GEOMORPHOLOGY
WEATHERING
-this is the breaking down or disintegration of rocks in situ
-this is the disintegration of rocks on or near the earth‟s surface to produce materials that are in
equilibrium with the newly imposed physico-chemical conditions
PHYSICAL / MECHANICAL WEATHERING
-is the breakdown of rocks into small fragments by mechanical processes e.g temperature changes,
pressure release etc
-there is no mineral or chemical change to the composition of the broken down rock
-mechanical weathering results in coarser material being produced
-it is common in the hot and dry areas e.g Savanna and cold and dry places e.g Polar
-it occurs at a shallow depth as it depends on the renewal of exposure
MECHANICAL WEATHERING PROCESSES
a) Frost Shattering / Freeze-Thaw
-occurs when water penetrates the joints and bedding planes of rocks during the day or in summer
-on freezing the water expands by about 10% and as it freezes in an enclosed crack or pore it exerts a
pressure of about 2100kg/cm at -22ºC
-successive frost-melt cycles will widen and deepen the cracks this gradually weakens the rock and
causes it to break down
-freeze-thaw will occur under the following conditions :
.the rock must have cracks and joints in order to collect water
.temperatures must fluctuate above and below freezing point
-freeze –thaw weathering is most common in regions of high latitudes e.g Poles,mid-latitudes e.g
Temperate, high altitude and in some deserts
b) Insolation / Thermal Shattering
-occurs as a result of rocks being subjected to large daily temperature ranges
-the high day temperatures cause exposed rock surfaces to expand and thus develop cracks that are
parallel to the rock‟s surface
-the low night temperatures cause rock surfaces to contract and therefore develop vertical cracks
-repeated expansion and contraction gradually cause the rock to disintegrate
-if the rock surface peels off in layers the process is known as exfoliation which results in the
formation of exfoliation domes like Ayers Rock (Australia) and Sugarloaf mountain in Brazil
-the process occurs mostly in rocks that are :
.polyminerallic (different minerals) with minerals that have different co-efficients of
expansion and contraction
.dark colored fine grained rocks like basalt
-this type of weathering is most common in the Hot deserts and Savanna areas
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c) Pressure Release / Dilatation
-occurs due to that rocks are elastic substances that can be compressed if sufficiently high force is
applied to them
- therefore the weight of overlying rocks compresses down those rocks beneath them,however due to
the denudation of these overlying rocks the once suppressed rocks expand sideways and recoil and
curve upwards
-the expansion brings with it the formation of a series of cracks parallel to the surface of the rock
thus weakening the rock and causing it to disintegrate
-these parallel joints then form sheet joints and this is very common in granite rocks where they form
„pseudo bedding planes‟
-if it happens that the rock surface peels off in layers the process is called exfoliation
-dilatation could also be a result of the withdrawal of a glacier
d) Salt Crystal Growth / Salt Weathering
-occurs as a result of the formation of salt crystals from a saline solution within the pores and
fissures of rocks through evaporation e.g sodium carbonate expands by 300% within these pores
-as the crystals grow they exert a pressure against the fissure and pore walls thus weakening and
breaking down the rock
-salt weathering is responsible for the formation of smooth rounded caverns at the base of moist
inselbergs these caverns are called “tafonis”
-salt weathering is most common in hot deserts and coastal areas
e) Wetting and Drying
-occurs mostly when clay rich sedimentary rocks disintegrate due to the alternate absorbing and
giving out of moisture
-on drying the once soaked rocks break down into smaller particles or simply flake at the surface
-wetting and drying is most common in those climates which have a marked wet and dry season like
the Savanna areas
f) Root Action
-occurs when rocks beneath the surface are jointed and then a small plant has its roots imbedded in
its joints
-when the plant grows so do its roots and when the roots grow they tend to prise and force open the
joints of the rocks thus widening them up and eventually causing their disintegration
g) Animal Action
-the burrowing of animals into the ground like rodents and termites promotes weathering by
loosening up rock particles beneath the soil and then the rock is easily attacked by chemical
reactions
CHEMICAL WEATHERING
-occurs when rocks decompose or decay to form new substances as aresult of chemical reactions that
would have taken place
-there is a mineral change to the composition of the rock
-it is most common in the hot and wet (humid) areas like Equatorial
-chemical weathering reaches greater depths
-there is formation of new substances
-water,gases and organic acids area the principal agents
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PROCESSES OF CHEMICAL WEATHERING
a) Oxidation and Reduction
-occurs when minerals in rocks either combine or disassociate from oxygen in the air or dissolved in
water like when oxygen reacts with iron compounds [ferrous] and changes them into iron oxide
[ferric] FeO + O =FeO
-the soil or rock that was blue or grey in colour during oxidation will change to a reddish brown
colour
-reduction on the other hand is the removal of oxygen from a mineral and occurs as a result of the
continuous presence of water which tends to reduce the amount of oxygen thus turning the rocks or
soil grey (gleying)
b) Hydrolysis
-occurs when hydrogen and the hydroxyl ions of water combine with the minerals of a rock
-like when feldspar is broken down into aluminosilic acid which in turn will break down into silic
