Gary Nichols. Sedimentology and Stratigraphy(Second Edition)

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between lithospheric behaviour, climate, weathering

and erosion are then considered in terms of the Earth

Systems that are the sources of sedimentary material.

6.2 MOUNTAIN-BUILDING PROCESSES

Plate tectonic theory provides a framework of under-

standing the processes that lead to the formation of

mountains, as well as providing an explanation for how

all the main morphological features of the crust have

formed throughout most of Earth history (Kearey &

Vine 1996; Fowler 2005). Plate movements and asso-

ciated igneous activity create the topographic contours

of the surface of the Earth that are then modified by

erosion and deposition. Areas of high ground on the

surface of the globe today can be related to plate

boundaries (Fig. 6.2). For example, the Himalayas is

an orogenic belt, a mountain chain formed as a

result of the collision of the continental plates of

India and Asia, and the Andes have a core of igneous

rocks related to the subduction of oceanic crust of the

east Pacific beneath South America. High ground also

occurs on the flanks of major rifts, such as the East

African Rift Valley, where the crust is pulling apart.

Interpretation of the stratigraphic record indicates

that the same mountain-building processes have

occurred in the past: the Highlands of Scotland and

the Appalachians of northeast USA are the relics of

plate collisions resulting from the closure of past

oceans. Similarly, past subduction zones and related

magmatic belts can be recognised in the Western

Cordillera of western North America. Plate tectonic

processes are therefore the principal mechanisms for

generating uplift of the crust and creating areas of

high ground that provide the source of clastic sedi-

mentary material.

However, not all vertical movements of the crust can

be related to the horizontal movement of plates. The

mantle has an uneven temperature distribution within

it, and there are some areas of the crust that are under-

lain by relatively hot mantle, and other places where the

mantle below is cooler. The hot regions are known as

‘plumes’, upwelling masses of buoyant mantle that in

some instances can be on a large scale – ‘superplumes’

that probably originate from the core–mantle bound-

ary. Above the hot buoyant mass of a superplume the

continental crust is uplifted on a vast scale to generate

high plateau areas, such as seen in southern Africa

today. Plateaux like these are distant from any plate

boundary, but are important areas of erosion and

generation of detritus for supply to sedimentary basins.

6.3 GLOBAL CLIMATE

The climate belts around the world are principally

controlled by latitude (Fig. 6.3). The amount of energy

from the Sun per unit area is less in polar regions than

in the equatorial zones so there is a temperature gra-

dient from each pole to the Equator. These temperature

variations determine the atmospheric pressure belts:

high pressure regions occur at the poles where cold air

sinks and low pressure at the Equator where the air is

heated up, expands and rises. These differences in

pressure give rise to winds, which move air masses

between areas of high pressure in the subtropical and

polar zones to regions of low pressure in between them.

The Coriolis force imparted by the rotation of the

globe influences these air movements to produce a

diagenesis and

lithification

uplift and

exposure of

bedrock

deposition

burial

erosion

transport

in situ weathering

processes

Fig. 6.1 The pathway of processes

involved in the formation of a succession of

clastic sedimentary rocks, part of the

rock cycle.

88 Continents: Sources of Sediment

basic pattern of winds around the Earth. The Coriolis

force is a consequence of the movement of any body

travelling towards or away from the poles over the

surface of a rotating sphere, such that any moving

object – an air mass, water in the ocean, or an air-

plane – will be deflected to the right in the northern

hemisphere and the left in the southern hemisphere.

The combination of temperature distribution and

wind belts gives rise to four main climate zones.

Polar regions lie mainly north and south of the

Arctic and Antarctic circles. They are regions of

high pressure and low temperatures with conditions

above freezing only part of the year, if at all. Between

about 608 and 308 either side of the Equator lie the

temperate, moist mid-latitude climate belts which

have strongly seasonal climates and moderate levels

of precipitation. The dry subtropical belts are vari-

able in width depending on the configuration of land

masses in the latitudes of the tropics of Cancer and

Capricorn. Over large continental areas these dry areas

are regions of high pressure, high temperatures and

low precipitation. In the middle lies the wet equato-

rial zone of high rainfall and high temperatures.

