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I am wondering if there are any theories about the formation of oases, and I am also curious about why an oasis can even last for a very long period of time.

I have heard that fresh water exists on the surface of the desert because of a difference in elevation of the desert, causing the underground water to pop up. Is it the reason why oases exists?

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Oasis are places where aquifers are connected to the surface. The source of the water in the aquifer however can be hundreds of miles away in areas that do get significant rainfall.

The trick is geologic strat have different properties; some allow water to flow easily, others are very water tight, a water accessible layer covered by a water proof layer creates a aquifer. Faults or erosion can breach this water seal allowing to water to the surface, as can human wells. Recharge areas are places at higher elevation where water can enter the aquifer and flow downhill, they can be hundreds of miles away and are often in the mountains which often get much higher rainfall. A picture is worth a thousand words.

enter image description here

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  • $\begingroup$ This should really be the accepted answer, although the note about palm trees (whether naturally-occurring or human-planted) is helpful, this answer illustrates the elevation/geologic formation differences that cause oases. Basically they are created the same as aquifer-fed lakes - the only difference is the land around it. $\endgroup$ – MandisaW Dec 8 '19 at 17:39
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There are two types of oases: natural and human made.

Natural oases form when springs, created when underground aquifers allow fresh water to pool or flow on the ground surface of deserts, creating a fertile region.

A human made oasis occurs when humans create fertile region in a dry or arid region, such as a desert. The supply of water in human made oases is from underground water supplied by human made wells or water bores. Because wind-blown desert sands can damage or destroy wells, sturdy trees (such as palm trees) are planted around the periphery of the oases.

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  • $\begingroup$ But why the fresh water in the area can last for that long of period, when the area is low in rain fall and full of human activities. Like the Siwa Oasis which known to settle since at least 10th millennium BC. Would the fresh water just dry out before 21st century. $\endgroup$ – C.Calvert Dec 6 '19 at 6:21
  • $\begingroup$ Some aquifers can hold a lot water & continually release water, release rates are generally low, the volume of surface water is generally low, any shade over the water reduced evaporation rates. $\endgroup$ – Fred Dec 6 '19 at 6:47
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    $\begingroup$ I wonder if natural oases do form in the middle of desert? I always pictured it the other way around: desert forming in once fertile areas, oases being the last fertile "pockets". They don't appear, they remain (for now). Is it a misconception? $\endgroup$ – Jean-Marie Prival Dec 6 '19 at 9:46
  • $\begingroup$ As long as there is fossile water in aquifers available, an oasis can exist. Also, sub-terranean water can be re-supplied. sciencedaily.com/releases/2013/07/130722123014.htm But in many cases, human water tap of fossil ground water outweighs the resupply by far, leading to a rapid fall of the water table until it gets out of reach (Yemen, Arabia/Rhiyad/others ?) $\endgroup$ – user18411 Dec 6 '19 at 11:35
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    $\begingroup$ @AaronF It's possible that natural ones would tend to look similar - any that don't happen to grow palm trees would slowly be dispersed and buried by the desert winds and sand, and for those that have trees that are not at the edge, anything outside the tree line would be damaged and eventually killed until the treeline becomes the outer edge. Noting that the longer-lasting oases tended to have trees around their edges, humans would then plant trees on oases they wanted to maintain. It's just my own speculation, but it would seem to make sense. $\endgroup$ – anaximander Dec 6 '19 at 14:15

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