GEOGRAPHY As LEVEL(FORM FIVE) NOTES THE DYNAMIC-EARTH AND CONSEQUENCE (1)

GEOGRAPHY FORM FIVE

TOPIC SEVEN NOTES

THE DYNAMIC-EARTH AND CONSEQUENCE

THEORIES

THEORY OF ISOSTASY

Denudation has been ongoing on the continents where tons of materials are removed from mountains and hills and deposited in oceans; however, the hills or mountains are not reduced to sea level.

WHY IS THIS SO?

This phenomenon can be explained by the theory of Isostasy.

The term originates from Greek, meaning “Equal standing.” It describes a state of equilibrium or balance in the earth’s crust where equal mass underlies equal surface area. Therefore, there is a state of equilibrium. The theory suggests that the continents and their major features are maintained in a sort of equilibrium or are moving towards that equilibrium.

ISOSTATIC EQUILIBRIUM

Isostatic equilibrium can be disturbed

How? Processes on the surface of the continents.

1. Denudation – Weathering/mass wasting, transportation, and erosion, e.g., removal of material which lowers the surface.
2. Deposition – Building up process (rising of the land).
3. Accumulation of ice masses and melting of ice masses.

For example, when denudation removes material from mountains, pressure is released, uplift occurs where the material is removed, and there is no isostatic balance.

If the material is deposited on the seabed, there is compression which results in sinking (vertical movements). These vertical movements cause horizontal movements of simatic material from where compression has taken place to where there was pressure release.

In such processes, a state of equilibrium is restored.

EFFECT OF DISTURBANCE AT SURFACE AND READJUSTMENT OF ISOSTATIC EQUILIBRIUM

EFFECTS OF DISTURBANCE OF ISOSTATIC EQUILIBRIUM

1. Earthquakes – Earth movements (gradual).
2. Subsidence and uplift (submerged coast / raised beaches).
3. Faulting – Results in rift valleys or block mountains, lift blocks.
4. Folding – Fold Mountains.
5. Volcanic eruptions forming different volcanic features (volcano).
6. Displacement of lithosphere leading to plate tectonics.

Evidences of Isostatic movements

1. The depression of the crust in the northern part of America and Europe was due to the weight of ice sheets of vast thickness during the ice age.
2. After the melting of these ice sheets, the crust has been rising.
3. Slowly, for example, there are numerous former beaches that occur around the coast of Scandinavia. They now lie between 8m-30m above the present-day beaches. These old beaches have been raised because of the uplift of the land.
4. The continental shelf around Antarctica is covered with water to a depth of about 750m compared with 180m around other continents.
5. The presence of Rias and Estuaries between the coastlands of Gambia and Sierra Leone.
6. The submergence of forests on the shores of Britain.

IMPORTANCE OF THE THEORY

1. It provides knowledge on the dynamic state of the earth’s crust; the earth’s crust is not static but always dynamic as it tends to balance itself after some disturbances with the influence of gravitational force.
2. The analogy that the crust floats on the mantle, just like an iceberg floats on ocean or seawater, is crucial in understanding the theories of plate tectonics and continental drift.
3. The theory helps in understanding how different landforms were formed.
4. It provides a basis for predicting the future state of the crust at any particular place on the earth’s surface.
5. It can help humans take precautions depending on the nature of phenomena observed over time, like the occurrence and melting of ice sheets.

2. THEORY OF CONTINENTAL DRIFT

There are 7 continents.

Origin of these continents

Propounders

• Francis Bacon (1620) – Expanded Earth – F. B. Taylor
• Alfred Wegener (1912)

According to Wegener’s theory, about 280 million years ago, the present-day continents were united in a single block called Pangaea and surrounded by an ocean called Panthalassa. He believed that Pangaea was located near the South Pole. Later, Pangaea split into two super-continents: Gondwanaland (near the South Pole) and Laurasia (along the equator in the northern hemisphere).

These two super-continents were separated by a narrow water body called the Tethys Sea. Laurasia split to form present-day North America, Asia, Europe, and numerous landmasses found in the northern hemisphere including Greenland, Iceland, and the United Kingdom.

Gondwanaland split to give present-day Africa, South America, Australia, the Indian subcontinent, Antarctica, and other islands in the southern hemisphere.

Drift

Since that time, the continents have been drifting apart to occupy their present positions. The drifting is very slow, about 2 cm per year, and is still in progress.

Evidence to support Wegener’s theory of Continental Drift

1. Structural evidence (Jigsaw fit): If the continents were brought together, they would form one single landmass called Pangaea. For example, South America fits into Africa; North America fits into Europe; Antarctica, Australia, India, and Madagascar formed a single landmass with South America.
2. Geological evidence: Similar rock types in the coastal margins of continents, e.g., rocks of West Africa and Eastern Brazil are similar in type, age, structure, and formation, indicating they were once connected.
3. Biological evidence: Similar fossils of animals and plants of different times are found in different continents, proving they originated from one landmass.
4. Geomorphologic evidence: Structures of mountains like the Alps and Atlas have similar features and were formed under similar conditions, supporting the drifting movement.
5. Paleoclimatic evidence: Discovery of ancient ice in the Congo Basin, where the climate is warm, suggests Africa drifted from cold to warm regions. Coal deposits beneath Antarctic ice caps and Greenland indicate these areas were once warm.
6. Paleomagnetism: Paleomagnetic dating shows rocks older than 200,000 years from different parts of the earth have shifted their relative positions, indicating continental drift.
7. Ocean floor spreading

Weaknesses of Wegener’s theory / critiques

1. He did not explain how the movement occurred or the continental drift processes.
2. He was a meteorologist, not a geologist, and was criticized for being less informed in the field.
3. Not all continents fit exactly as argued in the jigsaw fit theory.
4. Some scientists argue that plant remains might have been spread by wind from one continent to another.
5. Wegener failed to explain the development of glaciers in hot arid Australia.