These climate zones are not uniform in width

around the world and have different local climatic

characteristics that are determined by the extent of

continental land masses and the elevation of the land.

As both the positions and height of continents vary

through geological time due to plate movements,

palaeoclimate belts can be related to the modern

belts in only a relatively simplistic way unless com-

plex climate modelling is carried out.

6.4 WEATHERING PROCESSES

Rock that is close to the land surface is subject to phys-

ical and chemical modification by a number of different

weathering processes (Fig. 6.4). These processes

generally start with water percolating down into

joints formed by stress release as the rock comes

close to the surface, and are most intense at the

Eurasian Plate

African Plate

Indian-Australian Plate

North American

Plate

South American Plate

Nazca Plate

Cocos

Plate

Caribbean

Plate

Antarctic Plate

Arabian

Plate

Philippine

Plate

Scotia Plate

Pacific Plate

Pacific

Plate

Divergent Plate

Boundary

Convergent Plate

Boundary

Transform Plate

Boundary

North American

Plate

Pacific Plate

Fig. 6.2 The boundaries of the present-day principal tectonic plates.

Weathering Processes 89

surface and in the soil profile. Weathering is the

breakdown and alteration of bedrock by mechanical

and chemical processes that create a regolith (layer

of loose material), which is then available for trans-

port away from the site (Fig. 6.1).

6.4.1 Physical weathering

These are processes that break the solid rock into

pieces and may separate the different minerals with-

out involving any chemical reactions. The most

important agents in this process are as follows.

Freeze–thaw action

Water entering cracks in rock expands upon freezing,

forcing the cracks to widen; this process is also known

as frost shattering and it is extremely effective in

areas that regularly fluctuate around 08C, such as

high mountains in temperate climates and in polar

regions (Fig. 6.5).

Salt growth

Seawater or other water containing dissolved salts

may also penetrate into cracks, especially in coastal

areas. Upon evaporation of the water, salt crystals

form and their growth generates localised, but

significant, forces that can further open cracks in

the rock.

Temperature changes

Changes in temperature probably play a role in the

physical breakdown of rock. Rapid changes in tempera-

ture occur in some desert areas where the temperature

can fluctuate by several tens of degrees Celsius between

Mountains

Semi-arid

Tropical wet-dry

Tropical arid

Dry continental

Periglacial

100°

80°

40°

20°

0°

60°

80°

140°

0°

20°

40°

60°

40°

60°

Glacial

Humid temperate

Humid tropical

Fig. 6.3 The present-day world climate belts.

90 Continents: Sources of Sediment

day and night; if different minerals expand and contract

at different rates, the internal forces created could cause

the rock to split. This process is referred to as exfolia-

tion, as thin layers break off the surface of the rock.

6.4.2 Chemical weathering

These processes involve changes to the minerals that

make up a rock. The reactions that can take place are

as follows.

Solution

Most rock-forming silicate minerals have very low

solubility in pure water at the temperatures at the

Earth’s surface and so most rock types are not suscep-

tible to rapid solution. It is only under conditions of

strongly alkaline waters that silica becomes moder-

ately soluble. Carbonate minerals are moderately

soluble, especially if the groundwater (water passing

through bedrock close to the surface) is acidic. Most

soluble are evaporite minerals such as halite (sodium

chloride) and gypsum, which locally can form an

important component of sedimentary bedrock.

Hydrolysis

Hydrolysis reactions depend upon the dissociation of

O into H

and OH



ions that occurs when there

is an acidifying agent present. Natural acids that

are important in promoting hydrolysis include

carbonic acid (formed by the solution of carbon diox-

ide in water) and humic acids, a range of acids

formed by the bacterial breakdown of organic

matter in soils. Many silicates undergo hydrolysis

reactions, for example the formation of kaolinite (a

clay mineral) from orthoclase (a feldspar) by reaction

with water.

