FORCES THAT AFFECTS THE EARTH
s mainly act on the earth’s crust or close the surface of the earth. Often the features produced by these forces are seen on the surface of the earth. They include mountains, volcanoes, moraines and valleys, just to mention a few.
INTERNAL FORCES (ENDOGENETIC/ENDOGENIC)
These are forces that operate within (inside) the earth’s crust OR These are forces which operate beneath (under) the earth’s surface. These forces are generally referred to as TECTONIC FORCES. The internal forces (tectonic forces) are divided into the following. (1) Earth movement (Diastrophism) (2) Vulcanism/ Vulcanicity/Volcanic erruptions
Earth’s movement is the movement of the solid parts of the earth towards each other or away from one another or side way. These are also known as Diastrophism Types of Earth movement Earth movements are classified into two (2) main groups:- (i) Vertical or radial movements (ii) Lateral or horizontal movements or tangential.
I ) VERTICAL OR RADIAL MOVEMENTS.
(b) Changes sea level because of the upward lift of the land or sinking of the land. NB: This changes in the sea level is not eustatic changes but is due to vertical forces. The eustatic change is the changes of the sea level due to ice melt during ice ages
II. LATERAL/HORIZONTAL MOVEMENTS
Are the organic forces (movement) because they are on the process to build mountains. Orogenesis means the process of mountain building There are two (2) types of lateral forces
1) Compressional forces
2) Tensional forces.
Are forces which move towards each other i.e move against each other. – They tend to shorten the crust (the land) i.e. they squeeze the land. Compressional force causes the following.
(ii) Break the land to form faulting which may produce features like block mountains, rift valley and faults.
Are forces that tend to stretch the land i.e. the force move away from each other, they pull the land away. The forces cause faulting of the crust and produce features like faults, Block Mountains, and rift valley.
Features associated with earth movements
Rift valley is a trough or hollow which may result from both vertical and lateral movements of the earth’s crust. It is formed when two faults develop parallel to each other. It can develop either by tensional forces or compressional forces.
Formation of rift valley by tensional forces
This is formed when tensional forces move away from each other. These forces of tension produce faults and the block between two parallel faults subsides to form a rift valley.
Formation of the Rift Valley by compressional forces
This is formed when horizontal forces act towards each other. These forces of compression produce faults on the outside of the two parallel faults and the pieces of land on either side are lifted up above the general level of the ground to form a rift valley. Diagrammatically, formation of the Rift Valley occurs like this:
Block mountain (horst)
A block mountain refers to a table-like mountain formed due to the influence of faulting that leads to rising of crustal rocks. It is nearly a flat surface. A block mountain can be formed by either tensional or compressional forces. This is when the earth’s movements cause parallel faults which results into uplifting of some parts. Examples of Block Mountains are: Usambara and Uluguru, in Tanzania, Ruwenzori, in Uganda, Vosges and Black Forest, in Europe; and Mount Sinai in Asia.
A plateau is a large, extensive uplifted part of the earth’s crust which is almost flat at the top. The top of the plateau is mostly a plain. Plateaus were formed during Mesozoic and Jurassic eras. It was due to uplifting of the earth’s crust. Such landforms are East African and Brazilian plateaus. High plateaus especially in tropical latitudes are used for agriculture and settlement.
A basin is a large, extensive depression on the earth’s surface. Most basins are formed due to vertical movement of the earth. Examples of basins include: an inland drainage e.g. Congo basin, Chad basin; and Amazon basin.
