DESERTS AND WIND Chapter 15
A. Deserts
1.
Definition: A desert is any area that receives
less than 25cm/yr (= 10inches/yr) precipitation
Note: there are major differences in deserts that receive
8-10 inches precipitation/year such as those in the southwestern US and the
Chilean desert where areas typically go years or even decades between
rainfalls.
2.
Reasons for low precipitation: water vapor and air
a.
warm air
may hold more moisture that cold air
b.
hot air
rises and cools
c.
cold air
sinks and warms
3. Types of Deserts pages 354-356
a. Global air currents—cause dry areas at 30
degrees N & S
b. Topographic Deserts—rain shadow of high
mountains
c. Global
ocean currents---west coast of
d. Extreme distances from water source
(ocean) ---example
e. Polar deserts
4.
Characteristic of deserts in the
d.
Precipitation---varies but usually closer to 10
than 0 inches/year
e.
Vegetation---sparse but usually present
f.
Weathering—to dry for chemical weathering to be
efficient; physical weathering predominates; very poor soil development
g.
Drainage—poorly developed (not enough runoff);
often internal drainage, few streams flow year around
h.
Erosion—wind may be important, but most erosion
in SW-US is stream erosion (and mostly immediately after rainfalls. In humid climates erosion tends to be lateral
because of extensive chemical weathering but in arid climates erosion tends to
be “vertical” with very minor differences in erodiablity
greater exploited
5. Erosional Features of SW-US Deserts pp. 361-363
a. Plateaus, mesas, buttes See page 363, Fig. 15.21---erosional
remnants
b. Badlands topography: White River
c. Other features: alluvial fans, Bajadas, playa lakes, dry
gulches
1. Introduction: Vegetation is the single most important
deterrent to wind erosion. Wind tends to
be important where there is little or no vegetation; places such as:
a. Deserts
b. Beaches
(sometimes under water, sometimes above water, therefore few permanent plants)
c. Glacial
Outwash Plains (important given the recent glacial events)
2. Erosional
Processes p.
349-350
a. Deflations: know about “blowouts” and “desert pavement” p. 350; see Fig.
15.6, p.351
b. Abrasion ---“sandblasting” effect p349; See Fig 15.3
3. Transportation p.348-349
a. Lower
competency capacity than water (streams) or ice (glaciers)
b. Load
i. Suspended load: carried high in the atmosphere p. 349
Clay-carried very high—much like volcanic dust
Silt—carried as large dust clouds, but not
dispersed as widely as clay
ii. Bed load: Sand--even in a sand storm,
most sand is within a few feet of the ground
p.348
NOTE: Wind does NOT carry a dissolved load
a.
Suspended load (silt portion) becomes:
Loess, See pp.
354 Loess forms tan, unstratified layers that can
maintain a near vertical slope and
usually consists of silt sized quartz grains.
Most loess deposits in the
Midwestern
b. Bed load (sand) becomes sand dunes. All dunes have an asymmetric cross-section (see Fig. 15.8; p. 352) but different types of dunes
can form depending on: sand supply, wind
velocity, duration of wind, extent of limited vegetation
c. Types
of Sand Dunes See pp. 351-354 and look at all photos
(Figs.)
i.
Barchan Dunes see Fig. 15.19, p. 352
ii.
Transverse Dunes see Fig.
15.11, p. 353
iii.
Longitudinal Dunes see
Fig. 15.10, p. 353
iv.
Parabolic (or U-shaped) Dunes see Fig. 15.12, p. 354 (kind most commonly found on beaches)