Download GEOL 1e Lecture Outlines

GEOL: CHAPTER 15
The Work of Wind
and Deserts
The Mesquite Flat sand dunes in Death Valley, California, are
a mix of dominantly transverse-type dunes with some
crescent-type dunes and star-type dunes.
Learning Outcomes
LO1: Discuss the role wind plays in
transporting sediment
LO2: Explain the two processes of wind
erosion
LO3: Identify the types of wind deposits
LO4: Describe the air-pressure belts and
global wind patterns
Learning Outcomes, cont.
LO5: Describe the distribution of deserts
LO6: Identify the various characteristics of
deserts
LO7: Identify the different types of desert
landforms
More than 6,000 years ago,
the Sahara was a fertile
savannah supporting a
diverse fauna and flora,
including humans. Then the
climate changed, and the
area became a desert. How
did this happen? Will this
region change back again in
the future? These are some
of the questions geoscientists
hope to answer by studying
deserts.
By this point in the semester, you
should have some theories of your
own!
Desertification
• The expansion of deserts into formerly
productive lands
• Human agriculture has altered natural
vegetation patterns
• Sahel hard hit: south of Sahara Desert
– Famines
– Malnutrition
– Poverty
Wind Sediment Transport
• Wind is less dense than water, so can
only transport smaller sediments
• Bed load: moves by saltation or by
rolling or sliding
• Saltation: wind lifts sand grains that
dislodge other grains upon hitting the
surface
Most sand is moved near the ground surface by saltation. Sand grains
are picked up by the wind and carried a short distance before falling
back to the ground, where they usually hit other grains, causing them to
bounce and move in the direction of the wind.
This should look familiar. Think of stream transport.
Erosion Natural forces of wind and water have eroded the rocks in
Monument Valley, Arizona, to create “mitten” buttes such as this one.
Wind Sediment Transport, cont.
• Suspended load:
– Silt- and clay-sized particles
– Will usually stay at surface unless
physically disturbed
– Once in the air, smaller particles can travel
thousands of miles
Wind Erosion: Abrasion
•
•
•
•
•
Caused by saltating sand grains
Analogous to sandblasting
Rarely more than a meter above ground
Typically modifies existing features
Ventifacts: surfaces altered by
windborne particles
a
b
Ventifacts
a. A ventifact forms when windborne particles (1) abrade the surface of a
rock, (2) forming a flat surface. If the rock is moved, (3) additional flat
surfaces are formed. b. Numerous ventifacts are visible in this photo,
which also shows desert pavement in Death Valley, California. Desert
pavement prevents further erosion and transport of a desert's surface
materials by
Wind Erosion: Deflation
• Removal of loose surface sediment by
wind
• Deflation hollows/blowouts from
differential erosion
• Desert pavement: close-fitting pebbles
and cobbles caused by wind eroding
away smaller particles
A deflation hollow, the low area, between two sand dunes in Death Valley,
California. Deflation hollows result when loose surface sediment is
differentially removed by wind.
Dune Formation
• Mound or ridge of wind-deposited sand
• Form around an obstruction that stops
sand grains
• Shallow windward slope
• Steep leeward slope: angle of repose
• Dune can migrate in direction of strong
prevailing winds
Sand Dunes Large sand dunes in Death Valley, California. The prevailing
wind direction is from left to right, as indicated by the sand dunes in which
the gentle windward side is on the left and the steeper leeward slope is on
the right.
Dune Migration
Wind
Wind
Direction of
dune migration
Sand moves by saltation
Windward
side
a. Profile of a sand dune.
Leeward
slope
b. Dunes migrate when sand moves up the
windward side and slides down the leeward slope.
Such movement of the sand grains produces a
series of crossbeds that slope in the direction of
wind movement.
Stepped Art
Fig. 15-7, p. 308
Cross-Bedding Ancient cross-bedding in sandstone beds in Zion
National Park, Utah, helps geologists determine the prevailing
direction of the wind that formed these ancient sand dunes.
Dune Types
1.
2.
3.
4.
•
Barchan
Longitudinal
Transverse
Parabolic
Determined by:
– Sand supply
– Wind direction and velocity
– Vegetation
Barchan Dunes
•
•
•
•
•
•
Crescent shaped, tips point downwind
Form in areas with a flat, dry surface
Little vegetation
Nearly constant wind direction
Mobile
Up to 30 meters high
Barchan Dunes
Longitudinal Dunes
• Long parallel ridges of sand that are
parallel to prevailing winds
• Limited sand supply
• Winds converge from slightly different
directions
• 3-100 meters in height
• Up to 100 km long
a
b
Longitudinal Dunes
a. Longitudinal dunes form long, parallel ridges of sand aligned roughly parallel to
the prevailing wind direction. They typically form where sand supplies are limited.
b. Longitudinal dunes, 15 m high, in the Gibson Desert, west central Australia. The
bright blue areas between the dunes are shallow pools of rainwater, and the
darkest patches are areas where the Aborigines have set fires to encourage the
growth of spring grasses.
