Key unit competence:
By the end of this unit, I should be able to demonstrate an understanding of different features resulting from external processes and their relationships with human activities.
Observe the photograph below and explain the processes that affected the rock shown
4.1.1. Types and processes of weathering
Learning activity 4.1
a. Making good use of the diagrams below explain the processes involved in both physical and chemical weathering.
b. Make a research and compare the processes of soil formation and the processes of weathering
a. Definition of weathering
Weathering refers to the process of disintegration and decomposition of rocks ‘in-situ’ into small particles by the action of weather and living organisms.
Agents of weathering: temperature, rainfall (water), wind, animals and plants (vegetation).
b. Types of weathering
There are three types of weathering namely physical or mechanical weathering, chemical weathering and biological weathering which cuts across each of the physical and chemical weathering.
1. Physical weathering
Physical weathering refers to the breakdown or disintegration of rocks, without any change in the chemical or mineral composition of the rock being weathered. Rocks disintegrate into smaller particles but maintain their previous chemical characteristics. Only the physical size and shape change. Physical weathering is mostly influenced by temperature changes.
Processes of physical weathering include:
i. Thermal expansion or isolation weathering
This process is caused by the changing of temperature ranges which causes differential heating of minerals forming the rock. When heated dark minerals expand, faster than others resulting in cracking and fragmentation of the rock.
This occurs when there is expansion of rocks during the day and contraction of rocks during the night due to repeated temperature changes. It is common in arid and semi-arid regions. This results into rocks of a few centimeters thick to start peeling off (breaking away) leaving behind exfoliation domes.
ii. Freeze thaw
This process also called frost weathering (or frost shuttering) occurs due to water that enters into the cracks of the rocks; this water freezes and expands exerting pressure within cracks. Water from rain or melting snow and ice is trapped in a crack or joint in the rock.
If the air temperature falls below freezing point, the water freezes and expands. As a result, the rock becomes weak and breaks. This process is common in cold regions, especially glacial, periglacial and high mountainous zones. The figure below shows steps from infiltration of water into the rock to the condensation within rock fissure which result in the fragmentation.
iii. Pressure release
The process of pressure release known as the unloading or dilatation weathering occurs when materials on top are removed by erosion. This releases pressure, which causes the materials below to expand and crack parallel to the surface.
iv. Salt crystallization
The process of salt crystallization weathering illustrated on the figure below occurs when saline water (or water carrying salts in solution) passes through cracks and joints in rocks. As it evaporates, the dissolved salts change into salt crystals. These crystals expand within cracks as they are heated up and apply pressure on the rock leading to its breaking up.
v. Shrinkage weathering
Some clayey rocks expand after absorbing water. For instance, there are some clays which swell when they absorb water during rainy seasons. This results in increase in volume. During dry seasons, they massively lose this water through evaporation and they contract. This process is known as shrinkage. This alternating expansion of these rocks during the wet season, and contraction during the dry season, creates stresses and later cracks the rock.
vi. Granular disintegration
This takes place almost in the same way as exfoliation except that in this type, rocks disintegrate into small particles called granules. It is produced either by differences in thermal expansion and contraction, or through the frost heaving process (congeliturbation).
2. Chemical weathering
This is a type of weathering which involves a complete change in the chemical and mineralogical composition of the rock resulting into the disintegration of rocks. It is common in areas which experience alternating wet and dry seasons.
The following are the chemical reactions that take place during weathering:
i. Oxidation: oxidation is one of the varieties of chemical weathering in which oxygen dissolved in water reacts with certain rock minerals, especially iron, to form oxides.
ii. Carbonation occurs on rocks which contain calcium carbonate, such as limestone and chalk. This takes place when rain combines with carbon dioxide or an organic acid to form a weak carbonic acid.
H2O +CO2 H2CO3 (weak carbonic acid)
This reacts with calcium carbonate (the limestone) to form calcium bicarbonate which is soluble in water.
iii. Dissolution: Dissolution is one of the less important forms of chemical weathering, in which solid rocks are dissolved by water. When water (e.g. rainwater) mixes with carbon dioxide gas in the air or in air pockets in soil, a weak acid solution, called carbonic acid, is produced. When carbonic acid flows through the cracks of some rocks, it chemically reacts with the rock causing some of it to dissolve.
iv. Hydrolysis: Hydrolysis involves water combining with rock minerals to form an insoluble precipitate like clay mineral. Compared to hydration-a physical process in which water is simply absorbed – the hydrolysis process involves active participation of water in chemical reactions to produce different minerals.
