Physical Geography - Version 1 Unit 18 Shaped by Wind as a Geomorphic Agent

Figure Mesquite Dunes , Death Valley California . Image by Jeremy is used under a license . UNIT 18 SHAPED BY WIND AS A GEOMORPHIC AGENT Goals Objectives of this unit To identify and explain desert , and whether they are formed by erosion and deposition . Describe the main types of dunes and the conditions that form them . Understand how wind erosion changes and defines landscapes . Understand how sand supply and wind variability can design dunes and dune systems . Describe the factors that control and explain the processes of wind erosion , transportation , and deposition . GEOGRAPHY

PROCESSES processes are most effective where surface material is fine , dry , and loose . Vegetation imposes a frictional force on the wind to reduce its effectiveness . Thus , an environment devoid of vegetation is best . These conditions are met in deserts found on every continent of the world . In most cases , wind erosion over deposition leaving a surface of stones . Only of Earth deserts are partially or completely covered by sand . Though less extensive in area , coastlines of large bodies of water are another environment . Here , waves and currents supply weathered material susceptible to wind action . processes have been enhanced by human activity over the past few centuries , especially in Asia and Africa . Overuse of soil and grazing land resources in and arid and seasonally dry regions has led to extensive wind erosion and . EROSIONAL FEATURES The entire solid surface ofthe Earth is subject to wind erosion . It is the balance between the driving force of the wind and the resistance of the surface that ultimately determines whether surface materials are detached and transported away . The roughness length of the surface , a parameter based on the size and distance between objects in a group , has a critical control over the velocity of the wind . Grass , shrubs , and trees , all impart a drag on the wind to reduce its erosive force . Vegetation also acts to bind soil particles to the surface . A surface without a cover of vegetation exposes soil to the direct force of the wind making erosion more effective . Thus , dry regions lacking a protective cover of vegetation display the effects erosion more than humid climates . The cohesiveness of surface materials affects the resistance of the surface to erosion . Clay particles exhibit a greater degree of cohesiveness than sand . soils tend to resist erosion by wind more than less cohesive materials . Thus , clays require a much higher threshold velocity for detachment than one would expect . Cohesiveness may be provided by a cementing agent as well . Calcium carbonate and other salts common to desert regions deposited in voids harden and bind particles together . Features of Wind Erosion Landscapes formed from the work of wind result from either the removal of fine particles or the sculpting effects of material in movement . Deflation lifts and removes loose particles from GEOGRAPHY

the surface . Deserts , where soils of mixed particle size have been eroded of fines , leave a surface behind called desert lag or desert pavement . The interlocking pavement of stones protects the underlying surface from the wind . If disturbed , the surface becomes subject to erosion . Such has been the result of surface degradation from military activities in the desert regions of North Africa during World War II ( as well as later conflicts fought in the desert regions ofthe Middle East ) Deflation may also result in blowout depressions , basins ranging in size from less than a meter to many meters deep . Chemical weathering of cementing materials loosens a particles cohesiveness to enable wind erosion . Some extremely large depressions like the Depression in the western desert of Egypt are partially a result of deflation . Sand particles lifted free of the surface can sandblast rock surfaces in a process called abrasion . Abrasion shapes and polishes exposed bedrock . Abrasion is restricted to a distance of about a meter or two above the surface because sand grains are lifted a short distance . are smooth faceted rocks that often have a polished surface that results from abrasion . At a much larger scale , elongate ridges called form by the abrasion and streamlining rock structures oriented parallel to the prevailing wind direction . Abrasion occurs at the windward end while deflation removes material from the leeward end . Archaeologists have suggested that the Great Sphinx of , Egypt is partially formed from a yardang . Sand Transportation Much of our knowledge about sand transport dates back to the and 19405 when British Brigadier Ralph conducted experiments while stationed in the desert of North Africa . Sand generally begins to move when the wind achieves a velocity of about . At first , sand exhibits a rolling motion called traction or surface creep . Approximately 20 to 25 of total sand transport during sand storms occurs by traction . As wind speed increases , grains are lifted into the air by wind gusts . Figure Example of Sand Grains . image by is in the public domain . GEOGRAPHY