2. VOLCANIC ERRUPTIONS/VULCANICITY/VULCANISM VULCANICITY
is the range of processes by which molten materials and gases are either intruded (injected) or extruded (ejected) into the earth’s crust or into the earth’s crust respective Vulcanicity is the formation of various feature due to the intrusion or extrusion of molten materials, and gases. The molten materials are called magma when found within the earth’s crust and magma when poured on the earth’s crust. Vulcanism/Vulcanicity –Is a broader term which includes both extensive and intrusive igneous activities while volcanicity –Refers to the extensive volucancity in which the materials are forced at onto the surface. Types of vulcanicity There are two types of vulcanicity which are classified as follows
1. INTRUSIVE VOLCANIC FEATURES
This is when magma intruded within the earth’s interior. The features resulted due to the intrusive volcanic eruption is called intrusive features. The intrusive volcanic features are the features which are found within the earth’s interior. These include the following. Sill–is a rock sheet formed when the magma solidifies horizontally along the bedding plane. Eg:- Tyolo scarp in Malawi Dyke- is a rock sheet formed when the magima solifies vertically across the bedding planes. Eg:- Kinkon Laccolith is the cone dome shaped mass of rock with flat formed of viscus lava. It look like a mushroom. Eg:- Laccolith found in Morafonobe in Madagases. Lapolith –is a scar shaped mass of rock formed in glosyncline. It forms a saucer –like shape may be due to the increased weight of the deposits Phacolith –is a leans shaped strip of igneens rock formed when the magima solidies along the anticline or syncline. Eg: Cordon wills in U.K Phacolith Batholith is the large mass of solidified rock formed when magma cools plutonically at the great dept Eg: at the heart of the mountain ranges. Eg:- Chilu Batholic in Gabon
2. EXTRUSIVE VOLCANIC FEATURE
I. Active volcano: is the volcano which erupts frequently. Eg:- Oldonyo Lengai in Tanzania and Mount Cameroon.
II. Dormant volcano is the one which has stopped erupting but not extinct and it is expected to erupt. The dormant volcano is also known as sleep volcano. III. Extinct volcano. Is the volcano which has stopped erupting for a very long time in history and is not expected to erupt. It is also known as dead volcano.
INFLUENCES OF VOLCANIC ERUPTION TO MAN AND ENVIRONMENT
Earthquake these are sudden earth movements or vibration in the earth’s crust i. When one tectonic plate sliding over/or past another plate along the line of a faulty ii. Volcanic eruption- The movement of molten rock below or onto the earth’s crust which in turn is caused by the movement of plates. The Nature of Earthquakes •The point of which on earthquake is originate is called focus. And sometimes it is several kilometres below the surface. •The point on the earth’s surface immediately above the focus is called the Epicentre, This is where the shock waves first hit the surface. It is the shock waves which gives rise to an earthquake.
TYPES OF SHOCK WAVE
2. Surface wave. These travel through the surface and are of two types i. Love wave;- Which cause the surface rock to move side to side of right angles to the direction of wave movement. ii. Rayleigh wave;- Wave which cause the surface waves to have a circular movement very similar to that of water wave movement.
DETECTING MAGNITUDE AND INTENSITY
The intensity of an earthquake is measured by an instrument called seismograph. Seismograph record the vibration produced by an earthquake. The magnitude of an earthquake is the total amount of energy released and the scale which gives the magnitude is called the Richter scale. The scale range from 0 to 8.9 Effects of Earthquake. For example the earth quake occurred in Bukoba Tanzania Sept. 10, 2016 has 5.7 Richter scale. Rise and fall of the sea floor Eg. Agadir Earthquake in Morocco in 1960 Displacement of the earth’s crust, it can happen vertically or laterally. Land slides and open up deep cracks in the surface rocks eg. The El Asnam earthquake in Algeria 1954 destroyed an area of radius 40km and open surface cracks up to 3m deep. Destruction of infrastructure and properties Loss of life, can lead to death’s They can raise or lower erosion rocks eg. In Alaskan the earthquake of 189 lead to raise of some rock for about 16km.
THE CAUSES OF EARTHQUAKES
• Faulting of the lithosphere caused by tectonic movement where one plate slides over another plate. • Volcanism can cause occurrence of the earthquake. This is due to the fact that the magma moves under the influence of intense pressure from within the earth’s interior. • Mass wasting like land slide and rock fall can cause occurrence of earthquake, but this is for local scale. • Falling objects from the atmosphere such as meteorites may lead to the shaking earth’s crust. • Man’s influence through his activities such as mining using explosives like dynamites and transport vessels like trains and heavy trucks.
