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I posed here a question about a super-earthquake separating a Pangaea-like continent in a matter of days. Unfortunately, one of the answers rightfully points out that such an earthquake would generate a volcanic aftermath that would basically destroy all life on my planet.

So I thought of a different scenario. Imagine that the earthquake does not separate the continent through plate tectonics, but that it creates a tsunami of sufficient magnitude to alter the coastline in a way that effectively separates two landmasses.

I know that tsunamis can be strong enough to alter coastlines, as has happened in 2004. But these changes were relatively minor. I would like to know if they could be big enough to produce the desired effect, knowing full well that the earthquake producing such a tsunami would have to be much greater than the one in 2004. But it could not produce the kind of volcanic aftermath that would destroy all life on earth.

To clarify, this is an earth-like planet. The landmass that gets separated from the main continent need not be very big: it can be of the approximate size of Australia. But the two landmasses need to be separated by a big enough distance so that they cannot be sighted from the horizon of each other's coastline. Also, the sea forming in between must be deep enough so that one can only cross it by resorting to boats or rafts.

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    $\begingroup$ This is sort of how the English Channel was formed. One or two megafloods broke the Weald-Artois anticline. (In the case of the English Channel, it was not a tsunami, but rather a gigantic lake breaking free and liberating a huge amount of water, which simply broke through the chalk barrier.) $\endgroup$
    – AlexP
    Dec 12 '20 at 14:22
  • $\begingroup$ "need not be very big" and "it can be of the approximate size of Australia" are two very different statements. You do realize that Australia is as large as the continental USA? or 32 times as large as the whole UK? $\endgroup$
    – PcMan
    Dec 15 '20 at 6:12
  • $\begingroup$ Yes, but continent-wise it’s not very big. It is not Eurasia, or Africa, or America (not US, the continent) $\endgroup$ Dec 15 '20 at 10:37
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If there are two rockey landmasses connected only by a low lying narrow neck or isthmus of sandy soil, a tsunami striking the low sandy neck might possibly push all the sand out of the strait between the two land masses and distribute it over a wider area of sea floor.

Thus it might lower the ground level in the strait enough that it is below low tide level and even too deep to wade across at low tide.

The possibility of that does not depend on the relative or absolute sizes of the two landmasses but on there being only one connection between them which is above sea level, a connection which is both long enough to put each land mass out of sight of the other and narrow enough and low enough for the tsunami to move enough of the sand away and destroy the above sea level part of the connection.

Possibly the two land masses are a continent which is slowly splitting apart, or two continents slowly coming together. What had happened or will happen over geological eras is of very little importance to characters, even if they live in a society which knows about plate techtonics.

So the strait between the two land masses should be hundreds of miles wide in most places, but much narrower in one place. And by coincidence rivers in the two land masses flow down from heights carrying silt and have their deltas almost opposite each other. And the silt deposited in the deltas forms sandbanks above water level which get larger and larger until they join and there is one narrow path across them fron one land mass to the other.

And I suppose that a sufficiently large tsunami could scour away enough sand and silt that the connection would be lowered to at least ten feet below sea level, for hundreds or thousands of years until the two rivers build up their deltas again.

Hog Island was a mile long barrier island south of Rockaway, New York City which formed about 1863 and grew large enough to have a number of businesses and buildings on it. Most of Hog Island disappeared in a hurricane on Augyst 23, 1893, and it was damaged more in a storm in 1896. Hog island is believed to have disappeared entirely in 1902.

https://en.wikipedia.org/wiki/Hog_Island_(New_York)[1]

I believe that L. Sprague de Camp in Lost Continents: The Atlantis Theme in History, Science, and Literature discusses historic events which may have inspired Plato's account of Atlantis. I believe de Camp mentioned an island split in half by a tsunami, and a city that sank beneath the sea.

Helike was an importan city in ancient Greece that founded colonise in Asia Minor and Italy.

The city was destroyed in 373 BC, two years before the Battle of Leuctra, during a winter night. Several events were construed in retrospect as having warned of the disaster: some "immense columns of flame" appeared, and five days previously, all animals and vermin fled the city, going toward Keryneia.6 The city and a space of 12 stadia below it sank into the earth and were covered over by the sea. All the inhabitants perished without a trace, and the city was obscured from view except for a few building fragments projecting from the sea. Ten Spartan ships anchored in the harbour were dragged down with it. An attempt involving 2,000 men to recover bodies was unsuccessful.[7] Aigion took possession of its territory.

https://en.wikipedia.org/wiki/Helike[2]

It is believed that an earthquake produced a tsunami which flooded Helike and then receeded, but didn't receed all the way to the former shore because the rock layer beneath Helike had been titled and lowered by the earthquake. So Helike remained underwater, though in shallow enough water that for centuries tourists sailed over it to admire the ruins.

