The climatic extremes of deserts have created a wide range of landscapes. Try to picture a desert in your mind and the chances are you are imagining a vast featureless sandy plain as far as the eye can see, with only the odd cactus to break the monotony. Although this picture is not actually wrong, it only gives about a quarter of the story. Sand covers just 20 per cent of the world's desert.

Sand is formed by the erosion of rocks into tiny particles. This erosion is the result of a number of processes, the most important of which are the heating and cooling of rocks and the action of the wind and rain. The enormous temperature ranges experienced by most desert regions cause the rocks to expand and contract, eventually cracking and disintegrating in a process known as weathering. The smaller fragments of rock that break off are carried by the wind, and sometimes water, and they in turn erode other rocks. Gradually they become so small that they are merely grains.

The same wind that helps to form sand particles also shapes the landscape in other ways. Given the right conditions, the easily transported grains of sand will accumulate to form sand dunes. These extraordinary features can vary from small heap-like structures of only about a metre in height to enormous sand mountains of 1,000 metres. Some dunes often reach several kilometres in length. They are often found in large groups known as sand seas or ergs. The largest of these, covering around 560,000 square kilometres, is in the Arabian Desert. An important and intriguing characteristic of sand dunes is their tendency to move around. Typical annual movement of sand dunes is between 10 and 20 metres, depending upon size, but in extreme cases small dunes may travel up to 50 metres in a year.

But sand is just one of a number of landscape types to be found in the desert. Rocks and stones feature quite prominently in most of the world's arid places, often with vast plains covered in gravel, or large towering cliffs, eroded into extraordinary shapes by the action of wind and water. Elsewhere, the evaporation of ancient lakes has left enormous areas known as salt flats, which represent one of the greatest obstacles to plant and animal life to be found on the planet. Other areas are covered with clay or mud which has been dried hard by the heat of the searing desert sun. One feature that is common to many desert areas is the scarcity of soil, and consequently vegetation.

Deserts provide us with some of the most spectacular and stunning scenery on the planet. From the dramatic rock formations of Death Valley in the US, to the sand dunes of the Sahara, the world's arid zones constitute a natural gift that should be neither overlooked nor taken for granted.

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photo courtesy of DigitalVision