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McKnight's Physical Geography 11e The Topography of Arid Lands
Lectures Chapter 18 The Topography of Arid Lands Andrew Mercer Mississippi State University © 2014 Pearson Education, Inc.
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Learning Goals of This Chapter
Identify the special conditions and factors that influence landform development in deserts. Explain the influence of weathering, soil and regolith, soil creep, impermeable surfaces, sand, and vegetation on landform development in deserts. Explain the influence of rainfall, fluvial deposition, wind, and interior drainage basins on landform development in deserts. Differentiate between slope development in humid and arid climates. Explain the role of running water in erosion and deposition in deserts.
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Learning Goals of This Chapter
Define exotic stream, and cite two examples. Define and describe ephemeral streams. Differentiate between playa, salina, playa lake, and saline lake. Explain flash flood and debris flows in deserts and the associated hazards. Explain the relationship between escarpments and differential weathering and erosion as it impacts slope development in arid climates. Differentiate between inselbergs, bornhardts, pediments, and piedmont zones and explain their formation.
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Learning Goals of This Chapter
Define and explain desertification, and summarize its significance with examples. Explain wind transportation of particles, and name the process associated with wind as an agent of erosion. Identify and explain the two mechanisms of wind erosion and the landform features that result. Identify and describe the five types of sand dunes, and explain the conditions from which they form. Identify and describe the internal structure of a sand dune. Explain the formation and importance of loess, and identify major areas of loess deposits.
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Learning Goals of This Chapter
Describe and explain the formation of an erg. Describe and explain the formation of a reg, and identify alternative names. Explain desert varnish. Describe and explain the formation of hamadas. Explain the three principal features of basin-and-range terrain and their formation. Explain the prevalence of playas and alluvial fans in the basin-and-range desert region of North America. Explain the formation of alluvial fans. Explain the formation of mesa-and-scarp terrain and associated landforms.
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Learning Goals of This Chapter
Explain the formation of badlands. Identify locations of basin-and-range terrain, mesa-and-scarp terrain, and badlands.
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Karst and Hydrothermal Processes
A Specialized Environment Running Water in Waterless Regions The Work of the Wind Characteristic Desert Landscape Surfaces Two Representative Desert Landform Assemblages
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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
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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 – most streams are ephemeral, effective agents of erosion; alluvium unusually common in deserts Wind – wind action shifts particles
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A Specialized Environment
Desert special conditions (cont.) 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
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Running Water in Waterless 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
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Running Water in Waterless Regions
Surface water in the desert Exotic streams – permanent streams that originate outside of the arid land (e.g., Nile River) Ephemeral streams – periodically flow, result in intense erosion, transportation, and deposition Desert lakes – playas and Salinas (dry salt lake beds), saline lakes
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Running Water in Waterless 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 (e.g., bornhardts), pediments Desert stream channels – ephemeral stream beds
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Running Water in Waterless Regions
Characteristics of a desert pediment [Insert Fig p. 430]
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Running Water in Waterless 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
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The Work of Wind 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
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The Work of Wind 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
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The Work of Wind 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
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The Work of Wind Aeolian deposition Desert sand dunes
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
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The Work of Wind 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
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The Work of Wind Common types of desert sand dunes
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The Work of Wind Aeolian processes in nondesert locations
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
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The Work of Wind Major loess locations of the world
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Characteristic Desert Landscape Surfaces
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
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Characteristic Desert Landscape Surfaces
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 – a useful dating tool Hamada – barren bedrock Barren surface of consolidated material Exposed bedrock or cemented sedimentary material Regs and hamadas extremely flat
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Two Representative Desert Landform Assemblages
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
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Two Representative Desert Landform Assemblages
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
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Two Representative Desert Landform Assemblages
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 – a prime example of a basin and range terrain
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Two Representative Desert Landform Assemblages
Death Valley Excellent example of basin and range terrain Graben, large portion of valley 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
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Two Representative Desert Landform Assemblages
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
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Two Representative Desert Landform Assemblages
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
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Two Representative Desert Landform Assemblages
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
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Summary Desert topography is abrupt and stark.
Desert terrain has 10 primary characteristics that separate it from other terrain types. Running water is the most important land formation mechanism in 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. Regs are stony deserts, consisting of rocks, boulders, and pebbles.
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Summary 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. There are two primary desert land formations. The basin-and-range formation consists of a basin surrounded by mountain ranges.
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Summary 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.
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