acid and clay [kaolinite]
Feldspar + water = potassium hydroxide + silicic acid + clay
c) Carbonation
-occurs when rocks contain calcium carbonate (calcite) like limestone
-when rainwater falls it dissolves the carbon dioxide in the atmosphere to form a weak carbonic acid
-when this carbonic acid comes into contact with rocks that contain calcium carbonate the hydrogen
ions combine with the carbonate ions to form calcium bicarbonate which is easily dissolved and
removed as a solution by running water
CaCO + HCO = Ca(HCO)
d) Hydration
-occurs when minerals in rocks adsorb water in so doing they expand and set up stresses within the
rock thus causing it to disintegrate
-this process occurs where there has been some decay in the rocks e.g the changing of anhydrite into
gypsum
-the reaction gives out heat (exothermic reaction)
e)
Solution
-occurs when some minerals become soluble when they come into contact with water e.g rock salt
(gypsum)
-the rate of solution depends on the acidity or alkalinity of the groundwater
f)
Chelation / Humification / Cheluviation
-occurs when organic acids formed by the decaying vegetation and animals cause certain minerals in
the soil like iron to be soluble
-this then causes the minerals to be either leached downwards or taken up by the roots of plants
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DIFFERENCES BETWEEN CHEMICAL AND PHYSICAL WEATHERING

the products of chemical weathering are finer than those of physical
weathering

physical weathering involves the breaking down of rocks by mechanical
means eg heat or force whereas chemical weathering is a result of the minerals in rocks coming into
contact and reacting with substances like water

chemical weathering reaches greater depths than physical weathering which
depends on the rate of renewal of exposure

in physical weathering there is no change in mineral or chemical composition
of the rock the original rock simply breaks down into smaller particles whereas in chemical
weathering there is formation of a new substance

physical weathering is most common in the hot and dry like Desert or cold
and dry areas like Poles whilst chemical weathering is most common in the hot and wet areas like
Equatorial
VULNERABILITY OF PARTICULAR ROCKS TO CERTAIN WEATHERING TYPES
-HYDROLYSIS : is a major process in the disintegration of the mineral feldspar that is mostly found
in granite
-CARBONATION : is a major process in the breaking down of rocks that contain calcium carbonate
e.g limestone
-HYDRATION : is the main weathering process in rocks that contain mica and feldspar e.g
haematite
-OXIDATION : is the main process in the disintegration of sedimentary rocks that contain iron
compounds
FACTORS INFLUENCING THE RATE AND TYPE OF WEATHERING
1.CLIMATE
-the main climate elements that affect the rate and type of weathering are rainfalland temperatures
like freeze-thaw will only occur where there are freeze-thaw cycles like in Temperate areas and
chemical weathering will only occur where it is hot and wet like in Equatorial areas
-an increase in temperatures means an increases in the rate of chemical reactions thus the warmer
climates will experience more chemical weathering than the colder climates
-according to Van Hoff an increase in temperature of about ºC brings about a
in the rate of
chemical reactions
- water is the main reagent of chemical reactions therefore areas with high precipitation will
experience more chemical weathering like Equatorial areas whilst those with low rainfall like
Deserts will have very little chemical weathering
-therefore those areas like Equatorial places that have high rainfall and high temperatures will
experience more chemical weathering than those areas that are hot and dry like Desert areas
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PELTIER‟S WEATHERING / MORPHOGENETIC REGIONS
-the fact that a particular weathering type will occur in a certain climatic region results in the
formation of large weathering regions called morphogenetic or morphoclimatic regions
a)
Equatorial / Humid Tropics
-very high rainfall of more than 1500mm and uniformly high temperatures of more than 20ºC
throughout the year speed up chemical reactions thus making chemical weathering more dominant ,
this then causes chemical reactions to be 4-5 times faster than the decay of rocks in other areas
-also the high temperatures and rainfall encourage the growth of a very dense forest whose trees die
and decay quickly thus releasing humic acids that also help in the disintegration of rocks
-this then results in these areas having a very deep layer of weathered materials (regolith) of 30-60m
depth
-although chemical weathering is the dominant type there is some limited physical weathering in the
form of root action and burrowing animals
-physical weathering is reduced in these areas due to :
.presence of a protective layer of vegetation
.presence of a deep layer of weathered material (regolith)
b) Savanna / Seasonally Humid Tropics
-
-
these areas have a marked hot wet summer and a cool dry winter season
the alternating wet and dry seasons mean that both chemical and physical weathering are present
during the long dry winter season the scarcity of vegetation and rainfall cause a high daily range
of temperature thus encouraging physical weathering like freeze-thaw,expansion and contraction
etc
during the hot wet summers chemical weathering takes place due to the high rainfall and
temperatures n the form of oxidation,hydrolysis etc
-however the rate of physical weathering is not as high as that one in the hot desert areas neither
is the rate of chemical weathering faster and more than that of the Equatorial areas
-the depth of regolith is not very thick due to rapid surface erosion as a result of the scarcity of
vegetation
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c)
