Review Questions and Answers; Deserts and Winds

Steppes are vast, slightly dry to semiarid plains and grasslands that are transitional between humid lands and much drier, true deserts. Steppes and deserts generally lie between 15 and 35 degrees north and south latitude; the Sahara Desert (Africa) and its bordering plains and semiarid grasslands are a good example. The desert and steppe regions of North America and central Asia extend to higher latitudes (40 to 45 degrees). Desert lands and steppes comprise about 30 percent of Earth's land area (Fig. 13.2).

2. What is the primary cause of subtropical deserts? Of middle-latitude deserts?

Both result from prevailing high atmospheric pressures and descending wind patterns (Figs. 13.3 & 13.4). As air masses descend, they are compressed and warmed, resulting in a drop in relative humidity. Low relative humidities mean that evaporation potential is high and precipitation is unlikely. Thus descending winds are hot and dry. In the subtropical areas, the descending winds are part of the overall atmospheric circulation pattern. Humid air that rose in the tropics and flowed poleward at high altitudes sinks and flows back toward the tropics as trade winds or poleward as westerlies. In middle-latitude areas, surface winds crossing a mountainous area rise on the near (windward) side and sink on the far (leeward) side. Desert to semiarid conditions exist in valleys and basins where the prevailing winds descend after having first risen to cross a mountain range.

3. In which hemisphere (Northern or Southern) are middle-latitude deserts most common?

The Northern Hemisphere (Fig. 13.2) has, by far, larger areas of mountain-rimmed valleys and basins. Many are in the western United States, Mexico, and central Asia. Thus deserts and dry lands at middle latitudes (35 to 45 degrees) are much more prevalent than in the Southern Hemisphere; most are rainshadow dry lands.

4. Why is the amount of precipitation that is used to determine whether a place has a dry climate or a humid climate a variable figure? (Box 13.1)

Mean annual temperature alone is not a valid predictor of an area's moisture balance. A significant fraction of the precipitation runs off or evaporates; a smaller fraction infiltrates, adding to the soil moisture and contributing to groundwater recharge and perennial stream flow. Water lost through evaporation and runoff is effectively eliminated from the soil moisture budget.

Evaporation rates are directly related to air temperature and humidity, warmer air at saturation holding more water vapor than cooler air; thus evaporation increases as the air becomes warmer and dryer. Soil moisture content reflects the net difference between the quantity of water infiltrating the soil and that lost to evapotranspiration, runoff, and groundwater recharge. For a given, yearly precipitation, the evaporation potential (how much water would evaporate from a unit area each year given the area's mean annual temperature and humidity) largely determines if soils are usually moist or dry. If infiltration greatly exceeds evaporation, the region is humid; soil moisture and groundwater are plentiful enough to sustain abundant vegetation and perennial streams. Where the two are about equal, soil moisture is available for grasses and scattered trees, but little is left over to recharge the groundwater. This description characterizes steppe lands. In true desert lands, the evaporation potential greatly exceeds infiltration; soil moisture is quickly evaporated and the soil stays dry except for brief periods following storms. Thus 10 inches of annual precipitation in southern Arizona result in sparse vegetation, dry washes, and a desert landscape, while the same amount in central Siberia supports extensive forests, streams, and rivers.

The seasonal distribution of precipitation is also important; evaporation losses are much smaller if the rains come in the cooler season rather than during the hotter, summer months.

5. List four common misconceptions about deserts. (Box 13.2)

Deserts are infernally hot, lifeless, sand-covered wastelands dominated by wind erosion and deposition. Although containing some elements of truth, the preceding misconceptions fail to adequately describe the diversities in climatic conditions, living organisms, and geomorphic processes that characterize desert lands.

Many desert lands do have dunes fields of various sizes; some are quite extensive. However, bedrock, unconsolidated sediments, and weathered debris comprise the surface materials of most desert lands. Bedrock forms vast tracts of the Sahara and Arabian Deserts, and alluvium covers most of the low, desert valleys in the western United States. Daytime temperatures in excess of 130 degrees Fahrenheit have been recorded in Death Valley, CA, and in interior parts of Libya; but sub-zero, bone chilling temperatures are common during winters in the Gobi Desert. Animals and plants may be sparse and less diverse than in humid lands, but dry land organisms are well adapted to their harsh, environmental conditions. Seeds and eggs remain viable for decades, lying in wait for that next, rare, storm event to initiate a renewed generation of growth and reproduction. Wind action is certainly more important in dry lands than in humid areas. However, even in the driest deserts, running water associated with rare storm events plays an important role in shaping the landscape; accelerated runoff and sparse vegetation render the soils and regolith highly vulnerable to sheet erosion, rill cutting, and gully incision.

