diastrophism

Diastrophism

Definition

Diastrophism, also known as diastrophic forces, encompasses all processes that move, elevate, or build up portions of the earth's crust through orogenic and epirogenic processes.

Epirogenic Processes

Nature of movement

• Vertical forces are responsible for upward (emergence) or downward (submergence) movements.
• The vertical movement occurs along a radius from the earth's center to its surface (cymatogenic).

Effects

• Upward Movement (Emergence): Uplifts continental parts or coastal areas.
• Downward Movement (Submergence): Submerges parts of landmasses or coastal areas.

Orogenic Processes

Nature of movement

Endogenetic forces act horizontally, involving tensional and compressional forces.

Types of forces

• Tensional Forces (Divergent):
  • Operate in opposite directions.
  • Cause rupture, cracks, fractures, and faults in the earth's crust.
• Compressional Forces (Convergent):
  • Operate face-to-face, bending the crust.
  • Lead to crustal warping and the formation of folds.

Effects

• Upwarping: Bending upward due to compressional forces.
• Down-warping: Bending downward due to compressional forces.

Effects of earth movement

The effects of diastrophic processes can be categorized into two main groups:

1. Faulting
2. Folding

Faulting

Definition

Faulting is the process involving the breaking or fracturing of crustal rocks due to forces resulting from earth movements, forming faults. The movement involved in faulting can occur vertically, horizontally, or in any direction. In faulting, vertical displacement of rock blocks occurs, leading to landscape changes such as uplift, depression, and horizontal displacement of large crustal blocks.

Faults

Faults are fractures in the crustal rocks where displacement occurs along a fault plane. Various parts and features of faults include:

1. Fault Plane: The surface along which rock blocks are displaced due to tensional or compressional forces.
2. Upthrown Side: The uppermost block of a fault.
3. Downthrown Side: The lowermost block of a fault.
4. Fault Dip: The angle between the fault plane and the horizontal plane.
5. Hanging Wall: The upper wall of a fault.
6. Foot Wall: The lower wall of a fault.
7. Fault Scarp: A steep, wall-like slope caused by the faulting of crustal rocks.

Types of faults

1. Normal Faults

   • Formed due to displacement of rock blocks in opposite directions.
   • Result from tension forces (pulling apart of rocks) when the crust is stretched.
   • Characterized by an inclined fault plane and the direction of downthrow, which can be either right or left.
   • The middle part may rise while the hanging wall sinks.

2. Reverse Faults

   • Formed due to compressional forces.
   • Occurs when rock beds on one side of the fault plane are thrust over the other side.
   • The fault plane is typically inclined at an angle between 40° and horizontal.

3. Transform (Tear) Faults

   • Also known as strike, slide, shear, wrench, or transcurrent faults.
   • Formed when a vertical fracture is displaced parallel in opposing directions along the fault line.
   • Example: The San Andreas Fault in California.

4. Overthrust Faults

   • Formed when the top part of a fault overrides the bottom part along the fault plane due to compressional forces.

5. Monocline Faults

   • A tensional fracture where the strata bend, closely related to normal faults but involving horizontal strata.

Landforms resulting from faulting

Faulting influences the landscape through direct and indirect effects, resulting in the formation of distinct landforms.

1. Direct landforms
2. Indirect landforms

Direct landforms

These landforms are primarily shaped by faulting processes and appear as abrupt features. Over time, erosion may modify their appearance. The following are direct effects of faulting:

Step faulting

1. Occurs when several parallel faults form, creating a series of steps.
2. These steps appear on either one side or both sides of a rift valley.

Fault block (tilted block)

1. A block-like structure with inclined strata caused by uneven uplift on one side.
2. If the block is tilted, it is referred to as a tilt-block.
3. Example: Afar Triangle in Ethiopia and the basin and range country in the USA.

Horst mountains

1. A horst is a table-like mountain formed by the uplift of a rock block during faulting.
2. Block Mountain: Uplands bordered by faults on one or more sides.
   • Can form when land subsides between two faults or a block remains elevated while adjacent sides drop.
   • Example: Drakensberg Range in South Africa.

Fault scarp landform

1. An escarpment with a steep slope on one side and a gentle slope on the other.
2. Created by vertical movements along a fault line.

Rift valley

1. Formed when the Earth's crust splits apart due to tectonic activity.
2. Characteristics:
   • Narrow, with steep sides and a flat floor.
   • Distinguished from river and glacial valleys as they result from tectonic forces, not erosion.
   • Rift valleys are also known as grabens (a smaller rift valley).
3. Formation Process:
   • Caused by plates moving apart, leading to the crust separating or rifting.
4. Examples:
   • East African Rift Valley
   • Baikal Rift Valley
   • Many underwater rift valleys are found along mid-ocean ridges.

Indirect effects of faulting

Indirect effects result from the landforms formed through faulting processes. These effects influence the landscape and hydrological systems in various ways.