v. Hydration: Hydration is one of the major processes of mechanical weathering, involving the addition of water to a mineral, causing it to expand and thereby initiate stress within the rock. For example the conversion of hematite to limonite. Once minerals have experienced hydration, they become more susceptible to the effects of chemical weathering, especially those of carbonation and oxidation.
vi. Solution: is a process in which the minerals in the rock directly dissolve in water without their chemical and mineralogical composition being altered. e.g. olivine, Rock salt (calcium chloride) and calcium bicarbonate are easily weathered in solution.
e.g. NaCl + H2O → Na+, Cl- (dissolved ions with water).
vii. Chelation: Chelation is a complex organic process by which metallic cations are incorporated into hydrocarbon molecules. In fact, the word chelate means a coordination compound in which a central metallic ion is attached to an organic molecule at two or more positions. Chelation is a form of chemical weathering by plants.
3. Biological weathering
Biological weathering is a process of rock disintegration (decay) due to the influence of living organisms both growing plants and animals. The diversity of life in soil includes plants, algae, fungi, earthworms, flatworms, roundworms, insects, spiders and mites, bacteria, and burrowing animals.
Plants wear away the rocks by their roots which widen the rock joints hence allowing in other weathering agents like water to disintegrate the rocks. Some plant roots also have chemicals at the tips of their roots which are acidic and hence cause rock weathering.
Tree roots find their way into cracks or joints in the rocks. As they grow, they cause the joints to become bigger. The end result is that the rocks break into smaller pieces at some points.
Burrowing animals like rodents and moles, warthogs (wild pigs) and wild animals in game parks like the chimpanzee, excavate the rocks and as such, they break up the rocks hence weathering them. Man also disintegrates rocks through his activities.
Man’s activities such as mining, construction, quarrying, agriculture, etc. result in such a fast rate of disintegration of geo materials (rocks).
Application Activity: 4.1
Use your local environment to identify the evidences of biological weathering.
4.2. Factors influencing weathering and interdependence of physical and chemical weathering
Learning activity 4.2
Using the illustration below, identify the missing factor and explain how it influences the rate of weathering.
A number of factors are required for weathering to occur in any environment. The major factors of weathering include:
The term relief refers to the nature of landscape or topography. It influences significantly the weathering process because it controls the flowing of run-off and infiltration of water through slope exposition, steepness and length. In mountainous regions, the windward slopes receive heavy rainfall which may speed up chemical weathering, whereas the leeward sides receiving little amount of rain becoming arid. This favors physical weathering to dominate on the lee ward part.
ii. Living organisms
Living organisms include plants and animals. They both contribute to weathering in a number of ways. Growing roots of trees widen and deepen into the ground and open up joints. Animals ranging from the big to small, including man affect the rate of weathering both mechanically and chemically. Animals and micro-organisms mix soils as they form burrows and pores, allowing moisture and gases to move about
The longer a rock is exposed to agents of weathering, the more weathered it is likely to be and vice-versa. Young rocks such as solidified volcanic rock after a fresh volcanic eruption are likely to be less weathered than rocks formed long ago.
The key components of climate in weathering are moisture and temperature. The type and amount of precipitation influence soil formation by affecting the movement of ions and particles through the soil, and aid in the development of different soil profiles. High temperatures and heavy rainfall increase the rate of chemical weathering. Arid and semi-arid areas are associated with physical weathering since there is low rainfall and high temperature. As the rocks expand during a period of high temperature and contract during a period of low temperature they develop cracks. In addition, equatorial regions with high rainfall and high temperature experience fast and deep chemical weathering.
v. Nature of rocks
Nature of the rock determines the rate at which it may break down. Their nature depends on rock forming minerals. Some minerals are easily soluble. Also environmental condition such as organic acids and temperature may increase the rate of weathering of rocks. Soft rocks, for example, break down more easily than hard rocks. Similarly, jointed rocks (rocks with cracks) break down faster than rock substances without joints.
vi. The interdependence of physical and chemical weathering
There is interdependence between mechanical and chemical weathering. Chemical weathering to occur needs first mechanical process which provides fragmented pieces of rocks. These rock fragments are then attacked by the chemical process of weathering. Many reasons can be advanced to justify their interdependence:
• The joints and crack found in a rock as a result of physical weathering allow deeper penetration of water which leads to chemical weathering.