Oxidation

The most widespread evidence of oxidation is the

formation of iron oxides and hydroxides from min-

erals containing iron. The distinctive red-orange rust

colour of ferric iron oxides may be seen in many rocks

exposed at the surface, even though the amount of

iron present may be very small.

Fig. 6.4 The principal weathering

processes and their controls.

Chemical weathering

Solution

Hydrolysis

Oxidation

Physical weathering

Freezethaw

Salt growth

Temperature changes

Water chemistry

Acidity

Oxidising agents

Salinity

Temperature

Reaction rate

Fluctuations

Climatic regime Geomorphology

Water

Availability

Fig. 6.5 Frost shattering of a boulder (50 cm across) in a

polar climate setting.

Weathering Processes 91

6.4.3 The products of weathering

Material produced by weathering and erosion of mate-

rial exposed on continental land masses is referred to as

terrigenous (meaning derived from land). Weathered

material on the surface is an important component of

the regolith that occurs on top of the bedrock in most

places. Terrigenous clastic detritus comprises miner-

als weathered out of bedrock, lithic fragments and

new minerals formed by weathering processes.

Rock-forming minerals can be categorised in terms

of their stability in the surface environment (Fig. 6.6).

Stable minerals such as quartz are relatively unaf-

fected by chemical weathering processes and physical

weathering simply separates the quartz crystals from

each other and from other minerals in the rock. Micas

and orthoclase feldspars are relatively resistant to

these processes, whereas plagioclase feldspars, amphi-

boles, pyroxenes and olivines all react very readily

under surface conditions and are only rarely carried

away from the site of weathering in an unaltered

state. The most important products of the chemical

weathering of silicates are clay minerals (2.4.3). A

wide range of clay minerals form as a result of the

breakdown of different bedrock minerals under differ-

ent chemical conditions; the most common are kaoli-

nite, illite, chlorite and montmorillonite. Oxides of

aluminium (bauxite) and iron (mainly haematite)

also form under conditions of extreme chemical

weathering.

In places where chemical weathering is subdued,

lithic fragments may form an important component of

the detritus generated by physical processes. The na-

ture of these fragments will directly reflect the bedrock

type and can include any lithology found at the

Earth’s surface. Some lithologies do not last very

long as fragments: rocks made of evaporite minerals

are readily dissolved and other lithologies are very

fragile making them susceptible to break-up. Detritus

composed of basaltic lithic fragments can form around

volcanoes and broken up limestone can make up an

important clastic component of some shallow marine

environments.

6.4.4 Soil development

Soil formation is an important stage in the transfor-

mation of bedrock and regolith into detritus available

for transport and deposition. In situ (in place) physical

and chemical weathering of bedrock creates a soil

that may be further modified by biogenic processes

(Fig. 6.7). The roots of plants penetrating into bedrock

can enhance break-up of the underlying rock and the

accumulation of vegetation (humus) leads to a change

in the chemistry of the surface waters as humic acids

Weathering

products

Increasing resistance to chemical weathering

Rock-forming silicate minerals

Olivine

Pyroxenes

Amphiboles

Biotite mica

Muscovite mica

Ca-feldspars

Na-feldspars

K-feldspars

Clay minerals

Kaolinite

Illite

Montmorillonite

Chlorite

Quartz

Fig. 6.6 The relative stability of common silicate minerals

under chemical weathering.

HORIZON

Vegetation

Horizon with organic

matter

Horizon without organic

matter

Unstructured

sand

Fractured rock

Fresh source rock

Weathering

horizon

Organic particles

Fig. 6.7 An in situ soil profile with a division into different

horizons according to presence of organic matter and degree

of breakdown of the regolith.

