GEOGRAPHY As LEVEL(FORM FIVE) NOTES THE DYNAMIC-EARTH AND CONSEQUENCE (2)

1. 2. CLIMATE

Variation in climate causes differences in the rate and type of weathering. The main climatic controls are temperature and humidity. However, the role of climate in weathering varies greatly from region to region depending on temperature and rainfall patterns. Hence, the type of climate determines the rate and type of weathering.

1. Equatorial regions – Characterized by high temperature and rainfall throughout the year. Chemical weathering of rocks is very active in these latitudes due to high temperature and high rainfall totals.
2. Tropical (Savannah) regions – Characterized by seasonal variation of rainfall and temperature, i.e., there are dry and wet seasons. Both chemical weathering and mechanical weathering take place: chemical weathering during wet seasons and mechanical weathering during dry seasons.
3. Hot Deserts – Characterized by a large diurnal temperature range, i.e., day temperatures are extremely high while night temperatures fall rapidly. There is a low amount of rainfall and excessive evaporation. Both mechanical and chemical weathering take place: mechanical by exfoliation and (frost action) and chemical by salt crystallization due to excessive evaporation.
4. Mountain regions – Characterized by high humidity and low temperature. Frost action is active in tropical regions where there are mountains with heights above 430m like Elgon and Ruwenzori. Frost action is also common.

3. 3. RELIEF (Slopes)

At steep slopes, the rate of physical weathering is fast but chemical weathering is retarded because H₂O moves quickly with no penetration of water.

On lowlands, physical weathering is slow because weathering is protected by weathered materials, and chemical weathering is faster because water is present and influences weathering.

Under relief, there is aspect – the position of a place in relation to sun rays. North-facing slopes do not face sunlight; hence, they are less developed and weathering takes place slowly. In contrast, on south-facing slopes, weathering takes place faster with well-developed soil and good vegetation.

• In tropical regions, there is no aspect.
• In mountainous regions, frost action takes place actively.

4. 4. ROCK AGE

Old rocks are more susceptible to weathering as they have had enough time to be subjected to different weather forces compared to young rocks. Thus, the rate of weathering is higher in old rocks compared to young rocks.

5. 5. BIOTIC ACTIVITIES

Biotic activities contribute significantly to weathering in various ways and include:

• Penetration of plant roots causes physical destruction of rocks.
• Some plants and animals secrete acids from their bodies leading to decomposition of rocks.
• Burrowing of animals also causes mechanical weathering.
• When living organisms in the soil respire, they give out carbon dioxide gas. The gas dissolves in water to form carbonic acid, which causes decomposition of rocks.
• Thick vegetation cover, such as tropical forests, acts as protection against physical weathering and helps to slow down the removal of weathered materials.
• Human activities and poor methods of agriculture expose the bedrock to weathering.

Study Questions

1. Discuss exhaustively the role of water in the weathering process.
2. Where and for what reasons is mechanical weathering a dominant process?
3. “Mechanical weathering and chemical weathering processes are interdependent and complementary.” Discuss.
4. Weathering is not influenced by the force of gravity but mass movement is influenced by the force of gravity. Justify.

II. MASS WASTING (Mass Movement)

Mass wasting is the downslope movement of weathered materials under the influence of gravity. In this movement, there is no transporting agent. H₂O is involved as a lubricant and not a transporting agent. Water helps to reduce friction of particles within weathered materials and also adds to bulkiness, facilitating the process of mass wasting.

Types of mass wasting

Two types according to the speed of movement

a) Slow mass movement

1. Soil creep
2. Solifluction
3. Talus creep
4. Mudflow

b) Rapid movement

1. Slump
2. Rock slide (sometimes both slump and rock slide are called “landslide”)
3. Rock fall

SOIL CREEP

Soil creep is a steady downward movement of soil on all sloping land. Rainwater lubricates soil particles and enables them to slide over each other.

• It is the slowest and imperceptible movement of weathered material, mainly fine soil down a gentle slope.
• Soil creep can be manifested through mounds of soil behind walls, tilting and cracking of walls, bending of trees, fences, and telegraph poles, as well as cracking of roads.

In equatorial regions, creep is often disguised by dense vegetation cover.

SOLIFLUCTION

Movement of weathered materials under frost areas. It is limited to mountain and cold climate areas where thawing causes a saturated surface layer to creep as a mass over underlying frozen ground (saturated soil, gravels, and weathered rock).

