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Rock Weathering and Mass Wasting

Weathering and Erosion

Weathering is the breakdown of material at or near the Earth’s surface, due to the influence of physical,

chemical, and biological processes acting separately or, more often, together to achieve the disintegration

and decay of rock material. This distinguishes it from the physical and chemical alteration of rock

through metamorphism, which usually takes place deep in the crust at much higher temperatures.

Weathering differs from erosion in that erosion usually includes the transportation of the disintegrated

rock and soil away from the site of the degradation, by the agency of surface water, wind, ice,

groundwater, or gravity.

1. PHYSICAL WEATHERING

This is the gradual disintegration of rocks by mechanical processes into its constituent minerals or

particles with no decay of any rock-forming minerals. It requires the application of force. The principal

agents of physical weathering are:

i. Frost Action: the alternate freezing and thawing of water between cracks and fissures within rocks

and crystal growth within rocks. The force of crystallization at about 220C is about 22 bars

(kg/cm2), which is enough to burst steel pipes.

Frost Wedging: causes water contained in rock cavities to expand in volume on freezing,

breaking the rock.

Frost heave: takes place in fine-grained regolith when water contained in the void space

freezes in winter, causing the material to expand and bulge on the surface, which are clearly

visible at the end of winter.

ii. Exfoliation: is the separation, during weathering, of successive shells from a usually jointed rock,

with release of confining pressure. The peeled-off layers may be flat or curved and of variable

thickness. Exfoliation is a major factor of many dome-shaped rock outcrops, e.g. Zuma Rock. The

separated sheets may be that or curved, paper thin or metres thick.

CAUSES

Thermal expansion and contraction: repeated heating and cooling, caused by lightning strikes, forest

and bush fires may cause rocks to flake off in thin sheets repeated heating and cooling of a rock that

causes its break-up a process called spalling.

Biogenic activity: burrowing animals and plant roots create openings in rocks and regolith, enhancing

infiltration of water into the soil/rock. This aids disintegration. Potassium ions and other nutrients go

into solution in the openings so created, leading to further crowding by plants and also further

breakdown of rock material.

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Purpose of Physical Weathering: physical weathering causes the breakdown of rocks,

increasing the surface area and enhancing the activity of the agents of decomposition

(chemical weathering).

CHEMICAL WEATHERING

This is the chemical alteration (change, reorganization, or redistribution of component

minerals) of the rock that usually leads to the formation of new minerals. It is the major factor

in rock disintegration. It may occur at depth or at or close to the surface. The minerals are

exposed to solution, carbonation, hydration, and oxidation by circulating waters.

Examples:

A. Solution: rock material goes into solution and gets dissolved.

1. Dissolution:

(i): limestone + water + weak acid i.e.CaCo3 CaH +Co3

(ii): acid rain

2. Dissociation:

(i): halite + water

B. Hydrolysis: H+ and OH- ions reacting with other ions in mineral.

(i): alteration of feldspars to form clay minerals and quartz.

C. Oxidation: atmospheric oxygen reacting with a mineral to form an oxide.

Agents of Chemical Weathering

Minerals present in a “parent” rock are attacked by agents of chemical weathering to produce sediment

made up of new minerals, rock fragments and dissolved ions. The principal agents of chemical weathering

are rainwater, oxygen, carbon dioxide and organic acids.

Water:- this must be seen as crucial element in the process of chemical weathering.

Carbon dioxide: - A principal agent of mineral alteration reacts with rainwater to form carbonic acid, a

mild acid that dissolves calcite in a process called carbonation.

H2O + CO2 H2CO3 H+ + (HCO3)

Rainwater carbon dioxide Carbonic acid Hydrogen ion Bicarbonate ion

This process creates a sink for carbon dioxide by pulling it from atmosphere.

Other acids – organic (tannic, citric, etc.) acids are produced in soils through biogenic activities.

