I mean, a desert is basically just a huge amount of sand but where does it stop, what is beneath? I googled it and it says there’s groundwater but what does that mean, and (how) is that even possible?
Edit: So, actually the main question I wanted to know is if there’s a exact border between sand and rock, or if the sand just gets denser and denser until it’s rock (but I believe the weight must not be that big), like Saturn or something.
(Yes, I know what a desert is but I specifically need to know about this certain type of desert)
In: 1165
Everywhere on earth has a bedrock, and that bedrock ends at some place we call surface (even the bottoms of oceans have bedrock below).
In most places on earth, there is a (thin in geological terms) layer of loose materials, which are sediments (like at the bottom of the ocean) or soil (like in your backyard). The layer of unconsolidated materials (material that is not packed and turned into rock or rocky-ish stuff, so loose material like sand or silt or clay that a person can break into small bits fairly easily) depends a lot in its composition and grain size and thickness on what caused that material to get dropped where it now exists.
Can be zero thickness (no cover at all) to perhaps several kilometers (like the Mississippi delta deposits in the Gulf of Mexico), although commonly on land, the thickness is measurable in meters to tens of meters (a few hundred feet of loose sediment or “soil” is pretty uncommon in most of the world).
Deserts, like anywhere on earth, will have a cover of loose materials. Most people think of this in terms of sand (sand dunes and deserts are welded together in the imagination of most people) but this is hardly true. In many deserts, wind erosion exposes the bedrock to surface and only in those areas where the wind drops the sediment do we find sand dunes. The sand itself is there because it is, like with a beach, made up of mostly quartz grains, because quartz is very resistant to physical and chemical attack. It is the residue of rock that has been weathered and eroded away. And sediment (that is what the sand is, a sediment) gets dumped wherever it gits pushed to and the force moving it becomes too small to move it any further. Smaller stuff like clay keeps on going with the wind or water so all that is left is sand. That is why sand and sand dunes exist in desert areas, they are basically the wind equivalent of beaches and sandbars from water erosion.
What all is underneath the sand (when there is sand)? It depends very much on the local geology, what that area has for bedrock. Any rock can be found as bedrock if the geological history of the place made that type of rock be made at that place.
As to groundwater, this is also a very interesting thing. It turns out that soil/sediment particularly, and bedrock as well, still has open space and water, being liquid and subject to flowing down under the pull of gravity, will sink until it cannot sink further. All bedrock has groundwater passing through it. Some bedrock is pretty solid and has few fractures or holes so there is very little water, but there still is water.
Deserts generally do not have much groundwater near surface, but there is always water at some depth, even in desert areas. Oases are places where the water table (the top of the water surface that fills open spaces underground) actually makes it to the surface and can make small ponds. All ponds/lakes/oceans are places where the water table meets ground surface and fills beyond the surface of solid materials.
Whether or not the water is of drinkable quality is a bigger problem. High evaporation and low rainfall tend to cause water to become salty, but not all the water comes from local rain. Groundwater can flow over distances of many many kilometers, just like rivers. However, unlike rivers, there is a lot of resistance to migration so the “rivers” of groundwater (not usually actual rivers, more like a water-saturated wet cloth that slowly drips water from its bottom onto the bathroom floor) are very slow, and water you grab from the ground in some specific place might be many thousands, perhaps millions of years old, meaning it entered the ground some time very well in the past and only now is being brought back to surface.
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