Atalanti is a small island in Greece.

Anciently, the island was known as Atalanta or Atalante (Ancient Greek: Ἀταλάντη). It was noted by ancient geographers and historians as a small island off Locris, in the Opuntian Gulf, said to have been torn asunder from the mainland by an earthquake. In the first year of the Peloponnesian War this previously uninhabited island was fortified by the Athenians to prevent Locrian pirates attacking Euboea.1 In the sixth year of the war a part of the Athenian works was destroyed by the sea, with half the ships on the beach destroyed. Thucydides reports that following an earthquake, the sea receded from the shore before returning in a huge wave.2 Citing similar events at Peparethus and Orobiae, he suggests that earthquakes and such "sea events" are linked—we now know that such tsunami are in fact caused by earthquakes. In 421 BCE, the Peace of Nicias returned Atalanta to Sparta.3 Aside from Thucydides, the island is noted by Strabo,4 Diodorus,5 Pausanias,6 Livy,[7] Pliny the Elder,[8] Seneca,[9] and Stephanus of Byzantium.[10]

https://en.wikipedia.org/wiki/Atalanti_Island[3]

Port Royal, Jamaica, was the largest city in the caribbean until destroyed in 1692.

On 7 June 1692, a devastating earthquake hit the city causing most of its northern section to be lost – and with it many of the town's houses and other buildings. Many of the forts were destroyed, as well; Fort Charles survived, but Forts James and Carlisle sank into the sea, Fort Rupert became a large region of water, and great damage was done to an area known as Morgan's Line.3

Although the earthquake hit the entire island of Jamaica, the citizens of Port Royal were at a greater risk of death due to the perilous sand, falling buildings, and the tsunami that followed. Though the local authorities tried to remove or sink all of the corpses from the water, they were unsuccessful; some simply got away from them, while others were trapped in places that were inaccessible. Improper housing, a lack of medicine or clean water, and the fact that most of the survivors were homeless led to many people dying of malignant fevers.[18] The earthquake and tsunami killed between 1,000 and 3,000 people combined, nearly half the city's population.[citation needed] Disease ran rampant in the next several months, claiming an estimated 2,000 additional lives.[19]

https://en.wikipedia.org/wiki/Port_Royal[4]

Some types of soil, especially when saturated with water, such as the sand in a low sandy isthumus connecting two land masses, liquify during earthquakes and flow like mud. Thus the sandy isthmus could slind into the deeper water to eaither side during an earthquake and might not need any tsunami to scour it away.

But those examples may be on a much smaller scale than necessary to sink enough land that the low land masses are out of sight of each other.

How high are two land masses where the isthmus will sink? There is a distance X that is the maximum distance that a height Y can be seen from another height Z. I don't know how to calculate it, but there are examples of mountains which can be seen from other mountains hundreds of kilometers or miles away.

https://beyond-horizons.org/lines-of-sight/[5]

On the other hand, from places only slightly above sea level, the horizon appears much closer.

I have often stood on the boardwalk of Cape May, New jersey, and seen the lights of Lewes, Delaware across Delaware Bay at night. In the daytime I have clearly seen buildings, trees, and hills on the opposite shore thorugh binoculars. And sometimes I thought that with unaided vision I glimpsed tiny bumps on the horizon in the direction of Lewes, But I couldn't be certain.

I strongly suspect that Delaware would be visible to the unaided eye from the tops of tall buildings in New Jersey, such as the lighthouse at Cape May Point, and vice versa.

Apparently the direct distance across Delaware Bay from cape May to Lewes is 16.75 miles or 26.95 kilometers.

https://www.distance-cities.com/distance-cape-may-nj-to-lewes-de[6]

And people who have spent time at other straits around the world can say whether one shore is visible from the other shore and at what distance.

So you need to submerge a length of low, narrow isthmus between the two land masses which is long enough that the two land masses are far enough away not be be visible from the nearest points of their shores, and also make sure that the hills and mountains on those land masses are separated by enough space that peaks on one land mass can not be seen from peaks on the other land mass. And you will need to find the formula for calculating visibility at a distance.

So you need to design the geology of your two land masses so that a giant earthquake and tsunami can separate them.

