Transcript Wind - HCC Learning Web

Grotzinger • Jordan
Understanding Earth
Sixth Edition
Chapter 19:
WINDS AND DESERTS
© 2011 by W. H. Freeman and Company
Chapter 19
Winds and
Deserts
About Winds and Deserts
• Wind is a major agent of erosion and
deposition that moves vast quantities of
sand, silt, and dust over large areas.
• Eolian processes shape the land surface
in areas like deserts where few plants
live.
• Deserts expand and contract, depending
upon climatic changes.
Lecture Outline
1. Global wind patterns
2. Wind as a transport agent
3. Wind as an agent of erosion
4. Wind as a depositional agent
5. The desert environment
1. Global Wind Patterns
● Wind (the flow of air)
● wind is unconfined, except in
narrow valleys
● wind obeys all the laws of flow
● wind is parallel to surface, and
may go upward too
1. Global Wind Patterns
● Characteristics of wind
● turbulence
● wind belts
● trade winds
● westerlies
1. Global Wind Patterns
● Complications to simple circulation
● Earth’s rotation
● Coriolis effect
● equatorial upward movement
● dry, sinking air over deserts
Wind
as a
Flow
of Air:
Earth’s
Wind
Belts
North Pole
Westerlies
30°N latitude
NE trade winds
Equator
SE trade winds
30°S latitude
Wind
as a
Flow
of Air:
Earth’s
Wind
Belts
North Pole
Westerlies
30°N latitude
NE trade winds
Equator
SE trade winds
30°S latitude
At the equator,
the Sun’s rays are
perpendicular to
the surface,
concentrating heat.
Wind
as a
Flow
of Air:
Earth’s
Wind
Belts
North Pole
At the poles, the Sun’s rays
are spread over greater
areas, yielding colder
temperatures.
Westerlies
30°N latitude
NE trade winds
Equator
SE trade winds
30°S latitude
At the equator,
the Sun’s rays are
perpendicular to
the surface,
concentrating heat.
North Pole
There is little surface wind
at the equator, and the air
rises, forming clouds and
rain as it cools.
At the poles, the Sun’s rays
are spread over greater
areas, yielding colder
temperatures.
Westerlies
30°N latitude
NE trade winds
Equator
SE trade winds
30°S latitude
At the equator,
the Sun’s rays are
perpendicular to
the surface,
concentrating heat.
North Pole
There is little surface wind
at the equator, and the air
rises, forming clouds and
rain as it cools.
At the poles, the Sun’s rays
are spread over greater
areas, yielding colder
temperatures.
Westerlies
30°N latitude
NE trade winds
At 30°N and 30°S latitudes,
the cooled air sinks, warms
up, absorbs moisture, and
yields clear skies.
Equator
SE trade winds
30°S latitude
At the equator,
the Sun’s rays are
perpendicular to
the surface,
concentrating heat.
North Pole
There is little surface wind
at the equator, and the air
rises, forming clouds and
rain as it cools.
At the poles, the Sun’s rays
are spread over greater
areas, yielding colder
temperatures.
Westerlies
30°N latitude
NE trade winds
At 30°N and 30°S latitudes,
the cooled air sinks, warms
up, absorbs moisture, and
yields clear skies.
Equator
SE trade winds
30°S latitude
These two motions set up
the horizontal circulation
between the equator and
the North and South Poles.
At the equator,
the Sun’s rays are
perpendicular to
the surface,
concentrating heat.
North Pole
There is little surface wind
at the equator, and the air
rises, forming clouds and
rain as it cools.
At the poles, the Sun’s rays
are spread over greater
areas, yielding colder
temperatures.
Westerlies
30°N latitude
NE trade winds
At 30°N and 30°S latitudes,
the cooled air sinks, warms
up, absorbs moisture, and
yields clear skies.
Equator
SE trade winds
30°S latitude
These two motions set up
the horizontal circulation
between the equator and
the North and South Poles.