Temperate Regions
-these areas have moderate temperatures and moderate rainfall hence chemical and physical
weathering take place at a moderate rate
-however chemical weathering is more dominant than physical weathering because the soil is always
wet and due to the presence of thick coniferous forests that decay and release humic acids
-where the rocks are jointed and exposed they are broken down by freezethaw,carbonation,hydrolysis and oxidation especially in summer when the snow thaws and the
temperatures are high
d) Semi-arid and Arid regions
-these areas have extreme temperature changes,very low rainfall,very high evaporation and very
little vegetation cover
-in these areas the dominant type of weathering is physical due to the temperature variations and
high evaporation rates hence chemical weathering is very slow
-physical weathering is in the form of freeze-thaw,wetting and drying and insolation
-however due to the effect of dews,occasional rainstorm and water drawn up by capillary action
there is some chemical weathering in the form of hydration
-chemical weathering mostly attacks crystalline rocks and those that will be in shade and therefore
moist to form smooth caverns called „tafonis‟
-the layer of regolith is very thin due to the very slow rate of weathering
d)
Peri-glacial / Polar areas
-due to the very low temperatures the main weathering process is physical usually frost shattering
-rock disintegration in these areas occurs in two ways :
.on bare rock surfaces where rocks are attacked by frost action
.freeze-thaw that occurs in the active zones above the permafrost
-there is also very little chemical weathering in the form of carbonation (nivation)
e)
Mountain Regions
-the most active type of weathering is freeze-thaw due to the low temperatures
-the rate of physical weathering is faster than that of chemical weathering due to the low
temperatures and absence of liquid precipitation
-however there is some chemical weathering present in the form of carbonation
2. ROCK TYPE
a) Rock Strength and Hardness
-the strength of a rock is given by the minerals it contains and how cemented or bonded they are e.g
granite is stronger than sandstone
-also the strength of a rock is given by its age,the older the rock the more compacted and stronger it
is
-generally igneous rocks are harder and stronger than sedimentary rocks
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b) Mineral Composition
-rocks are made up of different minerals that expand and contract at different rates and also react
differently to weathering agents
-minerals like silica are more stable and therefore less affected by weathering hence rocks that
contain quartz like granite are more resistant to weathering whereas those that do not contain quartz
like basalt are easily weathered
-light coloured rocks like limestone weather slowly than dark coloured rocks that absorb more heat
like basalt
c) Rock Texture
-this refers to the crystalline nature of the rock whether the rock is coarse or fine
-coarse grained rocks weather faster than fine grained rocks as the fine grained have their crystals
compactly bonded
d) Joints and Bedding Planes
-joints are small cracks that are found in rocks when there is no displacement along the fault line
-joints are caused by the cooling and contraction of igneous rocks
-it can be a result of faulting,pressure release and may occur when sedimentary rocks are dry
-bedding planes are lines that separate the layers of sedimentary rocks
-rocks that have more joints and bedding planes are easily weathered along these lines of weakness
than those rocks that are compact or massive
- joints and bedding planes increase the surface area to be attacked by weathering agents hence a
well jointed and bedded rock allows the easy penetration of acids,roots and water
e) Relief
-areas of high relief and steep slopes which encourage mass wasting movements like soil creep are
always bare and hence encourage physical weathering and very little chemical weathering
-areas of gentle relief like at the foot of the mountain tend to have a thick layer of weathered material
that protects the rocks below from physical weathering however chemical weathering is increased
due to the presence of moisture within the accumulated regolith
f)
Vegetation and Animals
-a thick vegetation cover like that of the Equatorial forest protects the rocks from physical
weathering as it reduces the rate of renewal of exposure however vegetation increases the rate of
chemical weathering through the process of humification
-in desert areas and high mountain slopes the absence of vegetation increases physical weathering
because of exposure
-the presence of plants and burrowing animals also increases the rate of weathering as animals like
rodents mix up and loosen weathered rocks
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g) Geological Time
-the changing climate,vegetation and internal structure of the earth have an effect on weathering
processes e.g the presence of deep chemical weathering in hot deserts suggests tat these areas were
once wet
h) Human Influence
-people encourage weathering by carrying out the following activities : mining,road
construction,quarrying,deforestation etc
THE WEATHERED MANTLE / REGOLITH
-this is a layer of loose disintegrated rock waste overlying the parent rock
THE DEPTH OF REGOLITH ACROSS THE EARTH‟S SURFACE
-the thickness of the weathered mantle (regolith) depends on the balance between the rate at which
the weathered material is produced and the rate at which the same material is removed
-the depth of regolith varies from very shallow/thin in the Arctic and Desert areas to very deep in the
Savanna and Equatorial areas where it can reach a depth of 100m
-this variation in the global depth of regolith is a result of rainfall,temperature and vegetation