• Some rocks are dissolved in water and weathered away in solution. The solutions formed may later undergo precipitation leading to the formation of crystal. These crystals will exert a lot of pressure that will disintegrate the rocks physically.
• Hydration (chemical process) results in a high rate of absorbing water by rocks .e.g.: hematite, limonite which makes these rocks to peel off in a physical process called spheroidal weathering The physical process of frost shattering opens up cracks in the rock and when these cracks are occupied by water, chemical weathering process takes place. e.g. carbonation.Roots of plants which expand within bedding planes of rocks and burrowing animals which drill holes in rocks allow water entry into these rocks which accelerates chemical weathering.
Application activity 4.2
Make a field study around your school and explain how relief and nature of the rock have influenced the rate of weathering.
4.3. Weathering in limestone regions
Learning activity 4.3
1. Differentiate the types of weathering.
2. Describe the type of rock associated with limestone regions.
Limestone is a sedimentary rock in which calcite (calcium carbonate: CaCO3) is the predominant mineral, and with varying minor amounts of other minerals and clay. Limestone rocks are very sensitive to organic acids derived from the decomposition of living organisms.
The major land forms associated with weathering in limestone regions are Karsts land forms that include: caverns, stalagmites, stalactites, pillar, dolines, limestone pavements (uvalas), poljes.
Caverns or caves are also one of the important characteristic features of groundwater in limestone regions. Caverns are formed in several different ways. The rocks in which most caverns occur are salt, gypsum, dolomite and limestone, with the latter by far the most important.
Doline also called Dolina is a round or elliptical hollow on the surface of a limestone region which is formed when several small hollows merge. The small hollows are formed when water starts acting on the points of convergence of joints on the surface.
Uvala is a large surface depression (several km in diameter) in limestone terrain (karst region). It is formed by the coalescence of adjoining dolines and has an irregular floor which is not as smooth as that of Polje.
Polje is a large depression in a karst region with steep sides and flat floor. If it is drained by surface water sources, it is termed as open Polje.
Stalactites are protrusions on top of limestone cave formed as results of water dissolving some rocks which form a solution that leaks from the roof.
Stalagmites are formed like a columnar concretion ascending from the floor of a cave. It is formed from the re-precipitation of carbonate in calcite form perpendicularly beneath a constant source of groundwater that drips off the lower tip of a stalactite or percolates through the roof of a cave in a karst environment. It may eventually combine with a stalactite to form a pillar.
Pillars are formed within the weathered limestone cave after the joining together of stalactites from up and stalagmites from down. The two may finally meet forming a pillar.
For karst land forms to be formed the following conditions must be in place:
• Precipitation: the major types of precipitation which contribute to groundwater are rainfall and snowfall.
• Slope: infiltration is greater on flat areas since water is likely to remain in one place for a long time given that other factors are favorable. On steep slopes, a lot of water is lost through surface run-off with little infiltrating in the ground.
• Nature of the rock: For groundwater to percolate and accumulate there must be spaces within the rocks for it to pass through as well as to occupy further beneath.
• Vegetation cover: the presence of vegetation increases the rate of infiltration.
• Level of saturation of the ground: The rate of water infiltration is high when the ground is very dry and the soil is dry; all the air spaces in it are wide open.
Application activity 4.3
In groups make a field trip to any limestone region, observe karst land forms and present your findings in class.
4.4. Weathering in humid tropical and arid regions and resultant land forms
Learning activity 4.4
Choose any climatic region (Humid tropical/Arid) and identify the type of weathering which will dominate the area
4.3.1. Humid tropical regions
The tropical climate is characterized by high amount of rainfall (more than 1000mm)and high temperature of up to and (more than 18° C) respectively. Weathering is favored in equatorial and tropical regions where the wetness and high temperature are permanent. During the rainy season, chemical weathering dominates through the process of hydration, hydrolysis, solution, oxidation, and reduction. In areas with alternating seasons, chemical weathering is temporary interrupted during drought periods because of lack of moisture. Physical weathering processes such as exfoliation, granular disintegration and block disintegration dominate. Therefore, in tropical (savanna) climate, both physical and chemical weathering processes dominate in dry and rainy seasons alternatively.
4.3.2. Arid and desert regions and resultant land forms
The features formed in these regions as a result of weathering are both erosional and depositional.
1. Erosional features
An inselberg (island hill or mountain in German) called Monadnock in the United States, is an isolated hill, knob, ridge, or small mountain that rises abruptly from a gently sloping or virtually level surrounding plain. These forms are characterized by their separation from the surrounding terrain and frequently by their independence of the regional drainage network.
