General
- Social Studies for ECLPE Y2 SB File Uploaded 2/11/21, 14:01
UNIT 2 EXTERNAL LANDFORM PROCESSES (WEATHERING AND MASS WASTING)
By the end of this unit the student-teacher should be able to demonstrate an
understanding of the different features resulting from the external processesand their relationships with the human activities.
2.1. Weathering
2.1.1. Definition of Weathering
When rocks are formed, they are strong, consolidated and cohesive. With time
however, this strength, consolidation and cohesiveness reduces and hitherto
consolidated rock particles loosen and the rocks start to break down. When this
happens, we say that the rock has been weathered down. Weathering therefore
is an external process in which rocks are decayed or disintegrated in situ. Theterm situ means in one place without motion.
2.1.2. Types and processes of weathering.
Weathering is caused by physical, chemical and biological processes hence the
three types of weathering as described below
2.1.2.1. Physical/Mechanical weathering
This is a type of weathering where by rock are broken down into small pieces
without changing their chemical composition. What changes is only the physical
size but they maintain their chemical properties. It is mostly influenced by
temperature changes. Physical weathering takes the following forms:
i. Exfoliation. In arid regions, such as hot deserts, rock surfaces heat up rapidly
when exposed to the sun and the surface layers expand and break away. At
night when temperature falls rapidly the same layers contract and more
cracks develop. In time the layers of rocks peel off and fall to the ground. This
process is called exfoliation. Exfoliation results into formation of exfoliation
domes. Exfoliation domes occur in areas of with exposed granite like in
Mubende, Kitgum in Uganda, Kongwa, Serengeti, Iringa and Songea. Morepronounced ones are found in Kalahari, Sinai and Egyptian deserts.
An exfoliation dome with screes peeled off after mechanical weathering process
ii. Block disintegration
This is a type of weathering in which a rock is disintegrated not in small particles
as in exfoliation but in blocks. It is common in exposed plutonic rocks like
granite because these rocks develop cracks and joints as they cool down. After
exposure to thermal heating and cooling, the rock expands and cools along the
already developed cracks and they will be widened. Eventually the rock will fall
apart in blocks hence block disintegration. This type of weathering producesgranitic tors or inselbergs. Examples are Bismark rock in Mwanza, Tanzania.
Fig. showing blocks of rocks broken down along their joints
iii. Frost shattering. This type of weathering is experienced in areas that fall
below freezing point. In East Africa, they are found on mountain peaks
of mountain Kenya, Kilimanjaro and Rwenzori. Rainwater or snow-melt
collects in cracks in the rocks. At night the temperatures drops and the water
freezes and expands. The increases in volume of the ice exerts pressure on
the cracks in the rock, causing them to split further open. During the day
the ice melts and the water seeps deeper into the cracks. It is this expansion
in these cracks that makes rock particles to break off as screes and rock todisintegrate.
Plants and Animal action can also cause physical weathering as their roots grow.
Seeds of plants or trees can grow inside rock cracks where soil has collected.
The roots then put pressure on the cracks, making them wider and eventuallysplitting the rock. Even small plants can cause this kind of weathering over time.
Animals that burrow underground, such as moles, gophers or even ants, can
also cause physical weathering by loosening and breaking apart rocks. Dens
and tunnels are signs of this type of weathering. Other animals dig and trample
rock on the Earth’s surface, causing rock to slowly crumble apart. This process
exposes new parts of the rock to the elements, making them susceptible to
other types of weathering, such as chemical weathering
2.1.2.2 Chemical weathering
• Chemical weathering is the weakening and subsequent
disintegration of rock by chemical reactions. These
reactions include oxidation, hydrolysis, and carbonation. These
processes either form or destroy minerals, thus altering the nature
of the rock’s mineral composition. Temperature and, especially,
moisture are critical for chemical weathering; chemical weathering
of rock minerals generally occurs more quickly in hot, humid climatic
regions
• Carbonation is the process of rock minerals reacting with carbonic
acid. Carbonic acid is formed when water combines with carbon
dioxide. Carbonic acid dissolves or breaks down minerals in the rock.
The effect of carbonation on limestone rocks can be summarized usingthe equation below:
• Oxidation is the reaction of rock minerals with oxygen, thus changing
the mineral composition of the rock. When minerals in rock oxidize,
they become less resistant to weathering. Iron, a commonly known
mineral, becomes red or rust colored when oxidized.
• Hydrolysis is a chemical reaction caused by water. Water changes
the chemical composition and size of minerals in rock, making them
less resistant to weathering. For example, when feldspar mineral is
completely hydrolyzed, clay minerals and quartz are produced and
such elements as K, Ca, or Na are released.
• A hydrolysis reaction of orthoclase (alkali feldspar), a common mineral
found in igneous rock, yields kaolinite, silicic acid, and potassium.
