Revised 8 / 06 (Monroe 6th ed.)

Arid Lands - Chapter 18



What causes a desert

Moisture and the desert

Wind and the desert

A closing note...



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


What causes a desert

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


Moisture and the desert

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


Wind and 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


A closing note

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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