These are dome-shaped and steep-sided rocks that rise up to 30 meters. They are massive rock, commonly granite comprised of bare rock that stretches several hundred meters. They take many shapes such as oranges. A good example of where Bornhardts are found is Central Australia.
A tor is a pile like hill of rocks or rock peak. It is a product of massive weathering and comes in all manner of shapes.
This is a rock that is gently inclined at an angle of 0.5 to 7 degrees. It is concave in shape and is found at the base of hills where rainfall is heavy and falls over a short period of time.
v. Deflation basins
Depressions are formed in the deserts due to removal of sand through the process of deflation and are called deflation basins or blow-outs, or deserts hollows. The depth of deflation is determined by groundwater table.
vi. Mushroom rock
The rocks having broad upper part and narrow base resembling an umbrella or mushroom are called mushroom rocks or pedestal rocks. These undercut, mushroom-shaped pedestal rocks are formed due to abrasive works of wind.
Demoiselles represent rock pillars having relatively resistant rocks at the top and soft rocks below. These features are formed due to differential erosion of hard rocks (less erosion) and soft rocks (more erosion). The demoiselles are maintained so long as the resistant cap rocks are seated at the top of the pillars.
Rock masses of tabular form resembling a capped inkpot standing on softer rock pedestal of shale, mudstone is called Zeugen. The bases of such features are broader than their tops.
These are formed always in the same way as Zeugens except that yardangs only develop on landscapes which have alternating rock layers with different resistance to erosion parallel to the direction of prevailing winds. Winds enter and scour up rock particles from the soft bands, thus digging depressions within the soft bands. The resistant hard bands therefore remain standing high up as raised ridges.
Reg is a desert surface armored with a pebble layer, resulting from long continued deflation; found in the Sahara desert of North Africa. Often the winds blow off all the smaller fragments, and leave the bigger size pebbles and gravels over an extensive area.
These are depressions that have water in deserts. These are created by strong winds which remove rock particles from a particular place until a depression is excavated (created).
2. Depositional features in desert
Dunes are mounds or ridges of wind-blown sand. They are depositional features of the sandy deserts and are generally mobile. They vary in size and structure. The main types of sand dunes are Barchan, Transverse Dunes, and Seifs.
Also called Barchans, these are typical crescent shaped sand dunes. The windward slope of barchans is gentle and convex, and the leeward slope is steep and concave. Barchans move slowly, at a rate of meters per year in the direction of the prevailing winds.
These are long and narrow sand ridge which grow parallel to the direction of the prevailing or dominant wind.
- Transverse dune
Transverse dune is an alongated dune lying at right angles to the prevailling wind direction. They have a gentle sloping windward side and a steep sloping leeward side, they are commmon in areas with enough sand and poor vegetation.
Loess is a wind-blown deposit of fine silt and dust. It is unstratified, calcareous, permeable, homogenous and generally yellowish in colour.
Erg is also called sand sea or Dune Sea. It is a large, relatively flat area of desert covered with wind-swept sand with little or no vegetative cover.
Application activity 4.4
Explain the reasons why the erosive power of wind is high in arid regions than in tropical regions.
4.5. Weathering in the glaciated (cold) regions
Learning activity 4.5
Observe the photographs provided below and answer the questions that follow:
Photograph A Photograph B
1. Explain the difference between photograph A and B.
2. Why is the top of mountain in photograph A white in color?
A glacier is a mass of ice of limited width, which moves outwards from a central area of ice accumulation. In other words, a glacier is a mass of ice produced by the accumulation and compression of snow, which moves slowly downhill or sea ward due to its weight.
Glaciation or glacial activity refers to the work done by glaciers or moving ice. It is a process of movement of ice usually from mountain tops downhill which leads to erosional and depositional glacial land forms. Snow/ice is formed when temperature falls under 0oC.
The permanent ice sheets occur in Greenland, Antarctica, and on high mountain tops. The level above which there is perpetual snow cover is called a snow line.
In temperate regions, ice accumulation occurs in winter as the temperature falls under 0oC, and melts later in summer. In tropical regions, snow accumulates on top of mountains of about 4800m above sea level.
