Basin on a mountaintop plateau - how to keep the water salty?

Question

I have recently asked a question about creating a sea on a mountaintop. I have come to realize that I would run with some problems by doing this, namely about the salinity of the lake. I want that thread to stay on topic, so I'll post this question here, and possibly will edit the first question accordingly.

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So without further ado, let's try to provide some context:

• Cosmic background: An Earth-like planet, with a Moon like ours, orbiting a Sun like ours
• Geographical background: There's a continent (its size doesn't matter) on the Southern hemisphere with a north shore on the equator. On that shore, next to the ocean, there is a mountain. On the mountaintop there is a basin.

I want to create a permanent salt lake / sea on that basin... and to build a city on the shores of said lake / sea.

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Parameters:

• Mountain's altitude from sealevel to mountaintop: 1.000 meters
• Basin's radius: 100 Km
• Basin's maximum depth: 100 meters
• Water source: Primarily from rain (I guess that, being on the equator, it is quite rainy)

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Now, I have two questions:

1. Is such a lake possible? (Note: Not probable... possible)
2. How could such lake come to be formed? (Note: Having the lake form on the sealevel and then being lifted by some kind of geographical phenomenon is acceptable, as long as it remains steadily saline overtime)

To answer both this questions, it is acceptable to tamper with any of the parameters that I posted here, with two exceptions: a) The altitude must be suficiently high to be a mountain, but suficiently low to receive rainwater; b) the lake surface may be greater, but not lower than posted.

Other than that, feel free to change the altitude, radius, depth or even imagine alternative water sources (remember that you're on a mountaintop) to achieve the desired goal.

It is not acceptable to change anything on the Cosmic or Geographical background.

Answer 1

Catchment to lake ratio is relevant

Your lake is near the Equator, so there should be plenty of rainfall. We can calculate the ratio of catchment to lake surface, and then apply that ratio to your lake. The ratio we will calculate is total size of basin (catchment + surface) to surface. This will give us (if inverted) the percentage of the basin covered by lake surface.

We will restrict our list to lakes that are large, relatively close to the Equator, and not man-made (i.e. not a dam reservoir). Its a short list. All areas in square kilometers.

Lake             Catchment    Surface    (C + S)/S
[Victoria][1]    184,000      68,000     3.7
[Tanganyika][2]  231,000      32,900     8.0
[Kivu][3]        2,700        2,700      2.0
[Edward][4]      12,096       2,325      6.2
[Titicaca][5]    58,000       8,372      7.9
[Nicaragua][6]   41,600       8,264      6.0
[Toba][7]        2,550        1,103      3.3

There is also Lake Albert, but I couldn't find a reliable catchment for it.

There are two things we have learned here. First, all these lakes have outflows, and so your endoheric lake, with no outflow, could potentially have an even larger basin to surface area ratio. Secondly, there exists an example on Earth of a lake, Lake Kivu, that has relatively large, very deep, and even with an outflowing river it occupies about half of the area of its basin.

What this means for your basin

Your basin has a 100 km$^2$ radius and a total area of around 31,000 km$^2$. If you use Lake Kivu as a model, then you could easily have a 15,000 km$^2$ occupying half the basin, even if you still had a river flowing out. Since Lake Kivu is significantly smaller, you could use the Lake Victoria ratio, where the lake surface is about 8,500 km$^2$, taking up a little over a quarter of the basin. Again, this ratio is supportable even with water flowing out.

Alternately, if you want a 31,000 km$^2$ lake, you would need the basin to be 62,000 km$^2$ (140 km radius) using the Kivu ratio, or 115,000 km$^2$ (190 km radius) using the Victoria ratio.

Again, as mentioned repetedly in the other post, Lake Victoria is a giant lake on top of a giant mountain: the enormous chunk of land lifted a mile into the sky between the two branches of the African Rift Valley. Victoria's basin is bordered on one side by the Aberdare range and solitary volcanic summits like Elgon and Meru, and the other by the Ruwenzori and Virunga mountains. But on the north and south it is bordered by high plateau land before it drops off to lower altitudes. Whether you want high encircling mountains, or just a flat topped plateau, Lake Victoria's situation has the scale and geological foundation you are looking for.