Weathering on sulphide rock terrains (e.g. with pyrite present) produces sulphuric acid. This is common in

coal mining areas like Enugu (Usi and Okaba Coal Mines) where sulphides may accompany coal deposits.

When acids attack the primary silicate minerals in a process called hydrolysis, secondary clay minerals are

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produced. An example is the alteration of feldspars to form clay minerals (illite, montmorillonite) and

quartz.

Oxygen in a process called oxidation, iron-bearing primary minerals (e.g. pyrite, FeS,) may be oxidized to

form iron oxide (hematite, Fe2O3) and liberate sulphur, which forms sulfuric acid. This is typical in coal

mining areas (e.g.Enugu) where acid drainage may become an acute environmental problem.

CaCO3 Ca++

+ CO3

Calcite ion

FACTORS THAT AFFECT RATE OF EWATHERING

Factors affecting the rate of weathering are: Surface area, Rock type, Climate and Time.

Factor 1: Type of rock

- The mineralogical composition of a rock rate determines the rate of disintegration, while

the texture affects the type of weathering that us most likely to occur, Fine-grained rocks

are usually more prone to chemical alteration but less susceptible to physical

disintegration. The presence of joints, fractures and fissures in a rock may provide an

avenue for water to penetrate, making fractured rocks more prone to weathering than

massive rocks.

Factor 2: Surface Area and Slope

A steep sloping surface leads to a faster erosion of weathered material, thus exposing fresh

material to further weathering. On the contrary a gently sloping surface would retain the

weathered material, and this may accumulate as much as up to 50m thick.

Factor 3: Climate

Climate controls the type and rate of weathering by affecting the freeze-thaw and heat-moisture

cycles and the chemical reaction. Different types of weathered products characterize different

climate zones. In cold and dry climatic zones weathering by FROST action is a dominant, but in

the humid tropical region chemical decomposition is dominant.

Factor 4: Time

Weathering rates are quite variable, being dependent on rock type and climate, and even on

topography. Chemical weathering is , for instance, quite rapid in the humid tropics compared to

the arid tropical deserts where it proceeds much slower. In fact weathering is slowest in hot arid

climates.

The Role of Solubility in Chemical Weathering

Some elements are more soluble than others. Ca, Mg, Na, K, Fe2+

are very soluble while A1, Si,

Fe3+

are quite insoluble and form the major components of residual end product of chemical

weathering.

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Chemical Weathering of Granite and Basalt

EXAMPLE: GRANITE

Primary Constituents Weathering Products

Minerals Cations colloids Secondary minerals

formed from colloids and

ions

Persistent

Minerals

Feldspar K+, Na+ Silica

Alumina

Clay Minerals - Na+. K+

Quartz - - Quartz

Micas K+. Fe2+.

Mg2+

Silica,

Alumina

Clay Minerals Some Mica

Ferromagnesians Mg2+. Fe2+ Silica,

Alumina

Clay Minerals Mg2+

Ferromagnesians Mg2+. Fe2+ Iron oxides

Hematite, “Limonite” Mg2+

EXAMPLE: BASALT

Primary Constituents Weathering Products

Feldspar Ca2+. Na+ Silica,

Alumina

Clay Minerals Ca2+. Na+

Ferromagnesians Mg2+. Fe2+ Silica,

Alumina

Clay Minerals Mg2+

Magnetite Fe2+ Iron

oxides

Hematite, “Limonite” Mg2+

N. B.: “LIMONITE” Goethite = hydrous ferric oxide (Fe2O3.H2O): Hematite= Fe2O3

1. Soil is an organized body of weathered, unconsolidation rock material mixed with some

organic matter. It is divided into soil horizons, called a soil profile. The 0-horizon, found only in

moist climates, consists of the plow zone and contains plant and animal litter. The A-horizon or

“topsoil” is a zone of leaching that is rich in organic matter. The B-horizon or “accumulation

zone” is rich in clay. In dry climates it may be a zone of hardpan or caliche. The C-horizon is the

zone of fractured and weathered bedrock. Soil fertility is controlled by availability of essential

elements like N, C, Ca, K AND P. extensive weathering may cause residual deposits of economic

importance such as laterite or bauxite to accumulate.

