Why deserts are sandy

Shallow caves are sometimes formed at the base of cliffs by this means. As the desert mountains decay, large areas of shattered rock and rubble occur. The process continues and the end products are either dust or sand. Dust is formed from solidified clay or volcanic deposits whereas sand results from the fragmentation of harder granites, limestone and sandstone. As the mountains are eroded, more and more sand is created.

At high wind speeds, sand grains are picked up off the surface and blown along, a process known as saltation. The whirling airborne grains act as a sand blasting mechanism which grinds away solid objects in its path as the kinetic energy of the wind is transferred to the ground. Sand and dust storms are natural events that occur in arid regions where the land is not protected by a covering of vegetation.

Dust storms usually start in desert margins rather than the deserts themselves where the finer materials have already been blown away. As a steady wind begins to blow, fine particles lying on the exposed ground begin to vibrate.

At greater wind speeds, some particles are lifted into the air stream. When they land, they strike other particles which may be jerked into the air in their turn, starting a chain reaction. Once ejected, these particles move in one of three possible ways, depending on their size, shape and density; suspension, saltation or creep.

Suspension is only possible for particles less than 0. In a dust storm, these fine particles are lifted up and wafted aloft to heights of up to 6 km 3. They reduce visibility and can remain in the atmosphere for days on end, conveyed by the trade winds for distances of up to 6, km 3, mi.

The sunlight can be obliterated and it may become as dark as night at ground level. The mean particle size was 1. Sandstorms occur with much less frequency than dust storms. They are often preceded by severe dust storms and occur when the wind velocity increases to a point where it can lift heavier particles. These grains of sand, up to about 0.

Their weight prevents them from being airborne for long and most only travel a distance of a few meters yards. The sand streams along above the surface of the ground like a fluid, often rising to heights of about 30 cm 12 in. They are transported by creep, being rolled along the desert floor or performing short jumps. During a sandstorm, the wind-blown sand particles become electrically charged.

They are also unpleasant for humans and can cause headaches and nausea. Eolian processes are major factors in shaping desert landscapes. Some of the barren rock is to be found in the so-called Dry Valleys of Antarctica that almost never get snow, which can have ice-encrusted saline lakes that suggest evaporation far greater than the rare snowfall due to the strong katabatic winds that even evaporate ice. This is because they reflect more of the incoming light and their albedo is higher than that of forests or the sea.

Many people think of deserts as consisting of extensive areas of billowing sand dunes because that is the way they are often depicted on TV and in films, [43] but deserts do not always look like this. A sand sheet is a near-level, firm expanse of partially consolidated particles in a layer that varies from a few centimeters to a few meters thick. The structure of the sheet consists of thin horizontal layers of coarse silt and very fine to medium grain sand, separated by layers of coarse sand and pea-gravel which are a single grain thick.

These larger particles anchor the other particles in place and may also be packed together on the surface so as to form a miniature desert pavement. They form perpendicular to the wind direction and gradually move across the surface as the wind continues to blow. The distance between their crests corresponds to the average length of jumps made by particles during saltation. The ripples are ephemeral and a change in wind direction causes them to reorganise.

Sand dunes are accumulations of windblown sand piled up in mounds or ridges. They form downwind of copious sources of dry, loose sand and occur when topographic and climatic conditions cause airborne particles to settle.

As the wind blows, saltation and creep take place on the windward side of the dune and individual grains of sand move uphill.

When they reach the crest, they cascade down the far side. As this wind-induced movement of sand grains takes place, the dune moves slowly across the surface of the ground. When these are extensive, they are known as sand seas or ergs. The shape of the dune depends on the characteristics of the prevailing wind.

Barchan dunes are produced by strong winds blowing across a level surface, and are crescent-shaped with the concave side away from the wind. When there are two directions from which winds regularly blow, a series of long, linear dunes known as seif dunes may form. These also occur parallel to a strong wind that blows in one general direction. Transverse dunes run at a right angle to the prevailing wind direction.

Star dunes are formed by variable winds, and have several ridges and slip faces radiating from a central point. They tend to grow vertically; they can reach a height of m 1, ft , making them the tallest type of dune. Rounded mounds of sand without a slip face are the rare dome dunes, found on the upwind edges of sand seas. This exposes coarser-grained material, mainly pebbles with some larger stones or cobbles, [34] [45] leaving a desert pavement, an area of land overlaid by closely packed smooth stones forming a tessellated mosaic.

Different theories exist as to how exactly the pavement is formed. It may be that after the sand and dust is blown away by the wind the stones jiggle themselves into place; alternatively, stones previously below ground may in some way work themselves to the surface.

Very little further erosion takes place after the formation of a pavement, and the ground becomes stable. Evaporation brings moisture to the surface by capillary action and calcium salts may be precipitated, binding particles together to form a desert conglomerate. Other non-sandy deserts consist of exposed outcrops of bedrock, dry soils or aridisols, and a variety of landforms affected by flowing water, such as alluvial fans, sinks or playas, temporary or permanent lakes, and oases.