distribution that are different e.g the presence of thick forests in the Equatorial areas encourages
deep chemical weathering and the protection of the regolith by the vegetation therefore a very thick
layer of regolith accumulates
-the depth of regolith is shallow where we have the dominance of physical weathering processes and
very active erosion and is deep where there is a dominance of chemical weathering processes
WEATHERING FRONT / BASAL SURFACE OF WEATHERING
-is the narrow zone that separates the weathered rocks (regolith) from the unweathered rocks (parent
rock)
REGOLITH
is the loose,coarse unconsolidated layer of weathered materials
it is monominerallic and has fewer layers
it is uniform in structure
SAPROLITE
-is made up of regolith and humus
-it is made up of many layers and is polyminerallic
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-it is made up of fine particles
BLOCK DISINTEGRATION
-is the breakdown of rocks by both chemical and mechanical weathering into large blocks along
lines of weakness that is joints and bedding planes
GRANULAR DISINTEGRATION
-is a form of weathering where the individual grains of a rock are loosened and fall to the ground
-the grains fall out to leave a pitted,uneven surface
-it is a result of freeze-thaw,insolation,salt-crystal growth and oxidation
SLOPES
-are all the hard surfaces present on the earth‟s surface
-are part of the solid land surface
Slope Form
-refers to the shape of a slope in cross-profile
Slope Processes
-are the activities that take place on a slope
Slope Evolution
-is the change or development of slopes over time
FACTORS THAT AFFECT THE DEVELOPMENT OF SLOPES
a)Geology
-the greater the mechanical strength of the rock the higher will be the cliff because more
undermining will be needed to cause its failure
-therefore hard rocks like granite tend to have steeper slopes than weakly cemented rocks like
sandstones
-the closer the joints and the thinner the bedding planes the more easily the cliff fails and the smaller
the vertical slope whereas where the rocks are poorly jointed and bedded the slope remains steep
-permeable rocks allow infiltration to occur thus reducing run-off this causes the slope to be stable
and steep whereas where the rocks are impermeable there is a lot of run-off which promotes soil
erosion and this results in slope instability
-rocks that have large angular fragments form steeper slopes than those rocks that have finer
weathered materials
-the material that is weathered from rock beds that dip inwards remains in situ but where rocks dip
outwards the weathered material is removed leading to a decline in the slope angle
-earth movements like earthquakes and volcanic eruptions trigger off slope failure
b) Climatic Factors
-in humid climates the slopes are smooth,gentle and covered with soil and vegetation whereas in the
arid areas the slopes are rough,steep and lack vegetative cover
-this is due to the absence of slope wash processes in the dry climates
c) Biotic Factors
-where there is deforestation there is rapid removal of weathered materials thus forming gentle
slopes
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-a thick vegetation cover will anchor and protect the weathered materials beneath from erosion and
this causes the slope to remain steep
-humans by cultivating and building on slopes increase surface wash and mass movements through
the removal of natural vegetation and this leads to slope failure like the favelas on the slopes of Rio
de Janeiro
SLOPE ELEMENTS MODEL
a)Convex / Waxing Slope
-is the upper part of the slope profile (summital convexity)
-is formed due to structural control as on exfoliated granite hills or through transportational
processes like rainwash and soil creep
-it is most common in the humid Temperate areas especially where there are limestone and chalk
rocks
b)Free Face / Cliff
-is the steepest part of the slope always bare as it is too steep for any loose material to rest upon it
-cliffs develop on coasts,glaciated valleys and river valleys where lateral erosion undercuts the base
of the slope or where there are hard rocks on the valley side
-cliffs occur where there are very rapid mass movements like rock falls
c)Rectilinear / Debris / Constant Slope
-is more or less straight in profile
-is made up of weathered materials that accumulate at the foot of the cliff
-the angle of this slope is controlled by the size and the shape of the weathered materials
d)Concave / Waning Slope
-is found at the lower part of the slope profile
-is formed due to the deposition and erosion of sediments especially in arid areas where the
dominant action is surface wash
5.Complex Slope
-is a result from the combination of some or all of the individual forms of slopes like convexo –
concave
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THEORIES OF SLOPE EVOLUTION
1.SLOPE RETREAT / BACK WEARING / PEDIPLANATION : KING L.C
-occurs where there is a hard lateritic cap of rock that controls the rate of evolution
-is most in the arid and semi-arid areas where the dominating processes are physical weathering and
sheet wash which then result in the parallel retreat of the scarp slope
- the maximum slope angle and the shape of the slope do not change they remain constant the only
aspect that changes is the lower part of the slope that increases in concavity to form a pediplain
Geological and Climatic Conditions Associated With Slope Retreat
.presence of resistant rocks
.presence of freeze-thaw
.presence of steep slopes in areas of soft rocks
.excessively drained rocks like limestone
.arid and semi-arid conditions where there is limited chemical weathering
.sufficient mass movements and sheet wash to remove materials
Landforms Formed By Slope Retreat
-interfluvial inselbergs
-U-shaped valleys
-very wide valleys (pediplains)
2.DOWN-WEARING / EXHUMATION / PENEPLANATION : DAVIS W.M.