• Hydration is the absorption of water into the mineral structure. This
causes the rock to expand in size. This expansion reduces the
cohesiveness of the rock particles hence internal stress is caused in the
rock and therefore the rock crumbles. A good example of hydration is the
absorption of water by anhydrite, resulting in the formation of gypsum.
Hydration expands volume and also results in rock deformation.
• Solution. This is more-less a physical-chemical weathering process
which does not change the physical structure of the rocks very much.
This is a process by which soluble rock particles are dissolved and
weathered away in a solution form. Water being the major solvent, it
can dissolve soluble rocks and therefore form a solution. This therefore
reduces the size of the rock e.g. rock salt (calcium chloride) and calcium
bicarbonate are easily weathered in solution.
2.1.2.3. Biological weathering
This involves the weakening and subsequent disintegration of rock by plants,
animals and other living organisms. Growing plant roots can exert stress or
pressure on rock. Although the process is physical, the pressure is exerted by
a biological process (i.e., growing roots). Biological processes can also produce
chemical weathering, for example where plant roots or microorganisms
produce organic acids which help to dissolve minerals.
Burrowing animals like rodents and termites can move rock fragments to
the surface, exposing the rock to more intense chemical, physical, and biological
processes and so indirectly enhancing the process of rock weathering.
Although physical, chemical, and biological weathering are separate processes,some or all of the processes can act together in nature
2.2.1. Climate
This is the most important factor affecting weathering of rocks. The extent of
weathering is dependent on the climate of the area. There are two factors that
influence weathering, namely Temperature and Rainfall. For example chemicalweathering is highly pronounced in areas with high rainfall which facilitates
processes like carbonation, hydration and solution. Physical weathering by frost
action is most likely in cold climate where freeze and thaw occur alternately
during the cold weather.
In this case again precipitation is the main factor. In the absence of water ice
cannot form and frost action is not possible. Hence an effective frost action
occurs in the cold moist climate. At higher temperatures chemical reactions are
likely to take place faster. Chemical reactions in most cases need water which
is a reactant in hydration and carbonation. Water is also the medium in which
the reaction can take place. Exfoliation process also occurs due to temperature
fluctuations during day and night.
2.2.2. Relief
Relief refers to the nature of the landscape or topography and it has a direct
impact on weathering.in mountainous regions, the windward side receive high
rainfall and as such, chemical weathering is dominant on this slope. However,
due to arid conditions on the windward slope, physical/mechanical weathering
is more dominant on this slope.
Slope - On steep slopes weathering products may be quickly washed away by
rains. On gentle slopes the weathering products accumulate. On gentle slopes
water may stay in contact with rock for longer periods of time, and thus result
in higher weathering rates
2.2.3. Nature of the rock
We know, the chemical properties of a rock depend on the mineral composition
to a great extent. Mineral in a rock may readily react with acids, water or oxygen
causing considerable weathering. For example, limestone can get severely
acted upon by even very mildly acidic rainwater. Granite on the contrary mostly
containing silica remains unaffected by such agents.
2.2.4. The impact of living organisms
Plants and animals have a great role to play in rock disintegration and decay.
Plants add certain chemicals such as nitric acid which chemically weather
away the rocks. Their roots also break the rocks as they expand. Therefore, the
thicker and bigger the vegetation, the faster will rocks disintegrate.
Animals also cause weathering in different ways, for example big animals like
elephants trample on rocks causing weathering. Man through his activities like
agriculture can cause weathering. Decaying animals release certain acids like
ammonia, lactic acid and urea which react with and weaken rocks.
2.2.5. Time
Since a rate is how fast something occurs in a given amount of time, time
is a crucial factor in weathering. Depending on the factors above, rates of
weathering can vary between rapid and extremely slow, thus the time it takes
for weathering to occur and the volume of rock affected in a given time will
depend on slope, climate, and animals
2.3. Weathering in the humid tropical regions and the
resultant landforms
Humid tropical climate is characterized by high rainfall and temperature of up to
1500mm and 25 centigrade degrees respectively. This climate however has got
some periods of drought. Due to amount of rainfall especially during the rainy
season, chemical weathering processes are common. These include, hydration,
solution, oxidation and carbonation. During the dry season when temperatures
are high and rainfall is low, physical weathering processes become pronounced.
These include, exfoliation, block disintegration, crystallization etc.