EFFECTS OF EARTH QUAKES
5. Destruction of infrastructure and houses in Bukoba Tanzania 2016 earth quake causes more than 100 without settlement. Precautionary measures to avoid high damage from earthquakes • Refraining from building high-rising structures on the land vulnerable to earthquake as well as strengthening buildings by using reinforced concrete, steel frames, deep foundations and light roofs. • Geologists should detect epicentres and tell the people to evacuate the places likely to be affected by earthquakes. • To avoid constructing very large water bodies like Kariba dam which can cause the earthquakes due to the weight of water and other materials. • Discouraging the use of explosives like dynamites in breaking the rocks during mining and construction operations.
These are natural forces that operate on the earth’s surface. The forces mainly act on the earth’s crust or close the surface of the earth. Often the features produced by these forces are seen on the surface of the earth. They include mountains, volcanoes, moraines and valleys etc.
Mass wasting is the movement of the weathered materials downslope due to gravitational forces accompanied by rain action. Mass wasting also known as slope movement or mass movement Types of Mass Wasting Types of mass movement are distinguished based on how the soil, regolith or rock moves down the slope as a whole. Based on this factor, mass wasting can be categorized or grouped into two types. These are slow and rapid mass movements, each with its own characteristic features, and taking place over timescales from seconds to years. Slow mass movement This is the movement of soil at very slow speed, water acting as the lubricant. Slow mass wasting is categorized into several types.
These are as follows.
Soil creep Soil creep is the slow movement of the soil downhill after it gets soaked by water. This process is very slow and its evidence is provided by tilting of trees and falling of buildings and fences. Sol creep is activated by any process that loosens the soil, making it easy to move gradually down the slope. Factors influence soil creep: a. Alternate heating and cooling of the soil particles. b. The freezing of water in the soil causing frost heaving. c. Removal of the soil further down the slope. d. Percolation of water into the soil, acting as a lubricant. e. Ploughing of the soil, a fact which makes the soil loose and more mobile. Talus Creep It takes place due to the processes of thawing and freezing and is more pronounced in high latitude regions. It is very common on sides of mountains, scarps and valleys. This is also a very slow mass movement of screes. Rock creep It occurs commonly where individual rock blocks are lying over clay materials. In the presence of moisture, the clay surface becomes slightly slippery. The rock blocks may creep slowly down the slope under the influence of gravity. Individual rock blocks may move very slowly down a slope. Solifluction This is the slow movement or flowing of weathered materials, especially when mixed with water and gravels. It is limited on highlands and cold regions. Rapid mass wasting This involves the movement of materials in form of mud flow, land slide, rock fall and earth flow. Earth flow This type of movement occurs in humid regions. The materials on the earth’s surface gets so saturated with water that it gains much weight, and starts to move down the slope under the influence of gravity. This normally occurs on the slopes of the hills or mountains. The removed earth material leaves a shallow scar on its place of origin and it creates terraces or mounds in its destination.
Mudflow Mudflow is the movement of a large mass of unconsolidated rocks down the slope when saturated with water. It flows in semi liquid state. It is common in desert slopes, which are not protected by a cover of vegetation. This occurs, for instance, during a torrential storm when more rain falls than the soil can absorb. Land slide This is the rapid movement of surface rocks and soil down a steep slope such as a cliff face. It includes slumping and sliding of materials. During the movement, the block tilts and leaves holes. It is common in well jointed limestone rocks, shale or clays. The common forms of landslides are slump, debris slide, rock slide, rock fall, debris fall and avalanche. Rock fall This is the free-falling of a single mass of rock, common on steep slopes of mountains and along scarp slopes of the sea. This is the most rapid of all mass movements. If a rock fall occurs repeatedly, for a long time, the broken rocks collect at the bottom of the slope in a mound called talus.
The Factors which Cause Mass Wasting Mass wasting is caused by a number of factors which include the following:
9. Climate: Climate has a great influence on mass wasting. Areas that receive heavy rains often experience mass movements, such as landslides and soil creep, more often compared to dry areas. On the other hand, a little amount of rainfall does not wet the soil and so cannot cause the soil to move. In cold regions, alternate freezing and thawing triggers mass wasting. 10. Vulcanicity: Volcanic activity often causes huge mudflows when the icy cover of a volcano melts and mixes with the soil to form mud as the magma in the volcano stirs preceding an eruption.
