In arid climates, lakes may be ephemeral to the extent that they contain water only part of each year during the rainy season. A playa is a lake bed formed in an arid climate by rapid evaporation during dry season of wet-season stream inflow causing sediment and salt deposits (evaporites) to accumulate over time. One of the best known large playas is Owens Lake on the east side of California's Sierra Nevada:
Changes are frequent in playa lakes, as rainy seasons vary. Note the difference in the location of the shoreline in Australia's Lake Yamma Yamma:
The huge lake (more than 9,000 km2 [3,475 mi2]) in the image above is the Salar de Uyuni in Bolivia, the largest salt playa in the world. In this March 1973 image, the salar (salt pan) has about a meter of briny water (blue). Some of the surface is uncovered salt (white), which, in addition to sodium chloride, NaCl (rock salt) and calcium sulfate, CaSO4, also contains lithium chloride, LiCl, making this the biggest source of lithium in the world. This, and the Salar de Coipasa (upper left), were once part of a much bigger lake that had formed in this part of the Altiplano province of the Andes, during a rainier climate in the past.
The most caustic (Sodium carbonate-rich) lake in the world is Lake Natron in Tanzania. In the view below, the red in this lake is due to an alkaline-loving algae, Spiricum, which attracts more flamingoes to this lake than any other place on Earth:
Similar to some playas/salars are ephemeral lakes - those that fill to shallow depths during extended periods of wet seasons but during droughts dry up until the next periods of rainfall. A good example is Lake Carnegie in Australia; here is one of its filling phases but at other times of the year it dries partially into marshland.
Lake Disappointment in Western Australia is another ephermeral lake that can contain salt brine for years:
In an odd twist of vegetative activity, look at these two Landsat images of the Great Artesian Basin in east-central Australia: On the left, waters from the gently flowing Barcoo River has flooded the arid land to form a lake-like spillover, known when it is there as Lake Yamma Yamma. The land shows signs of widespread grasses and acacias. But in the dry season, the vegetation vanishes overland but widespread thick marshy vegetation now fills the lake:
Another lake that is dry most of the year is Lop Nor (nicknamed "The Great Ear of China" by the writer after seeing it in a Landsat image) in the Takla Maklan desert of Sinkiang Province in western China. The area is extremely dry but meltwater from the Tien Shan mountains to the north partly fill the depression each year. Multiple shorelines help to similate an earlike appearance:
A significant fraction of the Earth's land surface can be described as arid. This is primarily a climatic term, implying lower rainfall and distinctive vegetation adapted to the sparsity of water. Much of the arid terrains can also be called deserts. Deserts do not necessarily consist mainly of sand; some are mountaineous but with usually well exposed rocks and thinner soils. This map shows the main deserts of the World; note that most of Australia and a large part of Africa are arid lands:
Many landforms characterize arid regions and we would need numerous images to show their variety. We elect to start by showing 8 examples of what many people visualize as the hallmark of desert terrain - sand dunes, which people usually associate with the Sahara Desert that has been the locale for many movies (perhaps the best known: Beau Geste). But first, consider this sketch which shows the major types of dunes - given specific names - and the relation of their shapes to prevailing wind directions.
The dune forms are as follows: a) Crescentic dune; b) linear or longitudinal dune; c) star dune; d) parabolic dune; e) dome dune.
Our first Landsat example shows star dunes is found in the Grand Erg Orientale, a Sahara Desert member here at the border of Algeria with Libya. These appear as roughly equi-sided hills rising in some cases more than 100 m (330 ft) above the sandy base:
The largest active sand sea on Earth (560,000 km2 [216,237 mi2]) is the Rub'al Khali or Empty Quarter, shown above, in the southern Arabian Peninsula. Loose sand, up to several hundred meters thick, covers most of the solid rock landscape in this region. In the Landsat scene below, six distinct sand fields, each characterized by particular types of dunes (distinguished mainly by shape), occupy the entire area. Of these, we mention four. Most of the center consists of Complex Crescentic Dunes on the left and Complex Linear Dunes on the right. In the lower right corner are Star Dunes, and along the left is a separate sea consisting of small Linear and Complex dunes.
The ASTER sensor onboard Terra has generated false-color images of some of these Empty Quarter dunes )longitudinal type) in southern Saudi Arabia. In the version below, dunes are bright yellow and brown whereas the blue relates to the spectral response of interdune clays and silts.
For comparison, look at these more widely spaced longitudinal dunes in the Egyptian desert
Close-spaced longitudinal dunes are shown here; they are found in Oman in the eastern part of the Arabian Peninsula:
Elsewhere in the Rub' al Khali are these unusual seif dunes, with arrowlike point upwind, and again blue area which denote some surficial moisture.
A variant of these seif dunes, also found in Saudi Arabia, shows them to be extensively interlocked.
This next Landsat image shows three separate dune fields (note part of one in the upper right) in the Arabian desert. Each is a longitudinal type (the one in the lower right shows some curvature). The black smudges are smoke plumes from an oil well.