4.2 PLATE TECTONIC THEORY (Unifying theory)

It combines the theory of isostasy, continental drift, and ocean floor spreading.

According to this theory, the earth has an outer shell (lithosphere) made up of several rigid pieces called tectonic plates.

1. Geometrical part – The crust is made up of segments called plates, which vary in size. Examples include the Pacific, North American, Nazca, South American, African, Indian, and Antarctic plates.
2. Movements – The plates are in motion, either diverging or converging. The cause of movement is convectional currents from the earth’s interior, especially the asthenosphere.

The movements can cause plates to collide or slide past one another. The rate of motion is very slow, about 1 to 2 cm per year.

TYPES OF PLATE BOUNDARIES

1) Divergent boundary

A boundary created when plates move away from each other. This normally occurs in the ocean, where there are mid-oceanic ridges.

Examples:

• Mid Atlantic Ridge
• Island Arcs (volcanoes)
• Rift valleys (Graben)

2) Convergent boundary

A boundary where plates move towards one another.

3) Neutral / Transcurrent boundary

Occurs when plates slide past one another, normally along transform faults. There is no uplift or submergence of the land (crust).

MECHANISMS OF THE PLATE MOVEMENTS AND THE PLATE BOUNDARIES

Plates are either continental or oceanic. Oceanic plates are simatic (denser). Continental plates are sialic (lighter).

CONVERGENT PLATE BOUNDARY

1. Continental and continental – Both will be uplifted and result in folds and faults.
2. Oceanic and continental – The denser will subduct and the lighter will uplift, possibly resulting in volcanic eruptions and trenches.
3. Oceanic and oceanic – Convergent boundary with subduction resulting in trenches; both plates move down.

Plate Tectonic Theory states: “The lithosphere is made up of rigid segments called plates and the plates are in constant motion relative to one another.”

CAUSES OF MOVEMENTS

1. Convectional current – During mantle convection, some materials rise due to radioactive heat generation and move laterally below the lithosphere. The lateral movements drag the lithosphere leading to plate tectonic movements. On cooling, materials sink to the lower mantle where they melt again.
2. Upwelling of magma – Magma rises through lines of weakness, e.g., mid-oceanic ridges, forming new crust.
3. Isostatic adjustment – Causes slight movements when trying to restore balance.
4. Cooling and heating of crustal rocks – Expansion and contraction cause cracking and disturbance of the crust.

EFFECTS OF PLATE MOVEMENT

Changes on plate boundaries, which are lines of weakness, lead to the formation of major landforms on the earth’s surface.

1. Diverging plates

I) Oceanic

• Mid oceanic ridges
• Oceanic islands
• Rifts, e.g., Red Sea

II) Continental

• Volcanic mountains
• Block mountains
• Rift valleys

2. Convergent plate boundary – Collision may lead to subduction and uplift.

• Oceanic-oceanic trenches (e.g., Mariana trenches, Japan trenches)
• Oceanic-continental volcanic mountains on coastal boundaries and trenches
• Continental-continental fold mountains (e.g., Himalayas formed by Indian and Eurasian plates)

3. Neutral / Transcurrent boundary – No uplift or subduction; lateral displacement of plates (e.g., San Andreas fault in North America with displacement of about 1000 km).

Plate tectonic areas are zones of instability resulting in earthquakes and volcanoes. The theory explains almost all landforms on the earth’s surface.

Examples of Landforms

I. Deep sea trenches: Long, deep valleys along ocean floors formed along convergent destructive boundaries. Example: Mariana Trench in the western Pacific with a depth of more than 36,000 ft.

II. Mid Oceanic Ridge: Giant undersea mountain range made mostly of basalt, over 80,000 km long and 1500 to 2500 km wide, rising 2.3 km above the ocean floor. Associated with divergent plate boundaries where magma rises and cools to form the ridge.

Examples: East Pacific (Nazca and Pacific plates diverge), North Atlantic (North America diverges from Eurasia).

III. Island Arcs: Basalt eruptions along ridges or nearby build volcanoes protruding above sea level to become oceanic islands. Examples: Iceland, Japan, Hawaiian Islands, Mauna Loa, Easter Islands near the East Pacific Ridge, West Indies.

IV. Magmatic Arc: Island arcs at sea and belts of igneous activity on continental edges, such as batholiths in mountain belts. Examples: Aleutian Islands, Cascade volcanoes of the Pacific Northwest, Andes.

V. Mountain Belts: At convergent collision boundaries, oceanic lithosphere subducts, leading to continental collision and mountain formation. Examples: Himalayas, Alps, Atlas Mountains, Andes.

VI. Rifting and associated features: At passive divergent boundaries, continental crust stretches and thins, producing faulted landforms like the East African Rift Valley. Faults may be associated with volcanic landforms. Thermal expansion from rising mantle plumes causes landscape uplift.

VII. New Oceanic Crust: At divergent plate margins, gaps are filled with upwelled magma forming new oceanic crust.

4.3 MATERIALS OF THE EARTH’S CRUST

What is an element?

A substance which cannot be split into simpler substances by physical or chemical means. Elements are made up of atoms, which consist of protons (positive charge), electrons (negative charge), and neutrons (no charge).