Waterfalls

1. Formation: Vertical faulting across a river valley may create a waterfall (cataract).
2. Mechanism:
   • Waterfalls form as streams flow from soft rock to hard rock.
   • The soft rock erodes, leaving a hard ledge over which the stream falls.
3. Fall Line: An imaginary line along which parallel rivers plunge as they flow from uplands to lowlands.

River offset

Formation: Tear faulting with horizontal movement across a river causes the river to offset at the point it crosses the fault.

Lake formation

1. Formation: Rift faulting that creates enclosed basins may result in lake formation if rivers flow into these basins.
2. Examples:
   • Lake Tanganyika
   • Lake Nyasa
   • Lake Rudolf

River flow

Impact: Rivers may follow straight natural fault valleys, especially in areas with tilt-block faulting or differential movements that raise some parts while lowering others.

River reverse direction

1. Cause: Crustal tilting or upwarping across a river valley may force the river to reverse its direction if it cannot maintain its original flow.
2. Examples:
   • Kafu River and Lake Kyoga (Uganda)
   • Katonga River and Lake Victoria
   • Kagera River and Lake Victoria (Tanzania)

Occurrence of springs

Formation: Faulting creates areas of weakness where water emerges from an aquifer, leading to spring formation.

Obsequent fault line scarp

Description: A scarp that faces the opposite direction of the original fault line scarp.

Fault-guided valley

1. Formation: Faulting shatters and crushes rocks, making them more easily eroded compared to surrounding areas.
2. Example: Santa River in Guinea.

Folding

Folding is the process involving the wrinkling of the earth's crust, forming various geological structures such as folds and fold mountains. It occurs due to tremendous compressional forces resulting from the movement of tectonic plates towards each other.

Key concepts in folding

• Anticline: Upfolded rock beds forming an arch-like structure.
• Syncline: Downfolded rock beds forming a trough-like structure.
• Synclinorium: An extensive syncline that includes numerous minor anticlines and synclines.
• Limbs: The sides of a fold.
• Axial Plane: The plane that bisects the angle between the two limbs of an anticline or the middle limb of a syncline.

Types of folds

1. Simple Fold (Symmetrical Fold)

   • Formed when strata bend into upfolds (anticlines) and downfolds (synclines).
   • Limbs are similar in slope.
   • Caused by compressional forces.

2. Asymmetrical Fold

   • Formed when continued compression transforms a simple fold into an asymmetrical fold.
   • One limb is steeper than the other.
   • Z Folds: Axial plane rotated clockwise.
   • S Folds: Axial plane rotated anticlockwise.

3. Over Fold (Overturned Fold)

   • Formed when the axial plane inclines, causing both limbs to dip in the same direction, but not equally.
   • One limb is overturned or pushed over the other.

4. Recumbent Fold

   • Formed when an overfold is further compressed, causing one limb to lie horizontally over the other.
   • Characterized by an essentially horizontal axial plane.

5. Over Thrust Fold (Nappe Fold)

   • Formed under extreme pressure, causing one limb of the fold to thrust forward over the other.
   • Found in intensely deformed mountain belts, often at compressive tectonic plate boundaries.

6. Geosyncline

   • A large downfold caused by subsidence of the earth's crust due to the weight of deposited materials or warping movements.

Formation of folding

1. Compressional Forces: Tangential forces cause horizontal movements, leading to wave-like bends in the crustal rocks.
2. Endogenetic Forces: Originate deep within the earth to produce folding.

Mountain building and classification

Mountain building, or mountain formation, refers to the geological processes that result in the formation of mountains. These processes are driven by endogenic forces, such as epirogenic forces, orogenic forces, and volcanism, which produce various types of mountains.

Definition of a mountain

A mountain is a large natural elevation of the earth's surface that rises abruptly from its surrounding level. For example, Mt. Kilimanjaro rises to approximately 5,895 meters.

Classification of mountains

Based on height

1. Low Mountains: 700–1000 meters.
2. Middle Mountains: 1000–1500 meters.
3. High Mountains: 2000 meters and above.

Based on location

1. Coastal Mountains: Found near coastlines.
   • Examples: Appalachians, Rockies, Alps.
2. Inland Mountains: Formed on adjacent land masses.
   • Examples: Ural Mountains (Russia), Vosges, and Black Forest Mountains.
3. Oceanic Mountains: Found below the water surface, such as mid-oceanic ridges.

Based on mode of origin

1. Tectonic Mountains: Formed due to tectonic forces.
2. Fold Mountains: Formed due to compressional forces; classified into:
   • Young Fold Mountains: Recently formed (e.g., Himalayas).
   • Mature Fold Mountains: Moderately aged and eroded.
   • Old Fold Mountains: Highly eroded and older (e.g., Appalachians).
3. Block Mountains: Formed due to tensional forces, causing the uplifting of blocks.
4. Dome Mountains: Originated by magma intrusion and upwarping of the crust.
5. Mountains of Accumulation (Volcanic Mountains): Formed due to the accumulation of volcanic materials.