TALUS CREEP

This is a very slow movement of angular waste rock of all sizes (talus or scree) down a slope. It is common on the sides of mountains, hills, and escarpments. It takes place where freeze–thaw action is common, especially in highlands and high latitude regions.

Large talus sheets move mass especially in mountains where freeze–thaw is frequent. Talus moving down a valley in a long stream is a rock glacier.

MUD FLOW

Movement of large volumes of unconsolidated materials which are super-saturated with water. The materials flow as semi-liquid mud (slurry) with boulders and gravel embedded in mud.

Large volumes of unconsolidated material, super-saturated after heavy rain, become plastic and flow. Common in acid and semi-arid regions.

SLUMP

Massive rocks overlying weak rocks saturated by heavy rain, common on over-steepened slopes. Large masses of rock and debris move downslope.

ROCK SLIDE

Sliding movement of a slab of rock down a steep slope; no rotation is involved. It can be triggered by earthquakes or human activities like mining or cultivation.

Surface rocks slide over a slip surface formed by bedding or fault planes dipping sharply downslope.

ROCK FALL

Falling movement of individual rock blocks with boulders along a precipitous (steep) slope of a mountain or along road cuttings or cliffs.

Precipitous slopes in mountains where well-jointed rocks may be loosened by freeze–thaw.

Rocks accumulate as a talus slope along valley sides.

FACTORS WHICH AFFECT THE NATURE AND SPEED OF MASS WASTING

1. The degree of saturation and nature of weathered material
   The more saturated the weathered material is, the faster the rate of movement because there is less friction between particles. Depth of weathered materials, weakly bedded and steep dipping increase the rate of movement.
2. Gradient
   Steeper the slope, the faster the rate of movement and vice versa.
3. Climate
   Amount and nature of rainfall, annual and daily temperature ranges. Heavy rain or alternating freezing and thawing encourage movement. Heating and cooling also affect mass wasting.
4. Vegetation cover
   Absence of vegetation cover to hold the materials increases the speed of mass wasting.
5. Human activities
   Mining, overgrazing, and keeping of animals are among the ways humans affect surface stability and facilitate the rate of movement.
6. Earth movement
   Especially earthquakes which can disturb rocks and encourage mass wasting.

Effect of mass wasting

1. Loss of life.
   Example: Settlements built down hills may be affected by rock falls; mountain climbers may be caught in avalanches.
2. Destruction of property.
   Example: Farms on hills or slopes may be destroyed by mass wasting (soil creep, landslide, rock fall). Buildings, roads, railway lines, and rivers may become blocked.
3. Attract tourism.
   The resulting features after mass wasting can arrange rocks attractively. People also like to see disaster effects firsthand.
4. Land degradation.
   Removal of fertile soil leaves scars and reduces land value.
5. Formation of fertile soil on the foot of hills where weathered materials accumulate.
6. Can dam a river to form temporary lakes.
   However, weathered materials are loose, so water will remove them and the river will continue to flow.

PRECAUTIONS:

• Afforestation and reforestation on slope lands to stabilize weathered material and reduce movement rate.
• Control human activities, especially poor agricultural methods (contour cultivation stabilizes farms).
• Avoid establishing settlements in areas prone to mass wasting.
• Construct terraces across slopes.

Study questions.

Carefully distinguish mass wasting from weathering.

Mass wasting

1. Influenced by gravity.
2. Movement of weathered materials.
3. Types include slow mass movement and rapid.
4. Generally restricted to material breakdown in place.
5. Is a surface phenomenon.

EROSION

Is the detachment and removal of weathered materials from the earth’s surface by agents of erosion.

Or it is the process of breaking up and wearing away of exposed rocks by moving water, wind, and moving ice.

Agents of erosion: There are four agents of erosion:

i. Running water ii. Wind iii. Glaciers (moving ice) iv. Waves and tidal currents.

EROSION BY RUNNING WATER

What is running water?

Any water which falls on the ground and flows downslope under the influence of gravity.

When water runs on the surface, it performs three functions: erosion, transportation, and deposition.

What happens when water falls on the surface of the earth?

• Percolation – When H₂O is absorbed into the ground.
• Evaporation – Rain evaporates.
• Surface runoff – The most effective agent of erosion over the earth’s surface.

Surface runoff causes soil erosion

Types of erosion

1. Sheet erosion
   Uniform removal of upper soil without well-defined channels. Removal over a large area of a top layer of soil and other fine materials by a thin sheet of H₂O flowing over a fairly smooth surface.
2. Rills erosion
   Removal of upper soil from surface with well-defined channels called rills. The impact of rills is more effective than sheet erosion.
3. Gully erosion
   If rills are not checked, they will collide and become larger to form big channels called gullies. Gullies are grooves or V-shaped depressions.
4. Splash erosion
   Raindrop impact on the surface displaces particles (loose dry materials).

