1

Weathering and erosional

processes; deserts of

Egypt

Classification of arid landscapes Hyperarid P/ETP <0.03 Arid 0.03-0.20

Semiarid 0.20-0.5 Subhumid 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

2

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

Frost heaving and cracking

–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

3

THERMAL EXPANSION

Growth of crystals

Salt weathering

Frost heave and fracture

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

4

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.

5

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.

6

BIOLOGICAL ACTIVITY-

ROOT WEDGING

Chemical Weathering

Obelisk in Egypt Obelisk in New York

Chemical Weathering--Solution

CaSO4 . 2H2O Ca2+ + SO4

2- + 2H2O– Water not shown but must be present

– Reversible

– Congruent

CaCO3 + H2O Ca2+ + HCO3- + OH-

pH—negative log of hydrogen ion activity

7

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 Fe(OH)3 + 2SO42- +

4 H+

Oxidation—loss of electrons

Fe: Fe+2 – Fe+3 (1 electron) lost

S: S-1 – S +6 (14 electrons) lost

O: O0 – O-2 (3.75 x 2 x 2) = 15 electrons

gained

8

Oxidation-yellowish, reddish, brown colors

Chemical weathering--Carbonation

CO2 + H2O 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 Al2Si2O5(OH)4 + 4H4SiO4 + 2K+ + 2HCO3

-

– K feldspar (orthoclase) to kaolinite

– Incongruent

– Irreversible

– Products: silicic acid, potassium ion, bicarbonate

9

Geomorphic processesSediment yield in drainage basins

Langbein-

Schumm

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

Wadi Zin

Shortcut to En Gedi floods.lnk

Shortcut to Zin2.lnk

10

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

3 miles

El-Qaa Plain

Wadi Isla

11

El Qaa plain with Sinai massif in

background

Mouth of Wadi Isla “canyon”

View downstream with Red Sea in distance

12

Wadi Isla Fan

13

17 m

Cemented Alluvium in Canyon-Pleistocene?

Flash flood competence

14

Boulder berms

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.

~ 2m

~ 4 m

Estimation of velocity and

discharge in boulder berm reachManning Equation

V = R2/3 S1/2 n-1; whereR= hydraulic radius (A/P); where A = 65 x 4 = and P

=73 mS = 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

65 m

4 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.

15

Size measurements on boulder fan

Competence on boulder Fan

Clast imbrication and imbricate

cluster bedforms

Arrows show flow direction

16

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)

17

Wadi Isla

Geology

draped

over the

DEM

Preliminary Model Results

1. Storm Events for Wadi

Isla from 1998-2005

were correlated with

resultant discharge

amounts.

2. Forecasts show

precipitation amounts

required to achieve

discharge rates

determined by boulder

size.

Flood Analysis

R2 = 0.9127

0

400

800

1200

1600

2000

2400

0 20 40 60 80 100 120 140 160 180

Precipitation (mm)

Dis

charg

e (

m3/s

)

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.

18

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

19

Pediments

Mohave desert

Inselbergs, Joshua TreeHow do pediments form?

3 miles

20

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.

21

Reg surfaces

Giza Plateau El-Qaa Plain, Sinai

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

22

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

23

Wind erosion

Deflation

Abrasion

Forms

–Ventifacts

–Yardangs

Ventifacts (South Sinai)

Ventifacts-Iceland

24

Yardangs

Iran-largest on earth

Wind deposition

Sand shadows

Dunes

South Sinai

Dunes

25

Barchans

Barchans near the 3rd cataract: Dongola,

Sudan

1 mile

26

Longitudinal (linear) dunes

Great Sand Sea, Western Desert

5 miles

Transverse dunes

27

Parabolic (blowout) dunes