There are over 100 known elements; about 90 exist naturally. Some elements exist independently, while others combine to form compounds.

Of the 90 elements, 8 are most abundant in crystalline rocks:

1. Oxygen
2. Silicon
3. Aluminium
4. Iron
5. Calcium
6. Sodium
7. Potassium
8. Magnesium

What is a mineral?

A mineral is composed of atoms arranged in a specific three-dimensional (crystalline) structure. They are naturally occurring inorganic substances made up of elements or compounds.

Properties of Minerals

• Definite shape (crystalline)
• Inorganic (naturally occurring)
• Solid
• Made up of elements or compounds

Note: Only inorganic substances are minerals. Coal is not a mineral because it is organic.

Identification of Minerals in the field

1. Colour – Minerals have different colours.
2. Luster – How a rock reflects light (metallic luster glitters; non-metallic luster is dull).
3. Streak – Powder obtained by rubbing the rock with a harder substance.
4. Cleavage – How a mineral breaks into definite shapes (e.g., mica breaks into sheets).
5. Crystalline structure – Three-dimensional arrangement.
6. Specific gravity – Minerals have higher specific gravity than water (H₂O = 1 g/cm³).
7. Hardness – Measured by scratch test (Mohs scale from softest to hardest).

What is a rock?

• A more or less uniform mass made up of grains of one or more minerals found naturally on the earth’s crust.
• Aggregates of minerals.

ROCK CLASSIFICATION

Rocks can be classified according to:

• Mode of formation
• Geological age
• Structure

A. ROCK CLASSIFICATION ACCORDING TO MODE OF FORMATION

According to mode of formation/origin, rocks can be classified as Igneous, Sedimentary, and Metamorphic.

1. IGNEOUS ROCKS

These are crustal rocks formed by cooling either within or outside the earth’s crust. Formed by cooling and solidification of molten materials from the earth’s interior. Molten materials solidify intrusively or extrusively. When molten material is within the earth crust, it is called magma; when it reaches the surface, it is called lava. Examples include basalt, granite, quartzite, gabbro.

Igneous rock is referred to as mother rock because:

1. It is the rock from which other rocks owe their origin, e.g., weathering leads to sedimentary and metamorphic rocks.
2. It forms the base for soil formation (pedogenesis), with soil properties depending on the nature of the igneous rock.
3. Igneous rocks constitute about 99% of the earth’s crust.

Classification of Igneous Rocks

1. Extrusive rocks (volcanic): Formed when molten lava solidifies on the surface. They have small crystals due to fast cooling. Examples: Basalt, Andesite, Rhyolite, Obsidian.
2. Intrusive rocks: Formed when magma solidifies within the earth’s crust. Includes hypabyssal (near surface) and plutonic (deep inside crust) rocks. Examples: Granite, Gabbro, Pumice, Peridotite.

Categories of Intrusive Igneous Rocks

1. Hypabyssal igneous rocks: Formed when magma cools and solidifies near the surface. Medium-sized crystals. Examples: Granophyres, Porphyries, Dolerite.
2. Plutonic igneous rocks: Solidified deep in the crust, exposed after erosion. Examples: Granite, Gabbro, Pumice, Peridotite.

Classification of Igneous Rocks by Chemical Composition

• Felsic (acidic) igneous rocks: High silica and feldspar, little or no iron or metallic minerals. Examples: Granite, Granophyres, Rhyolite, Obsidian.
• Mafic (basic) igneous rocks: High magnesium, iron, and some aluminium. Examples: Gabbro, Basalt.
• Ultra-mafic (ultrabasic): Very high metallic minerals like iron and magnesium, less than 45% silica. Example: Peridotite.
• Intermediate igneous rocks: Silica content between basic and acidic. Examples: Diorite, Andesite.

Characteristics of Igneous Rocks

1. Hard and semi-precious.
2. Formed by cooling and solidification of magma or lava.
3. Chemical composition varies with silica content.
4. Crystalline in nature.
5. May undergo metamorphism to form metamorphic rocks.
6. May undergo weathering and sedimentation to form sedimentary rocks.
7. Contain minerals like iron and magnesium.
8. Do not contain fossils.

2. SEDIMENTARY ROCKS

Formed from sediments deposited by water, wind, or ice through sedimentation (deposition, accumulation, and lithification). Sediments are laid in layers (strata) and hardened by compression. These are stratified rocks.

– The plane between two layers is called the bedding plane.

– The angle of the tilted strata to the horizontal is called the dip.

Sedimentary rocks are non-crystalline and many contain fossils. Some form in water (e.g., sandstone, mudstone, chalk, limestone, coral, peat, coal) and some on land (e.g., boulder clay, moraines, loess).

Some sedimentary rocks form chemically, not from sediments.

Types of Sedimentary Rocks

1. Mechanically formed: Formed by compaction of rock fragments. Examples: Clay, gravels, alluvium, moraines, boulder clay, loess.
2. Organically formed: Formed by accumulation and cementation of remains of organisms. Examples: Limestone, chalk, coral reefs, peat, coal.
3. Chemically formed: Formed by chemical processes. Examples include carbonates (travertine), sulphates (gypsum), chlorides (rock salt), silicates (flint), ironstones (limonite, haematite).

Characteristics of Sedimentary Rocks

1. Stratified with young rock layers overlying old layers.
2. Non-crystalline.
3. Contain fossils from accumulated skeletons and shells.
4. May change to metamorphic rocks under pressure and temperature.
5. Consist of fragments deposited and cemented.
6. Soft (not hard).

