The Work of Wind

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McKnight's
Physical Geography
Lectures
Chapter 18
The Topography of
Arid Lands
Andrew Mercer
Mississippi State University
© 2014 Pearson Education, Inc.
A Specialized Environment
• Desert terrain stark and abrupt
• Desert special conditions
Top – thinner soil
Slope – very thin soil
Foot – thicker soil
– 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
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 – is the most important shaper of landscapes in
DESERT country
– Wind – also matters, but less so than even occasional rain
Even in very dry
country, water is still the
most important shaper
of topography
A Specialized Environment
Desert conditions
• Utah is semi-desert
• Basins of interior
drainage – most dryland watersheds do not
drain into any ocean –
• Water usually
evaporates before
reaching the ocean
or a large lake.
Running Water in Waterless Regions
• Running water most important external
landform agent in dry country and desert
• Erosion tremendously effective with little plant cover
• Intensity of rain combined with impermeable land
surfaces create intense runoff
• Unpredictable imbalance between erosion and
deposition
Running
Water
Waterless
Surface
water
in the in
desert
– Exotic streams – the Nile is
an example of a river that
originates in one region and
flows through an alien region
to some other destination.
– Ephemeral streams –
periodically flow, result in
intense erosion,
transportation, and deposition
Regions
C – more mild
B climate desert
– Perennial – fulltime flow,
year-round
– Desert lakes – playas and
Salinas (dry salt lake beds),
saline lakes
A climate
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
Consider Mr. Allred’s yard – the rocky top of a hill was
once the ‘armor-plated’ stream bed at the bottom of the
hill. Eventually, everything else washed away, leaving the
river bed as the new top of the hill.
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
The Work of Wind
• Wind as a sculptor is a relatively limited effect
• Air at the surface has nearly zero wind
• Wind speed increases with distance above ground
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
Work
of Wind
•The
Aeolian
(wind)
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
The Work of Wind
• Aeolian (wind) deposition
– Fine sand laid as thin coating;
• Desert sand dunes
– Some dune fields composed of unanchored sand, moved
by local winds; slip face
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
The Work of Wind
• Common types of desert sand dunes
Loess
–
–
–
–
Wind deposited silt
Lacks horizontal stratification
Great vertical durability
Formation not well understood
Millions of people have lived in
silt caves across thousands of
years.
However, when such soils are
WET they can collapse during
an earthquake – thousands of
people may get killed.
The Work of Wind
Major loess (SILT) locations of the world
Two Representative Desert Landform Assemblages
Notice the effect of water even in a desert –
occasional, sharp rain can result in vast erosion.
It is not a good idea to build or occupy ‘alluvial fans’
like the one below.
River deltas are similar.
Watch out.
Two Representative Desert Landform Assemblages
Death Valley
– Excellent example of
basin and range
terrain
– Surrounding
mountain ranges
– Basin filled with
alluvium
– Salt pans and mobile
dunes in the basin
– Notice that salt
collects in basins
after washing out of
mountains.
Two Representative Desert Landform Assemblages
Mesa-and-scarp terrain
•
•
•
•
Mesa – flat topped surface
Scarp – steep cliffs
horizontal strata
Some layers resist erosion well -- others erode
quickly
Harder rock lasts longer
Softer rock erodes,
allowing the whole
formation to fail
Two Representative
Assemblages
Monument Desert
Valley –Landform
Utah/Arizona
These features are the scattered remains of vast areas that were
eroded and washed away.
Only a few “hard spots” remain – the cap rock protected softer
rock nearby or underneath.
Once water got past
the cap rock, the
whole region
washed away.
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
– A small amount of cap rock
is all that remains of the entire
formation.
– Cap rock protects the softer rock
underneath.
– All of it will be gone very quickly.
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