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-this theory states that during the early formation and development (youth stage ) slopes were made
very steep due to vertical erosion by rivers on uplifted land but in time they became gentler due to
degradation until finally at the old stage (maturity stage) they will almost be flat (peneplain)
-slope decline is most common in the humid climates where there is a lot of chemical weathering
and erosion
Landforms Formed By Exhumation
-peneplains
-U-shaped valleys
-convexo-concave slopes
-residual hills and domed inselbergs
3.SLOPE REPLACEMENT : PENCK W
-this theory states that the angle of the slope decreases as the gentler lower slopes erode back to
replace the steeper slopes above
-this theory assumed that the landscape began with a straight rock slope with uniform weathering
-as scree (weathered materials) collected at the base of the slope it formed a gentler slope which as
the scree grew replaced the original slope above
-it is most common where there is tectonic activity e.g faulting and folding
MASS MOVEMENTS
-this is the downhill movement of weathered (regolith) and unweathered materials due to the pull of
gravity
CLASSIFICATION OF MASS MOVEMENTS
-mass movements are classified according to : a)-speed of movement e.g slow,fast and very fast
b)-nature of regolith e.g dry or wet
c)-type of movement e.g slide,heave or flow
FACTORS THAT AFFECT MASS MOVEMENTS
.angle of slope - the steeper the angle of the slope the more the mass movements that will occur than
on gentle slopes
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.climate - areas that have heavy rainfall and temperatures that fluctuate between freezing and
thawing encourage the action of mass movements
.vegetation - the presence of vegetation on mountain slopes reduces the occurrence of mass
movements but where it is absent there will be more mass movements
.earth movements - volcanic eruptions and earthquakes increase the occurrence of mass movements
by triggering off the movement of weathered materials
.pattern and degree of jointing – where the rocks are heavily jointed and the joints are parallel to the
slope plane mass movements will occur more than on where the rocks are poorly jointed and
massive
.depth of regolith – if the weathered material is very deep and unconsolidated mass movements will
occur more than in those areas where the regolith is thin
.degree of lubrication – where the regolith is moist there will be less resitance to friction and hence
more mass wasting occurs
.human action – increase the occurence of mass wasting processes by : blasting during
mining,quarrying,farming down the slopes,deforestation and reduce the occurrence of mass
movements by practicing afforestation and regrassing and draining of underground water
TYPES OF MASS WASTING
a) SOIL CREEP
-this is a very slow unnoticeable but continuous movement of soil particles down a slope due to the
total effect of individual displacement of its particles
-it is caused by freeze-thaw,wetting and drying,root action and burrowing animals
-it results in the formation of teracettes and convex slopes
b) DEBRIS FLOW
-is a form of mass movement in which coarse materials move downslope as slurry at rapid rates
-the material contains boulders and a fifth of its volume is water content
b) SOLIFLUCTION
-is a slow downslope flowage of masses of surface waste that are saturated with water that would
have been released by thawing in spring
-is common in peri-glacial areas where there are gentle slopes
-it results in the formation of solifluction sheets and lobes
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c)
MUDFLOW
-is a stream of semi-liquid mud which flows down valleys in mountainous areas after heavy rainfall
-its high water content makes it have a higher velocity than earthflows
d)
ROCK SLIDES
-occur in areas of unstable slopes and high relief
-there is very little disruption in the mobile mass
-slides are common in rocks that are jointed and have bedding planes that are parallel to the angle of
the slope and also where there is parallel slope retreat
-rock slides lead to the formation of structural slopes
e)
ROCK FALLS
-these are very rapid movements that occur on very steep slopes where there is very little friction
-the detached fragments fall and bounce rather than slide
-rockfalls are triggered off by freeze-thaw,thermal shattering,pressure release,earthquake and
volcanic action
-rockfalls result in the formation of freefaces,slope retreat and debris/scree at the foot of the slope
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ACTION OF RUNNING WATER ON SLOPES / SLOPE-WASH / DOWNWASH
Downwash
-is the movement of materials downslope by running water as a result of heavy rainfall
-the amount of transport achieved by wash related processes will depend on the slope angle,nature of
material eroded,vegetation cover and amount of water involved
-the load being transported by the water leads to slope erosion and thus shaping the slope
Forms Of Downwash
i)
Overland Flow
-this is the flow of water over the ground
-it is a result of :
.infiltration –excess flow - which is a result of rainfall intensity being greater than infiltration
capacity or
.saturation overland flow – which occurs when the water table reaches the ground surface
HORTON‟S OVERLAND FLOW MODEL
zone of zero erosion
-no erosion because
overland flow is thin
and not powerful to
overcome the soil‟s
resistance to erosion
-also the gradient will
be gentle
zone of erosion and active slope retreat zone of deposition
-as the volume of water increases
-the running water will be
downslope,the eroding power also
fully loaded with sediments
increases and so erosion will be
and the decreasing gradient
active and this causes the slope
reduces its velocity and
to retreat
deposition occurs
ii) Throughflow
-is the downward movement of water through the regolith
-this water will be transporting dissolved products (solutes) to the river
iii) Sheet-wash
-is the removal over a large area of the top layer of soil usually silt and clay particles by a thin sheet