Once airborne , sand grains travel downwind and then drop back to the surface several centimeters from their point of origin . Finer dust particles are lifted from the surface and suspended in the air at much greater heights than heavier sand grains . With strong winds and turbulence , sand grains can be lifted as high as two meters and travel a distance of 10 meters or more . When a settling sand grain impacts the surface , it sends another grain of sand into the air to travel in the downwind direction . Watching this process in action makes sand appear to be bouncing along the surface . Saltation accounts for over 50 of sand transport over dunes . Once saltation begins , transport can continue under somewhat lower wind speeds . Dust storms are primarily composed of fine material that easily reduces to a few meters and can persist for hours . During a prolonged dry period , extraordinary amounts of valuable topsoil were stripped from the Great Plains during the Dust Bowl period of the . conditions returned , and the application of soil conservation techniques have greatly reduced the erosive effects of wind . Sandstorms are raging systems of blowing , stinging sand traveling a meter or two above the surface . A strange phenomenon that has mystified generations is the sound moving sand can make as it is transported across the surface . In approximately 30 places around the world , a sustained hum rings out from this resonating dune when wind conditions are right . have long wondered what causes booming dunes . DEPOSITIONAL FEATURES DUNE TYPES A sand dune is an accumulation of loose sand grains piled up by the wind . Dunes are most likely to form where winds are strong and generally blow from the same direction . Some of the most extensive dune fields are found in the world great deserts like the Sahara . The dune fields of these great sandy deserts are called sand seas . Patches of dunes are found in the southwest desert of the United States . Ancient dune fields are found in regions that were dry in the past but now exhibit a more humid climate . Dune systems are found landward of beaches where sand is blown landward . Beach dunes are common along the coasts of the world oceans and large lakes like the Great Lakes of North America . Dune Formation Dunes are primarily made up of accumulated sand grains . Finer silt and clay are carried further by the wind leaving the heavier sand grains behind . Generally , a dune forms an asymmetric form with a gentle windward or stoss slope and a steep leeward slope called GEOGRAPHY

the slip face . grains move up the windward slope and then come to rest at their angle of repose on the downwind side in a zone of stagnant air . stoss or Windward I Sand Gram Leeward Side Side of Dune of Dune Sand Dune Figure Sand a Dune . Figure by Jeremy is licensed under SAND While deserts are defined by dryness , not sand , the popular conception of a typical desert is a sand sea called an erg . An erg is a broad area of desert covered by a sheet of sand often blown by forces ( wind ) into dunes . Probably the erg is the Empty Quarter ( Rub al ) of Saudi Arabia , but other exist in Colorado ( Great Sand Dunes National Park ) Utah ( Little Sahara Recreation Area ) New Mexico ( White Sands National Monument ) and California ( parts of Death Valley National Park ) It is not only deserts that form dunes the high supply of sand can form anywhere , even as far north as 60 in at the Sand Dunes Provincial Park . Coastal on the shores of lakes and oceans also do exist and can be found in places like Oregon , Michigan , and Indiana . The way dunes form creates an internal feature called . As the wind blows up the windward side of the dune , it carries sand to the dune crest depositing layers of sand parallel to the windward ( or stoss ) side . The sand builds up the crest of the dune and pours over the top until the leeward ( downwind or slip ) face ofthe dune reaches the angle of repose , the maximum angle which will support the sand pile . Dunes are unstable features and move as the sand erodes from the stoss side and continues to drop down the leeward side covering previous stoss and layers and creating the . Mostly , these are reworked over and over again , but occasionally , the features are preserved in a depression , then . Shifting wind directions and abundant sand sources create chaotic patterns of like those seen in the fossil represented by the Navajo Sandstone and Zion National Park of Utah . In the , Utah was covered by a series of , thickest in Southern Utah . Perhaps the best known ofthese sandstone formations is the Navajo Sandstone . The Navajo forms the dramatic cliffs and spires in Zion National Park and covers a large part of the Colorado Plateau . It is exposed beneath the Sandstone in Arches National Park , a later series of dunes in which the conditions of the rock allowed the formation of arches . GEOGRAPHY