Why does it have to be salty

As mentioned above, the catchment ratios I listed are from freshwater lakes with an outlet. If you want a freshwater lake, you can still have lakes that large with an outlet.

If you allow a surface outlet, you can have spectacular waterfalls. Angel falls, the highest in the world, is caused because of a flat-topped mountain like you mention. The Tepuis of Venezuela have their own unique geological orgins, though they are smaller than you are looking for. Auyan-tepui, the host of Angel Falls, rises vertically almost 1000m above the surrounding jungle, and is a flat topped mountain about 670 km$^2$ in area.

Also, if you do not want a surface outlet, you can have a subterranean outlet. If the river drains through a cave system, it can remain fresh while appearing to be in an endoheric basin.

Answer 2

Suppose the "mountaintop" is actually a plateau much larger than the proposed basin; the plateau might have an area of 300,000 square km at an altitude of 1000 meters. (For comparison, the Tibetan plateau has about eight times the area and five times the altitude.)

Now say nearly the entire top of this plateau is sloped inwards, with a low point a little over 100 km from the northern edge, and around the low point the plateau is bowl-shaped. So rain water flows from all over the plateau (eroding the surface and carrying salt that the water dissolves from the eroded material) toward this low point, collecting into a lake. Water will evaporate continually from the lake's surface, leaving the dissolved salt behind.

The deeper the lake gets, the more surface area it has (provided it doesn't overflow the edge of the plateau), and the faster water evaporates. The lake will remain a salt lake with no outlet river provided that as the lake fills in, the rate of evaporation rises to meet the rate at which water flows in before the lake spreads to the edge of the plateau.

So you just need that the amount of annual rainfall on the plateau (or at least the part that flows toward the lake) is the amount that evaporates in one year from a 100-km radius round lake at that latitude and altitude.

If you're really concerned about scientific realism you could work out the amount of rainfall and the rate of evaporation to see if they match. If the geography described above would collect too much rainwater, make the plateau smaller or make the lake larger; if too little, make the plateau larger or rainier, or lower the rate of evaporation, perhaps by being not so close to the equator.

Answer 3

Copied from: https://worldbuilding.stackexchange.com/a/68050/264 as requested by the poster.

This lake is on a mountaintop. That means its catchment area is little more than the lake itself—any other water goes down the outside of the mountain, not into the lake.

Thus you have in effect a giant-sized rain puddle that never dries up. I have a hard time picturing this.

As others have said, the normal means of making a salt lake can't work here. However, I don't consider this a showstopper, let's make a salt lake by a different method:

Long, long ago there was a massive magma intrusion in the area, perhaps there was some actual vulcanism but that's irrelevant. A huge area of granite was formed. As the millenia went by the material above this granite eroded away. (Granite only forms when the magma cools very slowly—which means it must be deep. The same material on the surface forms basalt—not nearly as hard. There's also an intermediate between these whose name I have forgotten in the decades since school.)

Now a supervolcano erupts, blowing a huge caldera—the size of your lake. This is lowlands, though, not a mountain. A salt lake forms, then it's opened to the sea and a fairly small amount of ordinary sedimentary rock is laid down on top.

Now the area is uplifted to your desired height, the wind chips away at the soft rock on top but it doesn't eat it all—the salt layer is still underneath. It reaches your desired height and the climate turns rainy for some reason. Now you have a basically freshwater lake on top of salt with a thin and damaged barrier—at some point the water reaches the salt and dissolves enough to make it salty.

If you will accept a somewhat greater deviation from your description:

While this is one mountain it's actually the foothills of an even higher mountain range that has arisen (uplift and vulcanism are often found together) The actual catchment area of the lake includes a decent chunk of those mountains, the water is flowing underground into your lake and through the salt.

Answer 4

Your real problem is keeping the lake a lake, without a wide basin surrounding the lake it is all but impossible to keep the lake from breaching and creating its own drain. Your best bet is to have it be a glacier carved lake on the plateau. such lakes have existed in the past. they do not survive for very long in a geologic sense but in the span of human civilization it could last. Many of our large inland lakes are glacier carved. All you need then is the right geology to get the shape, perhaps a cut off dome or old caldera, so you have a soft center and hard out regions. You will have a lot of land around your lake for settlement, basically it is surrounded by highland plains and will have feeder rivers.

Look up Lake Bonneville for ideas.