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Transcript of "Ch18"
1. Chapter 18: The Topographyof Arid LandsMcKnight’s Physical Geography:A Landscape Appreciation,Tenth Edition, Hess
The Topography of Arid Lands• A Specialized Environment• Running Water in Waterless Regions• Characteristic Desert Surfaces—Ergs, Regs,and Hamadas• The Work of Wind• Two Characteristics of Desert LandformAssemblages2© 2011 Pearson Education, Inc.
A Specialized Environment• Desert terrain stark andabrupt• Desert special conditions– Weathering: mechanicalweathering dominant,slower weathering andangular particle formation– Soil and regolith: soil is thinor absent, exposingbedrock– Soil creep: minor due tolack of soil and lubricatingeffects of water3© 2011 Pearson Education, Inc.Figure 18-1
A Specialized Environment• Desert special conditions(cont.)– Impermeable surfaces:caprocks and hardpans, highwater runoff– Sand: some deserts have sandabundance, allows for waterinput into the ground, easilymoved by rain and wind– Rainfall: limited, most streamsare ephemeral, effective agentsof erosion, alluvium unusuallycommon in deserts4© 2011 Pearson Education, Inc.Figure 18-2
A Specialized Environment• Desert special conditions(cont.)– Wind: wind action shiftsparticles– Basins of interior drainage:most watersheds do not draininto any ocean, watertransferred to basin or valleywith no external outlet– Vegetation: lack of continuousvegetative cover5© 2011 Pearson Education, Inc.Figure 18-4
Running Water in WaterlessRegions• Running water most important external landformagent• Erosion tremendously effective with little plant cover• Intensity of rain combined with impermeable landsurfaces create intense runoff• Unpredictable imbalance between erosion anddeposition6© 2011 Pearson Education, Inc.
Running Water in WaterlessRegions• Surface water in the desert– Exotic streams: permanentstreams that originate outsideof the arid land (i.e., NileRiver)– Ephemeral streams:periodically flow, result inintense erosion,transportation, and deposition– Desert lakes: playas andsalinas (dry salt lake beds),saline lakes7© 2011 Pearson Education, Inc.Figure 18-5
Running Water in WaterlessRegions• Fluvial erosion in arid lands– Occurs during small portion ofthe year, flash floods– Differential erosion: variationsin slope and shape of landformfrom rock type variations– Residual erosional surfaces:inselbergs (i.e., bornhardts),pediments– Desert stream channels:ephemeral stream beds8© 2011 Pearson Education, Inc.Figure 18-6
Running Water in WaterlessRegions• Fluvial deposition in arid lands– Talus accumulations at thefoot of steep slopes– Piedmont: zone at the foot ofa mountain range– Piedmont angle– Basins of interior drainagecovered with fine particlessince flow volume and speedare low9© 2011 Pearson Education, Inc.Figure 18-13
Characteristic Desert Surfaces:Ergs, Regs, and Hamadas• Ergs—seas of sand– Large area covered with sandin dune formation from wind– Hypothesized to haveoriginated in a more humidclimate– Drying of climate combinedwith wind created formationsseen today– Sahara and Arabian deserts10© 2011 Pearson Education, Inc.Figure 18-14
Characteristic Desert Surfaces:Ergs, Regs, and Hamadas• Regs—stony deserts– Tight covering of coarsegravel, pebbles, and/orboulders– Desert pavement or desertarmor– Desert varnish: dark, shinycoating consisting of iron andmanganese oxides– Desert varnish is a usefuldating tool11© 2011 Pearson Education, Inc.Figure 18-15
Characteristic Desert Surfaces:Ergs, Regs, and Hamadas• Hamada—barren bedrock– Barren surface of consolidated material– Exposed bedrock or cemented sedimentary material– Regs and hamadas extremely flat12© 2011 Pearson Education, Inc.