Other landforms include plains largely covered by gravels and angular boulders, from which the finer particles have been stripped by the wind. In some places the wind has carved holes or arches and in others it has created mushroom-like pillars narrower at the base than the top. Here the Colorado River has cut its way over the millennia through the high desert floor creating a canyon that is over a mile 6, feet or 1, meters deep in places, exposing strata that are over two billion year old.

One of the driest places on Earth is the Atacama Desert. The cold Humboldt Current and the anticyclone of the Pacific are essential to keep the dry climate of the Atacama. The average precipitation in the Chilean region of Antofagasta is just 1 mm 0. Some weather stations in the Atacama have never received rain. Evidence suggests that the Atacama may not have had any significant rainfall from to When rain falls in deserts, as it occasionally does, it is often with great violence.

The desert surface is evidence of this with dry stream channels known as arroyos or wadis meandering across its surface. These can experience flash floods, becoming raging torrents with surprising rapidity after a storm that may be many kilometers away. Most deserts are in basins with no drainage to the sea but some are crossed by exotic rivers sourced in mountain ranges or other high rainfall areas beyond their borders.

The River Nile, the Colorado River and the Yellow River do this, losing much of their water through evaporation as they pass through the desert and raising groundwater levels nearby. There may also be underground sources of water in deserts in the form of springs, aquifers, underground rivers or lakes.

Where these lie close to the surface, wells can be dug and oases may form where plant and animal life can flourish. A lake occupied this depression in ancient times and thick deposits of sandy-clay resulted. Wells are dug to extract water from the porous sandstone that lies underneath. Lakes may form in basins where there is sufficient precipitation or meltwater from glaciers above. They are usually shallow and saline, and wind blowing over their surface can cause stress, moving the water over nearby low-lying areas.

When the lakes dry up, they leave a crust or hardpan behind. This area of deposited clay, silt or sand is known as a playa. The deserts of North America have more than one hundred playas, many of them relics of Lake Bonneville which covered parts of Utah, Nevada and Idaho during the last ice age when the climate was colder and wetter. The smooth flat surfaces of playas have been used for attempted vehicle speed records at Black Rock Desert and Bonneville Speedway and the United States Air Force uses Rogers Dry Lake in the Mojave Desert as runways for aircraft and the space shuttle.

Plants face severe challenges in arid environments. Problems they need to solve include how to obtain enough water, how to avoid being eaten and how to reproduce. Photosynthesis is the key to plant growth. It can only takes place during the day as energy from the sun is required, but during the day, many deserts become very hot. Opening stomata to allow in the carbon dioxide necessary for the process causes evapotranspiration, and conservation of water is a top priority for desert vegetation.

Some plants have resolved this problem by adopting crassulacean acid metabolism, allowing them to open their stomata during the night to allow CO 2 to enter, and close them during the day, [62] or by using C4 carbon fixation.

Many desert plants have reduced the size of their leaves or abandoned them altogether. Cacti are desert specialists and in most species the leaves have been dispensed with and the chlorophyll displaced into the trunks, the cellular structure of which has been modified to allow them to store water. When rain falls, the water is rapidly absorbed by the shallow roots and retained to allow them to survive until the next downpour, which may be months or years away. Saguaro grow slowly but may live for up to two hundred years.

The surface of the trunk is folded like a concertina, allowing it to expand, and a large specimen can hold eight tons of water after a good downpour. Cacti are restricted to the New World but other xerophytic plants have developed similar strategies by a process known as convergent evolution.

Some are deciduous, shedding their leaves in the driest season, and others curl their leaves up to reduce transpiration. Others store water in succulent leaves or stems or in fleshy tubers.

Desert plants maximize water uptake by having shallow roots that spread widely, or by developing long taproots that reach down to deep rock strata for ground water.

Some desert plants produce seed which lies dormant in the soil until sparked into growth by rainfall. When annuals, such plants grow with great rapidity and may flower and set seed within weeks, aiming to complete their development before the last vestige of water dries up. For perennial plants, reproduction is more likely to be successful if the seed germinates in a shaded position, but not so close to the parent plant as to be in competition with it.

Some seed will not germinate until it has been blown about on the desert floor to scarify the seed coat. The seed of the mesquite tree which grows in deserts in American is hard and fails to sprout even when planted carefully.

When it has passed through the gut of a pronghorn it germinates readily, and the little pile of moist dung provides an excellent start to life well away from the parent tree. Even small fungi and microscopic plant organisms found on the soil surface so-called cryptobiotic soil can be a vital link in preventing erosion and providing support for other living organisms.

Cold deserts often have high concentrations of salt in the soil. Grasses and low shrubs are the dominant vegetation here and the ground may be covered with lichens. Most shrubs have spiny leaves and shed them in the coldest part of the year.

