Transcript No Slide Title

DESERT LANDSCAPES
Factors influencing desert landscapes:
1. lack of soil development - why? little chemical weathering (little
clay). Chemical weathering that does occur = slow oxidation;
hydrolysis is very limited due to lack of water. In humid areas much
chemical
weathering occurs
beneath the soil
due to soil
moisture/ground
water in contact
with rock; obviously
this is not the case in
deserts, where the
water table is
usually deep below
the surface.
Harry Williams, Earth Science
1
2. sparse vegetation - little organic matter for soil development;
little organic acids for chemical weathering; little cohesion due to
root mats.
Harry Williams, Earth Science
2
3. mechanical weathering produces uncohesive sand and silt
covered surface which can be moved by wind and running water,
and is of low permeability promoting surface run-off and erosion.
Harry Williams, Earth Science
3
Characteristic Desert Landforms. 1. Eolian Processes and Landforms:
Contrary to common belief, most deserts are not covered in sand
dunes; for example, only 1/9 of the Sahara is covered in sand. In fact,
fluvial processes do most geomorphic work in most deserts. However,
the wind is more effective in deserts than elsewhere, due to the loose
surface and production of sand and dust by mechanical weathering.
Harry Williams, Earth Science
4
a. Deflation - the erosion of a surface by wind is termed deflation.
Coarse particles are left behind to form a characteristic desert
pavement.
Harry Williams, Earth Science
5
Typical desert pavement.
Harry Williams, Earth Science
6
b. Sand Dunes - wind moves sand by creep and saltation; due to its
weight sand usually is not carried more than 2 m off the ground; fine
silt can be carried further aloft by wind turbulence. The sand is
deposited in various types of dunes.
Harry Williams, Earth Science
7
A dune is simply a deposit of wind-blown sand,with a
characteristic gentle windward and steep leeward slope. The
type of dune depends on sand supply and wind conditions.
Harry Williams, Earth Science
8
i. Barchan Dunes - limited sand supply, wind mainly from one
direction. Isolated dunes perpendicular to wind, with bare ground
between them.
Harry Williams, Earth Science
9
ii. Transverse Dunes - abundant sand supply, weak wind mainly
from one direction, dunes perpendicular to wind (a “sea of sand”).
Harry Williams, Earth Science
10
iii. Longitudinal (Seif) Dunes - moderate sand supply, strong winds
from one quadrant. Linear dunes parallel with prevailing wind.
Harry Williams, Earth Science
11
c. Loess - the fine silt carried in suspension for long distances forms
loess, a blanket of silt adjacent to the source region (Note: most
loess is relict, originating in the last Ice Age, when strong winds
blew across silty deposits left by glacial meltwater).
Harry Williams, Earth Science
12
A loess bluff, Kansas.
Dwelling carved into loess,
northern China. Loess here is up
to 250 m thick. Eroded loess
ends up in the Yellow River (the
name comes from the yellow
color of the loess-rich water).
Harry Williams, Earth Science
13
2. Fluvial Processes and Landforms
Due to the impermeable, uncohesive surface and infrequent, but
intense rains, most desert areas are dissected by numerous rills,
gullies and channels, and are also subject to sheetwash. Many larger
channels are dry most of the year and are called washes in the U.S.
southwest. Flash floods are common during thunder storms, due to
large amounts of run-off; the overland flow also transports large
amounts of sediment, so that many washes carry mud flows.
Where channels emerge, alluvial fans are built from mud flows;
these may coalesce to form BAJADAS.
Harry Williams, Earth Science
14
A dry wash
Harry Williams, Earth Science
15
Alluvial fans, Owens Valley, California.
Harry Williams, Earth Science
16
Bajada, Death Valley, California.
Harry Williams, Earth Science
17
Harry Williams, Earth Science
Basin and Range Province.
18
Death Valley, playa lake.
Harry Williams, Earth Science
19
Erosional Landforms In Deserts.
Humid areas = almost continuous cover of regolith, soil and
vegetation; regolith usually fine-textured due to clay content;
creep and solution produces smooth, curvilinear slopes which
become lower over time.
Dry areas = regolith/soil is thin and discontinuous, sparsely
vegetated; regolith is coarse due to lack of clay; rockfall,
sheetwash and mass wasting produce angular slopes which retreat
back over time.
Dry areas
Humid
areas
Harry Williams, Earth Science
20
Landscapes are angular due to differential erosion - resistant rock
forms steep cliffs; softer rock forms gentler slopes. There is little
soil creep or solution to "smooth out" the landscape.
Harry Williams, Earth Science
21
The effect of differential erosion is visible in the “stair-step”
topography of the Grand Canyon.
Harry Williams, Earth Science
22
As slopes are eroded back and maintain their angular form,
Plateaus, Mesas, Buttes and Pinnacles are formed where resistant
flat layers (cap rocks) are present e.g. lava flows in west Texas.
Time
Monument
Valley, Utah.
Harry Williams, Earth Science
23
Plateaus, mesas, buttes and pinnacles formed by stream erosion
of uplifted strata.
Harry Williams, Earth Science
24
Near Big Bend
Lava flow cap rock
Harry Williams, Earth Science
25