Based on period of origin

1. Pre-Cambrian Mountains: Formed during the Pre-Cambrian era (3 billion years ago).
   • Examples: Canadian Shield, Akwapim Hills (Ghana).
2. Caledonian Mountains: Formed during the Paleozoic era (500–350 million years ago).
   • Examples: Scottish Highlands, Scandinavian Mountains.
3. Hercynian Mountains: Formed between the late Paleozoic and Mesozoic eras (350–240 million years ago).
   • Examples: Cape Ranges (South Africa), Appalachian Mountains (USA), Ural Mountains (Russia), Welsh Mountains (Britain).
4. Alpine Mountains: Formed during the Cenozoic era (300 million years ago). These are the highest and most impressive mountains.
   • Examples: Atlas Mountains, Alps, Rockies, Himalayas, Andes Mountains.

Theories of mountain building

Mountain formation is explained through various theories. Below are the main theories that describe the processes responsible for mountain building:

1. Plate tectonic theory
2. Continental drift theory
3. Contraction theory
4. Convectional current theory
5. Geosyncline theory
6. Denudation theory

Plate tectonic theory

This theory explains the formation of mountains due to the movement of tectonic plates at plate margins.

1. Most mountains form in zones where plates converge, diverge, or slide past each other.
2. Example: Fold Mountains, such as the Himalayas.

Continental drift theory

1. Proposes that the Indian continent drifted towards Eurasia, squeezing the crust and sediments in between to form Fold Mountains.
2. Evidence: Marine limestone rocks found at the summit of Mount Everest, indicating marine origins.

Contraction theory

1. Suggests that mountains were formed during the Earth's cooling process.
2. As molten rocks cooled, the surface rocks contracted faster than the interior, causing wrinkling of the surface to fit over the cooling, contracting interior.

Convectional current theory

1. States that horizontal movements of convectional currents in the mantle exert a fractional pull on crustal rocks.
2. When currents move towards each other, Fold Mountains form.

Geosyncline theory

1. Introduced by German geologist Kober.
2. Mountain ranges form as a result of the compression of geosynclines located at the margins of ancient rigid land masses (referred to as forelands).
3. Mountains formed through this process are known as border ranges.

Denudation theory

1. Explains that some mountains are formed when resistant rock masses remain standing after denudation erodes the surrounding softer rocks.
2. Example: Residual mountains like Sekenke Hill in Singida, Tanzania.

Fold Mountains

Fold Mountains are formed due to the folding of the Earth's crust under compressional forces.

Classification of Fold Mountains

1. Based on the Nature of Fold

   • Simple Fold Mountains:
     • Characterized by well-developed systems of anticlines (upfolds) and synclines (downfolds).
   • Complex Fold Mountains:
     • Have intricate structures with recumbent folds caused by powerful compressional forces.
   • Pricline (Centroline) Fold:
     • A small anticline where the limbs pitch steeply along the axis, resembling an elongated dome.

2. Based on the Period of Origin

   • Old Fold Mountains:
     • Formed before the Tertiary period, about 3,000 million years ago.
     • These mountains have undergone extensive erosion.
     • Examples: Aravallis, Caledonian, and Hercynian mountains.
   • New Fold Mountains:
     • Formed during the Tertiary period in the Cenozoic era, about 300 million years ago.
     • They are higher and more prominent.
     • Examples: Rockies, Andes, Alps, Himalayas. Fold mountains

Distribution of Fold Mountains

Old Fold Mountains

1. Formed during the Paleozoic and Mesozoic eras.
2. Many of these mountains are remnants or roots of ancient fold mountains.
3. Examples:
   • North America: Appalachian Mountains.
   • Europe-Asia: Ural Mountains.
   • Africa: Cape Ranges in South Africa.

Young Fold Mountains

1. Formed during the Cenozoic era and are primarily found along the margins of continents.
2. These mountains align in either a north-south or east-west direction.
3. Examples: Alpine chains lying along the continental margins.

Global distribution patterns of Young Fold Mountains

1. Circum-Pacific System

   • Encircles the Pacific Ocean on both North and South America.
   • Includes:
     • Andes (South America).
     • Rockies (North America).
     • Cordilleran Ranges.
   • In the western Pacific Ocean, the belt takes the form of island chains.
     • Examples: Japan and the Philippines.

2. Eurasian–Indonesian Belt

   • Extends from:
     • West: Atlas Mountains.
     • East: Iran, Himalayas, and Southeastern Asia.
   • Continues to Indonesia, where it meets the Circum-Pacific Belt.
   • Includes:
     • Alps.
     • Caucasus Mountains.
     • Himalayas.