6. Why is rock weathering reduced in deserts?

Weathering rates are accelerated by persistently high levels of moisture, because many of the chemical weathering reactions involve solutions. Dry conditions, as in a desert, result in very slow rates of chemical weathering and in slow, overall weathering rates.

7. As a permanent stream such as the Nile River crosses a desert, does discharge increase or decrease? How does this compare to a river in a humid region?

Rivers flowing across humid lands generally show increased discharges downstream as they are joined by perennial tributaries. However, rivers that flow from humid lands into desert regions commonly show decreased discharges downstream, losing water to infiltration and evapotranspiration while intermittent tributaries seldom add discharge. The Nile has a large enough upstream discharge (supplied by the Blue Nile) to reach the sea, despite flowing across more than a thousand miles of desert lands in Egypt and northern Sudan. Other perennial rivers flowing into dry lands discharge into lakes or swamps, become intermittent, or eventually dry up completely.

One very interesting example is the Niger River in west Africa. It rises in the wet, tropical regions of Guinea and Ivory Coast less than a hundred miles from the Atlantic coast. However, the river flows northeastward and southward through the dry, sub-Saharan, steppe and desert lands of Mali and Niger. Along this route, the river loses discharge and shrinks to an extended series of shallow lakes and marshes during the dry season. As the river continues southward, it re-enters the wet, Atlantic coastal region (this time in Nigeria); and its discharge progressively increases downstream to the mouth. The northeastward-flowing, upper segment once probably ended in an interior lake or marsh system; it was captured and diverted to the Atlantic by headward extension of the present-day, lower, south-flowing segment of the river.

8. What is the most important erosional agent in deserts?

Although wind erosion is more prominent than in humid environments and rainfall events are rare, running water is still generally the dominant agent of erosion in semiarid areas and in most desert areas. In dry lands with unconsolidated surface materials and little or no vegetation, runoff from rare but often intense storms can effect extensive erosion.

9. Why is sea level (ultimate base level) not a significant factor influencing erosion in desert regions?

Sea level has no effect on the depth to which the wind can erode and excavate surface materials. The base level for wind erosion is marked by the water table or by layers of resistant material or bedrock. Many desert areas are in closed basins; in these and many other dry areas, river and stream channels may not necessarily be integrated and do not reach the sea. Thus base level for streams in many desert areas is independent of sea level.

10. Describe the features and characteristics associated with each of the stages in the evolution of a mountainous desert. Where in the United States can these stages be observed?

In the early or youthful stage, slopes are very steep, mountain uplands are extensive and relatively undissected, and canyon gradients are very steep. Alluvial fans are small and localized at the mouths of individual canyons; playas dot the relatively flat valley floors.

As the mature stage of erosion is reached, the uplands are dissected and the canyon systems extend to the crests of the mountains. The canyon gradients are less steep and alluvial fans merge into an alluvial apron (bajada) that slopes gradually toward the valley floor. A bedrock erosional surface (pediment) is cut upslope into the mountains.

As the terrain reaches the old-age erosional stage, the mountains are reduced in elevation and cover less area; low, isolated, bedrock ranges or knobs (inselbergs) rise above a vast expanse of gently sloping pediments and alluvium-floored valleys. Canyon systems in the mountains are short and gradients are low, individual alluvial fans are indistinct, and the valley floors grade imperceptibly upslope into bedrock pediments.

These various, evolutionary stages can be seen in the Basin and Range Province of the western and southwestern United States.

11. Describe the way in which wind transports sand. During very strong winds, how high above the surface can sand be carried?

In windstorms, sand rolls along the surface or moves by saltation, that is the sand grains bounce into the air and are blown a short distance downwind before falling back to the surface. In very strong winds, fine-sized sand can remain suspended and travel for fairly long distances before falling back to the surface. Even in the most intense windstorms, the blowing sand grains seldom rise more than a meter or so above the land surface.

Air temperature has an important effect on windblown sediment transport. Air density is inversely proportional to temperature; thus for a given speed, cold winds can move more and larger sediment grains than hot winds. Thus "hot desert" sands, such as those in Kuwait and Saudi Arabia, tend to be fine grained. Recall that during preparations for the Gulf War, concerns were expressed about how well mechanical equipment would perform in a "fine-grained sand" environment.

For example, in Great Sand Dunes National Monument, CO, rare, strong, extremely-cold northeast winter winds blowing over the Sange de Cristo range from the plains move enough sand to keep the dunes from piling up against the mountains, despite the southwest winds and to-the-northeast sand movements that prevail during most of the warmer months.

12. Why is wind erosion relatively more important in arid regions than in humid areas?

In humid areas, vegetation and soil moisture protect the surface particles from being picked up and entrained by the wind. Thus blowing sand and dust are relatively uncommon and require unusual weather and environmental conditions.

