Transcript Chapter19 Deserts - Personal.kent.edu

Chapter 19
Deserts and winds
What is a desert?
• Any region with an arid climate
and a rainfall less than 25
cm/year.
How deserts form?
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Descending warm and moist air.
Rain shadow
Distance from ocean
Coasts with cold ocean currents
Polar deserts
Distribution of deserts
• Most of the deserts lie along a narrow
belt 30° N and S of the Equator.
• These two belts have warmer air and
can hold a lot of moisture, leading to
very low rainfalls.
Distribution of deserts
• The two belts are characterized by
clear skies, lots of sunshine, little rain
and high evaporation.
Deserts due to Rain Shadow
• Rain shadow is a dry region
downwind from a mountain range.
• Example: Sierra Nevada range in
California causing desert formation in
Nevada and N. Arizona.
Formation of Rain Shadow Deserts
Moisture laden
winds come
onshore
Moist airmass
rise and cool,
causing
precipitation…
…resulting
in a rainy
windward
slope
Moisturedepleted airmass
sink and warm,
reducing relative
humidity…
…forming a
rain shadow on the
leeward side of the mountains.
• Distance from ocean: oceans are a great
source of evaporated water. At greater
distance from the oceans, deserts occur in
the continental interior.
• Example: Gobi desert in China.
• Coasts with cold ocean currents: warm air
which can hold more moisture and no
rains on the coasts.
• Example: Pacific coast of S. America.
• Polar deserts: cold air with little moisture.
No rains.
• Examples: Antarctica, Greenland, Alaska.
Wind erosion and transportation
• Wind can erode only finer particles
such as clay silt and sand.
• Deserts typically have strong
winds which can erode loose dry
sediments causing sand
storms/dust storms.
• Effects of wind action are strong
only close to the ground (upto one
meter from the ground surface).
Death
valley, Ca
Death
valley, Ca
Erosional features
• Desert pavement: thin surface
layer of closely-packed pebbles.
Desert Pavement
in the Sonoran
Desert, Arizona
• Ventifacts: rock fragments with flat, windabraded surfaces.
• Blowout: depression on the land surface
caused by wind erosion.
Wind deposition
• Loess
• Sand dunes
• Loess: deposit of wind-blown silt
and clay consisting of quartz,
feldspar and clay minerals.
• Weakly cemented by calcite.
• A desert or glacial outwash is
needed as source material.
Sand dunes
• Mounds of loose sand grains heaped up
by the wind.
• Composition of sand depends on;
– Sand source
– Chemical weathering
• Quartz, feldspar and calcite are generally
more abundant.
• Well sorted and rounded.
How a dune forms?
• A sand dune forms with a gentle upwind
slope and a steeper slip face on the
downwind side.
• Sand eroded from the upwind side is
deposited on the slip face, forming crossbeds.
Embryonic dunes forming on the lee-side of vegetation
Types of dunes
• Factors controlling dune type:
– Wind velocity and direction
– Sand supply
– Vegetation cover
• 4 types of dunes:
– Barchan
– Transverse
– Parabolic
– Longitudinal
Barchan: crescent shaped dune convex in the
upwind direction.
Transverse: relatively straight, elongate dune
oriented perpendicular to the wind direction.
Parabolic: similar to a barchan dune except that
it is convex in the downwind direction.
longitudinal: symmetrical ridge that forms
parallel to the wind direction.
Deserts in US
• Deserts in SW US are caused by;
– Proximity to the 30° N latitude.
– Rain shadow by Sierra Nevada.
• Colorado plateau
• Basin and Range Province, Nevada.
Associated features
• Plateaus: broad, flat-topped areas
elevated above the surrounding land and
partly bounded by cliffs.
Mesa: broad flat-topped hill bounded by
cliffs from all sides.
Butte: narrow hill of resistant rock with a flat
top and very steep sides.
• Runoff water in the valley may collect at
the floor forming a Playa lake.
• Mud cracks develop when a playa lake
dries.
Desert Weathering is
Characterized By:
• a dominance of physical weathering
• minimal chemical weathering, consisting
mainly of oxidation of mafic minerals
Desert Weathering is
Characterized By:
• formation of desert varnish: a
weathering veneer formed of a mixture of
clays and smaller amounts of iron and
manganese oxides.
Oxidation of mafic
minerals results in
the formation of iron
oxides and the typical
reds and browns of
a desert landscape
Petroglyphs scratched in desert varnish