1 | P a g e

مكتبة الشروقWeathering, erosion &

Soil

Dr. Aman Zalama

2.5

2 | P a g e

هرضاحملا نيوانع

Introduction

Alteration of Minerals and Rocks

Mechanical Weathering

Frost Wedging

Pressure Release

Thermal Expansion and Contraction

Growth of Salt Crystals

Organisms

Organisms and Weathering

Chemical Weathering Decomposition of Earth Materials

Oxidation

Hydrolysis

Erosion

Sand Dunes

Expansive Soils and Soil Degradation

Weathering and Natural Resources

Soil and Its Origin

The Composition of Soil and Soil Horizons

3 | P a g e

Exogenetic processes deals with

weathering, erosion, and soil.

Weathering is a pervasive phenomenon

that alters rocks and minerals so that

they are more nearly in equilibrium

with a new set of environmental

conditions.

Erosion removes weathered materials

which are transported elsewhere and

deposited as sediment that may be

converted to sedimentary rock.

Earth materials are not structurally

and compositionally homogeneous, so

they are attacked by weathering

processes at different rates, even in

the same area.

4 | P a g e

Differential Weathering In this example

of differential weathering, the rock

layers have been tilted so that the

planes separating adjacent layers are

vertical. Weathering and erosion along

these surfaces have yielded the

pillars, spires, and isolated knobs at

this rock exposure in Montana.

5 | P a g e

Mechanical weathering processes

break parent material into

smaller pieces but do not change

its composition.

Frost action, pressure release,

thermal expansion and

contraction, salt crystal

growth, and the activities of

organisms account for mechanical

weathering.

is a form of mechanical weathering

(that is, weathering that involves

physical, rather than chemical change).

Frost wedging is caused by the repeated

thaw cycle of water in extreme -freeze

6 | P a g e

ll cracks climates. Most rocks have sma

in them, called joints (or, tectonic

joints). When it rains, rainwater seeps

into these joints. As the day cools and

temperatures at night drop below

freezing, the water inside the joints

freezes. As water freezes into ice, it

ing ice places expands. The expand

pressure on the joints in the rock.

Finally, when the pressure is too much,

the joint expands. In some cases, the

rock will split, though this usually

happens after repeated freeze and

thaws. As new water is added during the

e ice is created at warmer days, mor

night, wedging the joints apart

.further

7 | P a g e

Some rocks form at depth and are

stable under tremendous pressure.

Granite, for instance, crystallizes

far below the surface, so when it

is uplifted and the overlying

material is eroded, its contained

energy is released by outward

expansion, a phenomenon called

pressure release.

Outward expansion results in the

origin of fractures called sheet

8 | P a g e

joints that more or less parallel

the exposed rock surface.

Sheet-joint–bounded slabs of rock

slip or slide off the parent rock—a

process known as exfoliation—

leaving large, rounded masses of

rock called exfoliation domes.

During thermal expansion and

contraction, the volume of rocks

changes in response to heating

and cooling.

In a desert, where the

temperature may vary as much as

30°C in one day, rocks expand

when heated and contract as they

cool.

9 | P a g e

Rock is a poor conductor of

heat, so its outside heats up

more than its inside; the

surface expands more than the

interior, producing stresses

that may cause fracturing.

Under some circumstances, salt crystals

that form from solution cause

disaggregation of rocks.

Growing crystals exert enough force to

widen cracks and crevices or dislodge

particles in porous, granular rocks

such as sandstone.

Even in crystalline rocks such as

granite, salt crystal growth may pry

loose individual minerals.

11 | P a g e

Animals, plants, and bacteria

all participate in the

mechanical and chemical

alteration of rocks.

Burrowing animals, such as

worms, termites, reptiles,

rodents, and many others,

constantly mix soil and sediment

particles and bring material

from depth to the surface where

further weathering occurs.

11 | P a g e

Chemical weathering processes include

solution, oxidation, hydrolysis, and

the activities of organisms. They

result in a change in the structure,

composition, or both of parent

material.

Particle size, climate, and type of

parent material determine how quickly

chemical weathering takes place.

To a chemist, oxidation is any

chemical reaction in which a compound

or ion loses electrons, whether oxygen

is present or not.

12 | P a g e

But in chemical weathering it refers to

reactions with oxygen to form oxides

(one or more metallic elements combined

with oxygen) or, if water is present,

hydroxides (a metallic element or

radical combined with the hydroxyl, OH

ion).

For example, iron rusts when it

combines with oxygen and forms the iron

oxide hematite:

13 | P a g e

When hydrolysis takes place, hydrogen

ions (H) or the hydrogen in (OH) of

water reacts with and replaces positive

ions in minerals, thereby changing

their composition and liberating

soluble compounds and iron that may

then be oxidized.

Potassium feldspars such as orthoclase

(KAlSi3O8) and plagioclase feldspars

(which vary from CaAl2Si2O8 to

NaAlSi3O8) are framework silicates, but

when altered by hydrolysis they yield

materials in solution and clay

minerals, which are sheet silicates.

14 | P a g e

Erosion is the removal of weathered

materials from their source area.

Erosion is the removal of weathered

rocks and minerals by moving water,

wind, ice, and gravity.

Deflation is erosion by wind. Silt and

sand are removed selectively, leaving

larger stones on the surface and

creating desert pavement.

The Western Desert of Egypt is the land

of wind.

15 | P a g e

Sand grains are carried short distances

and a meter or less above the ground by

saltation, but silt can be transported

great distances at higher elevations.

A dune is a mound or ridge of wind-

deposited sand. Most dunes are

asymmetrical, with gently sloping

windward sides and steeper slip faces

on the lee sides.

Types of dunes include barchan dunes,

transverse dunes, longitudinal dunes,

and parabolic dunes.

16 | P a g e

17 | P a g e

;wind eth yb dcreate dsan fo eridg a

dan slake rnea ro sdesert ni dfoun

socean.

18 | P a g e

Expansive soils contain clay minerals

that expand when wet and shrink when

they dry.

Soil degradation is any loss of soil or

decrease in soil fertility resulting

from erosion, compaction, and

contamination by pollutants.

Weathering is responsible for the origin of

some natural resources and for the

concentration of others, such as bauxite

and other residual concentrations. And of

course soils are one product of weathering.

19 | P a g e

Soil is made up of weathered materials,

air, water, and organic matter and can

support vegetation.

Soil profiles for pedalfer, pedocal,

and laterite show horizons designated

O, A, B, and C that differ in

structure, composition, and color.

Climate, parent material, organisms,

angle of slope, the direction a slope

faces, and time are the factors that

control the rate of soil formation.

21 | P a g e

A Horizon

B Horizon

C Horizon

Bed rock