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Ch18
- 1. Chapter 18: TheTopography of Arid Lands McKnight’s Physical Geography: A Landscape Appreciation, Tenth Edition, Hess
- 2. The Topography ofArid Lands • A Specialized Environment • Running Water in Waterless Regions • Characteristic Desert Surfaces—Ergs, Regs, and Hamadas • The Work of Wind • Two Characteristics of Desert Landform Assemblages 2© 2011 Pearson Education, Inc.
- 3. A Specialized Environment •Desert terrain stark and abrupt • Desert special conditions – Weathering: mechanical weathering dominant, slower weathering and angular particle formation – Soil and regolith: soil is thin or absent, exposing bedrock – Soil creep: minor due to lack of soil and lubricating effects of water 3© 2011 Pearson Education, Inc. Figure 18-1
- 4. A Specialized Environment •Desert special conditions (cont.) – Impermeable surfaces: caprocks and hardpans, high water runoff – Sand: some deserts have sand abundance, allows for water input into the ground, easily moved by rain and wind – Rainfall: limited, most streams are ephemeral, effective agents of erosion, alluvium unusually common in deserts 4© 2011 Pearson Education, Inc. Figure 18-2
- 5. A Specialized Environment •Desert special conditions (cont.) – Wind: wind action shifts particles – Basins of interior drainage: most watersheds do not drain into any ocean, water transferred to basin or valley with no external outlet – Vegetation: lack of continuous vegetative cover 5© 2011 Pearson Education, Inc. Figure 18-4
- 6. Running Water inWaterless Regions • Running water most important external landform agent • Erosion tremendously effective with little plant cover • Intensity of rain combined with impermeable land surfaces create intense runoff • Unpredictable imbalance between erosion and deposition 6© 2011 Pearson Education, Inc.
- 7. Running Water inWaterless Regions • Surface water in the desert – Exotic streams: permanent streams that originate outside of the arid land (i.e., Nile River) – Ephemeral streams: periodically flow, result in intense erosion, transportation, and deposition – Desert lakes: playas and salinas (dry salt lake beds), saline lakes 7© 2011 Pearson Education, Inc. Figure 18-5
- 8. Running Water inWaterless Regions • Fluvial erosion in arid lands – Occurs during small portion of the year, flash floods – Differential erosion: variations in slope and shape of landform from rock type variations – Residual erosional surfaces: inselbergs (i.e., bornhardts), pediments – Desert stream channels: ephemeral stream beds 8© 2011 Pearson Education, Inc. Figure 18-6
- 9. Running Water inWaterless Regions • Fluvial deposition in arid lands – Talus accumulations at the foot of steep slopes – Piedmont: zone at the foot of a mountain range – Piedmont angle – Basins of interior drainage covered with fine particles since flow volume and speed are low 9© 2011 Pearson Education, Inc. Figure 18-13
- 10. Characteristic Desert Surfaces: Ergs,Regs, and Hamadas • Ergs—seas of sand – Large area covered with sand in dune formation from wind – Hypothesized to have originated in a more humid climate – Drying of climate combined with wind created formations seen today – Sahara and Arabian deserts 10© 2011 Pearson Education, Inc. Figure 18-14
- 11. Characteristic Desert Surfaces: Ergs,Regs, and Hamadas • Regs—stony deserts – Tight covering of coarse gravel, pebbles, and/or boulders – Desert pavement or desert armor – Desert varnish: dark, shiny coating consisting of iron and manganese oxides – Desert varnish is a useful dating tool 11© 2011 Pearson Education, Inc. Figure 18-15
- 12. 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 flat 12© 2011 Pearson Education, Inc.