92 Continents: Sources of Sediment

form. Soil profiles become thicker through time as

bedrock is broken up and organic matter accumu-

lates, but a soil is also subject to erosion. Movement

under gravity and by the action of flowing water may

remove part or all of a soil profile. These erosion

processes may be acute on slopes and important on

flatter-lying ground where gullying may occur. The

soil becomes disaggregated and contributes detritus to

rivers. In temperate and humid tropical environments

most of the sediment carried in rivers is likely to have

been part of a soil profile at some stage.

Continental depositional environments are also

sites of soil formation, especially the floodplains of

rivers. These soils may become buried by overlying

layers of sediment and are preserved in the strati-

graphic record as fossil soils (palaeosols: 9.7).

6.5 EROSION AND TRANSPORT

Weathering is the in situ breakdown of bedrock and

erosion is the removal of regolith material. Loose

material on the land surface may be transported

downslope under gravity, it may be washed by

water, blown away by wind, scoured by ice or

moved by a combination of these processes. Falls,

slides and slumps are responsible for moving vast

quantities of material downslope in mountain areas

but they do not move detritus very far, only down to

the floor of the valleys. The transport of detritus over

greater distances normally involves water, although

ice and wind also play an important role in some

environments (Chapters 7 & 8).

6.5.1 Erosion and transport under gravity

On steep slopes in mountainous areas and along cliffs

movements downslope under gravity are commonly

the first stages in the erosion and transport of weath-

ered material.

Downslope movement

There is a spectrum of processes of movement of

material downslope (Fig. 6.8). A landslide is a coher-

ent mass of bedrock that has moved downslope with-

out significantly breaking up in the process. Many

thousands of cubic metres of rock can be translated

downhill retaining the internal structure and stratigra-

phy of the unit. If the rock breaks up during its move-

ment it is a rock fall, which accumulates as a chaotic

mass of material at the base of the slope. These move-

ments of material under gravity alone may be triggered

by an earthquake, by undercutting at the base of the

slope, or by other mechanisms, such as waterlogging of

a potentially unstable slope by a heavy rainfall.

Movement downslope may also occur when the rego-

lith is lubricated by water and there is soil creep. This is

a much slower process than falls and slides and may

not be perceptible unless a hillside is monitored over a

number of years. A process that may be considered to

be intermediate between creep movement and slides is

slumping. Slumps are instantaneous events like

slides but the material is plastic due to saturation by

water and it deforms during movement downslope.

Rock fall

Slide

Slump

Debris flow

Turbidity current

olistoliths

coherent slide unit

internally deformed mass

high density

(laminar flow)

low-density turbulent

mixture

Fig. 6.8 Mechanisms of gravity-driven transport on slopes.

Rock falls and slides do not necessarily include water,

whereas slumps, debris flows and turbidity currents all

include water to increasing degrees.

Erosion and Transport 93

With sufficient water a slump may break up into a

debris flow (4.5.1).

Scree and talus cones

In mountain areas weathered detritus falls as grains,

pebbles and boulders down mountainsides to accumu-

late near the bottom of the slope. These accumulations

of scree are often reworked by water, ice and wind but

sometimes remain preserved as talus cones, i.e. con-

centrations of debris at the base of gullies (Fig. 6.9)

(Tanner & Hubert 1991). These deposits are charac-

teristically made up of angular to very angular clasts

because transport distances are very short, typically

only a few hundred metres, so there is little opportu-

nity for the edges of the clasts to become abraded. A

small amount of sorting and stratification may result

from percolating water flushing smaller particles

down through the pile of sediment, but generally

scree deposits are poorly sorted and crudely stratified.

Bedding is therefore difficult to see in talus deposits

but where it can be seen the layers are close to the

angle of rest of loose aggregate material (around 308).

Talus deposits are distinct from alluvial fans (9.5)

because water does not play a role in the transport

and deposition.