Impact (result):

• Formation of small channels which lead to formation of badlands (smooth land – bad land which is less useful).
• Bad land
• Gullies
• Rills

EROSION BY RIVER, WIND, GLACIER, AND WAVES

RIVER

Mass of water flowing in a natural channel over the earth’s surface from highland to lowland under the influence of gravity.

TYPES OF RIVER

1. Perennial River: These rivers flow throughout the year. Their source is a region with abundant and well-distributed rainfall throughout the year. Examples: Nile River (Africa), Congo River (Africa), Amazon River (South America).

2. Intermittent Rivers: These rivers flow only during the wet season in regions with seasonal rainfall.

3. Ephemeral Rivers: These rivers appear during the rainy season in areas with very little rainfall, especially deserts. They disappear immediately after the rainy season ends.

River vs Stream

River and stream are used interchangeably.

Terminologies associated with rivers

1. River head (River source): The point where the river or stream begins; the highest point on a river system.

Possible river sources:

• Lakes – e.g., River Nile in Lake Victoria.
• Mountains with plenty of rainfall – e.g., Rockies and Appalachian (Mississippi River), Ganges, Indus (Himalayas).
• Springs – e.g., Thames River in England.
• Melting ice – e.g., Rhine River in France.

2. River mouth: The point where the river ends; the lowest point or base level of the river.

Possible river mouths:

• Oceans – e.g., River Rufiji into the Indian Ocean, River Nile into the Mediterranean Sea.
• Lakes – e.g., River Kagera into Lake Victoria, River Malagarasy into Lake Tanganyika.
• Swamps.
• Other rivers – e.g., Blue Nile meets White Nile.

3. Watershed/Catchment area/River basin: The collecting ground of a single river system where a river collects its water.

Tributary: A branch of a river pouring its water into a main river.

Distributary: A branch of a river which collects its water from the main river. This is more prominent in the lower stage and is associated with delta formation.

4. Divide: A highland separating two adjacent river systems.
5. River system: The main river and its tributaries (distributaries).
6. River valley: Lowland between two hills of a drainage basin where the river flows at the bottom.
7. River bed: The actual part of the river covered by flowing water.
8. River load: The materials carried by running water.

WORK OF RIVERS

Works of a river include three processes:

1. Erosion
2. Transport
3. Deposition

1. RIVER EROSION

Progressive removal of materials from the floor and sides of the river; progressive removal of materials from the river bed.

Vertical erosion deepens the river channel.

Section across a river channel.

Lateral erosion widens the stream.

Headward erosion takes place in the upper course, lengthening the stream.

The process of river erosion is accomplished through four interacting processes:

1. Hydraulic action: The force of moving water removes loose materials such as gravel, sand, and silt and weakens solid rock by surging into cracks in the rock from the sides and floor of the river.
2. Corrosion: Solvent action of water (solution). The process of removing soluble materials by moving water, e.g., limestone or calcium carbonate.
3. Corrasion: Wearing away of the river bed by the load of the river.
4. Attrition: Impact of the load of the river upon itself, where rock fragments collide with each other.

2. RIVER TRANSPORTATION

Movement of materials from one place to another by the river.

Mechanisms:

1. Suspension: Light material with specific gravity less than one is carried above the floor as suspended load.
2. Saltation: Larger particles are transported in a series of hops (bounces), touching the floor at intervals.
3. Traction: Load is dragged along the floor, continuously touching it.
4. Solution: Materials dissolve in water and become soluble, thus invisible.

Transportation of the load depends on the energy and power of the river.

Power is the ability to do work but depends on energy to perform work.

Energy of the river depends on:

1. Volume
2. Velocity

Volume + Velocity = Discharge.

i. The volume of the river is how large the river is (size).
ii. The velocity is how fast the river flows.

• Large volume rivers have more energy than slow-flowing rivers; the larger the velocity, the greater the energy.
• River discharge is the number of cubic meters per second passing through a particular section of the river (m³/sec).
• Discharge is measured by a current meter placed in the river, which automatically records the energy of the river at any point.