In the temperate zones,
the prevailing wind belts
come from the west.
At the equator,
the Sun’s rays are
perpendicular to
the surface,
concentrating heat.
North Pole
There is little surface wind
at the equator, and the air
rises, forming clouds and
rain as it cools.
At the poles, the Sun’s rays
are spread over greater
areas, yielding colder
temperatures.
Westerlies
30°N latitude
NE trade winds
At 30°N and 30°S latitudes,
the cooled air sinks, warms
up, absorbs moisture, and
yields clear skies.
At the equator,
the Sun’s rays are
perpendicular to
the surface,
concentrating heat.
Equator
SE trade winds
30°S latitude
These two motions set up
the horizontal circulation
between the equator and
the North and South Poles.
In the temperate zones,
the prevailing wind belts
come from the west.
In the tropics, the prevailing wind belts blow
from the east. The Coriolis effect is responsible
for the deflection of air eastward or westward
as illustrated by the curved arrows.
2. Wind as a Transport Agent
● Factors in how wind carries things
● wind strength
● particle size
● surface material
Wind
as a
Transport
Agent:
Rate of
Sand
Movement
versus
Wind
Speed
Wind
as a
Transport
Agent:
Sand Being
Blown from
the Desert
in Namibia
toward the
South Atlantic
2. Wind as a Transport Agent
● Materials carried by the wind
● windblown dust
● volcanic, organic, soil
● aerosols
● windblown sand
Wind
as an
Agent
of Erosion:
A Ventifact
Wind
as an
Agent
of Erosion:
Rounded
and
Frosted
Grains
of Sand
Wind
as an
Agent
of Erosion:
Desert
Deflation
Surface
Thought questions for this chapter
What evidence might you find in an ancient sandstone
that would point to its eolian origin?
3. Wind as an Agent of Erosion:
Formation of Desert Pavement
3. Wind as an Agent of Erosion
● What the wind does to erode
surficial materials
● sand blasting (ventifacts;
frosting on sand grains)
● deflation (desert pavements)
Thought questions for this chapter
You have just driven a truck through a sandstorm and
discover that the paint has been stripped from the lower
parts of the truck, but the upper parts are barely
scratched. What process is responsible, and why is it
restricted to the lower parts of the truck?
Compare the heights to which sand and dust are carried in
the atmosphere and explain the differences and
similarities?
4. Wind as a Depositional Agent
● Sand dunes
● deserts, beaches, lake shores,
floodplains, etc.
● need a ready supply of loose
sand
● vegetation stabilizes dunes
Wind
as a
Depositional
Agent:
Linear
Sand Dunes
of the
Southern
Arabian
Peninsula
Wind
as a
Depositional
Agent:
Wind
Ripples
at
Stovepipe
Wells,
California
Wind
as a
Depositional
Agent:
Sand Dunes
in Gusev
Crater, Mars
4. Wind as a Depositional Agent
● How sand dunes form and move
● saltation of sand
● transverse piling of sand or
sand piles up behind an
obstruction
● piling and avalanche of sand
Wind
as a
Depositional
Agent:
Sand Piling
Downwind
of an
Obstruction
4. Wind as a Depositional Agent:
Formation of Wind Shadow Sands
4. Wind as a Depositional Agent:
Formation of Sand Dunes
4. Wind as a Depositional Agent:
Formation of Sand Dunes
4. Wind as a Depositional Agent:
Formation of Sand Dunes
4. Wind as a Depositional Agent:
Formation of Sand Dunes
4. Wind as a Depositional Agent:
Formation of Sand Dunes
4. Wind as a Depositional Agent
● Types of sand dunes
● barchans
● blowout dunes
● transverse dunes
● linear dunes
4. Wind as a Depositional Agent:
Barchans
Wind
Barchans are crescent-shaped dunes,
always the products of limited sand
supply and unidirectional winds.
Wind
4. Wind as a Depositional Agent:
Blowout Dunes
Blowout dunes are almost the reverse of
barchans (the horns point “backwards”).
