Got Vinegar? EarthCache

Preparation

This dune is close to Cincol’s Steel Dune cache (GC21P25). You can access the site with a 4x4 or any sturdy car with sufficient ground clearance.

Next to a small bag to transport some sand back home please bring a magnifying glass along. If you don’t own one don’t forget your best estimating powers. Back home you’ll need a few drops of vinegar.

How are dunes formed

Dunes are mounds of windblown sand which can vary greatly in size and shape. Both size and shape depend on the supply of sand, and on how strong and steady the wind blows from one predominant direction.

The sand grains that form dunes are transported by wind. As the grains are usually too heavy to be lifted up permanently they skip and bounce over the desert floor. During transport the corners of the sand particles get chipped whenever they collide with the ground or other grains, and will slowly become more rounded with every little jump they make.

Similar to running water, wind drops its load of sand when the wind speed decreases. Whenever there's a wind shadow caused by an obstruction or a depression in the landscape, sand starts to accumulate. The more sand accumulates the larger the wind shadow behind this small mound becomes and the more sand gets trapped. If there is a sufficient amount of sand and the wind blows strongly over a long period of time the small mound will grow to become a dune.

Most dunes have an asymmetrical profile where the sheltered slope is steeper than the windward slope. The sand bounces up the shallower slope and gets deposited in the wind shadow just behind the dune’s crest. The more sand accumulates here and eventually slips down under its own weight, former the steeper leeward slope. This way dunes can migrate if the wind continues to blow from the same direction.

Barchan dunes of Qatar

Wind conditions have a strong influence on the overall shape of a dune. Constant winds form regular shaped dunes while irregular winds or changing directions tend to form irregular-shaped dunes (link dune types).

This dune, like many others in Qatar, is a barchan dune: a solitary crescent shaped sand dune, with tips pointing downwind. This type of dune is formed where the supply of sand is limited and the landscape is relatively flat, hard and lacking vegetation, and the wind blows from roughly the same direction. Quite often you see barchan dunes where one tip is larger than the other. This is because the wind is not always blowing from exactly the same direction.

Barchan dunes can migrate slowly with the wind at a rate of up to 15 meters per year.

The small ripples you see on the surface of the dune are oriented at a right angle to the wind. Like the dune, the windward side of these ripples has a longer, shallower slope and the leeward side a shorter, steeper slope. These small ripples show what the dominant dune form would look like when there was much more sand available in Qatar.

There are several other types of dunes. This website gives you a quick overview.

What dune sand can tell us

Sand, of which a dune is composed, is a granular material made up of finely divided rock and mineral particles, usually quartz. However any loose, granular material can be called sand, regardless of the composition as long as the particles size ranges from 0.0625 to 2.00 mm in diameter. Smaller particles are called silt or mud, while larger ones are called gravel.

In this part of the Middle East many dunes, but not all, are made up of sand sized carbonate rock particles, as the most common rocks here are limestone and dolomite. In other areas there is sand solely composed of black volcanic rock or pale pink gypsum.

Sand can be classified on the overall appearance of the grain, their size and shape, the amounts of larger and smaller grains (sorting), what material the majority of the grains is made up, their colour, and what additional particles are present. This information can mostly be derived by using a simple hand lens to describe the grains.

To determine if the sand contains carbonate rock a few drops of diluted hydrochloric acid or acetic acid (vinegar) can be used. Limestone, and to a lesser degree dolomite will fizz and dissolve, while other materials will not.

This information tells a great deal of the sand’s history. Round grains are considered to be mature. The corners of those grains are chipped off from spending a lot of time in the waves near a coast or bouncing over a hard surface. If many different grain sizes are present then the sediment was picked up from various places and most likely dumped very quickly. Think of a river that contains a lot of fast-flowing water after a strong rain fall or sediment transported within a glacier and left on the ground when the ice starts to melt. These sediments are often a big mixture of old rounded and younger angular grains. Sand that is composed of equally sized grains has been sorted by wind or waves.

Logging this earthcache

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References

Tarbuck, E.J. & F.K. Lutgens, 1999. Earth. An Introduction to Physical Geography (6th edition). Prentice Hall.