of water
-this is dominant in the lower part of the slope and most effective in arid areas
iv) Gullying
-occurs when the surface run-off is concentrated in rills which will later through erosion will deepen
and widen to form gullies
-once formed the gullies extend themselves headwards into the hillslopes
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GRADED SLOPE
-are those slopes that will be covered by a soil layer that is uniform in thickness
-they are characterized by neither bare rock outcrops nor by sections where the rock was deeply
buried beneath the regolith
SLOPE STABILITY
-the factors that affect slope stability are :
.cemented materials are more stable than uncemented ones
.fragments with rough surfaces are more stable than those with smooth ones
.angular fragments that are a mixture of various sizes are more stable than those that are rounded
and uniform
.dry sediments are more stable than moist ones
.slopes with vegetation are more stable than those with no vegetation as vegetation anchors the
slopes
.gentle slopes are more stable than those that are steep
.slopes that do not have farming,buildings are more stable than those where there is
farming,buildings and tourism
CAUSES OF SLOPE FAILURE
-extraction of mineral resources like mining on slopes
-dumping of waste materials on slopes e.g Arbefan
-rapid urbanization resulting in the building of houses on mountain slopes e.g the favelas of Rio de
Janeiro
-road construction through mountains and other steep areas e.g Swiss Alps
-farming down the slope
-recreational activities like skiing e.g Scarborough
-deforestation e.g Bangladesh
SOLUTIONS TO SLOPE FAILURE
-practise afforestation and reforestation
-practise contour ploughing and bunding
-terrace steep slopes
-practice slope stitching
-avoid building of houses on mountain slopes
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ROCKS
-are an agglomeration or an aggregate of various minerals
CLASSIFICATION OF ROCKS
1.ACCORDING TO ORIGIN
A] IGNEOUS ROCKS
-these rocks are formed from the cooling,crystallization and solidification of molten rocks (magma)
i.Plutonic Igneous Rocks
-these are those rocks that cooled and solidified very slowly at considerable depths under pressure
-the rocks are compact and coarsely textured because of the slow cooling process which enabled the
growth of crystals
-usually these rocks are massive
-examples are granite,gabbro diorite etc
ii.Hypabassal Igneous Rocks
-these are those rocks that were formed when magma cooled at shallow depths beneath the surface
-they are intermediate rocks that are formed between the plutonic and the volcanic rocks
-their crystals are medium grained because their cooling was faster than that of the plutonic rocks but
slower than that of the volcanic rocks
-examples are dolerite,porphyry,etc
iii.Volcanic Igneous Rocks
-these are those rocks that are formed after magma has been extruded or ejected onto the earth‟s
surface
-because of the very rapid cooling of the lava these rocks are finely textured and have a glassy
appearance
-examples are basalt,obsidian
Characteristics of Igneous rocks
.they are crystalline
.they do not contain fossils
B] SEDIMENTARY ROCKS
-are formed by the weathering,transportation ,deposition,compaction and cementation (diagenesis) of
sediments on the bed of oceans
-these sediments are laid down in layers called strata which are separated by bedding planes
-the sediments are then lifted up and out of the water by compressional forces to then dry and form
sedimentary rocks
i.Mechanically Formed
-are physically or mechanically formed rocks and consist of broken rocks which have been
eroded,transported and deposited at a different site
-such accumulations of mineral matter become compacted by pressure to form sedimentary rocks
-examples are sandstone,conglomerates etc
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ii.Organically Formed
-are rocks which are largely or wholly the remains of either plants or animals deposited either in situ
or have been transported to another site and redeposited
-organic sediments are either : .calcareous – those derived from animal remains e.g chalk,coral
or : .carbonaceous – those derived from plant remains e.g peat,lignite,coal
iii.Chemically Formed
-are those formed after the precipitation of various chemical reactions or by the evaporation of water
-they are compacted by the nature of their formation
-examples are limestone,dolomite,gypsum
Characteristics of Sedimentary Rocks
-they are formed in layers called strata which are separated by bedding planes
-they contain fossils
-they are not crystalline
C] METAMORPHIC ROCKS
-these are pre-existing rocks which have been altered from their original state by great heat and or
pressure on cooling and solidification they are changed into different rocks
i.Contact / Thermal Metamorphism
-occurs due to a rise in temperature when a mass of molten igneous rocks intrudes into existing
country rocks
-it creates a fusion or re-crystallisation of the minerals or grains in a rock and as there is no pressure
applied there is little deformation of the country rocks
-there is also no foliation,peeling or bending
-examples are when sandstone is changed into quartzite and limestone into marble
ii.Dynamic Metamorphism
-occurs as a result of great pressure being applied to the rocks
-this causes a molecular re-arrangement of the structure of the rock
-it occurs mostly along the thrust and sheer zones
-examples are milonites,cataclisites
iii.Regional Metamorphism
-this covers a large area and is associated with tectonic processe like mountain building periods
(orogenesis)
-this process produces mineral changes,re-shapes and re-arranges the rock
-example shale becomes slate
THE ROCK CYCLE
-takes place at or near the earth‟s surface where rocks of all types decompose and disintegrate when
exposed to the atmosphere
-the weathered material is picked up and carried away by streams and glaciers,wind and waves only
to be deposited to form sedimentary rocks through the process of lithification