The combined humid and dry conditions of tropical climate lead to the formation
of the following landforms:
• Exfoliation domes: these are smooth and round topped hills found in
regions that experience alternate heating and cooling during the day
and night respectively. This will result in the top layer peeling off in
form of screes leaving a dome shaped structure,
• Laterites: these are hardpan soil (duricrust) produced by concentration
of oxides in the soil giving it a red deep appearance,
• Grikes: these are landforms found in chemically weathered limestone
rocks. Chemical weathering especially carbonation weathers down the
surface of these rocks to produce deep and narrow groves called grikes;
• Clints: these are formed together with grikes. These are round ridges
that separate the two grikes;
• Stalactites: these are protrusions found on a roof of a chemically
weathered limestone cave. As water enters the limestone rock, it
dissolves some of its materials whose solution starts to leak from the
roof of the cave. Later when water evaporates, it leaves behind a columnof hard rock connected to the roof of the cave called a stalactite;
• Stalagmites: these are formed together with stalactites. Whereas
stalactites are connected on the roof of the cave, stalagmites are formed
on the base of the cave and therefore protrude upwards. They are
formed from the accumulation of leaking calcium carbonate solution.
When this calcium carbonate evaporates, it leaves this limestone rock
called a stalagmite.
• Pillars: these are formed within the weathered limestone rocks. When
a stalactite joins with a stalagmite in a limestone cave, they finallymake a pillar.
• Limestone Gorge: this is a deep narrow gorge produced when the roof
of the cave collapses.it is therefore a depression with almost vertical
sides that is drilled into the roof of the cave as the cave collapses.
• Dry valley: these are valleys in which there are no streams flowing.
Rivers flowing from non-limestone area may finally enter limestone
rocks, which are very permeable and may disappear underground only
to reappear on ground again at the end of the limestone rocks. Within
limestone permeable rocks where the river disappears, remains aformer valley where the river disappeared deep down as a dry valley.
Mass wasting is sometimes called mass movement or slope movement. Mass
wasting is defined as the large movement of rock, soil and debris downward due
to the force of gravity. In other words, the earth’s outer crust is being ‘wasted’
away on a ‘massive’ scale and falling to lower elevations. It is different from
erosion because in erosion, water physically transports away the soil particles
but in mass wasting, water does not wash away the soil but assists the slope toslide under gravity.
Mass wasting can be categorized under three major types, namely;
• Slow flowage (creep ) processes
• Rapid flowage processes
• Slide processes
2.5.1. Slow flowage
These are mass wasting processes which move slowly and are also called creep
movements. They are very slow in their motion and they may occur without
being noticed unless a very keen observation of certain features is made. They
include/
i. Soil creep - the very slow, usually continuous movement of regolith down
slope. Creep occurs on almost all slopes, but the rates vary. Evidence for
creep is often seen in bent trees, offsets in roads and fences, and inclined
utility poles.
ii. Solifluction - flowage at rates measured on the order of centimeters per
year of regolith containing water. Solifluction produces distinctive lobes on
hill slopes. These occur in areas where the soil remains saturated with water
for long periods of time.
iii. Rock Glaciers - a lobe of ice-cemented rock debris (mostly rocks with ice
between the blocks) that slowly moves downhill.
iv. Talus creep - this is the down slope movement of mainly screes that are
relatively dry.it moves almost in the same way as soil creep and it occurs
under tropical and temperate climate.
2.5.2. Rapid flowage processes
These include the following
i. Earth flows: this refers to the movement of saturated soil and other debris
on steep slopes under the influence of gravity. They usually occur after a
heavy down power where the rainwater thoroughly lubricates the clayish
materials making it easy to flow down along the rock beneath when friction
is thoroughly reduced. A sudden movement will occur and the weathered
lubricated materials flow rapidly down the slope.
ii. Mud flows: this refers to the movement of semi-liquid mud with
unconsolidated gravel and boulders. They move very fast to a speed of more
than 15km/hr.
iii. Debris avalanches: this refers to the sudden downfall of materials embedded
in the ice or glacier on mountain slopes under the influence of gravity. The
fact that slopes are very steep and there is enough rain to soak the slopes
makes avalanches run faster than other rapid flowage processes.
2.5.3. Slide processes
They are collectively called landslides. They are very fast and many often involve
dry materials. They occur on steep slopes. They include the following:
i. Rock slump. This is a type of slide whereby downward rotation of rock occurs
along a concave-upward curved surface. The upper surface of each slump
block remains relatively undisturbed, as do the individual blocks. Slumps
leave arcuate scars or depressions on the hill slope. Slumps can be isolated
or may occur in large complexes covering thousands of square meters. They
often form as a result of human activities, and thus are common along roads
where slopes have been over steepened during construction. They are also
common along river banks and sea coasts, where erosion has under-cut theslopes. Heavy rains and earthquakes can also trigger slumps.
ii. Rock falls occur when a piece of rock on a steep slope becomes dislodged
and falls down the slope. Debris falls are similar, except they involve a
mixture of soil, regolith, vegetation, and rocks. A rock fall may be a single
rock or a mass of rocks, and the falling rocks can dislodge other rocks as they
collide with the cliff. Because this process involves the free fall of material,
falls commonly occur where there are steep cliffs. At the base of most cliffsis an accumulation of fallen material termed talus.
iii. Rock slides and debris slides result when rocks or debris slide down a preexisting
surface, such as a bedding plane, foliation surface, or joint surface
(joints are regularly spaced fractures in rock that result from expansion
during cooling or uplift of the rock mass). Piles of talus are common at the
base of a rock slide or debris slide. Slides differ from slumps in that there is
no rotation of the sliding rock mass along a curved surface.
iv. Debris slide. This occurs in the same way as slumping except that under
the debris slide, only unconsolidated particles (debris) slide. Hence, it is therolling of unconsolidated earth debris from a vertical or overhanging face.
