Deserts cover huge areas, mostly between 10-30 degrees N and S of the equator. Many of these deserts are sandy, at least partly. Good example is the Sahara — the largest desert in the world. Ok, maybe it is the second largest after Antarctica. To describe a desert sand, we first have to make clear what we are talking about. When we imagine desert sand, we probably think of sand dunes, although dune fields cover only about 20 percent of modern desert areas. Dune sand is generally very well-sorted. It means that all the sand grains are roughly the same size. There is almost no dust. It is blown away and deposited elsewhere, there are also no gravel or boulders because wind is not capable of carrying such a heavy load.
The Majority of land surface in the UAE is a sand desert. Geologically the desert is a recent feature, caused by prolonged subaerial erosion and re-deposition in the desert environment. Throughout the UAE the sand differs in colour and composition, depending on its source. Near the coast the sand is white, made up of calcium carbonate, derived from the carbonate sediments, seashells and coral reefs of the coast. Further inland the sand consists of quartz crystals, a stable end-product of the chemical weathering of most types of rock. It can acquire various colours through impurities or coatings. For instance, red sand is quartz with amounts of oxidised iron. Black sand is derived from the igneous rocks of the Hajar mountains. A special type of white sand with perfect spheres called ooliths is formed in tidal channels between islands.
In many areas near the coast, sand has been stabilised by plant growth, although vegetation has been changed in recent times by the grazing of domesticated animals. Further inland the sands may be quite barren, this is because very few plants can successfully grow on mobile dunes
Sand dune formation is controlled by wind strength and direction, along with sediment supply. In reality, however, the formation of dune patterns remains largely unknown. Because dune patterns vary with wind direction, any variation in wind direction can add new elements into the overall pattern. These elements may reinforce or cancel each other, in the same manner as ocean waves. In addition, because sand dunes cannot move or change as quickly as ocean waves, past history may play a significant part in what we see today.
The desert sands vary in both composition and form. Near the coast they consist mostly of seashells and coral reefs of the coast. Further inland, the sand consists predominantly of quartz grains, the most common rock-forming mineral and is the end-product of the chemical weathering of most rock types. However, sand grains of the desert normally contain impurities or acquire fine surface coatings which may impart different colours.
Some basic dune forms descriptions
Barchan dunes; Are the simplest. These are individual crescent shaped dunes, convex in the upwind direction, having a relatively gentle upwind slope but a steep slip face downwind. They tend to form in areas where the sediment supply is limited and are most often seen atop gravel plains or salt flats. They may form fields of up to several dozen, advancing across the flats.
Transverse dunes Are elongated sand ridges that form perpendicular to the prevailing wind direction. Most of the active dune fields in the UAE are of this general type. Typically they occur in parallel arrays with elongated troughs between, but they may be quite variable. The crestlines may be relatively straight, or sinuous or cuspate (in which case they may be called barchanoid), and oblique elements are often present. Like barchans, transverse dunes are asymmetrical, having relatively gentle upwind slopes and steep slip faces on the downwind side.
Linear dunes, also called longitudinal or seif dunes Are oriented parallel to the prevailing wind direction. Their mode of formation is not well understood and various explanations have been proposed, including consistent high wind velocities, a bi-directional wind regime and helical air flow along the troughs between the dunes.
Of course, combinations of sand dune shapes can occur, so the patterns are often difficult to recognize.
The major dune ridges in the UAE were formed in the most recent ice age, some 18,000 years ago. Glaciation helped the sand dunes to develop because there were strong winds in the narrow area between the ice front and the equator. In addition the global sea level fell and this caused the Arabian Gulf to fall dry, exposing the masses of loose sediment that could be blown into dunes.
The prevailing wind direction in the UAE today is predominately from the north-west. Therefore active dune crests are usually aligned from the north-east to the south-west, with the steep face to the south-east.
A Sunset is well worth watching in the UAE for its geological significance. A rare and unusual statement I know but bare with me.... Fine-grained dust particles that have eroded from the Earth's surface are carried by the wind and suspended in its atmosphere. This suspension helps to refract the suns rays and produces the reds, oranges, and yellows of the Emirates' spectacular sunsets.
Let us not forget sandstorms. Take the warm Arabian climate and the volume of sand add a little wind and that very quickly produces airborne dust that produces a haze that makes driving (and breathing) difficult. This is the super fine grain that is found everywhere here. It is forever swept from portches and patios and even penetrates into the home. It is called "loess." It can travel for thousands of miles. On the upside, when it settles it produces good agricultural soil.
To log this earth cache you must answer the following questions and post a photo of you or GPS at GZ.
1. What causes the colour of the Dunes at GZ?
2. Give an example of a different colour (and cause) of sand in the UAE
3. What type of Dune is found at GZ?
4. Approximately how high is the largest Dune near GZ?
5. There is a monitoring station close to GZ what is it monitoring?
6. Name another location of dunes and describe any geographic or geological similarity to what is found at GZ.
This is an open desert ground so available at anytime but suggest daylight may assist a little with some of the questions. The sand just past the parking area looks quite stoney but be warned it is softer than it looks so use caution if in a 2x4.
Many thanks and happy caching :)