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Whenever you want to see if water floods can mess with a coastline, look at the Dutch history.

St. Lucia's flood (Sint-Luciavloed) was a storm tide that affected the Netherlands and Northern Germany on 14 December 1287 (OS), the day after St. Lucia Day, killing approximately 50,000 to 80,000 people in one of the largest floods in recorded history.

This disaster was similar to the North Sea flood of 1953, when an intense European windstorm coinciding with a high tide caused a huge storm surge. The St. Lucia flood had a major influence on the subsequent history of the Netherlands.

The name Zuiderzee ("Southern Sea", from the Frisian perspective) dates from after this event, as the water had before been a freshwater lake that was only directly connected to the North Sea by the former river Vlie. The St. Lucia's flood removed the last of a series of natural sandy (dunes) and boulder clay barriers, after which the new, now salty Zuiderzee came into existence and grew rapidly, since the peatlands behind the former barriers were now mostly unprotected against erosion from the sea. The coming into existence of the Zuiderzee was the undoing of the powerful medieval trading city of Stavoren on the right bank of the now disappearing river Vlie, and the making of first the IJssel Hanse-cities of Kampen, Zwolle, Deventer, Zutphen, and Doesburg, and later the anti-Hanseatic city of Amsterdam, which began its rise from nothing almost immediately after the St. Lucia's flood.

After the flood, Harlingen, about 25 kilometres southeast of Griend and formerly landlocked, came into existence as the new seaport of Friesland, a role it kept for seven centuries.

If a storm can do that, a tsunami for sure can do worse, displacing large amounts of loose ground like the peatlands quoted above and creating permanent waters were before they weren't.

If you look further back, when what is now the Mediterranean sea was flooded by the Atlantic waters, it probably started with an earthquake causing an initial flood which then kept carving the rocks with an immense waterfall where now we see Gibraltar strait.

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    $\begingroup$ The Mediterranean was already a depressed area before the flood. $\endgroup$
    – Mary
    Dec 12 '20 at 16:05
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I suspect that a variation on the Zanclean flood would meet your requirements. The historical event does not match your requirements exactly but is not far off and it would be relatively easy to modify the event to better match what you want.

Check out this video: https://www.youtube.com/watch?v=B5uW7Qg6rXM

Now imagine that this event had occurred with slightly different geology at the straits of Gibraltar – slightly wider with a thin crust of stable material and a wide band of sediment bridging the gap. Such a barrier could have very rapidly disappeared following an earthquake and instead of mere cubic kilometers of water per minute there could have been dozens or even more.

You wouldn’t want to be standing downstream.

In terms of the “other end” of the sea, perhaps it’s in the Arctic and when flooded rapidly breaks up is swept away and melts.

Further details here: https://www.sciencedirect.com/science/article/pii/S0012825219302521

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Yes, and a version of this may have happened around 8,000 years ago: look at the history of Doggerland and the Storegga Slide.

Doggerland was a large, low-lying, sandy landmass across what is now the North Sea and the English Channel, forming a land bridge from Great Britain to continental Europe. It was largest while sea levels were low, during the last glacial periods. As these ended, it was gradually diminishing and on its way to being submerged by rising sea levels, when the Storegga Slide happened — a massive submarine landslide (one of the largest we know of in any era) off the coast of Norway, triggering a tsunami in the Atlantic Ocean and North Sea.

Exactly how seriously the tsunami impacted Doggerland is unclear, and still debated (as far as I can see from some quick reading — I’m no expert and haven’t looked seriously into this). The latest consensus seems to be that Doggerland had already mostly been submerged by rising sea levels by the time of the slide. But some earlier models proposed that the tsunami was the decisive final blow — washing away enough of Doggerland to reduce it from an intact land bridge to a handful of islands. So the idea of a low-lying land bridge getting washed away by a tsunami is definitely within the realm of scientific plausibility.

The other half of the problem is whether it’s plausible to have this kind of land bridge joining two major continents, or two halves of a supercontinent, in the first place. This I’m not certain of; most major isthmuses today are, essentially, peaks of submerged mountain ranges, so are higher, rocky, and presumably less susceptible to getting quickly washed away. The nearest I can find today is the Isthmus of Suez linking Africa to Eurasia — suitable small and fairly low-lying, but still geologically rocky as far as I can tell. Speculating, I guess sandy landmasses aren’t likely to persist so long as isthmuses, exactly because they would erode more quickly. But at least this aspect seems not too implausible: suitable landmasses form and persist in many other places, and the Isthmus of Suez is not so far from being suitable.

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