– The velocity of a river varies from one place to another across the channel due to:

i. The middle part of the river has maximum velocity (maximum energy) because friction is less.

ii. Velocity varies with gradient. A river with a steep gradient has higher energy compared to one with a gentle gradient.

iii. The shape of the channel influences the energy of the river. A shallow and wide channel has less energy because friction is greater compared to a narrow channel.

A. Channel B loses more energy through friction than channel A, but channel C has the greatest available energy due to its shape.

Due to the large size of the channel.

It is important to note the difference between the river channel and the river valley.

THE HEIGHT OF A RIVER ABOVE THE BASE LEVEL OF THE RIVER

Base level of the river: The lowest level a river can erode. When a river ends in lakes or oceans, that is the base level.

– The height of a river above its base level gives it potential energy (P.E.) (energy due to position).

– When water flows, potential energy is converted into kinetic energy which does the work of the river.

A river uses its potential energy to:

1. Overcome friction in the river bed.
2. Erode.
3. Transport materials/load.

– The rate of erosion along the river channel depends on:

1. Volume of the river.
   The larger the volume, the higher the rate of erosion.
2. Velocity.
   The higher the velocity, the greater the rate of erosion (greater the rate of destruction).
3. Type of rocks over which a river flows.
   A river flowing over soft rocks will have more erosion and a higher rate of destruction than one flowing over hard rocks.
4. Type of cutting tools (type of load the river carries).
   If the river carries a large load, the rate of erosion will be high compared to a river carrying a small load.

RIVER COMPETENCE AND RIVER CAPACITY

River competence: The ability of a river to carry large load in terms of size of individual particles.

• At particular places and velocities.
• River competence is high where a river is narrow because energy is higher compared to large slow-moving water.

River capacity: The ability of a river to carry a large load in terms of volume.

• A large slow-moving river has high capacity but low competence and vice versa.
• The ultimate goal of a river is to bring the land above sea level to its base level, but this cannot be fully achieved due to adjustments.

3. RIVER DEPOSITION

What is river deposition?

Lay down or dropping of the load transported by a river.

Why deposition?

• It deposits its load when the energy of the river is insufficient to carry the load further.

When does this occur?

1. When the volume of the river decreases.
   This happens when the river enters arid and semi-arid regions (dry land/hot desert) because evaporation and percolation increase, reducing water in the channel. It also occurs in porous rock and limestone regions or during dry seasons (droughts) due to lack of rainfall.
2. When its velocity decreases (speed).
   Velocity decreases when the gradient falls, or when a river enters a lake or swamp where it meets resistance, or when it enters the ocean/sea encountering waves and tidal currents.

The stronger the encounter force, the more deposition occurs.

When the river channel widens, friction increases, energy decreases, leading to deposition.

DEVELOPMENT OF THE RIVER VALLEY

Long profile and cross profile of a river

Long profile of a river: The whole length of a river from its source to its mouth.

Cross profile of a river: The width across the river from bank to bank.

• River erosion leads to development of various features along the valley as it erodes from source to mouth. These features are studied according to the stages of the river.

THREE STAGES OF A RIVER

• Upper / torrential / youthful stage
• Middle / mature stage
• Lower / old / plain stage

LONG PROFILE OF A RIVER FROM ITS SOURCE TO ITS MOUTH

CROSS SECTIONS

CHARACTERISTIC FEATURES OF YOUTHFUL STAGE / UPPER

1. Deep, narrow valley (V-shape) because vertical erosion is dominant, deepening the valley.
2. Valley has step gradient – the speed of the river is very high.
3. Presence of potholes.
4. Presence of interlocking spurs.
5. Presence of waterfalls and rapids.

POT-HOLES

These are circular depressions on the river bed.

H₂O swirls when passing over the depression.

• Formed where the rock is softer than the surrounding rocks due to uneven river bed. Erosion by fast-flowing water swirls in the depression, deepening and widening it.
• A pothole can be much wider and deeper.
• Form at the base of a waterfall and form plunge pools.

Presence of interlocking spurs

What are spurs? A highland projecting into lower land. A river at the upper course cannot overcome obstacles and will swirl around them.

Spurs alternating on either side of the river interlock/overlap.

Waterfalls – A sharp break on the river channel where water falls from a higher level to a lower level.

Causes of waterfalls

1. Difference in rock hardness.
2. Uplift of land (tectonic forces) (earth movement).
3. Glaciated valleys.
4. At a cliff.
5. River rejuvenation.

Waterfalls formed by difference in rock hardness

1. Inclination of the hard rock. Rock layer is horizontal.
2. Rock layer dips upstream.
3. Rock layer is vertical (vertical dyke).

