remotely feasible? I know caves can be hundreds of miles long, but as far as I know this would be orders of magnitude wider than any cave on earth. Such a void would be a fantastic feat even if man-made, could such a thing happen naturally? $\endgroup$ Feb 29 at 20:53
  • $\begingroup$ @NuclearHoagie There’s a river under Antarctica currently carving its way toward the ocean. As it carves under the ice, it leaves a void that will eventually collapse. To best of my knowledge, same works for rock as long as you have a soft stone layer laying atop a slanted hard stone layer. $\endgroup$
    – SRM
    Feb 29 at 22:31
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    $\begingroup$ @SRM That doesn't really help, there is no such thing as a river that's >100 miles wide. This will create a long but narrow void, not a 100 mile wide gap the the breadth of a continent. $\endgroup$ Mar 1 at 14:11
  • $\begingroup$ @NuclearHoagie It's a river, but what is left behind in its wake is a sea. The "river" is a very strong current carving its way over. Think like the North Atlantic Current embedded in the ocean. $\endgroup$
    – SRM
    Mar 2 at 1:59
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You aren't going to even come remotely close in any time we'd consider human history. Even trying to do this in ten thousand years would be an ELE.

If you're talking about several hundred miles over a hundred years, then you're describing several miles per year of lava field being generated between the two continents. If these are actually continents, we're talking hundreds if not thousands of square miles of lava field being generated per year. The ex-gassing all by itself would be an extinction event.

This doesn't count the subduction that would occur somewhere on the far end of the plates. You're talking about a release of energy roughly ten thousand times all of Earth's current geological activity, continuing for a century.

I can imagine no scenario in which this would be survivable.

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Rather than the edges of the continents moving apart from each other, instead picture the area of the Mediterranean Sea as not being connected to the Atlantic, and rather than being completely filled by ocean water, it is instead a large depression with some (much) smaller separated lakes or seas, and containing quite a large amount of dry land allowing Europe and Africa access to each other by foot traffic.

It is unknown the actual time frame that it took the Mediterranean to fill, but it was a relative eyeblink in a geologic timeframe. You can easily just state that in your world, the amount of incoming water took 100 years to fully fill the basin.

This will give the population of the lowland time to migrate to above eventually stabilized tide line.

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About the only thing I can think of is that your continent stretches across 2 tectonic plates that are moving slowly apart.
This already has created a huge rift-valley (similar to the Great Rift Valley in Africa, but bigger) reaching almost from coast to coast, which (except near the coasts) is already below sea-level.

A massive earth-quake on the coast near one of the ends of the valley lowers terrain so the sea can rush in. (Or a big volcanic explosion blows away the dam on one end.)
Other possibility is that global warming or end of ice-age raises overall sea-levels high enough to flood the valley.

It will probably still take a couple of weeks/months/years for the entire valley to flood, depending on the size of the inlet that is opened and the height of the sea-level above the valley floor.

Please note that "opening the inlet" by earthquake of volcano makes the whole flooding event happen fast and without warning.
General sea-level rise due to end of an ice-age or global warming would only gradually bring the sea-level up and the locals will see it coming and can prepare in advance.
You can play with that to see what best fits your story.

As for the plausibility:
There are theories that our own Mediterranean has had such a major flood events several times as global sea-levels rose above the level of the Strait of Gibraltar between ice-ages. During ice-ages sea-level lowered again and closed Gibraltar off. The Med then gradually partially dried out (through evaporation) only to be flooded again later.

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An Industrial Revolution and Melting Ice Bridge

Instead of trying to displace all that rock, imagine a case where two continents are connected by an ice bridge. This was once the case for North America and Asia. During the last ice age, the ice bridge that connected these continents covered about 2 million square km of ocean with ice, then over the course of about 2000 years, the whole ice bridge melted away.

But... the end of the ice age was very slow compared to the global warming caused by modern civilization. 2 million square km is about how much arctic ice has melted away in just the last 60 years of global warming. So, if your planet features an ice age coming to an abrupt end thanks to fire happy humans, then you could see a similar event happen in a very short amount of time.

Now it could be argued that Global Warming IS an extinction level event, but for those of us in the middle of it, it's hard to tell.

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