4.5.2. Types of glaciers
The main types of glaciers include the following:
i. Valley glaciers: these are also called alpine or mountain glaciers. They move down slope and occupy former river valleys under the influence of gravity and size. e.g. Glaciers on Rwenzori, Kilimanjaro, and Mount Kenya.
ii. Continental glaciers: are alternatively called ice sheets or ice caps. These cover large areas of the plateau surface. They accumulate from a common area and spread towards continental margins with massive movement. e.g.: Glacier found in the polar regions of Greenland, Antarctica, Arctic, Northern Canada and north Western Europe.
iii. Piedmont glaciers: these are produced when mountain glaciers move down below the snow line and spread in the low lands of foot hills of glaciated mountains. They merge to produce large mass of ice.
iv. Cirque glaciers: these are small accumulations of ice which occupy Cirque basins on the mountain sides.
4.5.3. Types of glacial flow
Glacial movements are categorized into two types: gravity flow and Extrusion flow.
a. Gravity flow
In this process, glaciers move down slope under gravity and it usually affects low-lying valleys. This kind of gravity flow includes the following various types:
i. Plastic flowage: ice usually behaves as an elastic brittle solid. When more ice accumulates, internal stress forces the ice to spread and therefore to move like a highly viscous liquid.
ii. Regelation: when ice accumulates, pressure is created inside the ice sheet. This pressure forces some ice to melt and this molten water moves down-slope. When this water derived from the melting of ice reaches in an area of low pressure, it freezes again and solidifies to produce ice.
iii. Intergranular translation: this involves the movement of crystals or granules down slope due to pressure from overlying ice. Melt water lubricates these ice crystals making it easy for them to slide past each other.
b. Extrusion flow
As the accumulation of snow on ice caps increases, there will be an automatic side-ways displacement of ice in all directions following increased accumulation. Hence ice does not flow necessarily down slope as under gravity flow. It flows in all directions as a thick porridge spreads in all directions as more is added. This is how ice sheets which cover large areas of plateau surfaces move.
Application activity: 4.5
Using examples distinguish between valley glaciers and continental glaciers.
4.6. Factors influencing the formation and movement of glaciers
Learning activity 4.6
Why is glaciation dominant in high altitude regions?
There are many factors that influence the formation of glaciers in an area. The most important are briefly described in the following paragraphs:
The effect of altitude: Following the principle of altitude increase and temperature decrease, glaciers usually form in areas of higher altitudes. e.g. Everest, Kilimanjaro mountains.
• The factor of latitude: Areas that lie astride the equator within the tropics have high temperatures that limit ice accumulation. On the other hand areas far away from the equator have low temperature which favor ice formation.
• Precipitation of snow: Glaciers are formed from the condensation of water vapor. This results in the formation of ice crystals which fall as snow. The progressive accumulation and their compaction result in thick and continued glaciers that cover the surface.
The rate at which glaciers move is different from glacier to glacier and is determined by a number of factors. The most important are highlighted below:
- Nature of slope: when the slope is steep enough, glacier moves faster than when slopes are gentle.
- The amount of ice or size of the glacier: when the glacier thickness is big, there will be more pressure to generate quick motion than when the thickness is low.
- Temperature: The glaciers are faster in warm climate conditions due to the presence of enough melt water than in regions of low temperatures. High temperatures quickly produce melt water, which lubricates the ground for quick basal slippage and inter granular translation.
- The amount of load: Load is the eroded materials carried by a moving glacier. The more the load the slower the glacier due to increase in friction and the lesser the load the faster the glacier will be.
Application activity 4.6
Make research on other factors that influence ice accumulation and make a class presentation.
4.7. The work of glaciers and resultant land forms
Learning activity 4.7
From the experience you acquired in previous lessons, make a difference between ice and glacier
4.7.1. Processes associated with glacial erosion
Glaciers perform a triple function. These are erosion, transportation, and deposition.
Many processes are associated with glacial erosion but the most important are the abrasion, plucking and the frost shattering. They are detailed below:
- Abrasion also known as grinding process is the sandpapering effects of angular material embedded in glacier as it rubs the valley sides and floor. Glacial abrasion is caused by the rock debris embedded in the glacier.
- Plucking is also referred to as sapping or quarrying. It occurs when the ice at the base and sides of the glacier freezes onto rock outcrops. The rocks are then pulled and carried away by the moving ice.
- Frost shattering: this process produces much loose material which may fall from valley sides onto the edges of the glacier to form lateral moraine.