3. METAMORPHIC ROCKS

Formed when one type of rock changes form after being subjected to intense heat, pressure, or both. Any rock may undergo metamorphism.

Examples of Sedimentary to Metamorphic Rocks

1. Sandstone to Quartzite
2. Limestone to Marble
3. Coal to Graphite
4. Shale or Clay to Slate
5. Mudstone to Slate

Metamorphic to Metamorphic Rocks

1. Slate to Schist

Igneous to Metamorphic Rocks

1. Augite to Hornblende
2. Granite to Gneiss

Causes of Metamorphism

1. Great heat – High temperature acting on existing rock.
2. Pressure – Compression from earth movements.
3. Chemical reaction.
4. Combination of pressure and temperature can change sedimentary rocks to metamorphic.

Metamorphism – The process of changing rock from igneous, sedimentary, or metamorphic to metamorphic rock.

Types of Metamorphism

1. Contact metamorphism – Rock changes by contact with intrusive magma; localized effect.
2. Regional/Dynamic metamorphism – Result of pressure from internal movements compressing rocks over large areas.
3. Thermal/dynamic metamorphism – Combination of heat and pressure changing rock form.

Types of Igneous Rocks

ROCK FORMING MINERALS

Common minerals making up a large percentage of the earth’s crust are silicates.

1. Feldspar – Most abundant mineral in rocks. Two varieties:
   1. Orthoclase feldspar (KAlSi₃O₈) – Potassium, Aluminum, Silicate compound.
   2. Plagioclase feldspar (NaAlSi₃O₈ or CaAl₂Si₂O₈) – Contains sodium or calcium instead of potassium.
2. Quartz – Second most abundant mineral. Chemical formula SiO₂. Hardest common mineral, specific gravity 2.7, no cleavage. Used in making concrete, glass, and as a semi-precious stone.
3. Mica – Composed of elements like aluminum, silicon, oxygen, iron, magnesium, hydrogen, or potassium. Flat, shiny rock found in granite, gneiss, or schist.
4. Carbonates – e.g., CaCO₃ found in limestone and marble (Dolomite).
5. Hornblende – Dark crystalline mass associated with igneous and metamorphic rocks, composed of calcium, iron, and magnesium silicate.
6. Magnetite – Magnetic iron oxide Fe₃O₄.

ROCK CYCLE

The rock cycle describes how rocks change from one type to another repeatedly over time.

1. Igneous rocks form by cooling and solidification of magma or lava.
2. Igneous rocks can be weathered to form sediments, which compact to form sedimentary rocks.
3. Igneous or sedimentary rocks may undergo metamorphism to form metamorphic rocks.
4. Metamorphic rocks may weather and sediment to form sedimentary rocks.
5. Metamorphic rocks may undergo further metamorphism.
6. Sedimentary or metamorphic rocks may melt and cool to form igneous rocks.

This process is continuous and no rock type remains unchanged for long.

IMPORTANCE OF ROCKS

1. Rocks are important in soil formation essential for agriculture and plant growth.
2. Rocks store underground water in impermeable strata, which can emerge as springs.
3. Some rocks are used as fuel, e.g., coal and mineral oil.
4. Rocks are used in building and construction; limestone is used for cement manufacturing.
5. Salt extraction from rocks occurs in some places, e.g., Tunisia, Morocco.
6. Rocks provide minerals and ores like gold, lead, copper, tin, silver, zinc, aluminium, calcium, and manganese.
7. Some rocks attract tourists, generating foreign currency, e.g., coral reefs.
8. Marble is used for decorating floors of important buildings.

How does metamorphism manifest in rocks?

1. By foliation – alignment of mineral particles resembling stratifications, often wavy.
2. By cleavage – similar to foliation but cleavage direction may be independent of stratification.
3. By development of new materials not present in the original rock, including precious stones and ores.
4. By development of crystalline structure in originally amorphous rocks.
5. By change of crystalline rocks into amorphous rocks, e.g., marble from limestone, slate from shale.

B. ROCK CLASSIFICATION BY GEOLOGICAL AGE

Rocks can be classified by age:

1. Relative age – Determined by deposition sequence in sedimentary rocks (stratigraphy).
2. Absolute age – Determined by modern methods like radiometric dating.

1) RELATIVE AGE

Based on stratigraphy and paleontology (study of fossils).

Law of superposition: lower rock layers are older than upper layers.

Note: This is true where earth movements have not distorted rock layers.

What is a geological time scale?

A table dating past events of the earth’s history in chronological order.

Note: my = million years

• There are four eras: Cainozoic, Mesozoic, Palaeozoic, and Pre-Cambrian.
• Each era ends with geological evolution seen in rocks and fossils.
• Eras are divided into periods, which are subdivided into epochs, series, and formations.
• Processes in ERA, PERIOD, and EPOCH occur at some time and place worldwide.
• Pre-Cambrian takes about 70% of the geological time scale.
• Major geological events affect continents, including mountain building, volcanism, and glaciations.
• Geological time scale gives relative age but not exact years.

2) ABSOLUTE AGE OF ROCKS

Modern method of determining rock age is radiometric dating.

• Based on radioactive elements producing heat and daughter elements.
• Decay stops when daughter element is produced.
• Rate of decay from parent to daughter element is constant.
• New substances produced are used to calculate rock age.
• Time taken for half of parent element to decay is called half-life.

Example: 1 gm of uranium yields 1/760,000,000 of lead per year. Age of rock = (weight of lead / weight of uranium) × 7,600,000,000.