Examples of waterfalls

1. Gersoppa Falls, India
2. Victoria Falls, Zambia
3. Niagara Falls between Lake Erie and Ontario
4. Livingstone Falls, Zaire River

Rapids – A part of a stream where there is a sudden increase in speed/velocity of water.

Causes of Rapids/when do they occur

1. When the hard rock dips gently downstream.
2. During recession of waterfalls (retreats).

Gorges – An elongated steep-sided trough/hollow (deep) that always occurs where waterfalls retreat (waterfalls migrate up the river).

Canyons – Formed by H₂O recession or uplift of the land (areas of uplifting), e.g., the Grand Canyon.

CHARACTERISTIC FEATURES OF MIDDLE/MATURE STAGE

1. Valley has open V-shaped valley because lateral erosion is dominant.
2. The speed of the river has decreased because slope decreased; volume has increased because more tributaries join it, increasing the river load.
3. River beds are pronounced because of maximum erosion on the concave side and undercutting of the outside of the curve. The concave banks stand as river cliffs because there is little erosion or deposition on the inside of the bend. The convex bend slope is gentle as clipped-off slopes (smoothed ends of spurs). The river starts to meander and river plains are formed.
4. Bluffs – As spurs are removed, their remains form a line of bluffs on each side of the valley floor.
5. Deposition starts to take place on a fully mature valley.

Braided stream – Due to deposition on the river bed, some mounds build up causing the river to split into several channels which rejoin and split again.

Formation of oxbow lakes – Forms when a meander is so acute that only a narrow neck of land separates the two ends of the meander.

1. Neck of the land separates two concave banks where erosion is active.
2. The neck is ultimately cut through; this may be accelerated often during river flooding.

Deposition seals the cut-off which becomes an oxbow lake. Deposition takes place along the two ends of the cut-off and eventually seals it off.

Production of natural levees – Formed through successive flooding near the river because as the water floods out of the main channel, its speed is immediately checked by friction with the banks and the heavier sediments are dropped first.

– Ridge/embankment on the sides of the river formed by river deposition especially after flooding.

– Deferred tributary – A tributary which flows parallel to the main river and tries to enter it but the main river is high above the flood plain.

– Deferred junction – The point at which the tributary enters the main river.

Example: Yazoo streams flow for 200 km without joining the Mississippi River.

Formation of delta

What is a delta? A large, flat, low-lying plain at the river mouth where deposition takes place.

• Deposition continues at the river mouth causing the river to divide into various channels called distributaries.

Types of delta

There are many types of delta, but the main types are:

i. Arcuate delta
ii. Estuarine delta
iii. Bird foot delta

i. Arcuate delta

• Made up of many distributaries.
• The load is composed of coarse and fine materials.
• Triangular in shape.
• Examples: Nile, Ganges, Niger, Indus, and Hwan Ho.

ii. Bird’s foot (digital) delta

• Has few and long distributaries.
• Triangular in shape with fine and very fine sediments.
• Occurs where river energy is very low.
• Examples: Mississippi, Omo River in Ethiopia.
• Also occurs where wave energy is low, allowing deposition.

iii. Estuarine delta

• Has the shape of an estuary (submerged river mouth). Deposition takes place on the submerged part of the river (estuary). It is also triangular shaped.
• It has no distributaries.
• Examples: Elbe delta (Germany), Vistula delta (Poland).

Formation of floodplains

• A broad gently sloping surface of alluvium deposits immediately after the river channel.
• Produced by the deposition of alluvial and other materials on the floor of the river valley through which the river meanders.

Stages in the formation of delta

The stages in the formation of a delta in an ocean or sea are:

1. STAGE 1: Deposition divides the river mouth into several distributaries. Spits and bars arise and lagoons are formed. Depositions on the banks of the distributaries produce levees which extend into the river or sea.
2. STAGE 2: Some lagoons have begun to fill with sediments, causing further division of distributaries into smaller distributaries. The delta has a more solid appearance though it is still very swampy and usually well covered with water-loving shrubs and trees.
3. STAGE 3: Further filling of lagoons plus the growth of complete vegetation results in the older parts of the delta standing above water level and forming dry land.

Conditions necessary for the formation of delta

1. The river must have a large load, which happens if there is active erosion in the upper part of its valley.
2. The velocity of the river must be sufficiently low to allow deposition.
3. The rate of deposition must be higher than the rate of removal by tidal currents.
4. There should be no obstacles in the upper levels of the long profile.