4.7.2. Land forms produced by glaciers
There are two types of land forms performed by glacial processes:
1. Glacial erosional features
The most common glacial erosional land forms include:
- Cirque: also called corrie is a steep-sided rock basin with a semi-circular shape. It starts from a small depression which is gradually enlarged. Frost shattering helps shatter the rocks on the edges of the depression and as they break, the depression is enlarged.
- Arêtes: an arête is a narrow ridge with steep sides developing between two corries
.- Pyramidal peak: A pyramidal peak also called horn is a surviving top mountain mass that is not yet worn down by erosion. It is shaped like a pyramid hence the term “pyramidal peak”. It is formed at the junction of arêtes.
- Tan: also called tarn, is a cirque lake produced when the ice melts and the melt water occupy the cirque depression.
- U-Shaped valley or glaciated trough: is formed when a glacier passes through a preexisting river valley to a characteristic of U shape profile. The over deepening and widening of these former river valleys is a product of abrasive action of ice using large amounts of moraine as its tool.
- Hanging valleys: these are valleys associated with glacial troughs. They are small valleys whose floors are found at higher level than the floor of the main valley to which they are tributaries. The floor of the main valley is at a lower level due to greater erosion than the floor of tributary valleys where there is less erosive power.
• Ribbon lakes: the floor of a glacial trough is often eroded very unevenly, and long depressions may be formed at the U-shaped valley floor. These depressions may become sites of long narrow lakes called Ribbon lakes, for example, Lake Noir in France.
• Roche montane (roche moutonée): this is a mass of more resistant rock that projects above the general level of a glaciated valley floor. In most glaciated valleys, it is possible to find rock surfaces that have been grooved and scratched.
◊ Crag & Tail: This is an elongated rock mass which is formed when a flowing glacier meets a resistant rock protecting a soft rock on its leeward side. The soft rock on the leeward side is called a tail.
2. Glacial depositional features
Deposition of debris is among processes performed by glaciers. Debris are preferably deposited in depression or lowlands. Glacial deposits are generally called drifts. They include sands, gravels and rock boulders...
The major glacial depositional features are:
They refer to materials (debris) carried and later deposited by a glacier as it stagnates or decay. Moraines can be classified into the following types:
- Terminal moraine: these are deposited on the mouth of a glacier.
- Lateral moraine: these are deposited on the sides of a glacial trough and from elongated ridges on the sides of valley gorges.
- Medial moraine: These are materials that were originally carried by the valley sides of two small valleys which after emerge into one valley. These materials found themselves in the Center of a glacier.
- Ground moraine: This type of moraine covers the entire width of the valley floor.
• Till plains: these are extensive lowland areas covered by till or a till covered plain.Fluvial glacial deposits: Fluvial glacial deposits are those materials de-posited by melt water from a stagnant glacier. They lead to the formation of the following depositional land forms:
• Out wash plain: is a wide gentle sloping plain which is composed mainly of sand and gravel which were deposited by unevenly melt water
• Kame: is an irregular mound of sand and gravel deposited by melt water, they are short lived and can collapse any time.
• Kame Terrace: is a flat topped ridge formed between a valley glacier and the valley slopes. It is composed of materials deposited by melt water streams flowing laterally to the glacier.
• Esker: is an elongated, narrow ridge which is made up of sand and gravel.
• Kettle holes: is a depression or hole formed by glacial deposits when a block of ice detached from the main glacial while the latter is retreating.
• Drumlins: these are low, rounded smooth, elongated mounds or hills of till rising up to 50m or 1km long. They are products of glacial deposits which flattened the landscape
Application activity 4.7
a. Account for the limited coverage of glaciation in East Africa.
b. Make a research and illustrate the major glacial depositional features.
4.8. Impact of glaciation on the landscape and to human activities
Learning activity 4.8
Using the experience acquired in previous lessons, identify different human activities carried out in glaciated mountainous regions.
There are many impacts of glaciation on the landscape and human activities. Some are positive while others are negative. The main impacts are described below:
4.8.1. Positive impacts
- Crop farming: the till and out wash plains contain fertile soils. These are some of the richest agricultural areas in the world.
- Livestock rearing: the glaciated uplands provide suitable grazing lands since they form fine benches on which pastures thrive in summer.
- Tourism: glaciated landscape has features such as arêtes, pyramidal peaks and cirques that attract tourists.
- Natural harbors: fiords provide ideal sites for the development of natural harbors, for example, the port of Rotterdam in Netherlands, natural habors in Norway and Sweden.
- Fishing grounds: fiord coastlines such as those in Norway provide suitable fishing grounds since they are deep and well sheltered.- Provision of water: glacial lakes provide water for domestic and industrial use.
