Sand (and sandstone) formations display a range of colours from white to deep red and include yellows, reddish yellows, browns and buff. For example, the Saharan and Arabian deserts are mainly yellow and red. Depending on the light, dune sands can change colour from yellow to orange and red.
Virtually all sand grains are coated with a thin glaze (varnish) of iron-rich clay minerals; it is this that gives the sand its colour.
There is no consensus of opinion as to how or where this originated.
In addition to bright-yellow colours, the most widespread colour in desert environments is red. Field investigations have shown that red colour in desert sands is caused by the presence of hematite (iron oxide) coatings on individual grains. Reddened sands have been observed in deserts throughout the world, but their mode of formation is a matter of controversy.
Color Zoning in the Western Desert of Egypt
Farouk El-Baz and Hassan A. El-Etr
The source of the redness in dune sands puzzled early workers, who noted that it could not have come from within the sands themselves, which were largely quartz.
Desert Geomorphology. Cooke, Warren, Goudie, 1993 p314
Not only is there little research involving the nanoscale characterization of coating mineralogy, but also the formation mechanisms of these coatings are poorly understood.
Iron Oxide Coatings on Sand Grains from the Atlantic Coastal Plain
R.L. Penn, C. Zhu, H. Xu, D.R. Veblen
Geology; September 2001; v. 29; no. 9; p. 843–846;
The redness in an aeolian sand may derive from the parent sand (an already red alluvial sand or, more rarely, one with minerals which supply iron when weathered), from infiltrating waters, or perhaps most commonly, from dust added to them soil surface (walker 197, Wasson 1983a, Pye 1983h). Some authorities have found that source is the most important control on colour (Anton & Ince 1986).
Desert Geomorphology. Cooke, Warren, Goudie, 1993 p316
Most dune sands are primary said to be borne by water and later shaped by the wind. That is to say millions of years of rainfall gradually decouples individual grains (mainly quartz) from granitic basement rock and transports them via rivers systems either out to sea or deposited inland forming enormous piles of sand. Some becoming lithified to form sandstone.
The Namib Desert provides a good example.
The vast Namib Sand Sea, which covers roughly 13,125 square miles (34,000 square kilometers) along the coast of Namibia, is one of the world’s oldest and largest sand deserts. However, little is known about the origin of its sands — whether they come from remote sources or local sediments. This uncertainty holds true with other large deserts as well, largely because one sand dune looks much like another.
Sand Grains in African Desert 1 Million Years Old
LiveScience – Charles Q. Choi – 1 Nov 2010
More recent studies suggest the predominant source of the sand lies 3000 km away upstream of the Orange River!
Sourced as the Nile in distant basaltic rift highlands, the Orange River is the predominant ultimate source of sand for the Namib Desert dunes, as proved independently by bulk-petrography, heavy-mineral, pyroxene-chemistry, and U/Pb zircon-age datasets.
After long-distance fluvial transport, Orange sand is washed by ocean waves and dragged northwards by vigorous longshore currents. Under the incessant action of southerly winds, sand is blown inland and carried farther north to accumulate in the Namib erg, a peculiar wind-dominated sediment sink displaced hundreds of kilometres away from the river mouth. And yet changes in sand mineralogy along the way are minor. After a multistep journey of cumulative 3000 km from their source in Lesotho, volcanic rock fragments and pyroxene are found in unchanged abundance as far as the northern edge of the desert. (Author underline)
Petrology of The Namib Sand Sea: Long-Distance Transport and Compositional Variability in The Wind-Displaced Orange Delta
Eduardo Garzantia, Sergio Andòa, Giovanni Vezzolia, Michele Lustrinob, Maria Bonic, Pieter Vermeeschd,
Earth-Science Reviews – Volume 112, Issues 3-4, May 2012, Pages 173–189
The Orange River: which flows westwards through South Africa to the Atlantic Ocean.
Namib sand is thought to originate from the highlands in the east.
If this is how the Namib formed there are at least three glaring obvious and basic problems.
Firstly: A quick search reveals ages of between 104 to 55 million years down to a possible maximum age of 5 to 6 million years for the Namib Desert (think of a number and add a few zeros so as to keep uniformitarian minds happy).
So, are we to believe as the Namib gradually formed, it remained unaffected by tectonic activity, glacial and interglacial periods, sea level fluctuations and Milankovitch cycles?
In other words, when most major rivers are thought to be only 5000 years old the Orange River somehow remained virtually undisturbed for millions of years.
Secondly, the composition of the sand reveals an unusually high level of feldspars when compared to that of other sandy deserts such as the Sahara or Arabian deserts which are largely quartz (more than 90 percent quartz, the remainder feldspars).
Sand collected from the Namib dunes (Sossus formation) showed the sand to be composed of about 46 per cent quartz, 24 per cent feldspar, 13 per cent rock fragments and 12 per cent heavy minerals
Low Depositional Porosity in Aeolian Sands and Sandstones, Namib Desert
Warren W. Dickinson and John D. Ward
Journal of Sedimentary Research – Vol A64 . No 2 – April 1994 – p 226-232
This presents an impossible situation even by geologists own criteria, in that one of the most basic principles of geology teaches us that the primary source of quartz sand is granite rock which contains roughly 25 percent quartz (average) with the remainder feldspars – it is said that millions (billions, take your pick) of years of water erosion washes away the feldspars to form clay minerals (mud) – leaving behind the virtually insoluble quartz grains.
Question: How on earth did such a large percentage of feldspars survive the high energy 3000 km watery journey to the sea?
Having eroded from basement rock feldspars are said to turn to clay in the very first instance but here we are expected to believe this is not the case.
Geologists… which is it? Invincible feldspars or perhaps a complete rethink is in order?
Thirdly, the colour.
Assuming the impossible, that the Namib feldspars are indestructible insoluble grains… question: At what point during the 3000 km journey to the sea do the sand grains become individually coated with iron oxide?
Grains of Namib sand begin life devoid of coatings (quartz typically translucent), they are then washed, washed, and washed some more – indeed you couldn’t get a more abrasive soaking (just how did them feldspars survive?) and yet the sands somehow manage to secure a submicroscopic veneer of iron, staining the grains shades of yellow, red and orange.
It is well known and well understood that flowing water (especially sea water) would have the effect of bleaching fragments of rock, not coating them – yet another basic tenet of petrology.
Subsequently, if the coatings were introduced via groundwater or other subsurface fluids post deposition (as many propose) the evidence would be obvious. Clear quartz grains would dominate the region around the mouth of the river and increase in redness (oxidise more) consistent with the flow of the sand.
Clearly not the case.
“The Wave.” An area of fantastic, eroded Navajo Sandstone featuring beautiful swirls, striking colours (browns, beiges, yellows and buffs), countless striations, and bizarre shapes set amidst the dramatic surrounding North Coyote Buttes of Arizona and Utah. The Wave comprises of wind-driven sand dunes said to be laid down over millions of years. Groundwater is again thought to be responsible for the colours.
We ask the same basic question posed above; how does seeping groundwater (no matter what it consists of) manage to create such defined layers of bleached white sand the same time as creating interceding layers of red/brown/tan? Considering how some of the laminae are only a few inches thick, again this has to be impossible – seeping fluids cannot be so discriminate.
Could there be an alternative explanation for the origin of sand and how it got its colour?