Not far away is still another example of multiple types of dunes in an image of Landsat-1 size:
Perspective images of sand dunes provide a different impression of their appearance. The dunes in Namibia are believed to be the oldest field on Earth. They also can show up to 100 m (330 ft) of relief. Here is an oblique view made by combining ASTER imagery with DEM data:
The image below is part of the Dasht-E-Lut, or stony desert, located in southeast Iran. Dominating the scene is a large, but isolated, sand sea containing linear or elongated star dunes with a prevailing northeast trend. The surrounding lowlands have gravelly surfaces. Along the lower left edge of this Landsat image are dark streaks that orient towards the southeast. These streaks are the eastern end of a big complex of long, narrow ridges called yardangs, left behind when winds blew out soft clay/sand sediments, leaving trenches. As the trenches elongated, they controlled the eventual scooping out of the linear valleys that defined the residual yardangs. In this scene, exposed bedrock comprises the low mountains to the east of the sand sea, from which streams carry and deposit dark rocks in a series of distributaries and fans.
These two ASTER images show details of the yardang field (left) and the dunes (right; some as high as 300 m - among the tallest in the world) intermingled with white salt pans:
Further to the east, in the Thar Desert of western India, strongly crescentic dunes have formed, as well displayed in this ASTER subscene:
Here are two Landsat images of dunes in Africa that have enough vegetation to support wildlife:
One example of dunes hosting vegetation is found in the Kalahari Desert in Namibia. Here dunes are migrating northward, overriding natural vegetation and farms, leaving some growth to still persist as more sand continues to move to the grasslands beyond the image top.
Another dune region, in Mali, is crossed by the Niger River and its secondary streams, which provide enough water for vegetation to develop locally (red patches in this Landsat TM scene).
A similar situation exists within dunes around the rapidly drying up Lake Chad. Vegetation now has profuse growth that has stabilized the dunes.
We saw an example of vegetation gaining a foothold among dunes in the Great Sand Hills of Nebraska (Section 6). This Landsat image shows the same scene, now in color, showing the distribution of vegetation (the reds) within the Sand Hills; an aerial photo is beneath it:
Here is a similar case: the vegetated dunes are near the Caspian Sea where the Volga River is emptying its flow:
The vast majority of dune fields are made up of quartz sand. But there are exceptions: dunes made up of clastic particles of composition other than SiO2. Best known are the White Sands (National Monument) of New Mexico (see page 6-6). These sands are composed of Gypsum (hydrated Calcium Sulphate). At one time the gypsum deposits were laid down in lake beds. Once the lake dried up, this mineral was broken into sand-sized grains that have been dispersed by the wind into the deposits shown in this image made by the ALI (Advanced Land Imager) sensor on NASA's EO-1:
The above aeolian views feature sand dunes. But many have the misconception that deserts are mostly sand. In fact, most of the desert terrain in the world is rocky, often with gravel surfaces or thin fragmental soil. Some weird terranes can form in arid lands where the wind blows persistently. Sand blast sculpturing is a common process, leaving odd-shaped forms that in space imagery really stands out. Consider this Landsat image of the Tanezrouft Basin of southern Algeria. The Sahara, of which this a part, is mainly rocky rather than sandy in cover. Here sculpturing has carved out deflation basins within horizontal to gently dipping sedimentary rocks. Patterns that tend towards circular are the mark of this kind of terrain, but on the ground the effect would probably go unnoticed.
This color TM image is typical of the Sahara Desert region, being a mix of sand-filled valleys and low hard rock mountains:
Deserts can also contain other substances besides sand. Much of Iran southeast of Tehran is a desert region in which large stretches of land are periodically under water in a brief wet season. Upon drying, playas and salt flats result. Over time, some of these erstwhile lakes are marked by wide bands of sediments, as seen from space. The salt marshes are known locally as kavirs. Although rocky and sand surfaces exist in Iran's Dasht-e-Kavir (see above), salt (NaCl) is mixed with muds in the lower, playalike areas. Here are two Landsat images of this colorful land.
The next image is controversial. Its Internet source states the view to be part of the Kavir Buzurg and claims the feature to be a faulted tectonic fold. While this may be the case, the writer (NMS) believes it to be the markings of a dried-up lake rather than sedimentary layers; there is a fault sliver in the center but it seems likely to be the result of underlying ground displacements that have affected the lake beds. Judge for yourself.
There is obvious tectonic activity in the Kavir region. This is a typical landscape.
This next image contains a real oddity. The locale is the Atacama Desert that was discussed on page 6-11. Make an educated guess as to what the features within the rectangle might be. Hint: images elsewhere on this page and on page 6-10 afford clues. Look now:
These are salt pans similar to those in south San Franciso Bay, in terms of colors. The salars shown above also display evaporite beds. The greens are caused by algae.
Sand dunes usually are made up of sand- and silt-sized particles, most commonly of quartz. But it is possible for winds to produce regular-spaced dunes composed of snow flakes and particles, as shown in this image of a snow field (the full extent of which is almost as spacious as California) in the East Antarctic Plateau. The dunes are up to 8 meters (25 ft) high and 2 to 6 km (1.2 - 3.7 miles) from crest to crest. Although the region was traversed for years prior to this image, the true nature of this drift field was not realized until space imagery brought out the pattern seen below:
This last image takes us to the next topical landform: glaciers and glacial landscapes.