In Engineering, soil is any loose, unconsolidated, non-organic material, such as sands, silts and

clays in contrast to the stronger rocks, siltstones, sandstones, limestones basalts and many others.

The application of the term is related to the development of soil mechanics.

2. LATERITE is blackish-brown to reddish soil that is formed under strongly oxidizing and

leaching conditions and is rich in iron oxide. Other color variations are red, yellow, and brown. It

forms in humid tropical and subtropical environments. Due to extensive leaching, many plant

nutrients are lost; leaving quartz and hydroxides of iron, manganese, and aluminum that forms a

distinctive type of soil, called laterite. Lateritic soils may contain clay minerals, but are often poor

in silica, as percolating waters usually leaches this out. Laterite is typically porous and clayey, and

contains iron oxides such as goethite (Fe2O3.H2O) and hematite (Fe2O3). It is generally soft when

freshly quarried but hardens on exposure. It also contains titanium oxides and hydrated oxides of

aluminum, e.g. gibbsite (A12O3.3H2O). Laterites differ widely in composition but most contain

aluminum hydroxides and iron hydroxides and oxides, either separately or together, long with a

little residual quartz. A rare variety, bauxite, almost pure hydrous aluminum oxide (A12O3.nH2O),

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is the principal ore of aluminum. Residual laterites (many are transported, not residual) are

characterized by a pale zone of leaching just above the parent rock, and a dark brown

concretionary “limonite” zone at or near the surface.

Conditions (which must persist over thousands of years) necessary for the formation of laterite

include:

*A parent rock that contains iron.

* A well-drained terrain.

*Abundant moisture for hydrolysis during weathering.

* A relatively high oxidation potential.

3. Clays: Most clays result from weathering and have particle diameters usually less than 0.005

millimetre. Clay are plastic when wet but coherent when dry. Clays are the most widely used of all

earth materials. They are used in a wide variety of industries. Clays have a wide variety of uses in

engineering: as clay barriers to reduce porosity in earth dams and to minimize water loss in canals.

Clay; is also an essential raw material in the manufacture of Portland cement. Clays, treated with

acid, may be used as water softeners to remove calcium and magnesium and substitute them with

sodium. Drilling mud- a heavy suspension consisting of clays, chemical additives and weighting

materials is widely employed in rotary drilling. Clay bricks (baked and as adobe) are widely used

in building construction and impure clays may be used to make bricks, tile, and the cruder types of

pottery, while kaolin, or china clay, is used for the finer grades of ceramic materials.

- barriers

- impermeable liners

- soften for removal of Ca and Mg.

- clay bath

Mineral Stability Diagram

Olivine

Pyroxene

Hornblende

Biotite

Calcium-Plagioclase

Calcium-Sodium-Plagioclase

Sodium-Plagioclase

K-Feldspar

Muscovite

Quartz

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Arrows indicate direction of increasing stability to weathering.

The earliest formed minerals are the least resistant to weathering.

Why movement; do ferromagnesian minerals weather first?

These minerals are unstable at the low temperatures and pressures to which they are now exposed,

compared to the high temperature and pressure conditions under which they were formed. Also

they have a peculiar crystal chemical that contains Fe2+

and Mg2+

, which are readily removed

because they occupy weak spots in the crystal structure. Fe2+

is removed by oxidation while Mg2+

is removed by dissolution.