Just the opposite conditions exist in dry lands. Vegetation is sparse or absent, soil moisture is scarce, and surface particles can easily be picked up and entrained by the wind. Sparse vegetation provides few obstructions to slow down gusts and reduce ground-level turbulence. Thus sparsely vegetated lands are highly vulnerable to deflation.

13. What factor limits the depths of blowouts?

Blowouts (depressions excavated by wind erosion) will not be deepened below the water table or below a resistant stratum or bedrock surface. Vegetation also inhibits deflation but sea level (base level for most streams) is not important.

14. How do sand dunes migrate?

Dunes migrate sand grain by sand grain; yet the overall shape and profile of the particular dune type is maintained. Dunes have a longer, gentle slope facing into the prevailing wind and a shorter, steeper slope (the slip face) inclined in the downwind direction. Sand grains move up the gentle slope by rolling and saltation, then roll down the steeper, slip face or slide en mass when the angle of repose is exceeded. Thus a dune migrates in the downwind direction, always maintaining its characteristic form and profile.

15. List three factors that influence the form and size of a sand dune.

Speed, direction and variability of the wind, sand supply, and environmental conditions, such as topography and vegetative cover, control the form and size of dunes in a given region. For example, barchans (solitary, crescent-shaped dunes; Figs. 13.17A & 13.18) form in areas of consistent wind direction, low topographic relief, sparse vegetation and limited sand supply. Transverse dunes (Fig. 13.17B) form in areas of abundant sand and persistent, single-directional, prevailing winds. Longitudinal dunes (Fig. 13.17D) require higher wind speeds and a less abundant sand supply than transverse dunes, and star dunes (Fig. 13.17F) form only in areas where winds are strong and seasonally variable in direction. Parabolic dunes (Fig. 13.17E) form in coastal areas where beaches provide an abundant source of sand and persistent, onshore winds move the sand inland. Since parabolic dunes can form in other than desert areas, they may be vegetated and typically migrate into vegetated areas.

16. Six major dune types are recognized. Indicate which type of dune is associated with each of the statements below.

(a) Dunes whose tips point into the wind. - These are parabolic dunes; other tipped dunes (barchan and barchanoid) have tips that point downwind.

(b) Long sand ridges oriented at right angles to the wind. - transverse dunes

(c) Dunes that often form along coasts where strong winds create a blowout. -

parabolic

(d) Solitary dunes whose tips point downwind. - barchans (crescent-shaped)

(e) Long sand ridges that are oriented more or less parallel to the prevailing wind. - longitudinal dunes

(f) An isolated dune consisting of three or four sharp-crested ridges diverging from a central high point. - This is a star dune, formed where prevailing wind directions vary with the seasons.

(g) Scalloped rows of sand oriented at right angles to the wind. - This is a good description of barchanoid dunes; they can be visualized as combinations of transverse and barchan dunes.

17. Although sand dunes are the best-known wind deposits, accumulations of loess are very significant in some parts of the world. What is loess? Where are such deposits found? What are the origins of this sediment?

Loess is a weakly stratified, unlithified or weakly cemented deposit of windblown silt. The silt particles originate in one of two ways: 1) as rock flour (silt-sized, fairly fresh, rock particles produced by glacial erosion) carried down rivers and streams in the summer, deposited and left vulnerable to deflation during the other seasons, or 2) as weathered rock and soil particles picked up by the wind in desert areas and deposited in more humid areas bordering the desert.

The desert silt loess is common in central and western China, the silt having been blown eastward from the central Asian and Mongolian desert basins. Although not directly glacial in origin, much of the silt could have been delivered to the basins by streams draining from glaciated mountain ranges during the Pleistocene. The rock flour type is common along the eastern and southeastern sides of the Mississippi River and its major tributaries; these streams were supplied with ample quantities of rock flour during the major, Pleistocene, glacial stages.

18. What term refers to the process by which desertlike conditions expand into areas that were previously productive? Is this strictly a natural process? (Box 13.4)

The term is desertification. It describes the transformation of steppe lands (relatively dry grasslands) to sparsely vegetated, more desertlike lands characterized by exposed soil, regolith, and bedrock. Steppe lands, such as those bordering the Sahara Desert in Africa, are transitional between humid and desert areas; rainfall is typically seasonal and amounts may vary greatly from year to year. Extended, more humid periods and long droughts are common. In north Africa and elsewhere, gradual, long-term shifts in climate are probably involved in expansion of the desert areas, but inappropriate land-use patterns can greatly accelerate the process. Degrading of natural vegetative cover through overgrazing and cultivation leaves the soil highly vulnerable to deflation during drought and to sheet erosion, gullying, increased evaporation, and lessened infiltration when the rains finally come again.