Internal land-forming processes — folding and faulting (KCSE Geography Form 2)

TL;DR

Internal land-forming processes, like folding and faulting, are driven by forces from within the Earth, shaping its surface. Folding occurs when compressional forces bend rock layers, creating features like anticlines and synclines. Faulting happens when tensional or compressional forces cause rocks to fracture and move along a crack, forming features like rift valleys and block mountains.

1. The Mental Model

Imagine the Earth's crust as a giant, flexible but sometimes brittle skin. When powerful forces push or pull this skin, it either bends like a rug (folding) or cracks and shifts (faulting), creating mountains, valleys, and other landforms.

2. The Core Material

What are Internal Land-forming Processes?

These are processes that originate from within the Earth's crust and mantle. They are responsible for creating major relief features on the Earth's surface, such as mountains, plateaus, and rift valleys. The main forces involved are tectonic forces, which are generated by the movement of the Earth's tectonic plates.

Folding

Folding is the bending or crumpling of rock layers due to compressional forces. These forces push rock layers together, causing them to buckle upwards or downwards. This usually happens in areas where sedimentary rocks, which are often layered, are subjected to immense pressure.

Key Terms in Folding:

• Anticline: An upward fold in rock layers, resembling an arch. The oldest rocks are found at the core of the anticline.
• Syncline: A downward fold in rock layers, resembling a trough. The youngest rocks are found at the core of the syncline.
• Limb: The side of a fold. Each fold (anticline or syncline) has two limbs.
• Crest: The highest point of an anticline.
• Trough: The lowest point of a syncline.
• Axial Plane: An imaginary plane that divides a fold as symmetrically as possible.
• Axis: The line formed by the intersection of the axial plane with the folded rock layers.

Types of Folds:

The type of fold depends on the intensity and direction of the compressional forces:
* Simple Symmetrical Fold: Limbs dip at roughly the same angle away from the axial plane.
* Asymmetrical Fold: One limb is steeper than the other.
* Overfold: One limb is pushed over the other, making the axial plane inclined.
* Recumbent Fold: The fold is so intense that the axial plane is almost horizontal, and the limbs lie nearly parallel to each other.
* Nappe/Overthrust Fold: A recumbent fold that has been fractured and moved horizontally over other rock layers.

Faulting

Faulting is the fracturing and displacement of rock layers along a crack or zone of weakness called a fault line. This occurs when the stress on the rocks exceeds their strength, causing them to break. Faulting can be caused by both tensional (pulling apart) and compressional (pushing together) forces.

Key Terms in Faulting:

• Fault Plane: The surface along which movement occurs.
• Fault Line: The line visible on the Earth's surface where the fault plane intersects the ground.
• Hanging Wall: The block of rock that lies above an inclined fault plane.
• Footwall: The block of rock that lies below an inclined fault plane.
• Throw: The vertical displacement along a fault.
• Heave: The horizontal displacement along a fault.

Types of Faults:

The type of fault depends on the direction of movement of the rock blocks relative to each other:
* Normal Fault: Occurs due to tensional forces (pulling apart). The hanging wall moves down relative to the footwall. This lengthens the crust.
* Reverse Fault: Occurs due to compressional forces (pushing together). The hanging wall moves up relative to the footwall. This shortens the crust.
* Thrust Fault: A type of reverse fault where the fault plane has a very low angle (less than 45 degrees). The hanging wall is pushed a significant distance over the footwall.
* Strike-Slip Fault (Transform Fault): Occurs due to shearing forces (sliding past each other horizontally). The blocks move horizontally past each other, with little to no vertical displacement.

Landforms Resulting from Faulting:

• Rift Valley: A long, narrow depression formed when a block of land between two parallel normal faults sinks downwards due to tensional forces. The East African Rift Valley is a classic example.
• Block Mountains (Horsts): Uplifted blocks of land found between two parallel normal faults, often flanking a rift valley.
• Escarpment (Fault Scarp): A steep slope or cliff formed by faulting, representing the exposed fault plane.

Relationship between Folding and Faulting

Folding and faulting are both results of tectonic forces. Often, they occur in the same regions. When rocks are subjected to stress, they may first fold. If the stress continues to increase beyond the rock's ability to bend, or if the rocks are brittle, they will fracture and fault.

graph TD
    A[Internal Earth Forces] --> B{Stress on Rock Layers}
    B --> C{Rock Type & Intensity of Stress}
    C --> D{Ductile Rocks & Moderate Compression}
    C --> E{Brittle Rocks & High Stress (Compression/Tension/Shear)}
    D --> F[Folding]
    F --> F1(Anticlines)
    F --> F2(Synclines)
    E --> G[Faulting]
    G --> G1(Normal Faults - Tension)
    G --> G2(Reverse Faults - Compression)
    G --> G3(Strike-Slip Faults - Shear)
    G1 --> H1(Rift Valleys)
    G1 --> H2(Block Mountains/Horsts)
    G2 --> H3(Thrust Mountains)
    G3 --> H4(Offset Features)

3. Worked Example

Let's consider the formation of the Great Rift Valley in East Africa.

Observation: You see a very long, deep, and relatively narrow valley stretching for thousands of kilometres, flanked by steep cliffs and higher land on either side.

Analysis using Folding and Faulting concepts:
1. Initial State: The Earth's crust in this region was subjected to immense tensional forces. This means the tectonic plates were pulling apart.
2. Rock Response: The rocks in the crust, being relatively brittle, could not simply stretch. Instead, they fractured.
3. Fault Formation: Two major parallel normal faults developed.
4. Block Movement: As the tensional forces continued, the central block of land between these two normal faults began to sink downwards relative to the blocks on either side.
5. Landform Creation: This downward movement of the central block created the rift valley (a graben). The uplifted blocks on either side formed block mountains or horsts, and the steep sides of the valley are fault scarps.

This example clearly shows how tensional forces lead to normal faulting, which in turn creates distinctive landforms like rift valleys and block mountains.

4. Key Takeaways

• Internal land-forming processes are driven by forces from within the Earth, primarily tectonic plate movements.
• Folding is the bending of rock layers due to compressional forces, forming anticlines (upfolds) and synclines (downfolds).
• Faulting is the fracturing and displacement of rock layers along a fault plane, caused by tensional, compressional, or shearing forces.
• Normal faults result from tension, reverse faults from compression, and strike-slip faults from shearing.
• Major landforms like rift valleys and block mountains are created by faulting, especially normal faulting.
• The type of landform (fold or fault) depends on the type of stress, its intensity, and the nature of the rock (ductile or brittle).

Common Mistakes to Avoid:
- Confusing anticlines with synclines; remember "anti" often means up or against, so anticlines are upfolds.
- Mixing up the forces: tension causes normal faults, compression causes reverse faults.
- Thinking all mountains are formed by folding; many are formed by faulting or a combination.
- Forgetting that folding and faulting are often linked and can occur in the same region.

5. Now Try It

Imagine you are a geologist studying a new mountain range. You observe that the rock layers are severely bent and overturned in some places, and there are also large cracks where sections of rock have moved horizontally past each other.

Task:
1. Identify the primary internal land-forming processes that have likely shaped this mountain range.
2. Describe the types of folds and faults you would expect to find based on your observations.
3. Explain the types of forces that would have been responsible for these features.

What success looks like: You correctly identify folding and strike-slip faulting, describe features like recumbent folds and strike-slip faults, and link them to compressional and shearing forces respectively.