Rocky desert or Sand desert - Which would appear in this scenario?

Question

I've posed a question on how to build a continent with as many hot deserts as possible. On the basis of the answers I got on that question, I've drawn a map with the following desert.

1. Blue lines: Latitude and longitude (check right and bottom of the screen)
2. Black lines: Limits of the continent
3. Reddish thick lines: Mountain ranges
4. Blue area: Ocean
5. Green area: Fertile regions, with relatively high precipitation (irrespective of being tropical, temperate or whatever)

Let's forget about the plausibility of this desert as far as climate is concerned. That was addressed on another question.

Assuming that the yellow area has a desertic climate with low precipitation, I would like to know the type of ground that this desert would have. Would it be a rocky desert, like the ones in the USA? Or a sand desert, like the Sahara? And why?

Bonus question: Would it be possible to have different kinds of deserts with different features (rocky versus sand) dispersed through the yellow area?

Answer 1

The Sahara is actually mostly composed of gravel formed by the accretion of iron oxides exposed to the little moisture that does find its way into the region, there are large areas where sand overlies this gravel basement to considerable depths but most of the terrain is rockier than it is sandy this is generally true of most deserts.

The desert you've described has three borders that are going to be extremely rocky where the mountains drop colluvium generated by frost and wind erosion on the higher slopes. If we assume the same prevailing wind/latitude patterns as Earth then fines like sand and silt are going to be blown west in large linear, seif, barchan, or crescentic dune systems, depending on sediment supply. The silt will be blown right out of the desert basin to fertilise the sea while the sand will pile up on the western edge of the desert leaving desert pavement in the south east. On the far southern edge of the desert, where it dips into the roaring 40s, the reverse will be true due to the prevailing westerlies (westerlies blow west-to-east, easterlies east-to-west). In the northern half of the basin where the mountains block the prevailing winds the desert will make its own weather in the form of an almost permanent low pressure cell; this will draw in a little sea mist around the fringes allowing some plant and animal life to exist and creating heavy desert varnish. Due to continued erosion from frost and wind there will always be a little fresh sand in this central area which the internal wind systems will pile into large irregular and star dunes in the central desert, on Earth these can be hundreds of metres high. Most deserts have a number of different zones and being so vast and covering such a wide latitude and altitude range this one is going to cover almost all of the options.

Quick note parts of the areas near the mountains that you've shown as high rainfall are probably going to experience annual flood/drought cycles much like they see in northern Chile where it can rain metres in a couple of months but it's dry the rest of the year. It's an effect of the rain shadow and seasonal wind variations driving weather systems away from the coast instead of into the mountains.

Answer 2

This will mostly depend on the underlying geology, you need the right source rock for sand. You will certainly have rocky deserts in the mountains. Note that sandy and rock deserts often occur side by side with rocky headlands and sandy lowlands, see the image below.

Generally you need a quartz rich siliceous source rock, a large expanse of sandstone is a great source but any siliceous rock will work. Generally sand in deserts comes from non-deserts, either areas that were not desert in the past or chemical weathering of modern rock is the best way to produce sand. It can also be formed in the desert directly from weathering sandston. However, it will be a small area this way. Strong winds to remove dust and clay is also important, but that also keeps the sand moving.

Most importantly sandy deserts tend to be small compared to the area you have. The Sahara for instance is only sandy in a few places. The bulk of it is rocky or scrubland. Check out Ash's answer below for a more detailed description of what you can safely use, assuming your mountains are siliceous and your desert is old.

Answer 3

In general deserts are rocky by default.

You get sand via erosion, either wind or water based, which then moves the fine particulates to areas where it accumulates. For wind erosion those dry rocks get worn down and the sand blows and will accumulate in areas where the prevailing regional winds deposit it. For water erosion (your desert won't always have been a desert in geological time) you have rivers and streams eroding rocks depositing the fine particles along the waterways, especially in deltas or lake beds.

See: http://earthsky.org/earth/how-did-the-sand-in-the-desert-get-there

So rocky or sandy? The answer is yes it will likely be a bit of both.

It will be rocky with sandy deposits in areas that had ancient river or lake deposits and in wind catchments areas.

Answer 4

Read up on deserts and sand dunes.

You can get a bunch of different answers depending on * the amount of available sand, * the pattern of winds * the annual and decadal moisture regime

Desert is defined diferently by different groups.

Popular: Dry and dusty. No plants. Geographer: Less than 10 inches (250 mm) precip a year. Ecologist: Potential evapotransporation = X * annual precipitation, with X > 1

The geographer considers both the high arctic (north of tree line) and Antarctica to be cold deserts.

An ecologist's desert can be full of life. But water is the limiting factor in most organism's existance.

Most of Montana, Wyoming, Utah, southern Idaho, eastern Oregon eastern Washington, much of California, Nevada, Arizona, New Mexico, West Texas are desert.

Can desert types be mixed.

Very much so. Much depends on the geology and the prevailing wind. If you have easily eroded material, you get lots of sand. If you have a prevailing wind from one direction, the sand collects on the down wind side of the region.

Barach dunes (crescent shaped) can form sand islands in the middle of a cobbled plane.

Elevation changes can make for local wet areas. Much of the Western U.S. is 'Basin and Range' with fairly small (25-50 mile long x 10 mile wide) mountain ranges separated by fairly flat valleys. The valleys are desert. The mountain ranges catch what rain happens.

Because rain distribution can be heavily modified by mountains, cold currents etc , you can get lots of shadings in desert types, ranging from just barely a desert -- just very arid -- to places where no rain has fallen for years.

Because so much depends on wind you will get big differences depending on how the wind blows.

Most deserts of the world are neither sand nor cobble. They are dirt. Most deserts have some degree of life. That degree of life is both space dependent (Ravines running away from the equator have more shade, polar sides of mountains have more shade) and time dependent: Life is set up to take quick advantage of water when available: desert flowers that run through their entire life cycle in two weeks after a rain; brine shrinp that explode when the lake fills up, and leave a layer of eggs when the lake dries up.

Google desert on google images to see the impressive variety.

Answer 5

The real Sahara desert is about 3,000,000 square miles in area. It has vast ergs or seas of sand dunes. Most of it is covered by rocks and gravel. Some parts of the Sahara are very flat. Others are hilly or mountainous. There are some high mountains that have ancient trees thousands of years old growing on them. The Tenere region in Niger is so treeless that the famous tree of Tenere was the only tree for over 250 miles until it was struck by a careless driver in 1973. There are large areas of oasis. There were even reports of crocodiles in at least one body of water.

Maybe you want your fictional desert to be smaller than that. Then select some real Earth deserts of about the same size and read about them to see what kinds of landscape and ecology they have and what degree of variation.

Answer 6

There are other really good answers here so I'll just touch on where the sand is likely to be.

Sand and dust will be blown by the wind. So, check your prevailing winds. If your wind blows from west to east (like on Earth's northern hemisphere), the sand will blow with it. That means that the western side of the desert will be rockier than the eastern side.

One neat feature may be a low spot in the mountain ridge on the east side where some sand can spill out, making a small drift of sand stretching toward that sea.

One feature that doesn't really work is the mountain range crossing the top. It is possible to have that configuration if the top range is much younger but a more believable configuration would be to have the two side ranges form an inverted V.

I do see an issue with the desert that is below the mountain ranges. Maybe that is due to volcanic activity. That would give you 3 types of desert.

If it suppose to be non-volcanic, it is too symmetrical. The side that the winds are coming from (I'll assume west to be consistent with above) will have fertile lands extending further inward than the eastern side. If there are any sand drifts, they will form a crescent like shape with the bottom tip pointed toward the east.