As the cement that hold the grains together in these modern sand cliffs disintegrate and the freed grains gather at the base of the cliffs and move down the washes , sand grains may be recycled and . These great sand may represent ancient quartz sands recycled many times , just passing now through another cycle . One example of this is Coral Pink Sand Dunes State Park in Southwestern Utah , which is sand that is eroded from the Navajo Sandstone forming new dunes . Figure Enlarged Image Coral Pink Sand Dunes , Utah . Image by Utah State Parks is in the public domain . Dunes Sand dunes are formed by wind moving sand particles . The shape ofthe dune varies with the amount of sand available and the direction that the wind blows . If the wind blows steadily from one direction , linear , transverse , or dunes will form . If the direction that the wind blows shifts , star or network dunes will form . The diagram below notes the different types of dunes possible based on sand supply and wind variability . GEOGRAPHY

Transverse Dunes Linear or Longitudinal Dunes Network or Collection of Dunes Sand Supply Complex Wind Direction Variability Figure Image Separating Sand Dunes by Wind Variability and Sand Supply . Figure by Jeremy is licensed under A or crescent dune forms as individual units commonly found in dry regions lacking vegetation . The elongated tips , or horns , point in the downwind direction . are found moving across a surface of gravel or clay up of several tens of meters a year . Symmetrical dunes indicate winds blowing from a constant direction . Transverse dunes form perpendicular to the prevailing wind as accumulations of loose , sorted , very fine to medium sand . They have a gentle stoss slope ( usually less than ) and a steep ( slip face . Individual dunes whose horns point in the upwind direction are parabolic dunes . These like dunes form where sand accumulates in a moist environment with a cover of vegetation . Vegetation or dampness in the lower part slows the dune motion there , permitting the dry crest to push ahead of its base causing the horns to be anchored behind . Blowout dunes form from sand deposited at the end of an deflation hollow . Blowout dunes are common to many sandy coasts ( coastal blowout dunes ) The deflation hollow focuses the wind toward the center of the depression causing the sand to migrate landward as a dune . With little wind energy on the side of the hollow , the tails of the dune move much slower . As a result , beach plants like dune grass , sea oats or sand cherry can find a GEOGRAPHY

suitable habitat to flourish in . Once established , the plants help anchor the flanks to promote the formation of the characteristic parabolic shape Longitudinal dunes form where the wind blows from more than one direction in a region with an abundant supply of sand . In some places , longitudinal dunes form long smooth , while in other regions they display crests . Unlike transverse dunes , these do not migrate but lengthen in size over time . are elongate dunes that resemble great of sand . They form parallel to the direction of the wind , though are a product of shifting wind direction . The changing wind direction sends sand back and forth across the dune crest leaving a sharp profile . Pin It ! Sand Dunes ! View this website for more information on Great Sand Dunes National Park . Blowout dunes Parabolic dunes Transverse ridges Figure Five Common Dunes , Note Their Wind Direction . Image by , Colorado State University after , and . 1990 and , 1966 , is in the public domain . GEOGRAPHY