2.6. Causes of mass wasting, effects of mass wasting,
measures to control mass wasting.
2.6.1. Causes of mass wasting
Mass wasting is caused by both physical and human factors.
2.6.1.1. Physical factors
i. Slope gradient. Mass wasting is caused by gravitation pull of the slope. The
gradient of the slope determines the rate at which materials move downslope
under the influence of gravity.
ii. Types of rocks. The nature of the rock has a great influence on mass
movement. Rocks differ in terms of texture, permeability, joints and structure,
for example, if an impermeable rock underlies a permeable one, it is very
probable that the permeable rock on top will get properly soaked and will
slide off a permeable one causing landslides.
iii. Climate. Climate influences landslides in a number of ways. In areas where
rainfall is high, the rate of weathering is rapid and this facilitates mass
wasting to take place. Areas that receive low rainfall, the rate of weathering
is low and hence limited mass wasting. Some areas which undergo freezing
and thawing, frost heaving is common which trigger of rock debris and
avalanches.
iv. Earth movements like earthquakes and tremors trigger of mass movement.
When such tremors come and find already a steep slope, heavy and lubricated
slope, it will shake it and the debris will be forced to slide.
2.6.1.2 Human activities
i. Construction of transport and communication routes. Mass movement
is common on sides of road and railway cuttings especially in hilly and
mountainous areas. The vibrations caused by the moving traffic together
with the pull of gravity trigger off rock falls and movement of debris. Road
and railway construction also leaves rocks hanging hence rendering them
susceptible to movements
ii. Deforestation. This is the destruction of forests especially natural
vegetation. Tree roots bind soil particles together and firm. In many parts
of the world, trees have been destroyed through agriculture and settlement
activities. This leaves the soils bare and exposed to weathering process that
facilitates mass movement of rocks and soil.
iii. Mining and quarrying. Mining and quarrying lead to formation of steep
slopes which trigger off mass wasting. In addition, explosives used to blast
rocks cause vibrations that break the rocks which makes them prone to mass
wasting.
iv. Overgrazing. Grazing of large herds of animals can cause some tremors
on slopes and hence causing slope failure. This occurs on slopes which are
already overloaded especially in national parks
v. Poor cultivation methods: Poor cultivation methods such as ploughing up
and down hill slopes induce movement e.g. in Kondoa district of Tanzania.
This is exacerbated by the presence of steep slopes. Cultivation also involves
the remove of the protective cover of the vegetation hence leading to mass
movements.
2.6.2. Effects of mass wasting
Mass movement has been on increase in most areas of East Africa. This has had
both economic and social effects on the people in this region. Some of these
effects include the following:
i. Loss of life and property: For example, in May 2018, a landslide buried 18
people in Rwankuba sector Karongi District in Western Rwanda. Landslides
also destroy property like settlements which are buried during the mass
movement.
ii. Destruction of agriculture land and crops. When sliding occurs, the would
be land for agriculture is destroyed. Besides, the top fertile soil is swept away
leaving bare scars on hill slopes unfavorable for agriculture. Crops can also
be destroyed in the process of mass movement.
iii. Destruction of infrastructure especially roads, bridges, electric
transmission poles etc. which are buried by rock debris hindering transport
and communication along these routes. This is common in steep slope areas
of northern and western Rwanda.
iv. Loss of vegetation. Mass movements particularly landslides lead to the
destruction of valuable forest resources. These are totally buried anddestroyed.
2.6.3. Measures to control mass wasting
i. Afforestation and re-afforestation. This increases the cohesiveness of
rocks. Roots of trees bind the soil particles together. Afforestation program
is being encouraged in all districts of Rwanda.
ii. Use of better methods of farming which encourage slope stability
like terracing and contour ploughing are being practiced in all hilly and
mountainous areas of northern and western Rwanda. Terracing involves the
cutting of slopes into a series of steps on a steep slope. This reduces thesteepness of the slope hence controlling movements.
iii. Sensitization and mass education of people about avoiding any
activities that might trigger mass movement like settlement on steep slopes
and agriculture activities. This can help people to avoid any practices that
might cause mass wasting.
iv. Engineering works on steep slopes should respect slope stability. Forexample, need to construct embankments on the roadsides.
END UNIT ASSESSMENT