6.5.2 Erosion and transport by water

Erosion by water on hillsides is initially as a sheet wash,

i.e. unconfined surface run-off down a slope following

rain. This overground flow may pick up loose debris

from the surface and erode the regolith. The quantity

of water involved and its carrying capacity depends

not only on the amount of rainfall but also the char-

acteristics of the surface: water runs faster down a

steep slope, vegetation tends to reduce flow and trap

debris and a porous substrate results in infiltration of

the surface water. Surface run-off is therefore most

effective at carrying detritus during flash-flood events

on steep, impermeable slopes in sparsely vegetated

arid regions. Vegetation cover and thicker, permeable

soils in temperate and tropical climates tend to reduce

the transport capacity of surface run-off.

Sheet wash becomes concentrated into rills and

gullies that confine the flow and as these gullies coa-

lesce into channels the headwaters of streams and

rivers are established. Rivers erode into regolith and

bedrock as the turbulent flow scours at the floor and

margins of the channel, weakening them until pieces

fall off into the stream. Flow over soluble bedrock such

as limestone also gradually removes material in solu-

tion. Eroded material may be carried away in the

stream flow as bedload, in suspension, or in solution;

the confluence of streams forms larger rivers, which

may feed alluvial fans, fluvial environments of deposi-

tion, lakes or seas.

6.5.3 Erosion and transport by wind

Winds are the result of atmospheric pressure differ-

ences that are partly due to global temperature

distributions (8.1.1), and also local variations in pres-

sure due to the temperature of water masses that

Fig. 6.9 A scree slope or talus cone in a

mountain area with strong physical

weathering.

94 Continents: Sources of Sediment

move with ocean currents, heat absorbed by land

masses and cold air over high glaciated mountain

regions. A complex and shifting pattern of regions of

high pressure (anticyclones) and low pressure

(depressions) regions generates winds all over the sur-

face of the Earth. Winds experienced at the present

day range up to storm force winds of 100 km h

1

hurricanes that are twice that velocity.

Winds are capable of picking up loose clay, silt and

sand-sized debris from the land surface. Wind erosion

is most effective where the land surface is not bound

by plants and hence it is prevalent where vegetation is

sparse, in cold regions, such as near the poles and in

high mountains, and dry deserts. Dry floodplains of

rivers, sandy beaches and exposed sand banks in

rivers in any climate setting may also be susceptible

to wind erosion. Eroded fine material (up to sand

grade) can be carried over distances of hundreds or

thousands of kilometres by the wind (Schutz 1980;

Pye 1987). The size of material carried is related to

the strength (velocity) of the air current. The pro-

cesses of transport and deposition by aeolian processes

are considered in Chapter 8.

6.5.4 Erosion and transport by ice

Glaciers in temperate mountain regions make a very

significant contribution to the erosion and transport

of bedrock and regolith. The rate of erosion is between

two and ten times greater in glaciated mountain areas

than in comparable unglaciated regions (Einsele

2000). In contrast, glaciers and ice sheets in polar

regions tend to inhibit the erosion of material because

the ice is frozen to the bedrock: movement of the ice in

these polar ‘cold-based’ glaciers is mainly by shearing

within the ice body (7.2.1). In temperate (warm-

based) glaciers, erosion of the bedrock by ice occurs

by two processes, abrasion and plucking.

Glacial abrasion occurs by the frictional action of

blocks of material embedded in the ice (‘tools’) on the

bedrock. These tools cut grooves, glacial striae,in

the bedrock a few millimetres deep and elongate par-

allel to the direction of ice movement: striae can

hence be used to determine the pathways of ice flow

long after the ice has melted. The scouring process

creates rock flour, clay and silt-sized debris that is

incorporated into the ice.

Glacial plucking is most common where a glacier

flows over an obstacle. On the up-flow side of the

obstacle abrasion occurs but on the down-flow side

the ice dislodges blocks that range from centimetres to

metres across. The blocks plucked by the ice and

subsequently incorporated into the glacier are often

loosened by subglacial freeze–thaw action (6.4.1).