Individual sand seas are referred to as . Small are present in most desert regions . North Africa and the Arabian Desert contain the most spectacular forms on Earth with waves of sand covering tens of thousands of square kilometers . are also found in India , western China , and Australia . Star dunes are mounds of sand with on three or more arms radiating from the center and common to . They tend to accumulate in areas where the wind comes from several different directions . Star dunes grow vertically rather than laterally . The Sand Hills of Nebraska are remnants of an ancient sand sea . ALLUVIAL FANS Rivers are important features in any ecosystem . Apart from providing water for the different needs , it can also be dragged by gravity across the Earth to form some ofthe most beautiful landmarks , especially in the valleys . The flowing waters carry with it the alluvium across its path . The alluvium consists of soil , and small rock particles picked up by the river as it flows . The alluvium is deposited on flat plains where smaller streams fan out across the plain . Overtime the deposits stand out from the surrounding landscape to form a unique feature called alluvial fans . The alluvial fans formed range from just a minuscule to a massive sculpture that can be seen from the space . Alluvial fans are either cone or in nature . When an alluvial fan is built by debris the flow , then it is referred to as the debris cone or colluvial fan . Figure Alluvial Fan in Death Valley . Image on Commons is in the Public Domain . Alluvial fans are mainly formed by rivers but can also be formed by streams . The rivers and streams gather sediments or particles of soil and small rocks as it flows towards a particular direction . As the gradient of the river or stream decreases , it drops small rock particles reducing GEOGRAPHY

the capacity ofthe channel which forces the river to change course . The stream or river gradually builds a slightly conical fan shape with the sediments deposited at the apex of the fan . Alluvial fans are mostly , especially at their mouth but are relatively at their edges . Alluvial fans are common in desert areas characterized by flash floods in the nearby hills . The in desert areas are large basin at the top which opens up to the alluvial fan . Alluvial fans are also common in a wet climate . The River in Nepal has built a mega alluvial fan which covers an area of square kilometers at the point where the river traverses into India . The streams flowing into California Central Valley in North America have also deposited some sediment forming alluvial fans . The largest alluvial fan in the world is formed in the desert in China . The alluvial fan is wide and long with parts of the fan alive with water which flows from the River . Apart from the Earth surface , alluvial fans are also found on Mars and other parts ofthe solar system . BAJA DAS Alluvial fans formed on plains may also coalesce along a mountain to form a feature that is commonly referred to as Bajada . Bajada is a Spanish word that is often used to describe a landscape or geomorphology and mean inclination or descent . Bajada is common in a dry climate such as the Southwestern US where the flash floods deposit alluvium over time to form a series of alluvial fans that coalesce to form the Bajada . can also be formed on wetter climate with streams continuously depositing sediments . Figure A Collection Fans are Called , Death Valley National Park , California . Used under Google Earth reproduction guidelines GEOGRAPHY

OTHER GEOMORPHIC ARID FEATURES Rain does fall occasionally in deserts , and desert storms are often violent . A record 44 millimeters of rain once fell within hours in the Sahara . Large storms may deliver up to millimeter per minute . Normally dry stream channels , called or , can quickly fill after heavy rains , and flash floods make these channels dangerous . More people drown in deserts than die of thirst . Though little rain falls in deserts , deserts receive runoff from ephemeral , or , streams fed by rain and snow from adjacent highlands . These streams fill the channel with a slurry of mud and commonly transport considerable quantities of sediment for a day or two . Although most deserts are in basins with closed , or interior drainage , a few deserts are crossed by rivers that derive their water from outside the desert . Such rivers infiltrate soils and evaporate large amounts of water on through the deserts , but their volumes are such that they maintain their continuity . The Nile , The Colorado , and The Yellow Rivers are exotic rivers that flow through deserts to deliver their sediments to the sea . Lakes form where rainfall or meltwater in interior drainage basins is sufficient . Desert lakes are generally shallow , temporary , and salty . Because these lakes are shallow and have a low bottom gradient , wind stress may cause the lake waters to move over many square kilometers . When small lakes dry up , they leave a salt crust or hardpan . The flat area of clay , silt , or sand encrusted with salt that forms is known as a playa . There are more than a hundred in North American deserts . Most are relics of large lakes that existed during the last Ice Age about years ago . Lake was a lake almost 300 meters deep in Utah , Nevada , and Idaho during the Ice Age . Today the remnants of Lake include Utah Great Salt Lake , Utah Lake , and Lake . Because are arid from a wetter past , they contain useful clues to climatic change . In geomorphology , a butte is an isolated hill with steep , often vertical sides and a small , relatively flat top are smaller than mesas , plateaus , and . The word butte comes from a French word meaning small hill ' its use is prevalent in the Western United States , including the southwest where mesa is used for the larger . Because of their distinctive shapes , are frequently landmarks in plains and mountainous areas . In differentiating mesas and , use the rule of thumb that a mesa has a top that is wider than its height , while a butte has a top that is narrower than its height . GEOGRAPHY