-the solid sedimentary rock may then be weathered to form new substances or may be
metamorphosed by heat and pressure into metamorphic rocks
-the metamorphic rocks will then be melted once more to become magma and form igneous rocks
which in turn will be weathered and deposited once more to form sedimentary rocks
-e.g granite – sandstone – quartzite – granite
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2.ACCORDING TO MINERAL COMPOSITION
-rocks are also classified according to their mineral composition like the type of igneous rock is
determined by the mineral percentage composition e.g granite is made up of 31% quartz,52%
feldspar,11% mica and 6% other minerals
3.ACCORDING TO GEOLOGICAL AGE
-rocks are also classified according to the geological age during which they were formed e.g
Caledonian sandstones are those that were formed during the Caledonian epoch and older than those
sandstones that are formed during the Alpine-Himalayan period
ROCK CHARACTERISTICS WHICH INFLUENCE LANDFORM FORMATION
-the factors that determine the landforms formed by rocks are : rock hardness and strength,mineral
composition,nature of jointing and bedding planes,climate and also permeability and porosity
Permeability
-is the ease by which water passes through rocks
-permeable rocks are more resistant to erosion than impermeable ones as they allow water to
percolate through them hence are not greatly affected by wash processes that then cause erosion
Porosity
-is the number of pores or spaces between the minerals of rocks
-the more pores the rock has the more water it can hold (porous) thus making it susceptible to
chemical weathering processes and salt action
LIMESTONE / KARST TOPOGRAPHY
-the term karst is applied to those areas that have limestone and dolomite rocks
-karst areas are characterized by an absence of surface drainage,presence of hollows,collapsed
structures and subterranean drainage
Properties of Limestone




is a soft sedimentary rock-erodable
is made up of 80% calcium carbonate (calcite)-susceptible to carbonation
is layered and has bedding planes-well jointed
is highly permeable and porous-allowing ingress of acidulated water
Conditions Leading To Development Of Karst Relief
-presence of soluble rocks e.g limestone and dolomite at or near the surface
- the rocks must be thick,densely jointed and thinly bedded
-the area must receive moderate rainfall
-the water table should be low to allow the percolation and infiltration of acidulated water from
above
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LANDFORMS FOUND IN LIMESTONE AREAS
1.Doline
-is a shallow shaped depression with gentle sloping sides,a flat floor and is oval in plan
-is formed where water percolates underground where there is a major intersection of joints
-the limestone rock is then dissolved slowly in a downward direction forming a small basin initially
but which gradually will turn into an enlarged depression
-dolines are also caused from the collapse of underground caves
2.Uvala
-is a large closed depression which is formed by the union of several dolines
-usually it has a diameter of more than 500m
3.Poljes
-is a very large,shallow,steep sided depression with a flat floor that is covered by red clays (terra
rosa)
-most poljes coincide with structural basins formed by folding and faulting
4.Collapse Sink
-is a deep hole with very steep vertical sides
-is formed as a result of the collapse of the roof of an underground cave or may occur in chains
above the course of an underground river
-it is mostly found on the floors of poljes,uvalas and dolinas
5.Limestone Pavements
-these are flat areas of exposed rock
-where the joints have been widened by acidic water they form deep furrows called grikes which
separate flat topped blocks called clints
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6.Limestone Cavern
-is a widened underground river channel formed through solution and river erosion by underground
rivers and also by the collapse of the roofs of these underground rivers
Stalactite-is a column of calcite hanging from the roof of the cave, formed due to the evaporation of
water drops containing dissolved calcite
Stalagmite-is a column of calcite that grows from the floor of the cave upwards,formed by the
dripping of water drops containing calcite from the stalactites
Pillars-are formed when stalactites and stalagmites join to form one column that extends itself from
the floor to the roof
7.Conekarst
-are a series of cone-shaped hills separated from one another by multi-sided closed depressions that
produce a hummocky landscape
-it is also known as „cockpit‟ karst
8.Towerkarst
-are a series of steep-sided hills that are undercut at the base and separated from each other by areas
of level clay plains
-it is common where the limestone is hard and has vertical jointing
LANDFORMS RESULTANT FROM THE DISSECTION AND STRIPPING OF REGOLITH
PROPERTIES OF GRANITE
.coarse grained plutonic igneous rock
.crystalline and made up of quartz,feldspar and mica
.has a well jointed system which divide the rock into cuboidal blocks
.has great physical strength and is resistant to erosion
.is impermeable
Factors Affecting The Weathering Of Granite
a)
Climate : deep weathering occurs where there are high temperatures that
accelerate the rate of chemical reactions and high rainfall which is the main reagent in chemical
reactions
b)
Nature of jointing : poor jointed (massive) roks are affected by spheroidal
weathering (exfolication) e.g. ruwares and bornhardts
- Well jointed and thinly bedded rocks from blocky inselbergs and castle koppies
c)
Climate – hot and dry areas are affected by physical weathering e.g. insolation,
freeze – thaw
- Hot and wet areas are affected by chemical weathering e.g. hydrolysis
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EP WEATHERING ALSO REGOLITH STRIPPING IN HUMID TROPICS
Conditions that lead to deep weathering
1. Climate condition – high temperatures (Vant Hoff) and very high rain fall (reagent) result in very
deep chemical weathering.