- 13. The Work ofWind • Wind as a sculptor is a relatively limited effect • Air right near surface has zero wind • Wind speed increases with distance above ground • Effects of wind shear • Aeolian processes 13© 2011 Pearson Education, Inc. Figure 18-16
- 14. The Work ofWind • Aeolian erosion – Two effects, deflation and abrasion – Deflation: shifting of loose particles via the wind, blowouts – Abrasion: requires tools such as airborne sand and dust, sculpts landforms already in existence, ventifacts 14© 2011 Pearson Education, Inc. Figure 18-17
- 15. The Work ofWind • Aeolian transportation – Only finest particles are carried in suspension as dust – Dust storms – Larger particles moved by saltation (curved trajectory) and traction (rolled or pushed) – Creep by saltation 15© 2011 Pearson Education, Inc. Figure 18-18
- 16. The Work ofWind • Aeolian deposition – Fine sand laid as thin coating, no landform significance – Coarser sand deposited locally, sand plains or sand dunes • Desert sand dunes – Some dune fields composed of unanchored sand, moved by local winds, slip face 16© 2011 Pearson Education, Inc. Figure 18-19
- 17. The Work ofWind • Desert sand dunes (cont.) – Three most common dunes • Barchan—individual dunes migrating across landscape, crescent shaped • Transverse—supply of sand greater than for barchans, crescent shaped, but entire landscape made of these dunes • Seifs—long, narrow dunes that are parallel, orientation seems to represent an intermediate direction between two dominant wind directions 17© 2011 Pearson Education, Inc. Figure 18-20
- 18. The Work ofWind • Coastal dunes – Ocean waves deposit sand on beaches – Prominent onshore winds move sands inland • Loess – Wind deposited silt – Lacks horizontal stratification – Great vertical durability – Formation not well understood 18© 2011 Pearson Education, Inc. Figure 18-24
- 19. Two Characteristic Desert LandformAssemblages • Basin and range terrain – Largely without external drainage – Numerous fault-block mountain ranges – Three principle features • Ranges – Surface features shaped by weathering, mass wasting, and fluvial processes – Long, narrow ranges of different elevations 19© 2011 Pearson Education, Inc. Figure 18-27
- 20. Two Characteristic Desert LandformAssemblages • The piedmont zone – Sharp break in slope that marks change from range to piedmont – Underlain by erosional pediment – Alluvial fan: channels on piedmont break into distributaries, deposit new material on old material – Coalescing alluvial fans – Piedmont alluvial plain, bajada 20© 2011 Pearson Education, Inc. Figure 18-28
- 21. Two Characteristic Desert LandformAssemblages • The basin – Flattish floor, very gentle slope on all sides towards a low point – Shallow, ill-defined drainage channels – Salt accumulations commonplace on playa due to evaporation of water – Playa lakes – Basin floor covered in fine grain material – Death Valley is a prime example of a basin and range terrain 21© 2011 Pearson Education, Inc.
- 22. Two Characteristic Desert LandformAssemblages • Death Valley – Excellent example of basin and range terrain – Graben, large portion of valley is below sea level – Surrounding mountain ranges – Piedmont at foot of the mountains is alluviated into a complex fan structure – Basin filled with alluvium – Salt pans and mobile dunes in the basin 22© 2011 Pearson Education, Inc. Figure 18-29
- 23. Two Characteristic Desert LandformAssemblages • Mesa-and-scarp terrain • Mesa—Spanish for “table,” flat topped surface • Scarp—short for “escarpment,” pertains to steep cliffs • Associated with horizontal strata • Variable resistance to erosion in strata • Plateaus and stripped plains 23© 2011 Pearson Education, Inc. Figure 18-32
- 24. Two Characteristic Desert LandformAssemblages • Sapping—groundwater seeps out of the scarp face and erodes soluble material • Buttes—small surface areas and cliffs that rise above surroundings • Pinnacles • Buttes, mesas, and pinnacles typically found near a retreating escarpment face 24© 2011 Pearson Education, Inc. Figure 18-34
- 25. Two Characteristic Desert LandformAssemblages • Badlands – Overland flows from occasional rains develop tiny rills that expand into ravines or gullies – Characterized by maze of ravines and gullies, lifeless and nearly impassable • Arches and natural bridges – Arch formation – Natural bridge formation – Pillar formation 25© 2011 Pearson Education, Inc. Figure 18-35
- 26. Summary • Desert topographyis abrupt and stark • Desert terrain has ten primary characteristics that separate it from other terrain types • Running water is the most important land formation mechanism is deserts • Surface water is relatively uncommon in desert regions but does exist in isolated lakes and streams • Fluvial erosion and deposition result in most of the land formations that exist in arid regions • Ergs are vast expanses of sand in desert regions 26© 2011 Pearson Education, Inc.
- 27. Summary • Regs arestony deserts, consisting of rocks, boulders, and pebbles • Hamadas are regions of barren, exposed bedrock that is subject to mechanical weathering • Wind acts as a sculptor of arid rock formations, although it plays a minor role in the formation of arid topography • There are two primary effects of aeolian erosion • Different sized particles interact with desert winds in different ways 27© 2011 Pearson Education, Inc.
- 28. Summary • There aretwo primary desert land formations • The basin-and-range formation consists of a basin surrounded by mountain ranges • Death Valley is a classic example of a basin-and-range desert land formation • The mesa-and-scarp land formation is made up of a flat-topped terrain surrounded by steep slope • Different compositions of the rock material result in different orientations of the land form structure 28© 2011 Pearson Education, Inc.