Weathering and erosional
processes; deserts of
Egypt
Classification of arid landscapes




Hyperarid P/ETP
Arid
Semiarid
Subhumid
<0.03
0.03-0.20
0.20-0.5
0.5-0.7
Characteristics of arid areas
Lack of continuous ground cover
Discontinuously flowing rivers (decrease in discharge
downstream)
Accumulation of secondary minerals in soils (lack of
recharge, upward movement of gw when wt is near
surface)
Distribution of arid areas
1
Soils and weathering

Accumulation of salts
–
–
–
–
–

Upward movement of ground water
Lack of infiltration and leaching
Source of wind blown salts
Calcic horizons
Main orders-aridisols, vertisols, entisols
Rock (desert) varnish
– Manganese and iron oxide coating produced by
manganese-oxidizing bacteria in alkaline environment

Mass wasting
– Angular, steep slopes; soil creep less important; talus
common
Weathering
 Mechanical
(Physical)
– Pressure Release
– Thermal expansion
– Growth of crystals
 Salt
weathering
heaving and cracking
 Frost
– Plants and animals
Thermal expansion


Thermal stress fatigue or
shock
Factors
– Differential expansion of
minerals
– High temperature
gradients-rapid heating
during day raises T in
outer layer—rapid cooling
at night relative to interior


Cause tensional stresses in
rock—microfractures,
which gradually expand
and lengthen
Moisture accelerates the
process
Spalling caused by brush fire-granite
2
THERMAL EXPANSION
Growth of crystals
 Salt
weathering
heave and fracture
 Frost
Salt weathering
 Volume
expansion on oxidation
 Hydration pressure
– Anhydrite to gypsum and others
– CaSO4 + 2H2O
CaSO4.2H2O
 Precipitation
of salt crystals in voids
and later expansion—crystallization
pressure
3
Dendara Temple, Egypt
Tafoni
Frost action

Water expands 9% upon
freezing, and even more if
confined in a joint or
fracture: however, this
probably does not create
enough pressure to
fracture rocks. The process
may involve freezing of a
capillary film of adsorbed
water and capillary
movement through the
rock toward the freezing
front. May be able to
expand microcracks and
fractures.
4
Andesite slabs and fragments produced by frost shattering-southwestern
Montana
Canadian Rockies
 Plants
and animals—root growth in
fractures; combination of physical
and chemical process.
5
BIOLOGICAL ACTIVITYROOT WEDGING
Chemical Weathering
Obelisk in Egypt
Obelisk in New York
Chemical Weathering--Solution
CaSO4 . 2H2O
Ca2+ + SO42- + 2H2O

CaCO3 + H2O
Ca2+ + HCO3- + OH-

pH—negative log of hydrogen ion activity

– Water not shown but must be present
– Reversible
– Congruent
6
Solution of limestone and dolomite; formation of sinkholes
and caves in humid climates. May play a role in formation
of depressions in the western desert.
Dahkla Oasis
5 miles
Chemical Weathering-Oxidation
FeS2 + 7/2H2O + 15/4 O2
4 H+
Fe(OH)3 + 2SO42- +
Oxidation—loss of electrons
Fe: Fe+2 – Fe+3 (1 electron) lost
S: S-1 – S +6
(14 electrons) lost
O: O0 – O-2
electrons
gained
(3.75 x 2 x 2) = 15
7
Oxidation-yellowish, reddish, brown colors
Chemical weathering--Carbonation
 CO2
+ H 2O
H2CO3
– Water not a strong acid by itself
– Much more aggressive when it reacts to
form carbonic acid
– Reversible
– Origin of CO2 in subsurface: aerobic
decomposition of organic matter
 O2
+ CH2O
H2O + CO2
Chemical Weathering--Hydrolysis

MgSiO4 + 4H+ + 4OH-
Mg2+ + 4OH- + H4SiO4
– Dissolution of olivine
– Reaction with dissociation products of water
– Congruent

2KAlSi3O8 + 2H2CO3 + 9H2O
2HCO3-
Al2Si2O5(OH)4 + 4H4SiO4 + 2K+ +
– K feldspar (orthoclase) to kaolinite
– Incongruent
– Irreversible
– Products: silicic acid, potassium ion, bicarbonate
8
Geomorphic processes
Sediment yield in drainage basins
LangbeinSchumm
curve
Erosion by water

Precipitation in desert
– Very infrequent
– Occurs in thunderstorm, cloudburst events
– Rapid runoff because of lack of vegetation and
steep slopes
– Dry valleys called wadis were originally formed
in a wetter climate but are not dry.
– Flash floods in these wadis present a major
geologic hazard to inhabitants.
Flash floods