Figure Devils Tower , Wyoming . Image by Bradley Davis . Mesas Mesa ( Spanish , Portuguese and Sardinian for table ) is the American English term for tableland , an elevated area of land with a flat top and sides that are usually steep cliffs . It takes its name from its characteristic shape . It may also be called a table hill , hill or , Table Mountain . It is larger than a butte , which it otherwise resembles closely . It is a characteristic of arid environments , particularly the Western and Southwestern United States in badlands and mountainous regions ranging from Washington and California to the , Wyoming , Utah , Oklahoma , and Texas . Examples are also found in many other nations including Spain , North and South Africa , Arabia , India , and Australia . Mesas form by weathering and erosion of horizontally layered rocks that have been uplifted by tectonic activity . Variations in the ability of different types of rock to resist weathering and erosion cause the weaker types of rocks to be eroded away , leaving the more resistant types of rocks topographically higher than their surroundings . This process is called differential erosion . The most resistant rock types include sandstone , conglomerate , quartzite , basalt , chert , limestone , lava flows , and . Lava flows and , in particular , are very resistant to weathering and erosion , and often form the flat top , or , of a mesa . The less resistant rock layers are mainly made up of shale , a softer rock that weathers and erodes more easily . GEOGRAPHY

. Figure Aerial View of Mesas in Monument Valley , Arizona . Image on Wikipedia by . The differences in the strength and resilience of various rock layers are what give mesas their distinctive shape . Less resistant rocks are eroded away on the surface into valleys , where they collect water drainage from the surrounding area , while the more resistant layers are left standing out . A large area of very resistant rock , such as a sill may shield the layers below it from erosion while the softer rock surrounding it is eroded into valleys , thus forming a . Differences in rock type also reflect on the sides of a mesa , as instead of smooth slopes , the sides are broken into a staircase pattern called topography . The more resistant layers form the cliffs , or , while the less resistant layers form gentle slopes , or benches , between the cliffs . Cliffs retreat and are eventually cut off from the main cliff , or plateau , by basal sapping . When the cliff edge does not retreat uniformly , but instead is indented by headward eroding streams , a section can be cut off from the main cliff , forming a mesa . Basal sapping occurs as water flowing around the rock layers ofthe mesa erodes the underlying soft shale layers , either as surface runoff from the mesa top or from groundwater moving through permeable overlying layers , which leads to slumping and flowage of the shale . As the underlying shale erodes away , it can no longer support the overlying cliff layers , which collapse and retreat . When the has caved away to the point where only little remains , it is known as a butte . GEOGRAPHY

Plateaus In geology and physical geography , a plateau is also called a high plain or a tableland , is an area of highland , usually consisting of relatively flat terrain , which is raised significantly above the surrounding area , often with one or more sides with steep slopes . Plateaus can be formed by several processes , including upwelling of volcanic magma , extrusion of lava , and erosion by water and glaciers . Plateaus are classified according to their surrounding environment as intermontane , piedmont , or continental . Figure The Volcanic Plateau in New Mexico . Image by , UNIT 18 SUMMARY The arid environment can be defined as one in which the amount of precipitation an area receives , divided by the amount , which is lost to . The arid environment can be organized into three zones , arid , and . The zones are characterized by low annual precipitation . Dunes are made of particles , and may consist of quartz , calcium carbonate , as sand is just a ofthe grain size . The upwind side ofthe dune is called the stoss side the downflow side is called the lee side . Sand is pushed ( creep ) or bounces ( saltation ) up the stoss side and slides down the lee side . A side of a dune that the sand has slid down is called a slip face ( or ) The differences between , mesas and plateaus are their surficial size . GEOGRAPHY