- Transportation: glacial lakes provide natural waterways, for example, the Great Lakes of North America.
- Mining: glacial erosion exposes minerals to the surface making their exploitation easy, for example, gold and copper in the Canadian Shield of North America.
- Generation of hydro-electric power: waterfalls formed by rivers flowing through hanging valleys are suitable for the generation of hydro-electric power, for example, in Switzerland.
4.8.2. Negative impacts
- Production of bare land: in some instances, the land surface has been scrapped and polished to bare rock. Such regions are of no economic use.
- Discourage settlement: the cold temperatures especially at high altitude limit settlement and other economic activities. They therefore remain as wastelands.
- Transport barrier: the rugged landscape produced by glaciers makes it difficult to establish infrastructure such as roads and railways.- Hindrance to agriculture: sand and gravels deposited on out wash plains make the land unsuitable for agriculture.
Application activity 4.8
Make research using geographical documents and internet on negative effects of glaciation apart from those mentioned in the content.
4.9. Mass wasting
Learning activity 4.9
Make research using geographical documents and internet on negative effects of glaciation apart from those mentioned in the content.
4.9. Mass wasting
Learning activity 4.9
Observe and explain the phenomena that occurred in the photograph below
Mass wasting or mass movement is defined as the creeping, flowing, sliding or falling of rocks and weathered materials down slope under gravity. It is different from erosion in a sense that, in erosion water physically transports away the soil particles, in mass wasting water does not wash away but assists the rock to slide down under the influence of gravity. Mass wasting is classified into two major categories: Slow movement and rapid movement
.4.9.1. Slow movement
Also called creep movements, they are very slow in their motion and they may occur without being noticed. These slow movements’ include:
• Soil creep: This is the most common and the most widely spread type, because it is found in both tropical and temperate climate. The movement of materials is so slow that they may move a few centimeters per day. It can be detected by leaning of trees, electric poles and fencing poles in the direction of the slope.
• Solifluction: This is limited to glaciated mountainous regions and cold climatic areas where thawing causes the saturated surface layer to creep as a mass over underlying frozen ground.
• Talus creep: This is a down slope movement of mainly screes that are relatively dry. It occurs almost in the same way as soil creep and it also occurs under tropical and temperate climate.
• Rock glacier creep: This is a slow process of slope failure in which individual rock boulders with very little soil but with some ice embedded within them slowly move down slope confined within a channel.
• Rock creep: This is the movement of individual rock boulders slowly down slope.
4.9.2. Rapid movement
• Earth flows: These are the rapid down ward movements of clayish or silty soils along a steep slope.
• Mud flows: These are similar to earth flows but they are muddy and occur on slopes that receive heavy rainfall. They are very fast. In Rwanda they are common in the Northern and Western-provinces.
• Debris avalanches: This is the most form of rapid flowage due to the fact that slopes are very steep and there is enough rain to soak slopes. It occurs on very steep slopes that occur in humid climate.
• Slumping: This is the downward slipping of one or several units of rock debris, usually with a backward rotation with respect to the slope over which movement takes place. Undercutting of slopes by streams and man are the main causes of slumping. The surface of the slumped mass has a number of step-like terraces.
• Rock slide: This is the type of sliding in which individual rock masses fall from vertical cliffs or faces of slopes or jointed cliffs.
• Rock fall: Here, individual boulders fall freely from a steep rock face.
Landslides: These are also called landslips. They are down-slope gravitational movements of a body of rock or earth as a unit. It may be induced by natural agencies (like heavy rain, earthquake) or it may be caused by human interference with the slope stability.
Application Activity 4.9
Make research and analyze the types of mass wasting common on hilly areas of Rwanda.
4.10. Causes, effects and control measures for mass wasting
Observe the photograph below taken in Gakenke district and describe the phenomenon that took place.
4.10.1. Causes of mass wasting
The following are the major causes of mass wasting:
- The degree of slope: The steeper the slope, the higher are the chances of material movement. Mass wasting is almost nil in gentle and flat areas.
- The structure and lithology of rocks: Alternating hard and soft rock layers on a slope can be a cause of slope fall. For example, a layer of clay on top of limestone layer can easily slide down.
- The degree of lubrication: Most mass wasting processes occur after a heavy down pour. Water assists to lubricate rock particles and the layers of rock on top of a slope. Therefore, water provides a medium of sliding because it reduces internal friction between rock particles and layers.