The landforms created by this combination of glacial

abrasion and plucking are called roche moutone

apparently because they resemble sheep from a

(very) great distance.

6.6 DENUDATION AND LANDSCAPE

EVOLUTION

The lowering of the land surface by the combination

of weathering and erosion is termed denudation.

Weathering and erosion processes are to some extent

interdependent: it is the combination of these pro-

cesses that are of most relevance to sedimentary geol-

ogy, namely the rates and magnitudes at which

denudation occurs and the implications that this has

on the supply of material to sedimentary environ-

ments. Rates of denudation are determined by a com-

bination of topographic and climatic factors, which in

turn influence soil development and vegetation, both

of which also affect weathering and erosion. In addi-

tion, different bedrock lithologies respond in different

ways to these combinations of physical, chemical and

biological processes.

6.6.1 Topography and relief

A distinction needs to be made between the altitude

of a terrain and its relief, which is the change in

the height of the ground over the area. A plateau

region may be thousands of metres above sea level

but if it is flat there may be little difference in the rates

of denudation across the plateau and a lowland

region with a comparable climate. With increasing

relief the mechanical denudation rate increases as

erosion processes are more efficient. Rock falls

and landslides are clearly more frequent on steep

slopes than in areas of subdued topography: stream

flow and overland water flow are faster across steeper

slopes and hence have more erosive power. A deeply

incised topography consisting of steep sided valleys

separated by narrow ridges provides the greatest

area of steep slopes for bedrock and regolith to be

eroded.

Denudation and Landscape Evolution 95

Relief tends to be greatest in areas that are under-

going uplift due to tectonic activity and thermal dom-

ing due to hot-spots in the mantle (Kearey & Vine

1996; Fowler 2005). Rejuvenation of the landscape

by uplift occurs mainly around plate boundaries, par-

ticularly convergent margins such as orogenic belts.

In tectonically stable areas the relief is subdued due to

weathering and erosion resulting in a low, gentle

topography. The cratonic centres of continental plates

are typically regions of low relief and hence rates of

denudation are low.

6.6.2 Climate controls on denudation

processes

Chemical weathering processes are affected by factors

that control the rate and the pathway of the reactions.

First, water is essential to all chemical weathering

processes and hence these reactions are suppressed

where water is scarce (e.g. in deserts). Temperature

is also important, because most chemical reactions

are more vigorous at higher temperatures; hot cli-

mates therefore favour chemical weathering. Finally,

water chemistry affects the reactions: the presence of

acids enhances hydrolysis and dissolved oxidising

agents facilitate oxidation reactions (Einsele 2000).

The rates and efficiency of the reactions vary with

different bedrock types.

Rates of erosion are climatically controlled because

the availability of water is important to the removal of

regolith by sheetwash and the extent to which rivers

and streams erode soil and bedrock. Temperature is

also significant: the presence of ice is important in

mountains because wet-based, rapidly moving gla-

ciers are more efficient at moving detritus than rivers.

Denudation rates are therefore related to climatic

regime, and general patterns can be recognised in

each of the main global climate belts.

Wet tropical regions

In hot, wet, tropical areas, chemical weathering is

enhanced because of the higher temperatures and

abundance of water. Bedrock in these areas is typi-

cally deeply weathered and highly altered at the sur-

face: seemingly resistant lithologies such as granite

are reduced to quartz grains and clay as the feldspars

and other silicate minerals are altered by surface

weathering processes. In general, chemical weather-

ing results in fine-grained detritus and partial solution

of the bedrock. High rainfall gives rise to high dis-

charge in streams, although the dense permanent

vegetation in these settings reduces soil erosion by

surface water, even on quite steep slopes.

Arid subtropical regions

The limited availability of water in arid regions means

that chemical weathering processes are subdued.

The bedrock is frequently barren of soil or vegetation

cover, so when rainfall does occur it has little residence

time on the land surface, and hence little time for

chemical alteration to take place. Mechanical break-

down can be significant, especially in desert regions

where cold nights and warm days promote freeze–

thaw action, using whatever water is available. Exfo-

liation also occurs as a result of temperature changes.