The Effects of Mass Wasting to the Environment Mass wasting has significant effects to the environment.
8. Loss of life: The more populations expand and occupy more and more of the land surface, mass movement processes become more likely to affect humans. The table below shows the impact of mass movement processes on human life over the last century. Year Location Type Fatalities 1916 Italy, Austria Landslide 10,000 1920 China Earthquake triggered landslide 200,000 1945 Japan Flood triggered landslide 1,200 1949 USSR Earthquake triggered landslide 12,000-20,000 1954 Austria Landslide 200 1962 Peru Landslide 4,000-5,000 1963 Italy Landslide 2,000 1970 Peru Earthquake related debris avalanche 70,000 1985 Columbia Mudflow related to volcanic eruption 23,000 1987 Ecuador Earthquake related landslide 1,000 1998 Nicaragua Debris avalanche and mudflow triggered by heavy rains during Hurricane Mitch ~2,000 2001 El Salvador Earthquake-induced landslide 585 2006 Philippines Rain triggered debris avalanche >1100 2009 Taiwan Typhoon Marakot triggered landslide 397 2010 Gansu, China Rain triggered mudflows 1287 2013 Northern India Heavy rain triggered landslides 5700
Weathering refers to a processes where by rocks disintegrate into small particles due to the agents of weathering such as water, ice, wind, wave, etc. The process results from the forces of weather, that is, changes in temperature, frost action and rain action.
Types of Weathering
The main forms of weathering include:
• Mechanical weathering;
• Chemical weathering; and
• Biological weathering.
Mechanical weathering This is also referred to as physical weathering. It is a type of weathering caused by changes in temperature. It is common in areas where there are extreme changes in temperature such as hot deserts, arid and semi-arid regions. Mechanical weathering includes the following types: Exfoliation This process occurs due to temperature change. During the day time rocks expand due to high temperatures and contract during the night due to low temperatures. Alternate heating and cooling set up powerful internal stress in the top layer of the rocks. The stress produces fractures which cause the outer layer to pull away leading to the cracking and disintegration of rocks into small particles. The peeled off rock fragments fall to the bottom of the standing rocks and are subjected to further alternate expansion and contraction and disintegrate to even smaller fragments. The fragments collect at the base of the standing rocks to form mounds of steeply sloping rock fragments called talus or sometimes screes, but the term is better used for angular rock particles produced by the action of frost. The rocks that remain standing as exfoliation takes place are called exfoliation domes. Exfoliation domes occur in desert, semi-desert and monsoon regions. There are many exfoliation domes in the Egyptian, Kalahari, Sahara and Sinai deserts. Frost action This is common in temperate regions where temperature falls up to freezing point. When temperature falls (freezing point) water collects in the rocks and it freezes, its volume increases causing the crack to deepen and widen. Usually it involves the freezing of water in the cracks during the night and thawing (melting) during the day in mountainous areas. This action of thawing (melting) and freezing of water in the cracks cause the rocks to shatter (break) into angular fragments which form screes and talus. After thawing the cracks deepen further. Alternate wetting and drying This usually occurs in tropical regions. These areas have seasonal rainfall and they get rain during summer season and during winter season they are dry. This causes the blocks to disintegrate. Self-check: Differences between Weathering Processes Chemical weathering Refers to the weathering involves the decomposition of some of the minerals contained in a rock. Some rocks decompose when they come into contact with water (H2O), or oxygen (O2) and carbon dioxide (CO2), two of the gases that make up air.
Chemical weathering includes the following processes:
5. Hydrolysis– This process involves the reaction of hydrogen (in the water) with certain mineral ions (in a mineral). This gives rise to the formation of different chemical compounds that can be easily weathered through other weathering processes.
