A view overlooking Death Valley, California.
Fig. 14-CO, p.333
Why do
deserts
exist?
Falling air
creates
deserts at 30
degrees north
and south
latitudes. The
red arrows
inside the
globe show
surface
winds; the
blue arrows
(right) show
air flow on
the surface
and at higher
elevations.
Fig. 14-1, p.335
The major deserts of the world. Where are they concentrated?
Deserts cover 25% of the Earth’s land surface where 13% of the world’s population
lives. A desert is any region that receives less than _______ inches of rain per year.
Fig. 14-2, p.335
Rain-shadow deserts form where warm, moist air from the
ocean rises as it flows over mountains. As it rises, it cools and
water vapor condenses to form rain. The dry descending air
on the lee side absorbs moisture, forming a desert.
Fig. 14-3, p.335
Rainfall patterns in the
State of California,
where prevailing winds
carry moist Pacific air
eastward over the
mountains. Note that
rain-shadow deserts lie
east of the mountain
ranges.
What about the Atacama
and Gobi deserts?
Rainfall given in
cm/year.
Fig. 14-4, p.336
Water and Deserts. Water reaches the desert from three sources:
streams (from wetter regions), groundwater and rain/snowfall. The
Colorado River (above) flows from the Rockies through the arid SW
United States and empties (where?). Fig. 14-5, p.337
Desert Streams: Courthouse
Wash, Utah. In the spring, when
rain and melting snow fill the
channel with water…
Fig. 14-6a, p.337
…same wash, in the summer,
when the creek bed is dry…
A stream bed that is dry for most
of the year is called a wash (or
arroyo)…
where does the water go? what is
a “water table” and where is it in
this photo?
Fig. 14-6b, p.337
Desert Lakes:
Mud cracks pattern
the floor of a playa
in Utah.
An intermittent
desert lake is called
a playa…how does it
form?
Is the Salton Sea a
playa lake?
Fig. 14-7, p.338
When lakes evaporate, ions precipitate to deposit salts on the playa.
Economically valuable mineral deposits can accumulate over the
years. Here, a mule team is hauling valuable mineral deposits from
Death Valley during the 1800s.
Fig. 14-8, p.338
Flash Floods:
In August 1997, 11 hikers
perished when a flash flood
filled a slot canyon similar to
this one in the Utah desert. The
steep walls made escape
impossible.
Fig. 14-9, p.338
Pediments and Bajadas: An alluvial fan in Death Valley forms
where a steep mountain stream deposits sediment where is enters a
valley. A Bajada is a depositional surface.
Fig. 14-10, p.339
The Bajada in the foreground merges with a gently sloping pediment
to form a continuous surface in front of mountains in Mongolia. The
basin has no external drainage. A pediment is an erosional surface
where sediment is transported from the mountains to the bajada. p.339
Fig. 14-11,
Two American Deserts:
The Colorado Plateau;
Death Valley and the
Great Basin
Features of the Colorado
Plateau include spires and
buttes formed when
streams reach a temporary
base level and erode
laterally. The streams
transport the eroded
sediment away from the
region. Bottom photo is
spires and buttes in
Monument Valley, Az. A
plateau is a large elevated
area of fairly flat land. It
is a larger area than mesas
and buttes.
Fig. 14-12, p.340
Location of Great Basin shown in red…Colorado Plateau is to the right of
the great basin and takes in parts of Utah, Colorado, Arizona and New
Mexico. Fig. 14-13b, p.341
Fig. 14-12a, p.340
Fig. 14-12b, p.340
Death Valley and the Great Basin: sediment eroded from
surrounding mountains is slowly filling Death Valley, in the
rain shadow of the Sierra Nevadas. DV has no external
drainage, unlike the Colorado Plateau which drains to the
Gulf of California. Fig. 14-13, p.341
Fig. 14-13a, p.341
Bajadas and pediments are common features of the Great Basin. The form from a
combination of tectonic, erosional and depositional processes. Mountains a valleys
commonly form by block faulting (horst and graben or basin and range topography).
Mountains slowly drown in their own sediment…
Fig. 14-14, p.341
Fig. 14-14a, p.341
Fig. 14-14b, p.341
Fig. 14-14c, p.341
Wind: it blows across and
erodes bare, unprotected desert
soil and forms features such as
dunes, desert pavement and
loess.
To right, wind erodes silt and
sand (called deflation) but
leaves larger rocks behind to
form desert pavement. This is
a continuous cover of stones
that protects the desert surface
from further erosion.
Fig. 14-15, p.342
Fig. 14-15a, p.342
Fig. 14-15b, p.342
Wind also moves sand
grains by saltation
(usually not lifted more
than 1 meter off the
ground) which can carve
features by abrasion such
as the pinnacle to right in
the Grand Canyon.
Fig. 14-16, p.343
Dunes: a
mound or
ridge of wind-
deposited
sand. They
form when
wind erodes
sand from one
location and
deposits it in
another.
To right,
dunes near
Lago Poopo,
Bolivia.
Fig. 14-17, p.343
Blowouts can form (saucer or trough-shaped depression). They can
very large, such as the Qattara Depression in Egypt (100 meters deep,
10 km in diameter).
Fig. 14-18, p.343
Most dunes are assymetrical; wind erodes sand from the windward
side of a dune, carries it up to the dune crest and the sand slides down
on the sheltered leeward side (slip face) at the angle of repose for sand
(approx. 35 degrees).
Fig. 14-19, p.343
What’s this? When dunes become buried by younger sediment and
lithified over geologic time, the sandstone retains the original
sedimentary structures of the dunes. Steeply dipping layers of the
dune face are preserved here as cross-bedding (Zion National Park).
Fig. 14-20, p.344
Types of Dunes: When sand
supply is limited (rocky
deserts), the tips of Barchan
dunes travel faster than the
center and point downwind.
Barchans migrate
independently.
Fig. 14-21, p.344
Fig. 14-21ab, p.344
Fig. 14-21c, p.344
If sand is plentiful and evenly
dispersed, it accumulates in long
ridges called transverse dunes
aligned perpendicular to the
prevailing wind.
Fig. 14-22, p.346
Fig. 14-22a, p.345
Fig. 14-22b, p.345
If sparse desert vegetation is
present, like along a seacoast
or semiarid desert, a blowout
might form at bare areas
between vegetation as sand
is eroded while the tips are
anchored by plants
surrounding the blowout. A
parabolic dune is similar to a
barchan, except that the tips
of a parabolic dune point
into the wind.
Fig. 14-23, p.346
Fig. 14-23a, p.346
Fig. 14-23b, p.345
If the wind direction is
erratic but prevails from
the same general
compass direction and
sand supply is limited,
then long, straight
longitudinal dunes form
parallel to the prevailing
wind direction. In the
Sahara Desert, they can
reach 100 km long.
Fig. 14-24, p.346
Fig. 14-24a, p.346
Fig. 14-24b, p.346
Loess: wind can carry silt for hundreds or thousands of miles and deposit it as silt.
Silt is porous, uniform and typically lacks layering, and particles can interlock. It’s
not cemented, but can form vertical cliffs and bluffs. The largest deposits in China,
more than 300 meters thick, were used as dwellings. In 1920, a great earthquake
collapsed the cave system and killed an estimated 100,000 people. Above are loess
caves in Pakistan.
Fig. 14-25, p.346
Loess deposits in the United States. Soils formed on Loess are
generally fertile and make good farmland.
Fig. 14-26, p.347
Desertification:
What is it?
Fig. 14-27, p.348
p.349