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)
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Usually a limestone bedrock layer.. that’s what allows the sand to remain in place despite winds. The desert sand is like an iceberg – it’s got a sturdy layer of limestone beneath it representing the mostly unseen bulk of it.
Basically rock. The earth has a hard shell of rock called the crust all over. Under the soil, under the sea, under the desert, under the ice; it’s rock everywhere.
So first off, most deserts aren’t sand. The largest sand desert in the world is the Rub Al Khali on the bottom half of the Arabian peninsula.
As a geologist, I’ve drilled in the area on some geotech projects so I can tell you what that’s like.
Depending on what you’re dealing with, you can have a sharp contact between rock and soil, or something more gradual. Generally, we’d drill in the sand and it would grade into a carbonaceous mudstone. Slowly becoming harder and more cohesive, there was also a fair amount of gypsum.
And yes, we did encounter groundwater on site. You go deep enough anywhere and it’s going to be rock and wet. To be clear, you’re not hitting an underground river or lake (they do exist in some places) but generally groundwater exists in the spaces between the material. In sand, there’s a surprising amount of space that holds water and let’s it move about easily. In clay there is much less space, so you’ll get much less water.
I’m guessing you are thinking of the classic sand dunes you see in movies set in the desert. Like you get hills of sand with little or no vegetation and can often see the sand blowing around in little wisps up to full-blown sandstorms, depending of the amount of wind. *Please correct me if I’m wrong.*
You see these dunes in parts of New Mexico USA, the Arabian Peninsula, North Africa, western South America, etc. You also get dunes and similar landforms in other places where the sand blow around enough that plants can’t get established to hold it down.
And that’s the key. This type of sand dune desert is formed by sand that is blown around by the wind. The dunes actually move slowly across the landscape with sand being eroded away on the upwind side of the dunes and deposited on the downwind side. So this means the sand forms younger deposits on top of whatever older rock was there before. The older rock is commonly sedimentary rock like shale, sandstone, limestone or whatever. But it could be any type. The sand blows over top so there is typically a sudden change from the loose sand to the older harder rock.
One thing that can happen is that the wind-deposited sand stops moving, maybe due to a change in rainfall. Then with time the sand beneath the surface can be slowly cemented into sandstone. In that case the sand might grade down from loose near the surface to rock below.
There are other types of desert sediments and other things that can occur but I think that is the ELI5 version.
I can’t speak to what specifically lies beneath deserts, but I think you would find it interesting to know that there is no distinct point where sand (soil in general) becomes rock. So theoretically, if it does just continually get more and more compact as you go down, then it is a case like Saturn where there’s no real line where it becomes something else.
Given that earth is more complicated than that though, I’d hazard a guess that there are more distinct layers
It depends where you are. The present desert you see is all “modern” sediments that are blown around. It depends where you are in the world. In the Sahara sometimes between the dunes you can find lake sediments (and ancient human artifacts who lived around the lake) from when the Sahara had lakes. Below those sediments the Sahara has a lot of different bedrock, some is lithified Cambrian aged hard rocks.
The same stuff that’s under non desert lands – layers of rock. Here and there there might be caverns, but mostly it’s fairly solid, and if there is water, its either seeping though cracks in the rock or its higher up in the bottom sand layer, where it is basically creates wet sand.
Imagine if you filled a glass with dry sand and then poured in some water. It wouldn’t form it’s own clear water layer, it would seep down and mix into the sand at the bottom, so near the top of the glass you would have dry sand, and towards the bottom, wet sand.
For anyone struggling with dessert vs desert: “dessert has two S’s because you always want seconds” – my first grade teacher
OP says he knows what a desert is. But he says deserts are just huge amounts of sand… hmmm…
From geotechnical persepct: the soil or sand void ratio(voids for liquid to get into) to solid particulates in a volume), along with porosity, is directly related to ground water flow. Water pressure is proportional with height, more water column, more water pressure.
So all that boils down to, deoending on how the sand layers are, and other comments and a google search can tell you. The diff layers of sand, soil, dirt, rocks, clay, all can contain water. Ground usually sits on clay, and stones. All together, there can be hundredsnof meters of soils and rock layers that can allow for water ‘flow’. In sand its like wet sand at the beach, water can flow through due to porosity, and that is done through sand that evebtually can get compacted by the flow, and we get stuff lile ‘quick’ sand, or like low bearing pressure sand. There are tons of engineering courses on groundwater flow, geotechnics, etc. I did this as part of my civil enginerring, but i am a structurak engineer atm. So ground water flow, is for geotechnic guys. Thank god, as ‘getting’ geotechnical engineering isnt easy, often cited as harderst units for eng degrees are the geotechnics. I failed one out of three units, but i improved immensly the second time around which is great as i do retaining walls and sometimes dams too. But think of it like a system, not like individual layers, as ‘layers’ often intersect, there are disctinct lines and levels of soils and rocks. But water flows through them differently due to their physical composition and porosity.
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