MASS WASTING

This is the downslope movement of weathered material, controlled primarily by gravitational forces, and without the aid of a wind, river or glacier. However water, e.g. moisture, plays an important role because

the rate of movement downslope is influenced by the degree of saturation of the regolith. For this reason

some mass-wasting activities are quite common after a long rainy period. Slope movements may sometimes produce much damage such as in cases of landslides or rockfalls, which

have been known to wipe out entire towns and killing several hundreds of people. Investigation and

delineation of areas prone to damaging slope movements is of importance in order to minimize damage that

may results to engineering infrastructure from such movements.

Factors Responsible For Mass Wasting Natural surface that are entirely horizontal are rare, thus mass wasting occurs globally, wherever there is a

slight sloping of the earth’s surface. The variety of downslope mass movements reflects the diversity of

factors that are responsible for their origin. Such factors include:

i. Weathering or erosional debris cover on slopes, which is usually liable to mass movement; ii. The character and structure of rocks, such as resistant permeable beds prone to sliding because of

underlying impermeable rocks;

iii. The removal of the vegetation cover, which increases the slope’s susceptibility to mass movement by reducing its stability;

iv. Artificial or natural increases in the slope’s steepness, which will usually induce mass movement;

v. Earthquake tremors, which affect the slope equilibrium and increase the likehood of mass movement vi. Flowing ground water, flow which exerts pressure on soil particles and impairs slope stability.

These factors affecting slope conditions will often combine with climatic factors such as precipitation and

frost activity to produce downslope mass movement.

Types of Mass Wasting

Mass wasting types are characterized by the speed of occurrence of the mass wasting process and may be classified as follows:

GROUP 1: RAPID PROCESSES

(a). LANDSLIDE: This is the multiplicity of downslope movements of bedrock and other debris due to

the separation of a slope section along a plane of least resistance or slip surface.

( b). FALLING AND SLIDING: Rock/Debris fall and Rock/Debris slide is the rapid, vertical fall of loosened block of solid rock along a cliff surface or by leaps down a steep slope.

(c). SLUMP: This is the separation of coherent mass of rock or regolith and its downward movement

along a curved slip surface and subsequent accumulation at the foot of the slope. (d). DEBRIS FLOW AVALANCHE: This is very rapid downslope plastic flow of mass of debris in a

mountainous region.

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(e). MUDFLOW: This is similar to debris flow, but the substance has the consistency of mud i.e. it is

characterized by higher water content up to 30% and a large proportion of fine particles. Mudflow is very destructive because it has a high density that enables it to move large and heavy objects (e.g. houses,

bridges, etc).

GROUP 2: SLOW PROCESSES (usually imperceptible downslope motion of material)

(a). CREEP: This is the almost imperceptible downslope movement of surficial soil particles and rock debris. The effects of creep are observed in the tilting of old fences and poles and in the misalignment of

road.

(b). BULGING: This is the subsurface creep of rock material.

Factors responsible for creep and bulging are: cycles of freezing/thawing, wetting/drying, heating/cooling,

solution, and the activities of plants and burrowing animals.

(c). SOLIFLUCTION: This is the extremely slow downslope movement of moisture-saturated surficial

material. The main difference between solifluction and creep is that the regolith in solifluction contains a

higher quantity of water, while creep can occur even in a dry regolith.

Clay: - weathering and frost weathering (removal of clay may be necessary for economics special

measures.

EFFECTS OF WEATHERING

i. Causes disintegration and decomposition of rock.

ii. Compressive strength of hard rocks after weathering is lowered due to drop in cohesion, caused by

increase in jointing of the rock man. – present of clay content.

iii. Effects on the bearing captivity of structure foundation i.e. bridge abatements composed of weathered

rocks.

iv. They also increase rock pressure in tunnels driven in weather beds.

Solution: drop in Bearing capacity of foundations composed of weathered rocks require that pressure

on the sub-grade be reduced or that the depth of the foundation be measured to ensure total stability

of bridge abatements and lower settlement.

v. Weathered metamorphic rocks exhibit low bearing capacity i.e. shales etc.

vi. Weathering process leads sudden and dramatic drop in the strength bearing capacity.