However, the absence of soil and vegetation means

that infrequent but violent rainstorms can be very

effective at removing surface detritus: flash-floods

carry higher amounts of detritus than equivalent

volumes of water occurring steadily over a longer

time. Fine-grained debris is removed from the regolith

by wind ablation, which is significant in barren

desert areas.

Polar and cold mountain regions

Chemical weathering is less significant in cold, dry

regions where chemical reactions are slower. In

these areas physical weathering processes are more

effective, although these too rely on the presence of

water. The products of weathering in cold mountains

are typically debris of the bedrock, broken up but with

little or no change in the mineral composition. A

granite breaks down into gravel clasts, plus grains of

quartz, feldspar and other rock-forming minerals.

Most of the products of physical weathering are

hence coarse material with little clay generated or

solution of the rock. Mountain glaciers are very

powerful agents of erosion as they move downslope

over rock, but in polar regions the ice is permanently

frozen to bedrock and erosion due to glacial action

is minimal (Chapter 7). Periglacial regions (areas

that border glaciers) have a seasonal cover of snow

that melts in the summer months. However, the

ground may remain frozen at depths of a few metres

all year round (permafrost – 7.4.4) and water

accumulating near the surface may eventually

96 Continents: Sources of Sediment

saturate the regolith and promote slumping on

slopes. Repeated freezing and thawing of the regolith

may also lead to creep downslope. Wind ablation is

important because of the sparse vegetation cover in

subarctic areas.

Temperate regions

In temperate climates both physical and chemical

weathering processes tend to be subdued. Erosion

is generally more vigorous under wetter climates,

but on the other hand, vegetation, which is usually

denser in humid climates, tends to stabilise the surface

and can reduce erosion. The rate of denudation of

limestone terrains is strongly climate-controlled, for

in humid temperate or tropical regions the rate of

denudation is ten times higher than in arid subtropi-

cal and subarctic regions (Einsele 2000).

6.6.3 Bedrock lithology and denudation

The type of bedrock is a fundamental control on the

rates and patterns of denudation. The main factor is

the rate at which weathering processes break down

the rock to make material available for erosion. The

greatest variability is seen in humid climates where

chemical weathering processes are dominant because

different lithologies are broken down, and hence

eroded, at widely different rates. The proportions of

the rock-forming silicate minerals (Fig. 6.6) are the

main factor: quartz-rich rocks are least susceptible to

breakdown, whereas mafic rocks such as basalts are

rapidly weathered and eroded. Large amounts of clay

minerals are generated by the denudation of terrains

such as volcanic arcs, which are composed mainly of

basaltic to andesitic rocks. Under extreme chemical

weathering of silicate rocks deep lateritic soils develop:

laterites are red soils composed mainly of iron oxides

and aluminium oxides.

Limestone bedrock is primarily weathered by disso-

lution, and the pattern of denudation is therefore

dominated by development of karst scenery

(Fig. 6.10). Solution related to joints and fractures in

the rock leads to the formation of deep, steep-sided

canyons on the surface and cave systems under-

ground. Little clastic detritus is generated from the

denudation of limestone terrains: conglomerates of

limestone clasts may form near the site of erosion,

but most of the material is in solution, with sand-

sized detritus largely absent.

A characteristic scenery also forms where the bed-

rock is poorly lithified: badland terrains (Fig. 6.11)

form by the deep erosion of weakly consolidated sand-

stones and mudstones as large amounts of detritus are

carried away.

6.6.4 Soils and denudation

Soil development has an important role in weathering

processes. First, water is retained in soils and hence

the thickness of the soil profile influences how much

water is available: if the soil profile is too thin it does

Fig. 6.10 Erosion by solution in beds of

limestone results in a karst landscape.

Denudation and Landscape Evolution 97