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Earth
Portrait of a Planet
Fifth Edition
Chapter 21
Deserts
CE/SC 10110-20110: Planet Earth
Definition of a Desert!
It is NOT defined by temperature - it is defined by ardity.! Distribution : Zones of high pressure at ~30˚N & S, as well as 90˚N & S. Also leeward sides of mountains.!
Area with less than 25 cm of precipitation annually.!
Deserts have no permanent surface water,<15% vegetation cover.!
Arid nature produced by high and low temperatures as well as orographic lifting.! Little chemical weathering - bare bedrock, wind blown sand, cobbles, and salt precipitation.! 2
Definition of a Desert!
Desert land surfaces often include:
- Exposed bedrock
- Accumulated clasts
- Unweathered sediment
- Precipitated salt
- Windblown sand
Mirages, which look like distant water, result from light interacting with heated air just above the ground surface.
Each desert has unique characteristics of landscape and vegetation. Geologists group deserts into one of five classes.
Types of Deserts! Five Categories:! Subtropical Deserts : (Sahara, Arabian, Kalahari, Australian) form where convection cells diverge. Has very little water, which has condensed out after rising at the equator. Dense air mass moves to the equator with high evaporation rates as sinking dense air heats up.!
Rain Shadow Deserts : (Washington State, east of the Cascades)!
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Found from 20 o^ to 30o^ N and S latitude across geologic time.
Weathering & Erosion in Deserts!
Weathering!
Physical weathering occurs along joints - expansion and contraction due to temperature changes. Lack of soil allows these blocks to build up ( as TALUS ) at the bottom of slopes and keeps bedrock exposed on slopes.!
Chemical weathering does occur, but slowly. Dew and some rain percolates in cracks and fractures and leaches material out of the rock, reducing its integrity.! Amount of water is not enough to flush it out of the system. Deposits material lower down - if calcite has been dissolved calcrete is deposited because it cements loose grains together.!
Calcrete!
Weathering & Erosion in Deserts!
Weathering!
Desert Varnish : wind-borne dust settles on rocks. When dew precipitates, bacteria metabolize these particles and deposit Fe- and Mn-oxides. Takes a long time to form, but Native Americans made use of it for artwork (petroglyphs).!
Weathering & Erosion in Deserts!
Lack of vegetation means variations in bedrock color stands out (e.g., Painted Desert, N. Arizona - variations in the amount of Fe and/or the amount of oxidation).!
Water Erosion!
Deserts get most of their rain all at once. Lack of vegetation means loose material is easily moved. Streams are ephemeral.! Deep channels with steep sides are carved, called Dry Washes or Arroyos in the US - Wadis in the Middle East. Flash floods are common.! 9
Weathering & Erosion in Deserts!
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Though rare, water is the dominant force shaping landscapes. Desert landscapes reveal dry drainages. Sediment erodes quickly when torrential rains generate dangerous flash floods characterized by rapid flow of thick, muddy, and viscous water. Flash floods quickly infiltrate dry streambeds.
Weathering & Erosion in Deserts!
Desert Pavement!
Mosaic of tightly fitting stones above a finer fraction that form a relatively smooth surface.!
- They are a lag deposit.!
2)!They are a product of sheetwash erosion.! 3)!Wetting of soils causes clays to expand pushing larger stones upward. Drying shrinks the clays and the stones settle down fitting together tightly.!
Theories of formation:!
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Weathering & Erosion in Deserts! 4)!Bubbles formed by bacteria metabolism gradually buoy stones upward.!
- Wind blown fine sediment is deposited between the stones and washes down. The stones were never buried, but this is a way to increase the thickness of the finer layer beneath them.!
Ventifacts
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Yardangs
Perched rocks. Pedestal formed by sand blasting (abrasion).!
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Development of Landforms in Basin & Range Topography
Alluvial Fan : deposit of sediment at the mouth of a valley.! Bajada : wedge of sediment along a mountain front formed by calescence of alluvial fans.! Pediment : bedrock surfaces that extend out from the mountain front to the alluvium- filled valleys. Formed by sheetwash.! Pediment + Bajada = PIEDMONT!
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Playas & Salt Lakes! Playas : remnants of temporary lakes. Salt encrusted clay deposits. Very flat! Salts include halite, gypsum, borax, etc.!
When it does rain, the surface becomes slippery - rocks can be blown over the surface.!
If there is sufficient water flowing into a desert basin, a salt lake may form due to great evaporation concentrating the dissolved ions. The Great Salt Lake in Utah is the remnant of a much larger lake - Lake Bonneville.! 20
Development of Landforms in Basin & Range Topography
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Mud-cracked playa surface
Playas & Salt Lakes!
Playa Lake Bed - Salt Encrusted
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Desert Landscapes: Inselbergs
EOLIAN LANDSCAPES!
Plateaus : broad, flat-topped areas elevated above the surrounding land and bounded, at least in part, by cliffs.!
Mesa : broad, flat-topped hill bounded by cliffs and capped with a resistant rock layer - wider than it is high.!
Butte : narrow hill of resistant rock with a flat top and very steep sides - about as wide as it is high.!
Monument/Chimney : pillar-like spire of resistant rock with a flat top and steep sides, much taller than it is wide.!
Plateau > Mesa > Butte > Monument/Chimney!
Rock Features of the Southwest!
Apart from climate, what is the main control on the development of these features?! (^26)
EOLIAN LANDSCAPES!
Cuesta : Asymmetric ridge developed due to gently dipping strata;! Hogback : Symmetric risge developed due to steeply dipping strata.!
Differential Erosion of a Monocline
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Eolian Deposits!
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Dune Formation & Migration!
Formation : movement of sand is retarded by an obstacle (wind shadow) & low pressure eddies result in deposition.!
Dunes are typically a few meters high, but can be 200 m by 100 km!
Gentle slope windward; steep slip-face ~34˚ leeward.!
Migration: can be >15 miles/year, but this depends upon wind velocity, persistence, obstacles. Migration produces cross bedding.! What is the transportation process depicted in these diagrams?!
Sand is blown up the gentle upwind slope.
Transport
Ripples
(i) !Velocity and persistence of wind;! (ii) Abundance of sand;! (iii) Vegetation.!
Dune shape is dependent upon:
Sand Dunes!
1) Irregular: Beach
Dunes – hummocky,
variable onshore/
offshore winds,
vegetation present.!
Can produce star dunes if wind direction is variable.!
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Sand Dunes!
3) Elongate :!
Transverse Dunes – perpendicular to wind, steep slip face; abundant sand.!
Sand Dunes!
3) Elongate :!
Longitudinal or Seif Dunes– parallel to wind; limited sand.!
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Sand Dunes!
Fossil Sand Dunes!
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Environmental Aspects of Wind! Unprotected fields – soil blown away during dry periods (e.g., dust bowl of the thirties – dust storm in Colorado drops 166,000 tons of dust on Kansas. Dust clouds up to 12,000 feet. Dust blocks out sun in New York city.!
Changing climates cause desert encroachment on cities = desert- ification; caused by overuse of land.! Violent storms especially in tropics.! Wind power.! 40
