Revised 8 / 06 (Monroe 6th ed.)
Including...
Regions of low moisture
Cover 1/3 of earth's land area (fig. 18-18; pg. 581)
Moisture is the key concept
Like on Arrakis (Dune)
Temperature is not a factor
Can be hot or cold
Rarely sand covered - classic dune stereotype not accurate for most
Commonly barren rock and/or rubble
Vegetation is generally sparse - again from lack of moisture
Also, from very thin soil horizons
No water to create soils
Chemical weathering processes almost nil
Interior drainage common
Not enough runoff to establish major, through-going streams
Results in seasonal lakes, often salty
Extreme climatic conditions
Temperature fluctuations can be intense
Lack of moisture accentuates diurnal (and seasonal?) temperature fluctuations
Long periods without rain
And often heavy when it comes
Generally a combination of factors
Latitude (fig. 18-17; pg. 580)
Differential heating at equator
Results in air movement - winds
Deserts 30° north and south, and at poles
Where dry air descends and warms up
Tends to evaporate any available moisture
DIGRESS TO: poor Australia! Come back in 10 million years.
Local topographic effects
Rain shadow (fig. 18-19; pg. 582)
DIGRESS TO: Wet & dry adiabatic rates
Cold coastal currents (west coast US and Africa)
Winds blowing onshore are cooled and drop moisture
Warm up over land and evaporate
Fog common
Also results in redwoods!
Desertification (fig. 18-1; pg. 568)
Caused by human misuse of semi-arid lands
EXAMPLE: Sub-Sahara Africa
Once started, deserts tend to build on themselves
Bare sand/rock reflects additional solar energy
Lack of vegetation
Nothing to hold soil
Local population pressures
Severely stress available water/plant/animal resources
By definition, almost non-existent
DIGRESS TO: water vs. moisture: a fundamental difference
Still plays an active role in shaping landform evolution
Physical weathering more active than chemical or biological
Large amounts of angular debris - minimal clay
Braided streams common
Majority of streams seasonal
Exception: headwaters in humid regions (Colorado, Nile)
These actually serve as recharge areas for groundwater
Are these influent or effluent streams?
Precipitation usually comes in short, intense storms
Many inches in short time
The full year's total in just a few storms
Results in flash floods
Very effective in transporting debris but usually for a short distance
Seasonal lakes - Playas (fig. 18-22; pg. 587)
Evaporite deposits common
In actuality, erosion rates are most rapid in semi-arid regions
Arid lands: not enough rainfall to do much
Humid lands: Excess vegetation holds soil in place
Depositional features associated with stream flow
Alluvial fans (fig. 18-23; pg. 588)
Direct result of Q=AV, with a twist
Percolation of flow into fan reduces discharge
As evolution of the landscape proceeds, fans grow and merge into bajadas (fig. 18-24; pg. 588)
Erosional features
Pediments (fig. 18-25; pg. 589)
Gently sloping bedrock surfaces
Essentially a small-scale peneplain in the desert
Both book and lab manual say water is most important
Don't loose sight of the effect of winds!
DIGRESS TO: I feel wind is at least the equal to water, if not more important
Lots of wind - several reasons
Remember latitude effects (above)
Extreme diurnal temperature fluctuations
Lack of vegetation
Wind not restricted to a defined channel
Results in immense, broad areas being set in motion during major storms
Lots of material moved!
Wind much more selective than water
Nowhere near the potential energy
Also less dense and lower viscosity
Therefore, only moves the small stuff (no boulders!)
Sand limited to 6' or so
High erosional capacity
DIGRESS TO: metal shields around utility poles
Silt & dust can go much higher into the atmosphere
Much less erosional capacity
Terms similar to stream flow
Bed load and suspended load
Deflation - general term for wind erosion
Removal of smaller materials
Deflation basins - wind-cut depressions (fig. 18-6; pg. 572)
Occasionally cut to water table
Springs around margins - oasis
Leaves big stuff behind
Desert Pavement (fig. 18-7; pg. 573)
Desert Varnish (fig. 18-21; pg. 586; Rock Art for the Ages, pg.584)
DIGRESS TO: Selective removal of stones to create patterns
Abrasion - all this moving material can erode
Like sandblasting - but limited to 6' above the ground
Ventifacts (fig. 18-4; pg. 571)
Yardangs (fig. 18-5; pg. 572)
Can result in some pretty unusual landforms
Hoodoo rocks (fig. 18-3; pg. 571)
Sand Dunes (figs. 18-8 thru 18-15; pg. 574+)
Piles of sand transported and deposited by the wind
Possibly eroded by the wind, too
Actually not that common of a feature in arid lands
The vast majority of arid lands do not have dunes
But all (most?) dunes occur in deserts
Many different types of dunes (will study in lab)
Loess deposits (fig. 18-16; pg. 579)
Wind-blown silt
Not unique to arid lands
Also associated with margins of continental ice sheets
Of all earth's landforms, arid lands may be the some of the most fragile
Lack of water retards the healing process (water again!)
EXAMPLE: petroglyphs & South American figures (Rock Art for the Ages, pg.584)
We are currently using the deserts as a vast playground and waste dump
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