Animals adapted to live in deserts are called xerocoles. There is no evidence that body temperature of mammals and birds is adaptive to the different climates, either of great heat or cold. In fact, with a very few exceptions, their basal metabolic rate is determined by body size, irrespective of the climate in which they live. One well-studied example is the specializations of mammalian kidneys shown by desert-inhabiting species. The cream-colored courser, Cursorius cursor , is a well-camouflaged desert resident with its dusty coloration, countershading, and disruptive head markings.

Deserts present a very challenging environment for animals. Not only do they require food and water but they also need to keep their body temperature at a tolerable level.

In many ways birds are the most able to do this of the higher animals. They can move to areas of greater food availability as the desert blooms after local rainfall and can fly to faraway waterholes. In hot deserts, gliding birds can remove themselves from the over-heated desert floor by using thermals to soar in the cooler air at great heights.

In order to conserve energy, other desert birds run rather than fly. The cream-colored courser flits gracefully across the ground on its long legs, stopping periodically to snatch up insects. Like other desert birds it is well-camouflaged by its coloring and can merge into the landscape when stationary.

The sandgrouse is an expert at this and nests on the open desert floor dozens of kilometers miles away from the waterhole it needs to visit daily. Some small diurnal birds are found in very restricted localities where their plumage matches the color of the underlying surface. The desert lark takes frequent dust baths which ensures that it matches its environment. Water and carbon dioxide are metabolic end products of oxidation of fats, proteins, and carbohydrates.

Kangaroos keep cool by increasing their respiration rate, panting, sweating and moistening the skin of their forelegs with saliva. The arctic weasel has a metabolic rate that is two or three times as high as would be expected for an animal of its size. Birds have avoided the problem of losing heat through their feet by not attempting to maintain them at the same temperature as the rest of their bodies, a form of adaptive insulation.

Being ectotherms, reptiles are unable to live in cold deserts but are well-suited to hot ones. They have few adaptations to desert life and are unable to cool themselves by sweating so they shelter during the heat of the day.

In the first part of the night, as the ground radiates the heat absorbed during the day, they emerge and search for prey. Lizards and snakes are the most numerous in arid regions and certain snakes have developed a novel method of locomotion that enables them to move sidewards and navigate high sand-dunes. These include the horned viper of Africa and the sidewinder of North America, evolutionarily distinct but with similar behavioural patterns because of convergent evolution.

Many desert reptiles are ambush predators and often bury themselves in the sand, waiting for prey to come within range.

Amphibians might seem unlikely desert-dwellers, because of their need to keep their skins moist and their dependence on water for reproductive purposes.

In fact, the few species that are found in this habitat have made some remarkable adaptations. Most of them are fossorial, spending the hot dry months aestivating in deep burrows. While there they shed their skins a number of times and retain the remnants around them as a waterproof cocoon to retain moisture. Heavy rain is the trigger for emergence and the first male to find a suitable pool calls to attract others.

Eggs are laid and the tadpoles grow rapidly as they must reach metamorphosis before the water evaporates. As the desert dries out, the adult toads rebury themselves.

The juveniles stay on the surface for a while, feeding and growing, but soon dig themselves burrows. Few make it to adulthood. Invertebrates, particularly arthropods, have successfully made their homes in the desert. Flies, beetles, ants, termites, locusts, millipedes, scorpions and spiders [88] have hard cuticles which are impervious to water and many of them lay their eggs underground and their young develop away from the temperature extremes at the surface. Humans have long made use of deserts as places to live, [93] and more recently have started to exploit them for minerals [94] and energy capture.

People have been living in deserts for millennia. These are the words typically used to describe the desert. But the endless, wind-swept dunes that come to mind only make up a small percentage of the deserts on the planet. A desert is actually just a place that has very little precipitation. Subtropical deserts like the Sahara are what people generally imagine when they think about the desert. The Sahara has rocky plateaus as well as sand dunes.

Despite these temperatures, the Sahara is home to olive trees, antelope , jerboa , scorpions, jackals, and hyenas. The largest desert on Earth is Antarctica, which covers The ice that covers the area is on average 2.

There is almost no vegetation in Antarctica, so the animals that live there are mostly carnivores, such as penguins, albatrosses, whales, and seals. How can both the Sahara and Antarctica be deserts? The reason is that they both see little precipitation during the course of a year, typically around 25 centimeters 10 inches , or less. This makes them both difficult places for plants and animals to live. Both deserts have fossil evidence suggesting that this was not always true.

Based on fossil evidence, the Sahara and Antarctica appear to have been the homes of many plants and animals in the past. Also called a midlatitude desert. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited. Tyson Brown, National Geographic Society. A ridge of curved sand is called a sand dune.

Sand dunes form because the wind can blow the sand about making shapes that are not unlike waves. That is why many sandy deserts are called sand seas. The sand is formed as the rocks of rocky deserts break up into small pieces.

The world's largest sandy desert is in North Africa, and is part of the Sahara Desert.