This is tied to a previous question of mine Does atmospheric pressure relate to altitude or sea level, what would happen if sea level rose, but the land was not submerged?

Given the same scenario of a continent surrounded by mountains and the sea level being a couple thousand feet high stopping short of going over the mountains, a commenter pointed out the problem of sea water still being able to seep through and flood the continental valley.

What natural material could reasonably make up massive mountains that stops sea water from seeping through? If there is no real-life material that could do this what properties would that material need, and would it make sense for it to make up the mountains?

  • $\begingroup$ As I mentioned in my answer to your previous question, with a continent below sea water, meaning water that invades the continent has nowhere to go, the continent will eventually become submerged regardless of seepage. Rain, if nothing else, will do this. Given that you need to handwave the continued survival of the continent, why isn't it enough to declare the mountains made of basalt, claim there's no seepage, and move on with your story? $\endgroup$
    – JBH
    Aug 6 at 4:22
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    $\begingroup$ The Dead Sea has a surface elevation of about 430 meters (1400 feet) below sea level. It is only about 77 km (48 miles) away from the Mediterranean. The Mediterranean does not flood it and never did. $\endgroup$
    – AlexP
    Aug 6 at 6:08

2 Answers 2


Granite is a material that both makes mountains and is not permeable to water unless fractured.

Granite (/ˈɡrænət/) is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies underground. It is common in the continental crust of Earth, where it is found in igneous intrusions. These range in size from dikes only a few centimeters across to batholiths exposed over hundreds of square kilometers.

Granite has poor primary permeability overall, but strong secondary permeability through cracks and fractures if they are present.

Granitic rock is widely distributed throughout the continental crust.[17] Much of it was intruded during the Precambrian age; it is the most abundant basement rock that underlies the relatively thin sedimentary veneer of the continents. Outcrops of granite tend to form tors, domes or bornhardts, and rounded massifs.

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    $\begingroup$ That being said, onetiny crack in OPs "wall" of mountains would wash out over time $\endgroup$
    – Hobbamok
    Aug 6 at 15:01

The answer is that it is not credible that a belt of un-fractured basalt or granite would surround a continent. If you want a continent below sea level then some seepage is bound to happen. The amount will depend on the exact circumstances and how credible you wish to make the scenario.

The best bet would be to have a thick belt of mountains around the continent, minimise the depth below sea level and accept some seepage. Seepage and rain fall could be removed by evaporation depending on the climate, but a small saline sea, lake or salt flat would form at the lowest point.

The best real world example is when the Mediterranean sea was cut of by closure of the straits of Gibraltar around 5 million years ago and much of it dried out.



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