11 Deserts and Wind Erosion GLY 2010 – Summer 2012 Lecture 22 Photo by Bruce R. Thomas. copyright ©

22 What Is a Desert? May be hot, warm, or cold Vegetation is generally sparse Must be arid or semi-arid

33 Desert and Steppes Deserts are arid, steppes are semi-arid Wind Circulation

Coriolis Effect 4

5 Wind Circulation 5 Map of desert regionsRain Shadow effectAridity Index

6 Rain Shadow Deserts On the leeward side of mountains, particularly those near the coast, there is often an arid region 6

77 Pacific Northwest

8 Rain Shadow on the Olympic Peninsula 8 Return to Wind Circulation

99 Aridity Index Aridity Index = area’s potential annual evaporation divided by recorded average annual precipitation  Humid - up to 1.5  Semi-arid 1.5 to 4.0  Arid greater than 4.0  Hyper-arid - no exact definition - numbers over Eastern Sahara and Atacama Deserts can reach 200

10 Physical Weathering Dominates With little available moisture, chemical weathering rates quickly slow to near zero Physical weathering also slows, but not to zero Therefore, physical weathering becomes the dominant process

11 Evaporite Deposits in Basins Interior drainage possible  Streams run into a basin, which may accumulate some water  Evaporation rates are so high, the lake never gets deep enough to spill over the lowest point in its rim  Rather, the water simply becomes more saline, or dries up entirely  Salt deposits (evaporites) form

12 Playa Lake A playa lake is a naturally occurring wetland that contains water only seasonally Playas often form when water collects in a natural depression Over time, a chemical reaction dissolves the underlying caliche layer, the land surface subsides (sinks), and a playa is created Ten’s of thousands of these lake exist on the western edge of the Great Plains 12

13 What Are Playas? 13

14 Playas and the Ogallala Aquifer 14

15 Salina A salina is an ephemeral lake, created by interior drainage and evaporation Near Shiprock, New Mexico

16 Bonneville Salt Flats The area west of Salt Lake Utah to near the Nevada border is a large, flat expanse of evaporite salt It supports almost no vegetation The salt flats are a remnant of Pleistocene glacial Lake Bonneville, which covered a much larger area As the glacial lake evaporated, salts began to accumulate, and are still accumulating today

17 Glacial Lake Bonneville Glacial Lake Bonneville was one of several large Pleistocene lakes Great Salt Lake and the Bonneville Salt Flats are remnants

18 Erosional Landforms Physical weathering dominates, so landforms in arid environments are much different than those in humid areas Arid regions, such as Monument Valley in Utah/Arizona, have often been used as sites for commercials Utah’s Red rock country has been used in many western movies

19 Pediment Gradual headward erosion of streams in arid regions producing a gently sloping plane called a pediment at the foot of the mountain Pediments enlarge as mountains retreat, due to a thin covering of loose material which moves downslope with each flood Eventually whole mountain may erode

20 Pediment Tucson, is situated in a valley, bordered by bajadas and pediments which form where eroded bedrock extends out from the mountains

21 Mesa Isolated, nearly level landmass standing distinctly above the surrounding country, bounded by abrupt or steeply sloping erosion scarps on all sides, and capped by layers of resistant, nearly horizontal rock (often lava) A mesa has a more extensive summit area than a butte, and is a common topographic feature in the arid and semiarid regions of the U.S

22 Checkerboard Mesa, Zion National Park Cross bedding in the Navajo Sandstone Massive sandstone layers

23 Mesa in Utah Mesa in the Red Rock country of eastern Utah

24 Inselberg Knobby remnant of mountain left by pediment formation, consisting of harder, more resistant rock Next three slides show the development of an inselberg

25 Inselberg Formation

26 Initial Exposure of Inselberg

27 Fully Exposed Inselberg

28 Spitzkoppe, Namibia The Spitzkoppe, an inselberg in the eastern part of the Namib, is probably the best known and most famous landmark in Namibia Known as the Matterhorn of Africa, the majestic form of the Spitzkoppe towers 1,784 metres above the flat surrounding plains

29 Desert Landforms, Early Stage

30 Desert Landforms, Middle Stage

31 Desert Landforms, Late Stage

32 Desert Landforms

33 Aeolian Erosion The lack of moisture means that water is not available to act as a glue, holding sediment together Lack of vegetation means there are few roots to hold sediment Therefore, the wind becomes a very important agent of erosion

34 Deflation and Desert Pavement Winds blowing across dry, treeless expanses lift and remove clay, silt and fine sand (dust storms) Landscape level is lowered, at rates of centimeters per century Dust Storm in Senegal

35 Initial Aeolian Erosion

36 Desert Pavement Development

37 Final Stage of Desert Pavement

38 Sediment Transportation Animation

39 Aeolian Load Diagram

40 Disturbance of Dead-Air Layer

41 Dust Bowl Movie Dust Bowl Era (1930’s) movie of aeolian erosion in the Great Plains

42 Desert Pavement Desert Pavement is formed when winds remove the sand and smaller particles, leaving gravel behind, via deflation Closeup of desert pavement, Russell Spit, Nevada