2. Vegetation – anchors soils from rain wash causing accumulation of regolith, also releases humic
acids for chelation
3. Rock Type – permeable and porous rocks allow acidulated water to penetrate.
4. Tectonic stability – occurs in stable areas where ther is no river rejuvenation
RUXTON AND BERRY MODEL OF DEEP WEATHERING
A.
B.
C.
D.
Finely decomposed rock – residual debris
Debris with smaller core stones
Larger core stones with debris
Partially weathered rock with very large core stones.
THE STRIPPING OF DEP WEATHERED LAYERS
- Where rate of erosion is more that of weathering
- Climatic change towards dryness (aridity)
- Reduced vegetation cover (deforestation)
- Land uplift and river rejuvenation
- This strippig will continue until the basal surface of weathering (bsw) is reached to expose
features loike inselbergs and laterites
- The stripping also causes pressure release or dilation of the rocks
LAND FORMS BY THE STRIPPING OF DEEP WEATHERED OF DEEP WEATHERED LAYERS
1. DURICRUST / CUIRASSES
-
is a hard layer formed at or near the regolith surface in tropical, arid and semi arid areas
is formed by the concentration of aluminous calcareous, ferruginous and siliceous elements drawn to
the surface by capillary action and continuous wetting and drying
these elements are then cemented by weathering into a concrete like layer.
If the caprock is breached by fluvial action (river action) it becomes undermined by slope retreat in
the weaker materials.
Durictrrusts are very important in that they protect the sofy unconsolidated (loose) weathered rocks
beneath the surface by forming a hard cap over them, thus forming musus and battes.
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-
Where slope retreat has led to the destruction of the laterite cap the hill slope declines in angle and
finally the isolated hills are destroyed and the individual pediments unite to form an extensive ped
plains.
DWALA
-
This is a low back exposure in the flat alluvial plains of tropical areas.
It represents either the initial exposure of a domed inselburg during its early stage of the regolith
stripping or represents the final stage in the destruction of and inselberg.
It could also be a result of pressure release / dilatation.
TORS
-
Is a small hill of spheroidically weathering boulders rooted
Are formed in any massively jointed rock where deep weathering reduces the jointed rock into core
and the rain wash selectively removes the finer sands leaving the core stones exposed (common in
temperature and humid tropics)
CASTLE KOPJE
-
This is a hill or a ridge of large angular boulders which resulted from deep weathering and the
subsequent removal of the finer sand and clays by erosion
The rock piles that compose a castle kopje seem to have suffered spheroidal weathering and have a
cube like blocky appearance
Castle kopjes are as a result of sub aerial decline in seasonally humid tropics
Are a form of collapsed or degraded form of inselburg
INSELBURGS
- This is important or outstanding steep sided hill rock rising abruptly from a plain of low relief
- Is usually made up of a resistant rock like granite, gneisses e.t.c
- - inselburgs are formed either by
- i). Parallel slope retreat; in which pediments encroach into residual uplands during the process of
pediplanation
- ii) Exhumation or the process of regolith stripping
there are two types of inselburgs:
domed Inselburgs / Bornhardrts
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-
this is an isolated hill with near vertical / over hanging sides and a rounded smooth summit
is a product of massive sheet jointing in granite rocks caused by the process of pressure release
(dilatation)
- is made up of poorly jointed granite rock which has resisted reduction by chemical weathering and
has only been exposed by regolith stripping also known as a shield inselberg.
BLOCK INSELBERGS
-
Is a result of block disintegration
Represents large scale tors or castle kopjes
Develop where rectangular jointing is more dominant in granite rock
Blocky inselburgs are thought to result from the process of parallel slope retreat instead of regolith
stripping
ETCHPLAINS
- are a land surfaces which have been subjected to one or more phases of deep chemical weathering
followed by partial or complete removal of the saprolite layer (regolith)
PEDPLAINS
These are plains that are formed as a result of scrap retreat and the extension of the pediment
They occur as a result of the fusion or the coalescence of pediments to form continues plain.
d.d. Maphosa 2011
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