4. Wind as a Depositional Agent:
Transverse Dunes
Transverse dunes form in arid regions
where there is abundant sand.
4. Wind as a Depositional Agent:
Linear Dunes
Linear dunes occur in areas that have a
moderate sand supply, a rough pavement,
and winds in the same general direction.
4. Wind as a Depositional Agent
● Dust falls and loess
● fallen atmospheric dust creates a
deposit of loess (loess blanket)
● loess covers 10% of Earth’s
surface
Wind
as a
Depositional
Agent:
Pleistocene
Loess
in the
Catalina
Mountains,
Arizona
Wind
as a
Depositional
Agent:
Pleistocene
Loess
in Central
China
Thought questions for this chapter
What factors determine whether sand dunes will form on a
stream floodplain?
Trucks continually have to haul away sand covering a
coastal highway. What do you think might be the source
of the sand? Could its encroachment be stopped?
Which of the following would be a more reliable indication
of the direction of the wind that formed a brachan: crossbedding or the orientation of the dune’s shape on a map?
Why?
Thought questions for this chapter
There are large areas of sand dunes on Mars. From this
fact alone, what can you infer about conditions on the
Martian surface?
What aspects of an ancient sandstone would you study to
show that it was originally a desert sand dune?
What evidence would cause you to infer that dust storms
and strong winds were common in glacial times?
5. The Desert Environment:
Where the Deserts Are
5. The Desert Environment
● Where deserts are found
● areas of low rainfall
● rain shadows
● far from oceans
● in polar regions
5. The Desert Environment
● Various factors play a role
in the formation of deserts
● plate tectonics
● climate change
● human actions
● in polar regions
5. The Desert Environment
● Desert weathering phenomena
● desert colors (rusty, orange
brown)
● desert varnish
● stream erosion (when water
is present)
The
Desert
Environment:
Rusty Colors
and Desert
Varnish
(carved by
aboriginal
people),
Canyonlands,
Utah
The
Desert
Environment:
Desert
Stream
Erosion
(when water
is present),
Saguaro
National
Monument, Arizona
The
Desert
Environment:
Desert
Stream
Erosion
(when water
is absent),
Saguaro
National
Monument, Arizona
The
Desert
Environment:
Desert Playa
Lake, Death
Valley,
California
5. The Desert Environment
● Desert sediments; sedimentation
● alluvial sediments
● eolian sediments
● evaporite sediments (playa
and playa lakes)
5. The Desert Environment
● Desert landscapes
● playa
● desert pavements
● dune fields
● dry washes (wadis)
● pediments
The
Desert
Environment:
Desert
Pediment,
Cima Dome,
Mojave
Desert,
California
5. The Desert Environment:
Formation of a Pediment
Elevated
mountains
Downfaulted
lowlands
Fault
Time 1
The lowlands are downfaulted, and the
mountains are elevated.
5. The Desert Environment:
Formation of a Pediment
Alluvial fans
Early
erosion of
mountains
Stream
floodplain
Time 2
Erosional debris is deposited as alluvial
fan and stream floodplain sediments.
5. The Desert Environment:
Formation of a Pediment
Alluvial
deposits
Pediment
Time 3
Erosion produces a pediment with thin
covering of alluvial deposits.
5. The Desert Environment:
Formation of a Pediment
Mountain
remnants
Pediment
Time 4
Continued erosion produces a more
extensive pediment.
Thought questions for this chapter
What features of a desert landscape would lead you to
believe it was formed mainly by streams, with secondary
contributions from eolian processes?
How does desert weathering differ from or resemble
weathering in more humid climates?
What kinds of landscape features would you ascribe to the
work of the wind, to the works of streams, or to both?
Key terms and concepts
Deflation
Desert pavement
Desert varnish
Desertification
Dry wash
Dust
Eolian
Loess
Pediment
Playa
Playa lake
Sandblasting
Slip face
Ventifact
Wadi