In this article we shall discuss the forces that shape deserts, and discuss how geologists can use this knowledge to identify rocks that are the lithified remains of former deserts. We shall use some terms that have been introduced in previous articles on sedimentary rocks and mechanical weathering and erosion; the reader who has not already read these articles may find it useful to go back and do so.
Definition of a desert
A desert is an area of low rainfall.
A desert by this definition is not necessarily hot: there are some areas of Antarctica that are considered deserts. Nor does a desert necessarily conform to our stereotype of being sandy: a sandy desert is known as an erg.
Causes of deserts
The immediate cause of a desert is, by definition, lack of rainfall. This itself can have a number of causes, which are not mutually exclusive: an area can be a desert for more than one reason.
- High-pressure deserts. In zones of high atmospheric pressure, the ability of air to contain moisture is increased, resulting in little rainfall. Examples include the Sahara, Arabian, Thar, and Kalahari deserts, and the desert regions within the Arctic and Antarctic circles.
- Mid-continent deserts. Areas in the middle of a continent can receive little rainfall simply because rain, originating from evaporation of seawater, will tend to fall before it can reach the middle of a large continent. Modern examples are the Turkmenistan, Gobi, and Great Australian deserts (the Great Australian Desert is also in a region of high pressure).
- Rain-shadow deserts. Rain will tend to break over mountains, so the presence of mountains can prevent rain from the sea from coming inland. Examples of rain-shadow deserts include the Mojave desert in the rain-shadow of the Sierra Nevada, the Patagonian desert in the rain-shadow of the Andes, and the Iranian desert in the rain-shadow of the Zagros mountains.
- Upwelling deserts. Finally, a desert may be by the coast and not in the rain-shadow of any mountains, but be adjacent to where a cold current of water rises to the ocean surface, reducing evaporation. Examples include the Atacama desert, the Western Sahara, and the Namib desert; these are all also in high-pressure zones.
Deserts and water
Although rainfall is rare in deserts, its effects are important in understanding the geology and ecology of deserts. It is characteristic of deserts that they exhibit internal drainage: that is, the water input to the desert in the form of rain or runoff from mountains does not flow out of the desert in the form of a river, but rather evaporates within the desert: this produces some highly typical depositional features.
Alluvial fans and bajadas. Deserts are often found associated with mountains: indeed, as we have seen, in some cases the mountains are the indirect cause of the desert. Runoff from rain on the mountains erodes them and transports the sediment downslope: when the sediment-rich water loses energy at the foot of the mountains, it forms a braided stream which rapidly deposits all but the lightest sediment in an alluvial fan ("alluvial" because it is formed by water, and "fan" because this is the shape in which it is deposited). Two or more adjacent alluvial fans can merge to form what is known as a bajada.
Playa lakes. These are temporary lakes, which, being shallow and fed only by intermittent rainfall, dry up in the arid climate of the desert, leaving behind a flat, heat-cracked bed of clay, known as a playa. One such is pictured to the right.
If the water contains significant quantities of dissolved minerals these will be deposited on top of the clay layer as the water evaporates, leaving an evaporite bed of such minerals as halite, calcite, gypsum, borax, or trona, depending on the rocks that were their source. Repetition of this process builds up alternating layers of clastic and evaporite sediments.
Oases. Oases are small lakes in the desert. These have a number of causes: they can be fed by springs; they can be deflation lakes, where erosion has caused a hollow the bottom of which lies below the water table; or they can be intermittent lakes filled by occasional rainfall and runoff.
Deserts and wind
Deserts are the only places where wind is a major factor in erosion and deposition: soil bound by moisture and vegetation is harder for the wind to budge than loose, dry particles. The reader should recall from earlier articles that processes associated with the wind are known as aeolian; the same term is used to distinguish sediment transported by the wind, or sedimentary rocks formed from such sediment: so we can talk, for example, of aolian sandstone: i.e. sandstone formed from sand heaped into dunes by the wind.
Wind has a number of effects. First, it can remove the sand and other light particles from the surface. leaving bare rock or the stony mosaic surface known as a desert pavement as shown in the photograph to the right.
Thirdly, it can pile sand up into dunes, giving us our stereotypical image of a desert: an erg. The dynamics and shapes of dunes vary depending on the regional winds; however, a typical dune is shaped something like a triangular prism with its long axis at right-angles to the direction of the wind. The wind transports the sand up the windward (stoss) side of the dune, to build up just over the crest of the downwind (lee) side of the dune. It accumulates until it reaches a critical angle at which it must avalanche, at which point the accumulated sand slides down the lee face of the dune to form a grainflow lamina.
The result of the transport of grains from stoss to lee is that the dune will move downwind. However, it will not always erode completely on the stoss side, instead leaving behind it a set of cross-beds composed of the bottom of the grainflow laminae. Then the next sand dune to pass that way can deposit another discontinuous set of cross-beds on top of that set (and on top of any sand that may have been deposited by the wind between the passing of the two dunes). By this process, set after set of cross-beds build up: the lithified results are shown in the photograph to the right; similar patterns of deposition can be seen by taking a cross-section of modern dunes.
The cross-bedding is informative in a number of ways. First, it shows us that the sand was deposited in dunes.
Secondly, the gradient of a grainflow lamina tends to become shallower at the bottom: this feature can be seen in the cross-beds in the photograph. Now, tectonic events are quite capable of turning rocks sideways or upside-down, as will be discussed in later articles. The curves of the cross-beds are one of a number of features, known as way-up structures, that allow geologists to determine which way was originally up: the curves are concave in the up direction, convex in the down direction. In the case shown in the photograph, we can see that the sandstone is still the right way up.
Thirdly, the crossbeds show us the prevalent direction of the winds, as we shall discuss later on in the article on paleocurrents.
Lithified deserts: how do we know?
Desert sand is quartz, and the resulting sandstone will be quartz sandstone. This is not a definitive criterion for recognizing aeolian sandstone, since quartz sandstone can be formed under other conditions. We may, however, safely say that arkose or graywacke is not aeolian sandstone.
Much, though not all, desert sand is colored, the grains being stained on the outside with iron oxides such as hematite and goethite, giving it a range of colors through red and orange to yellow. However, some desert sands are not stained in this way, so we cannot definitively say that sandstone lacking this feature is not desert sand.
Cross-bedded features reveal transport by a current of wind or water. We can distinguish between the two cases by various features: for example, wind-formed dunes typically result in much bigger sets of cross-beds; also, aeolian sandstone uniquely exhibits pinstripe laminae, very narrow stripes only a few grains thick consisting of finer grains than are found in grainflow laminae.
Playa deposits are extremely distinctive, and leave geologists no doubt that what they are looking at was once desert: they can only be formed by repeated episodes of deposition in and evaporation of a playa lake.
Finally, we can look at the fossils, which, in a desert should be of land plants and animals, and of freshwater organisms in those rare places where the geology indicates a former oasis. No marine fossils should be found.
These features, especially in combination, allow geologists to identify a lithified desert.