43 Desert Varnish Bandelier National Monument, New Mexico Over time, the very slow chemical weathering may also lead to a deposition of a thin coating of manganese compounds Photographer: Justin Gould (used by permission)

44 Abrasion Wearing away of hard materials but the impact of hard mineral grains Ventifacts are wind erosional features produced in areas where the wind is predominantly from one direction - they have flat sides, and sharp edges, with surfaces toward wind pitted Ventifact photo - "Photograph ©2000 Tom Schweich. Used by permission,

45 Ventifact Formation

46 Ventifact Facet Development

47 Continued Development of Facets

48 Loess Winds may deposit the fine “dust” as they die Aeolian dust accumulations are known as loess, which is often a post-glacial feature, but thick accumulations of loess can develop in deserts Glaciers grind the rock down into very fine particles (“rock flour”) and this becomes the aeolian dust

49 Vertical Loess Bluff Near Mississippi River, in Southern Illinois Wind blown material has a high cohesiveness, and retains vertical slope About 3 meters high

50 Depositional Landforms Erosion isn’t the only agent creating desert landforms Deposition also plays a role Alluvial fans and sand dunes are examples

51 Alluvial Fans Semicircular deposits at the base of mountain streams, where the gradient decreases as they flow onto the valley floor They violate the Law of Original Horizontality, because deposits at the edge of the fan aren’t horizontal

52 Alluvial Fans, Death Valley, Califormia Multiple alluvial fans are visible

53 Sand Dunes There are several kinds of sand dunes Important factors in their creation are sand supply, wind direction, and obstructions Sand dunes are another major exception to the Law of Original Horizontality Sand is blown up and over the dune, and deposited on the lee side, on a non-horizontal surface

54 Sand Dune Formation

55 Dune Migration

56 Saltating Sand Grains

57 Slipface Deposition White Sand Dunes NM, New Mexico Sand is sliding down the steep face of the slipface

58 Dunes and Mountains The Sangre de Cristo mountains, in the background, reach 14,000’ a.s.l., so they tower over the dunes, and act as an obstruction, which keeps the sand from moving further east Dunes reach nearly 700 feet in height Valley floor is about 7500’ a.s.l.

59 Barchan Dunes Crescent shaped ridges, perpendicular to wind Form where sand supply is limited and terrain is flat Horns of crescent point downwind

60 Barchan Dune Formation

61 Transverse Dunes Ample wind produces dunes perpendicular to wind direction, with a constant wind direction May reach 100 km in length, meters high, and 1-3 km in width Vegetation scarce, sand supply plentiful

62 Transverse Dunes Sand Hills, Nebraska

63 Florida Dunes Transverse sand dunes, anchored in place along the Florida coastline by grasses

64 Photo courtesy Philip Greenspun Great Sand Dunes National Park, Colorado Dune Ridge of Transverse Dune

Barchanoid Dunes Intermediate between isolated barchan dunes and transverse dunes Form where sand supply is more plentiful then is the case of barchan dunes 65

Barchanoid Dune Photo White Sands National Monument, New Mexico 66

67 Seif (Longitudinal) Parallel to prevailing wind direction Moderate sand supply Wind direction varies within a narrow range May reach 100 km long, 100 meters high, but often much smaller 60 m long by 5 meters high Etymology: Arabic sayf, = sword

68 Longitudinal Dune Formation

69 Seif Dunes

70 Blowout, or Parabolic Horseshoe shaped, horns point upwind Develop from transverse dunes where erosion increases, especially after vegetation is removed Horns are vegetated, rest is not, and center moves downwind

71 Parabolic Dune Formation

72 Parabolic Dune Photo courtesy Philip Greenspun Great Sand Dunes National Park, Colorado

73 Star Dune Formation

74 Star Dunes Star dunes form when the wind direction is variable

75 Paleowind Direction Determination Cross-bedding is a remnant of non- horizontal deposition in sand sediment or sandstone When wind directions change, the direction of deposition changes Together with changes in thickness of loess deposits (thicker near the source), the cross- bedding can be used to determine the direction of dominant paleowinds

76 Cross-Bedding Formation

Desertification Definition “Desertification means degradation of land and vegetation, soil erosion and the loss of top soil and fertile land in arid, semi-arid, and dry sub-humid areas, caused primarily by human activities and climate variation” - United Nations Convention to Combat Desertification (UNCCD) 77

78 Desertification Effects Invasion of non-desert regions by desert o Example: Sahal, immediately south of the Sahara Drought from 1970 to mid-90's - semi- arid lands are about 35% of total land area, and are subject to desertification

79 Sahara and Sahal

Desertification Video 80

81 Reversing Desertification Reversing the desertification process is difficult Involves  Wind breaks (planting forests in China)  Leveling dunes, covering with topsoil  Replanting, and providing water to establish cover crop of vegetation