En Gedi
Short cut to En Ge di flo ods.lnk

Wadi Zin
Short cut to Zin2.lnk
9
These pictures are from Wadi Hatzera, a small tributary of Wadi Zin in the
2004 flood. Water depth was 8 m, 4 m boulders were transported and the
flood was reconstructed at 600 m3/s—less than a 1% probability
Flash floods-Wadi Isla case history
Wadi Isla
El-Qaa Plain
3 miles
10
El Qaa plain with Sinai massif in
background
Mouth of Wadi Isla “canyon”
View downstream with Red Sea in distance
11
Wadi Isla Fan
12
17 m
Cemented Alluvium in Canyon-Pleistocene?
Flash flood competence
13
Boulder berms
~4m
~ 2m
Linear ridges of coarse clasts deposited by debris torrents (not debris
flows) along channel margins. Often located in zones of flow separation
near channel bends or changes in channel width (Carling, 1987).
Boulder berm height is an approximation of flow depth during flood.
Estimation of velocity and
discharge in boulder berm reach
Manning Equation
V = R2/3 S1/2 n-1; where
R= hydraulic radius (A/P); where A = 65 x 4 = and P
=73 m
S = slope (0.038)
n = Manning’s roughness coefficient (0.05), which is
a reasonable value from previous studies such as
Costa (1983) and Williams (1983);
V ~ 9.2 m s-1
Q = VA = 2392 m3/s
4m
65 m
Depth estimates in boulder berm
reach
Costa (1983) used four methods to estimate depth for flash floods in
small drainage basins in the Rocky Mountains: the Manning Equation,
stream power, the Shields function, and relative roughness.
For a velocity of 9.1 m3/s, the average of three of the four methods (one
was dropped for boulders over 0.6 m in diameter), the depth ranged from
5.9 m – 3.8 m, when slope was between 0.02 and 0.05, and n was
between 0.068 and 0.095. The height of the boulder berm falls within this
range.
14
Size measurements on boulder fan
Competence on boulder Fan
Clast imbrication and imbricate
cluster bedforms
Arrows show flow direction
15
Traverses and measurement
stations
Located by
GPS
 Intermediate
axis of 5
largest clasts
was measured
at each
location and
averaged.

Velocity estimates

Mean value for each
station was used to
estimate velocity with
the following equation,
which is the average of
four methods applied
to Rocky Mountain
flash floods.
V = 0.18 dI0.487,
in which dI is expressed
in mm. (Costa, 1983).

Mean velocity for each traverse
(m sec-1)
16
Wadi Isla
Geology
draped
over the
DEM
Preliminary Model Results
Flood Analysis
2
R = 0.9127
1. Storm Events for Wadi
Isla from 1998-2005
were correlated with
resultant discharge
amounts.
2000
1600
Discharge (m3/s)
2. Forecasts show
precipitation amounts
required to achieve
discharge rates
determined by boulder
size.
2400
1200
800
400
0
0
20
40
60
80
100
120
140
160
180
Precipitation (mm)
Observed Data
Artificial
Linear (Observed Data)
Paleoflood data from the Negev Desert, Israel. Curve A is
envelope curve for late Holocene paleofloods. (From Greenbaum
et al., QSR, 2006.
17
Dangers of flash flooding
A diversion ditch in Qena to convey floods through the
city to the Nile
Desert landforms
Stream valleys-wide shallow channels,
high bedload transport
 Pediments
 Desert plains and plateaus

– Bare surfaces
– Stony-lag concentrate from wind erosion (reg)
– Barren rock (hammada)
– Desert pavement, rocks in close contact, may
be covered with desert varnish
– Sand seas (ergs)
Mountain landforms
18
Pediments
Mohave desert
How do pediments form?
Inselbergs, Joshua Tree
3 miles
19
Landscapes formed by 2 stages of
weathering
 Deep
chemical weathering under a
humid climate
 Removal of weathering products in
an arid climate
Joshua Tree National Monument

Landscape that
formed in two stages;
deep weathering of
granitic rocks along
joints (humid climate)
with spheroidal
weathering; followed
by climate change to
arid climate; exposure
of core stones
Inselberg
2-stage weathering in Aswan
granites

Importance;
weathered core
stones provided an
easy source of
granite boulders for
temples and
statues, etc.
20
Reg surfaces
El-Qaa Plain, Sinai
Giza Plateau
Hammadas
Desert pavement

Theories for origin
– Winnowing of fines
by wind or water
– Heave of larger
clasts due to
swelling of clays or
formation of salt
crystals.
– Others
21
Desert varnish

Coating of
manganese oxides
on exposed rock
surfaces. Involves
oxidation and
precipitation by
microorganisms
that can live in
very alkaline
conditions.
Wind erosion and transport
V*= √τ/ρ Drag velocity, proportional to log of height;
function of wind velocity and surface roughness; Τ= shear
stress
Threshold velocity of particle motion
22
Wind erosion
 Deflation
 Abrasion
 Forms
– Ventifacts
– Yardangs
Ventifacts (South Sinai)
Ventifacts-Iceland
23
Yardangs
Iran-largest on earth
Wind deposition


Sand shadows
Dunes
South Sinai
Dunes
24
Barchans
Barchans near the 3rd cataract: Dongola,
Sudan
1 mile
25
Longitudinal (linear) dunes
Great Sand Sea, Western Desert
5 miles
Transverse dunes
26
Parabolic (blowout) dunes
27