Example: Lead-Uranium ratio in uraninite crystal is 0.10. Age = 0.10 × 7,600,000,000 = 760,000,000 years.

This method provides the absolute age of rocks.

Importance of the Geological Time Scale

1. Depicts the age of rocks and when and how they were formed.
2. Helps understand formation of different landforms like mountains.
3. Allows prediction of crystal deformation and faulting.
4. Reveals life records of plants and animals, aiding understanding of their relationship with geological processes.

Weaknesses

1. Methods used to determine rock age were largely estimates due to limited instrument power.
2. Crustal deformation like overfolding, unconformities, and magma intrusions complicate age determination.
3. Local modifications exist in geological time charts.

C. ROCK CLASSIFICATION ACCORDING TO STRUCTURE

This involves description of rock properties such as colour, size, hardness, and permeability.

Permeability refers to the rate at which rocks can store or transmit water. It is divided into:

Primary Permeability or Porosity

Rocks with pore spaces; size and alignment determine water absorption. Greatest in coarse-grained rocks like gravels, sands, sandstone; lowest in fine-grained rocks like clays and granite. When pores are filled with water, rock is saturated. Permeable rocks storing water are called aquifers.

Secondary Permeability or Pervious

Rocks with joints and fissures allowing water flow. Most pervious rocks have widened joints, e.g., Carboniferous limestone, basalt.

Porous or pervious rocks allow water to leave surface dry, e.g., chalk and limestone regions. Impermeable rocks like granite neither absorb nor allow water to pass.

GEOMORPHOLOGY

Science dealing with development of landforms (relief features).

MAJOR PROCESSES WHICH SCULPT THE EARTH’S CRUST

Two major forces:

1. Endogenetic / internal processes
2. Exogenetic / external processes

I. ENDOGENETIC PROCESS

Forces beneath the earth’s surface, categorized as:

• a. Earth movement
• b. Vulcanism

II. EXOGENETIC PROCESS

External forces operating on the earth’s surface, including:

• Destructive process (denudation)
• Constructive process (aggradation/deposition)

A. INTERNAL FORCE

Earth movements

a) Lateral and Vertical movements

Diastrophism: Movement of the solid crust (lithosphere, made of upper mantle and crust). Movements can be gradual or rapid. Gradual movements occur in isostasy; rapid movements occur during earthquakes.

Effects of diastrophism

1. Tension: Forces moving away from each other, causing extension of the crust producing joints and normal faults.
2. Compression: Forces moving towards each other, causing contraction producing folds and thrust faults.
3. Shear: Forces acting parallel but in opposite directions causing lateral displacement.

Causes of diastrophism

1. Convectional current in the asthenosphere.
2. Isostasy – maintenance of equilibrium.
3. Expansion and contraction of rocks due to heating (e.g., intrusion of magma).

A) FAULTING

What is a fault?

• A fracture or crack in crustal rocks caused by tensional or compressional forces.
• Compression causes bending and breaking of rocks.
• Faulting results in vertical or horizontal displacement of crustal rocks (shear, normal faults).

Types of faults

1. Normal fault

Caused by tensional forces. Foot-wall is the upper rock face on the lower side of the fault, marked by a low angle (< 90°). Hanging wall is the upper rock face on the upper side of the fault, with a high angle (> 90°). Hanging wall moves down relative to foot-wall.

2. Thrust/Reversed fault

Opposite of normal fault. Caused by compressional forces. Hanging wall moves upward relative to foot-wall. Low angle of dip.

3. Tear / Strike / Transcurrent faults

Vertical fault plane caused by shear forces. Lateral displacement of crustal rocks in parallel but opposite directions.

Terminologies associated with faulting

1. Shift – Total movement of rock along the fault line, involving slip (movement along fault) and throw (vertical change in strata level).
2. Heave – Lateral rock block displacement.
3. Hade – Angle of inclination of the fault plane from vertical.
4. Hanging wall – Rock face on the upper side of the fault.
5. Foot wall – Rock face on the lower side of the fault.
6. Up throw – Mass of rock moved upward along the fault.
7. Down throw – Mass of rock moved downward along the fault.

Landforms resulting from faulting

1. Rift Valley
2. Block mountains
3. Plateaus and basins
4. Fault scarps
5. Tilt blocks
6. Depressions

1. RIFT VALLEY

An elongated trough bounded by in-facing fault scarps along more or less parallel faults.

Formation of a rift valley

Theories on formation of rift valley include:

1. Rift valley by tension
2. Rift valley by compression
3. Rift valley by plate tectonics

i. Rift valley by tension

• Rock layers are subjected to tension.
• Faults develop.

ii. Rift valley by compression

• Rock layers are subjected to compression.
• Side blocks are unstable and thrust upward over the middle block.
• Rift valley forms as sharp edges are eroded.

iii. Rift valley by plate tectonics

Rift valley forms from two blocks with a gap between where plates diverge and magma wells up, forming the rift valley.

Examples of Rift Valleys

1. The Great African Rift Valley
2. Rhine Rift Valley (between Vosges and Black Forest block mountains)
3. Mid-Atlantic Rift Valley
4. Red Sea Rift Valley
5. Jordan Rift Valley

Great African Rift Valley

• One of the most outstanding physical geographic features on the continent.
• Extends from the Middle East (Jordan River) southwards to River Zambezi (Mozambique), about 7200 km long, 5000 km in Africa.
• In East Africa, the Rift Valley splits into two branches:
  • Eastern branch marked by lakes: Turkana, Magadi, Victoria, Natron, Manyara, Nyasa.
  • Western branch marked by lakes: Tanganyika, Albert, Edward.
• The western branch disappears in Uganda and is not noticed in Sudan.