The Work of Wind• Wind as a sculptor is arelatively limited effect• Air right near surface haszero wind• Wind speed increaseswith distance aboveground• Effects of wind shear• Aeolian processes13© 2011 Pearson Education, Inc.Figure 18-16
The Work of Wind• Aeolian erosion– Two effects, deflationand abrasion– Deflation: shifting ofloose particles via thewind, blowouts– Abrasion: requires toolssuch as airborne sandand dust, sculptslandforms already inexistence, ventifacts14© 2011 Pearson Education, Inc.Figure 18-17
The Work of Wind• Aeolian transportation– Only finest particles arecarried in suspension asdust– Dust storms– Larger particles movedby saltation (curvedtrajectory) and traction(rolled or pushed)– Creep by saltation15© 2011 Pearson Education, Inc.Figure 18-18
The Work of Wind• Aeolian deposition– Fine sand laid as thin coating,no landform significance– Coarser sand depositedlocally, sand plains or sanddunes• Desert sand dunes– Some dune fields composedof unanchored sand, movedby local winds, slip face16© 2011 Pearson Education, Inc.Figure 18-19
The Work of Wind• Desert sand dunes (cont.)– Three most common dunes• Barchan—individual dunesmigrating across landscape,crescent shaped• Transverse—supply of sandgreater than for barchans,crescent shaped, but entirelandscape made of these dunes• Seifs—long, narrow dunes thatare parallel, orientation seemsto represent an intermediatedirection between two dominantwind directions17© 2011 Pearson Education, Inc.Figure 18-20
The Work of Wind• Coastal dunes– Ocean waves deposit sandon beaches– Prominent onshore windsmove sands inland• Loess– Wind deposited silt– Lacks horizontal stratification– Great vertical durability– Formation not well understood18© 2011 Pearson Education, Inc.Figure 18-24
Two Characteristic DesertLandform Assemblages• Basin and range terrain– Largely without externaldrainage– Numerous fault-blockmountain ranges– Three principle features• Ranges– Surface features shaped byweathering, mass wasting,and fluvial processes– Long, narrow ranges ofdifferent elevations19© 2011 Pearson Education, Inc.Figure 18-27
Two Characteristic DesertLandform Assemblages• The piedmont zone– Sharp break in slope thatmarks change from rangeto piedmont– Underlain by erosionalpediment– Alluvial fan: channels onpiedmont break intodistributaries, deposit newmaterial on old material– Coalescing alluvial fans– Piedmont alluvial plain, bajada20© 2011 Pearson Education, Inc.Figure 18-28
Two Characteristic DesertLandform Assemblages• The basin– Flattish floor, very gentle slope on all sides towards a lowpoint– Shallow, ill-defined drainage channels– Salt accumulations commonplace on playa due toevaporation of water– Playa lakes– Basin floor covered in fine grain material– Death Valley is a prime example of a basin and rangeterrain21© 2011 Pearson Education, Inc.
Two Characteristic DesertLandform Assemblages• Death Valley– Excellent example ofbasin and range terrain– Graben, large portion ofvalley is below sea level– Surrounding mountainranges– Piedmont at foot of themountains is alluviatedinto a complex fan structure– Basin filled with alluvium– Salt pans and mobile dunesin the basin 22© 2011 Pearson Education, Inc.Figure 18-29
Two Characteristic DesertLandform Assemblages• Mesa-and-scarp terrain• Mesa—Spanish for “table,” flattopped surface• Scarp—short for “escarpment,”pertains to steep cliffs• Associated with horizontalstrata• Variable resistance to erosionin strata• Plateaus and stripped plains23© 2011 Pearson Education, Inc.Figure 18-32
Two Characteristic DesertLandform Assemblages• Sapping—groundwater seepsout of the scarp face anderodes soluble material• Buttes—small surface areasand cliffs that rise abovesurroundings• Pinnacles• Buttes, mesas, and pinnaclestypically found near a retreatingescarpment face24© 2011 Pearson Education, Inc.Figure 18-34
Two Characteristic DesertLandform Assemblages• Badlands– Overland flows from occasionalrains develop tiny rills thatexpand into ravines or gullies– Characterized by maze ofravines and gullies, lifeless andnearly impassable• Arches and natural bridges– Arch formation– Natural bridge formation– Pillar formation25© 2011 Pearson Education, Inc.Figure 18-35
Summary• Desert topography is abrupt and stark• Desert terrain has ten primary characteristics thatseparate it from other terrain types• Running water is the most important land formationmechanism is deserts• Surface water is relatively uncommon in desert regionsbut does exist in isolated lakes and streams• Fluvial erosion and deposition result in most of the landformations that exist in arid regions• Ergs are vast expanses of sand in desert regions26© 2011 Pearson Education, Inc.
Summary• Regs are stony deserts, consisting of rocks, boulders,and pebbles• Hamadas are regions of barren, exposed bedrock thatis subject to mechanical weathering• Wind acts as a sculptor of arid rock formations,although it plays a minor role in the formation of aridtopography• There are two primary effects of aeolian erosion• Different sized particles interact with desert winds indifferent ways27© 2011 Pearson Education, Inc.
Summary• There are two primary desert land formations• The basin-and-range formation consists of a basinsurrounded by mountain ranges• Death Valley is a classic example of a basin-and-rangedesert land formation• The mesa-and-scarp land formation is made up of aflat-topped terrain surrounded by steep slope• Different compositions of the rock material result indifferent orientations of the land form structure28© 2011 Pearson Education, Inc.