Transverse Dunes
• Long ridges perpendicular to prevailing
wind direction
• Abundant sand with little or no
vegetation
• “Sand seas”
• Up to 200 meters high
• Up to 3 km long
Transverse dunes form long ridges of sand that are
perpendicular to the prevailing wind direction in areas of
little or no vegetation and abundant sand.
Parabolic Dunes
•
•
•
•
Common in coastal areas
Abundant sand
Strong onshore winds
Form where vegetation cover is broken,
from a deflation hollow or blowout
• Tips point upwind
Parabolic Dunes
a. Parabolic dunes typically form in coastal areas that have a partial cover
of vegetation, a strong onshore wind, and abundant sand. b. A parabolic
dune developed along the Lake Michigan shoreline west of St. Ignace,
Michigan.
Loess
• Wind-blown silt and clay deposits
• Quartz grains, feldspar, micas, calcite
• Three sources:
– Deserts
– Pleistocene glacial outwash deposits
– River floodplains in semiarid regions
Loess, cont.
•
•
•
•
•
Easily eroded
Steep cliffs and rapid stream erosion
10% Earth surface
30% U.S. surface
Fertile soils
Global Air-Pressure Belts
• Air pressure: density of air exerted on
surroundings (weight)
• Heated air = lower surface air pressure;
much solar heating
• Cooler air = higher surface air pressure;
less solar heating
Global Air-Pressure Belts, cont.
• Equatorial zone receives most solar
energy
• Surface air rises, cools, releases
moisture
• Rising air is drier as it moves poleward
• At 20-30 degrees latitude, it sinks,
compresses, and warms to form a highpressure area conducive to deserts
Global Wind Patterns
• Winds: air flows from high-pressure
areas to low-pressure areas
• Coriolis effect: apparent deflection of
moving objects because of Earth’s
rotation
– Winds deflected to the right in the Northern
Hemisphere
– To the left in the Southern Hemisphere
The General Circulation Pattern of Earth’s Atmosphere
Distribution of Deserts
• 30% of land surface
• Low and middle latitudes
• Potential evaporation greater than
yearly precipitation
• Semiarid: more precipitation than arid
Distribution of Deserts, cont.
• Arid = desert
• Deserts
– Less than 25 cm precipitation per year
– High evaporation rates
– Poorly developed soils
– Mostly devoid of vegetation
The Distribution of Earth’s Arid and Semiarid Regions
Semiarid regions receive more precipitation than arid regions,
yet they are still moderately dry. Arid regions, generally
described as deserts, are dry and receive less than 25 cm of
rain per year. The majority of the world’s deserts are located
in the dry climates of the low and middle latitudes.
Climate Characteristics
of Deserts
•
•
•
•
Hot summer days: 32ºC – 50ºC
Cooler on winter days: 10ºC - 18ºC
Precipitation variable and unpredictable
Plants are small, widely spaced, grow
slowly
– Stems and leaves minimize water loss
Desert Vegetation Desert vegetation is typically sparse, widely spaced, and
characterized by slow growth rates. The vegetation shown here is in Death
Valley, California.
Weathering and Soils
• Mechanical weathering dominates
– Temperature fluctuations
– Frost wedging
• Some chemical weathering, though
minor
• Desert soils are thin, patchy and subject
to erosion
Desert Streams
• Most precipitation comes from brief and
heavy cloudbursts
• Rapid runoff quickly fills channels, with
rapid sediment transport and much erosion
• Most streams flow intermittently and don’t
reach the sea (internal drainage)
• Some permanent streams: Colorado River
Playa Lakes
• Excess water from rainstorms
accumulates in low-lying areas
• Temporary: hours to months
• Shallow and saline water
• Evaporates to leave a playa (salt pan)
A playa lake formed after a rainstorm near Badwater, Death Valley
National Park. Playa lakes last from a few hours to several months.
Salt deposits and salt ridges cover the floor of this playa in the
Mojave Desert. Salt crystals and mud cracks are characteristics
features of playas.
Alluvial Fans and Pediments
• Sediment-laden streams leave mountains
and create fan-shaped sediment deposit on
flat desert areas
• Common in the Basin and Range province
in western North America
• Pediment: an erosion surface of low relief
gently sloping away from a mountain range
Remember the Alluvial fans from Chapter 12?
Mesas and Buttes
• Mesa: broad and flat-topped; erosional
remnant bounded by steep slopes
• Buttes: mesa eroded into a pillar-like
structure
• Both have an erosion-resistant cap rock
underlain by more easily eroded rock
Buttes Right Mitten Butte and Merrick Butte in Monument Valley Navajo
Tribal Park on the border of Arizona and Utah.
Virtual Field Trip
• The effects of wind erosion
• The features of wind deposition
• A desert landform