Characteristics of Great African Rift Valley

1. Bounded by fault scarps evident in parts of Kenya and northern Tanzania (Manyara).
2. Floor is almost flat with some features higher than sea level like Ruwenzori Mountains (~5000 m).
3. Deep depressions like Lake Tanganyika (650 m below sea level), second deepest lake in the world.
4. Lakes are narrow and deep, following fault shapes.
5. Width varies from 50 km to 100 km.
6. Sides have highlands and volcanic mountains.

2. BLOCK MOUNTAIN (HORSTS)

Local or isolated landforms elevated above the general land level. Flat-topped with steep sides, common in fault areas.

Formation

Two theories:

1. By tension
2. By compression

i. By Tension

1. Rock layers subjected to tension.
2. Development of parallel faults.
3. Side blocks subside leaving middle block higher.

ii. By Compression

1. Rock layers subjected to compression.
2. Development of parallel faults.
3. Middle block squeezed up; side blocks thrust upward.

Examples of Block Mountains

1. Uluguru Mountains
2. Usambara Mountains
3. Rwenzori Mountains
4. Black Forest and Vosges Mountains
5. Sinai Mountains

FAULT SCARPS (Escarpments)

Steep slopes where land falls abruptly from higher to lower levels caused by vertical crustal movements along fault lines. Can be caused by tension or compression but modified by denudation.

Example: Chunya, Tanzania.

A fault scarp across a river results in waterfalls.

TILT BLOCKS

Landscape of angular ridges and depressions formed by series of tilted fault blocks.

Examples

• U.S.A Rocky Mountains
• Somali

PLATEAU

Extensive raised land with steep sides (table land). Example: East Africa generally is a plateau.

BASINS

Downwarping (sagging) areas, e.g., Lake Victoria, Lake Kyoga, Great Basin in Nevada, Zaire Basin.

Study Question

With examples, discuss the effects of faulting in East Africa.

Fault is a fracture or crack on crustal rock caused by tensional or compressional forces. Faulting results in vertical or horizontal displacement.

Positive effects

1. Formation of rift valleys (e.g., East African Rift Valley), block mountains (Uluguru), plateaus and basins (Lake Victoria), escarpments (Chunya/Kalambo Falls in Zambia).
2. Formation of waterfalls where fault scarps cross rivers.
3. River course changes due to faulting.
4. Development of rectangular drainage patterns.
5. Occurrence of hot springs (e.g., Mbeya, Tanzania).
6. Displacement of features (e.g., San Andreas fault).
7. Leads to earthquakes.

b) FOLDING

Compression forces cause folding of rock layers (strata). Degree of folding depends on force intensity and rock nature.

Types of folds

1. Simple fold – Symmetrical fold with equal limbs; upthrow called anticline, downthrow called syncline.
2. Asymmetrical fold – One limb longer and gentler, the other shorter and steeper.
3. Overfold – One limb pushed over the other.
4. Recumbent fold – One limb almost inverted over the other, nearly horizontal.
5. Overthrust (napped) fold – Very high pressure causes fracture and one limb pushed over the other (e.g., NW Highlands of Scotland).

Effect of folding on the earth’s crust

Formation of fold mountains with anticlines and synclines (geosynclines).

Fold Mountains

Categorized into old and young fold mountains.

• Old fold mountains formed during Pre-Cambrian era, e.g., Appalachian, Cape Ranges, Ural Mountains.
• Young fold mountains recently formed, e.g., Rocky, Andes, Atlas, Alps, Himalayas. Highest peaks like Mount Everest (over 8000 m).

Global distribution of Fold Mountains

Fold mountains are distributed along continental margins where plates collided. Young fold mountains form alpine chains, active mountain-making belts formed during Cainozoic era.

1. Circum-Pacific Belt: Rings the Pacific Ocean, includes Andes and Cordilleran ranges in Americas, island arcs in Aleutians, Japan, Philippines.
2. Eurasian-Indonesian Belt: From Atlas Mountains in North Africa through Near East, Iran, Himalayas, to Southeast Asia.

4.4 THE IMPACT OF EARTHQUAKES

• Shaking of the ground due to sudden vibrations.
• Caused by volcanic eruptions or rupture and sudden movements of strained rocks.
• Duration is short, rarely exceeding 5 minutes.

Causes of earthquakes

1. Diastrophic movement: Movement of tectonic plates sliding past each other.
2. Volcanism: Intrusion or extraction between crustal rocks causing vibrations.
3. Human activities: Explosions (e.g., atomic bombs), large transportation, dynamites.

Major cause is diastrophic movement explained by the theory of elastic rebound. Compression forces bend crustal rocks, building strain until breakage releases energy causing seismic waves and shaking.

The origin point of an earthquake is called the focus; the point on the surface vertically above is the epicenter, where effects are greatest.

Types of seismic waves

Two main types:

1. Body waves
2. Surface waves

i) Body waves

ii) Surface waves

Travel through surface rocks and include:

a) Love (L) waves – Move side to side at right angles to wave direction.

b) Rayleigh (R) waves – Move in vertical circular motion, similar to sea waves, moving up and down.

Energy moves from one point to the next.