NOTE: Usually two or more chemical weathering processes take place at the same time. Chemical weathering is most marked in hot wet regions. Biological weathering: It resulted when plants grow on rocks and their roots penetrate into rock joints which later force the rocks to break apart. Also man contributes much to rock disintegration through farming activities, mining, quarrying and construction. Macro- and microorganisms also disintegrate rocks through burrowing and by mineralization process. Bacteria, for example, in the presence of air, break some minerals which are dissolved in the soil. Plants also absorb minerals from the soil by their roots. Decayed vegetation produce organic acid which remain in the soil. All of these actions help to weaken the rocks. The Significance of Weathering
5. Weathering serves as carbon sink. Any process that reduces the amount of carbon dioxide from the atmosphere is termed as carbon sink. Some processes of weathering involve absorption of carbon dioxide from the atmosphere. This helps to remove excess carbon dioxide from the atmosphere. Limestone and other carbon-based sedimentary rocks are important carbon sinks.
EROSION AND DEPOSITION BY RUNNING WATER, ICE, WIND AND WAVE ACTION
4. Solution– this involves moving some materials that dissolve in water, which are carried away in solution form. A river transports its load until it has insufficient energy to transport it any further. When this happens, the load is deposited. River deposition A river deposits its load when its volume and speed decrease.
A river volume decreases when:
1. It enters an arid region especially a hot desert;
2. it crosses a region composed of a porous rocks e.g. sand and limestone; and
3. During the dry seasons or in a period of drought.
A river speed decreases when:
1. it enters a lake or sea; or
2. when it enters flat or gently slopping plain such as a valley bottom. Deposition takes place when the river has insufficient energy to carry its entire load. The first part of the load that is dropped consists of boulders and pebbles. The last part to be dropped is the fine sediment, called silt. Deposition takes place at any point in a river’s course.
THE LONG PROFILE OF A RIVER
The long profile of a river is the line following the course of a river from its source to its mouth. Three courses or sections of a river can be distinguished.
These are: • The upper course. • The middle course. • The lower course.
The upper course/section
This is the first stage of a river. It is sometimes called the youth or torrent course. Its characteristics are as follows:
1. It is the river source.
2. The speed of a river is high. 3. Most of the works of the river include vertical erosion.
4. The cross-section of a river valley in this section of a river is V–shaped.
5. The slope of a profile is very steep.
6. It is sometimes utilized for hydroelectric power (H.E.P) generation. Erosional and Depositional Features for each Agent The main features of the upper section are deep and narrow, V-shaped valley; a steep gradient; pot holes on the river bed; interlocking spurs and waterfalls and rapids, often with plunge pools.
• Gorge: It is a steep, narrow and elongated valley. A gorge often is formed when a waterfall retreats upstream, e.g. a gorge found in Victoria Falls. The middle/maturity stage This is the second stage of a river. The main features of this section are bluffs and waterfalls and rapids.
The characteristics features of the middle course of a river valley
1. The speed of a river is fairly low.
2. Most of the work of a river is transportation.
3. The cross–section of a valley in this section is an open V.
4. The slope of a relief is gentle
5. The volume of a river increases.
6. Lateral erosion predominates.
Features associated with the middle course of a river valley
2. Waterfalls and rapids: Waterfalls and rapids can also be found in the middle stage of the river valley. This is mainly caused by river rejuvenation which increases erosive activity and transportation, hence development of waterfalls. The old/lower stage Third is the third stage of a river. The main features of the lower section of a river valley are a flood plain; braided river; ox-bow lake; levee and deferred tributary and delta.
Characteristics of lower stage 1. It is the river mouth.
2. Always there are gradient falls or slope falls.
3. The main work of a river is deposition.
4. The cross–section of a valley is a U–shaped valley.
5. The speed of a river is decreased.
6. The river valley is very wide.
The Importance of Erosional and Depositional Features to Human Beings
The following is the importance of features resulting from erosion and deposition
a) Loess form very fertile soil in desert land,
b) Water falls attract tourists; headlands in coastal areas are natural ports.
c) Coastal features form breeding places for fish,
d) Coral reefs are used as building materials and for settlement.
• Transport: Vibrations from machinery, traffic, weight loading, stockpiling of rock or ore from waste piles and from buildings and other structures loosen the soil and make it prone to soil erosion and weathering.
2. Man-made rivers in the form of canals such as Suez and Panama Canals. 3. Wells and boreholes
4. Roads, harbours, railways, airports, bridges, etc.