- The amount of load on a slope: Slopes which are light rarely fall compared to those which are heavy. Therefore, additional load on a slope increase chances of slope fall.
- Tectonic movements: Earthquake and Volcanic eruptions cause vibrations of the earth which often trigger off widespread movements of materials such as landslides
- Climate: The amount and nature of rainfall received in an area determines the kind of movement that occurs.
- Grazing: The grazing of cattle, movement of elephants and other animals can cause some tremors on slopes hence making them fall.
- Nature of soil: soils which are infertile and therefore unable to support vegetation in enough quantities, are more susceptible to mass wasting compared to soils, which are fertile and therefore able to support dense vegetation.
- Influence of vegetation: Vegetation help to hold rock materials together thus reducing their movement on the surface.
- The work of animals: Animals and micro-organisms facilitate deep weathering which results into the reduced cohesion of the rock particles on slopes. This therefore leads to easy movement.
- Vulcanicity: Volcanic eruption on the ice capped highlands cause ice to melt and therefore soak the slopes. This lubrication greatly increases the chances of slope movement.
4.10.2. Effects of mass wasting
The following are some of the effects of mass wasting:
- Threat to life and property: There are several serious incidents of landslides and rock slides every year. They cause loss of life and property. In a minor incident they may block only one line of a road, but in severe cases entire blocks of buildings collapse.
- Loss of vegetation: Mass wasting and soil erosion result in the loss of surface topsoil which is essential for vegetation. As a result, more areas become barren.
- Scars and Gullies: In areas where topsoil and vegetation are removed, bare spots form scars in the landscape. Gullies form on weathered slopes through rain action and mass wasting in areas with little or no vegetation. Intense gully cuts up the landscape into large scale gullies and ridges and destroys the area. Gullying is common in the bare, granitic areas.
- Pollution of water: large amounts of geologic materials enter streams as sediments as a result of this landslide and erosion activity, thus reducing the potability of the water and quality of habitat for fish and wildlife.
- Wildlife destruction: Although most kinds of wildlife are able to retreat fast enough to avoid direct injury from all but the fastest-moving landslides, often are subject to habitat damage by landslides.
It is noticed that mass wasting especially landslides, has severe impacts on humans and environments. For this reason, measures have to be taken for preventing or mitigating them. Some of the measures are highlighted below:
- Gradients of steeper slopes could be reduced by constructing terraces.
- Retaining walls can be built to stabilize the slope.
- Steep slopes should be inspected regularly, especially during periods of intense or prolonged rainfall to identify areas prone to mass wasting for preventive measures.
- More surface drainage channels and ditches can be constructed to reduce overflowing discharge
- Legislation can restrict development and building in zones prone to mass wasting.
- Trees can be planted on steeper slopes to stabilize the soil and the slope.
- Appropriate instruments can be installed to monitor slope stability, providing early warning in areas of concern.
- Mass education of people
Application Activity 4.10
Make a field trip to observe different cases of mass wasting in your area. Analyze its causes and propose the sustainable measures to control it.
4.11. The relationship between weathering land forms and human activities
Learning activity 4.11
Are land forms resulting from weathering important in your area? Support your answer.
Weathering affects human activities in various ways as follows:
• Weathering provides a basis for the development of construction industry in an area. e.g. marrum soil and laterite are good for road construction.
• Weathering can also produce land forms that offer important touristic opportunities.
• Weathering facilitates soil formation, this directly provides a basis for the development of agriculture in the region.
• Weathering affects limestone regions (calcium carbonate) that are important for cement production.
• Building stones in urban areas are subjected to the weathering processes as natural outcrops but with additional influences.
• Weathered shales also produce good brick clays, whereas the weathered ba-salt produces fertile soils based on montmorillonite.
• On weathered rocks, weathering often improves the grade of economic de-posits by concentrating desirable elements such as copper around the water table.
Application Activity 4.11
Make research in your area; describe how weathering land forms have benefited the people for their sustainable development.
End unit assessment
1. Describe the main causes of mass wasting that usually occur in north- western part of Rwanda. How does the community work (umuganda) contribute to the reduction of mass wasting in your area?
2. With reference to East Africa explain the formation of glacial land forms in mountain areas.
3. How have topography and parent rock influenced the rate of weathering in your area?
4. Make a field trip in your local environment and explain how the weathering land forms identified in your area affect positively and negatively human activities
.5. Referring to above questions suggest ways of sustainable environmental protection.