THE STRUCTURE OF THE EARTH SHOWING PATHS OF EARTHQUAKE WAVES

MEASUREMENT OF SEISMIC WAVES

1. Magnitude: Size of quake measured on Richter scale (0–8.9), indicating total energy released.
2. Intensity: Effect/damage on surface measured on Mercalli scale (1–12), where 1 is smallest effect and 12 is most catastrophic.

Global distribution of earthquakes

Earthquakes occur mainly in narrow belts marking tectonic plate boundaries:

1. Mid-ocean ridges
2. Ocean deeps and volcanic islands
3. Regions of crustal compression

Major earthquakes caused by tectonic plate movements, e.g., North American and Pacific Plates along San Andreas Fault in California.

In Africa, earthquakes mainly occur in the Great Rift Valley region of East Africa and parts of Northwest Africa. Most are mild, but serious ones occurred in El Asnam (1954) and Agadir (1960).

THE MAJOR EARTHQUAKE AND VOLCANIC BELTS ON THE WORLD

Effects of Earthquakes

1. Destruction of life (e.g., Iran, Morocco, Chile).
2. Destruction of property and infrastructure.
3. Causes faulting and joints (e.g., San Andreas fault).
4. Displacement of crustal rocks vertically or laterally.
5. Causes landslides blocking transport or rivers.
6. Devastation especially in cities.
7. Tsunamis caused by seismic waves in oceans.

Precautions against Earthquake

1. Discourage settlement in earthquake-prone areas.
2. Run to open spaces during earthquakes.
3. Build shock-absorbent houses.
4. Avoid tall buildings in earthquake zones.
5. Avoid use of explosives like atomic or nuclear bombs.
6. Seismologists should detect and inform people about earthquakes.

B. EXOGENETIC FORCES WHICH SCULPT THE SURFACE OF THE EARTH

External forces operating on the earth’s surface include:

a) Denudation

Destructive forces lowering the earth’s surface:

• Weathering / mass wasting
• Erosion
• Transportation

b) Deposition

Constructive process raising the land.

4.5 VULCANICITY (VULCANISM)

• Process where molten (magma) material from the earth’s interior is injected between crustal layers or ejected on the surface.
• Material can be gaseous, liquid, or solid.
• Intrusive magma forms intrusive igneous rocks; ejected magma forms extrusive igneous rocks.
• Shape of features depends on nature and strength or weakness of bedrock.

Vulcanicity includes extrusive and intrusive features; volcanicity is the surface manifestation of vulcanicity.

Causes of Volcanism

1. Intensive Pressure: High pressure finds lines of weakness on tectonic plate boundaries, causing high temperature and release of molten material.

Intrusive features of vulcanism include dykes, sills, lacoliths, batholiths, lopoliths, phacoliths, and minor features. Shape depends on bedrock nature.

• Fluid magma moves far forming linear features.
• Thick, viscous magma accumulates and solidifies forming features.

1. Dykes

Intrusion of magma solidifying vertically or inclined across bedding rock strata. Pillar-like structures. Exposed by denudation; harder dykes form ridges, softer dykes form depressions.

Examples: Howick Falls (River Mgeni, South Africa), dyke ridges in Kaap Valley, Lake Turkana.

2. Sill

Horizontal sheet of intrusive rock where magma solidified between rock layers. Magma is fluid and moves far. Can cause waterfalls if crossing rivers.

Examples: Buttes with sill capping in Cape Province, South Africa; Kinkon Falls in Guinea.

3. Lacolith

Dome-shaped intrusive magma caused by viscous magma pushing overlying rock layers upward. Near surface.

Examples: Fonjay Massif and Ambereny Massif in Madagascar, Henry Mountains in Utah.

4. Batholiths

Very large masses of igneous rock formed deep in the crust (plutonic). Examples: Granite batholiths in England, Tanzania, Zambia, Ghana, Uganda.

5. Lopolith

Saucer-like intrusion formed by sagging. Large saucer-shaped intrusion may cause sinking. Upturned edges form scarps. Examples: Bushveld Basin (South Africa), Sierra Leone Peninsula.

6. Phacolith

Intrusive solidification of magma on anticlines and synclines of rock strata.

Extrusive features of volcanism

• Magma reaching the surface is called lava.
• Lava can erupt through fissures (linear) or vents (central openings).

Types of materials during eruption

1. Gaseous materials – Gases emitted include sulphur compounds, hydrogen, carbon dioxide.
2. Liquid – Lava, which can be mobile (flows fast) or viscous (accumulates and flows slowly).
3. Solid – Fragments of country rock like scoria, pumice, cinder (lapilli), volcanic bombs.

Explosive materials spread far and build landforms; slow lava builds volcanoes.

VOLCANO

A cone-shaped or circular feature built by volcanic activity.

Types of Volcano

• Active Volcano
• Dormant Volcano
• Extinct Volcano

Volcanic Landforms

Ash and Cinder cone (scoria cones)

• Built by pyroclastic material solidifying around the vent.
• Formed by vent eruption; cinders are small round particles from earth’s interior or solidified crust.
• Examples: South of Lake Turkana (Kenya), Likaiyu, Teleke, Nabuyatom.

1. Lava Cones

1. Acidic Lava – Viscous, accumulates around vent forming steep-sided cones. Example: Mount Pelee, Martinique, West Indies.
2. Basic Lava Cone – Very fluid, spreads far forming gentle sloping cones. Example: Mauna Loa, Hawaii.

iii. Composite cone

Formed by alternating layers of ash and lava. Forms high composite peaks, e.g., Mount Cameroon, Mount Kilimanjaro, Mount Vesuvius.

iv. Plug Volcano (Volcanic neck)

Cylinder-shaped feature occupying the vent of dormant or extinct volcano. Solidifies and blocks the vent.

Caldera (Basal Wreck)

Large shallow cavity (depression) on top of a volcano formed by violent eruptions removing the former top.

Water can accumulate forming caldera lakes, e.g., Lake Toba (Indonesia), Crater Lake (USA), Bosumtwi (Ghana), Ngorongoro, Mount Meru, Longonot (Kenya).

Lava Plateau

Result from fissure eruptions where lava spreads over wide areas and solidifies forming high-level lava plateaus (basalt plateaus). Examples: Sahara (Algeria, Morocco), Drakensberg Plateau (South Africa), Snake Plateau (North America), Deccan Plateau (India).

Other minor features associated with volcanism

• Hot springs – Natural outflow of hot water, e.g., Mbeya, Arusha, Mara (Majimoto).
• Geysers – Superheated water and steam drawn out explosively and periodically.
• Solfatara – Volcano releasing steam and gases, mainly sulphur.
• Fumaroles – Emission of steam.
• Mofatte – Emission of carbon dioxide.

Stages / life cycle of Volcano

1. Active volcano – Erupts periodically in recent times. Examples: Oldonyo (Tanzania), Mufungiro (Uganda), Vesuvius (Italy).
2. Dormant volcano – Has erupted before but inactive for a long time; may erupt again. Examples: Kilimanjaro, Mount Meru.
3. Extinct volcano – Dormant with no signs of eruption.

Global Distribution of Volcanoes and Lava Plateaus

Volcanicity occurs where the earth’s crust is disturbed, especially along tectonic plate boundaries, zones of divergence, fault regions like the East African Rift Valley, and fractured crustal rocks.

Economic importance of Vulcanicity

Positive importance

1. Volcanic soils are fertile and important for agriculture (e.g., Deccan Plateau, India).
2. Volcanic activity forms precious stones and minerals (e.g., diamonds in Kimberley, copper in USA).
3. Hot springs provide heating and hot water (e.g., New Zealand, Iceland).
4. Volcanic eruptions provide geothermal power (e.g., Kenya, Ethiopia).
5. Volcanic features attract tourists, boosting tourism industry.
6. Calderas form lakes useful for fishing and irrigation.
7. Volcanic mountains are tourist attractions.

Negative significance

1. Volcanism causes migration from affected areas.
2. Leads to loss of life and destruction of property.
3. Causes environmental degradation.
4. Earthquakes occur due to magma movement through fault lines.
5. Volcanic features create barriers, complicating construction.
6. Rugged volcanic landscapes discourage agriculture and settlement.

4.6 DENUDATION AND DEPOSITION

Denudation refers to all processes involving breaking, wearing away, and lowering of the earth’s surface, including weathering, mass wasting, erosion, and transport of materials.

Deposition is the laying down or release of rock particles on the surface by water, ice, living organisms, wind, or evaporation.

• Water produces features like flood plains, natural levees, alluvial fans, beaches.
• Ice produces features like outwash plains, clay plains, moraines, eskers, drumlins, kames.
• Living organisms produce features like coral reefs.
• Wind produces features like loess plains and sand dunes (barchans and seifs).
• Evaporation and precipitation produce features like salt deposits.

I. WEATHERING

Physical disintegration and chemical decomposition of rocks in situ when exposed to weather. Does not involve transport.

Types of weathering

Two types:

i. Mechanical / Physical weathering

Breaking down of rocks into small particles without chemical change.

Mechanisms

1. Temperature change: Occurs mostly in hot deserts with large diurnal temperature range causing expansion and contraction, leading to cracks (exfoliation). Screes collect at the foot of hills.
2. Frost action: Dominant in temperate and mountainous regions; freezing and melting cycles expand cracks, leading to rock disintegration.
3. Action of living organisms (biotic/biological): Plant roots penetrate and crack rocks; animals burrow loosening soil and rocks.
4. Alternating wetting and drying: Causes contraction and expansion weakening rocks, common in coastal areas.
5. Salt crystallization: Salt crystals expand cracks during evaporation and contraction cycles.
6. Pressure release or unloading: Removal of overlying materials releases pressure causing rock rebound and joint formation.

ii. Chemical weathering

Chemical decomposition of rocks when exposed to weather, weakening them.

Processes include oxidation, carbonation, hydration, hydrolysis, and solution.

i. Oxidation

Addition of oxygen to rock minerals, weakening them. Common in iron-rich clays where rusting occurs.

ii. Carbonation

Reaction of weak carbonic acid with calcium carbonate forming soluble calcium bicarbonate removed by groundwater, weakening rocks like limestone.

iii. Hydration

Rocks absorb water molecules causing expansion and internal stress, leading to fractures and weakening.

iv. Hydrolysis

Water reacts chemically with minerals forming new compounds like clay minerals, silica, and potassium carbonate, which leach out, weakening rocks.

v. Solution

Dissolution of salts like NaCl in water forming salt solutions, leading to rock disintegration.

vi. Biotic weathering

Caused by plant roots secreting acids and decaying organic matter producing acids that disintegrate rocks.

FACTORS WHICH INFLUENCE THE RATE OF WEATHERING

1. The nature of the rock – soft or hard.
2. Mineral composition – Stable minerals like quartz resist weathering; unstable minerals like basalt weather quickly.
3. Plane of weakness or rock structure – Joints and cracks allow acids to penetrate and accelerate weathering.
4. Colour – Dark minerals heat faster, increasing weathering rate by temperature change.