Comprehensive Study Notes on Geomorphology, Rock Classification, and Weathering

Geomorphology and the Internal Structure of the Earth

Definition of Geomorphology: Derived from the three terms:
- Geo: Earth.
- Morphe: Shape or form.
- Logos: The study of.
- Full definition: Geomorphology is the study of Earth’s landforms, their origin, and the processes that created them.
The Structure of the Earth: The Earth is divided into four distinct sections based on thickness, texture, temperature, and composition:
- Crust: The outermost shell of the Earth.
- Mantle: The layer located beneath the crust.
- Outer Core: The liquid layer above the inner core.
- Inner Core: The solid center of the Earth.

Classification and Characteristics of Rock Families

Rocks are classified into three distinct groups or families based on their formation processes and characteristic features.

Igneous Rocks:
- Formation: These rocks are formed when magma (underground) or lava (on the surface) cools and solidifies.
- Examples:
  - Granite: Formed deep within the crust.
  - Dolerite: Forms closer to the Earth's surface.
  - Basalt: Forms from lava directly on the Earth's surface.
  - Kimberlite: Notable example found in South Africa.
- Characteristics:
  - Contains crystals of varying sizes depending on cooling speed.
  - Described as "massive rock" because it does not contain bedding planes.
  - Extremely difficult to weather.
  - Has the ability to crack and contains structural joints.
- Uses: Often contains precious metals and ores. Granite is ideal for building roads and kitchen countertops. Basalt weathers to form highly fertile soil.
- Landforms: Granite domes, tors, dolerite sills, ridges, and poorts.
- RSA Examples: Paarl Rock, Parys Granite, Karoo Dolerite, and Drakensberg Basalt.
Sedimentary Rocks:
- Formation: These are created when deposited particles are compacted and cemented into solid rock over time.
- Examples:
  - Sandstone: From sand-sized particles.
  - Shale: From smaller silt or clay particles.
  - Limestone: From the remains of shells.
  - Coal: From organic vegetable matter.
- Characteristics:
  - Arranged in layers or strata.
  - Separated by bedding planes between the layers.
  - Can contain fossils (preserving geological history).
  - Shale weathers easily.
  - Sandstone is porous, meaning it can hold water.
- Uses: Coal serves as a primary fossil fuel. Limestone is used extensively in the building industry.
- Landforms: Cuestas, hogsbacks, and caves.
- RSA Examples: Karoo Shale, Table Mountain Sandstone, Sterkfontein Dolomite (Limestone), and Witbank Coal.
Metamorphic Rocks:
- Formation: Created when existing igneous or sedimentary rocks are subjected to extreme heat or pressure, causing them to change form.
- Examples (Parent Rock $\rightarrow$ Metamorphic Rock):
  - Sandstone becomes Quartzite.
  - Shale becomes Slate.
  - Granite becomes Gneiss.
  - Coal becomes Anthracite.
  - Limestone becomes Marble.
- Characteristics:
  - Extremely difficult to weather.
  - Usually display attractive colors.
- Uses: Marble is used for monuments. Slate is used for flooring and roofing.
- RSA Examples: Magaliesberg Quartzite, Barberton Schist, and KZN Anthracite.

External Forces of Gradation and Landscape Development

Landscape development is primarily influenced by two factors:

Rock Type: Whether the rock is resistant or erodible.
External Agents: The specific way in which water, wind, ice, and biological influences break the rock down.

Exogenic Forces (External Forces):
- 1st Stage - Weathering: Materials constituting mountains, ridges, and heights are softened and broken down.
  - Definition: The disintegration and decomposition of rock in situ (in its original place). It takes place on the Earth's surface over hundreds of years. The end result is soil. There is no movement of the material during this process.
- 2nd Stage - Erosion (Degradation): Surface agents remove finer materials.
  - Definition: The removal and transportation of weathered material from one place to another by agents such as moving water, moving ice, and wind.
- 3rd Stage - Deposition (Aggradation): The eroded materials are laid down in a new location.
  - Definition: The process by which weathered and eroded material is deposited.
Key Lithification Processes:
- Compaction: The compression or "squishing" of sediment layers to form sedimentary rock.
- Cementation: The adherence or "sticking together" of sediments to form solid rock.

Chemical Weathering Processes

Chemical weathering involves the decomposition of a rock due to chemical changes, often involving reactions between rock minerals, water, and atmospheric gases. This process changes the rock's chemical composition.

Oxidation:
- Oxygen reacts with minerals in the rock.
- Iron specifically reacts to produce rust.
- This weakens the rock, making it less resistant to further weathering.
Solution and Carbonation:
- This is the most common chemical process.
- Minerals in a rock dissolve into water.
- Rainwater contains $\text{CO}_2$ in solution, which produces weak carbonic acid.
- This acid reacts to break down rock, especially those containing calcium carbonate.
- Feature: Crucial in the formation of underground solution caves.
Hydrolysis:
- The chemical reaction between water and mineral elements.
- Silicate minerals specifically weather by hydrolysis to form clay.
- Clay is easily washed away, leaving the rock unstable and crumbling.
Hydration:
- Water is absorbed into the rock and combines with the minerals.
- This causes the rock to swell and creates cracks, leading to the rock breaking apart.
- Rocks containing salt are particularly susceptible as they absorb water and expand.

Mechanical (Physical) Weathering Processes

Mechanical weathering involves the physical breakdown of rocks into smaller fragments without changing the rock's chemical composition.

Granular Disintegration:
- Occurs in rocks composed of coarse grains or crystals.
- Repeated heating and cooling causes individual grains to expand and contract at different rates.
- The absorption of moisture can also cause particles to come away from the main body of the rock.
Exfoliation:
- Caused by repeated daily (diurnal) heating and cooling.
- Process: Heating causes the surface of the rock to expand, while cooling causes it to contract.
- Cracks develop parallel to the rock surface, causing the outer layer to peel away like an onion.
- Features: Results in the formation of domes and tors.
Thermal Shattering (Freeze-Thaw Weathering):
- Occurs in rocks with joints, cracks, or crevices with limited vegetation.
- Condition: Temperatures must fluctuate around $0^{\circ}\text{C}$ .
- Process: During the daytime, water enters the joints. At night, the water freezes.
- Volume Expansion: Ice occupies approximately $9\%$ more volume than liquid water.
- This expansion exerts pressure that widens the joints. Repeated diurnal repetition leads to the shattering of the rock.
- Effects:
  - Steep Slopes: Results in block disintegration.
  - Gentle Slopes: Results in the formation of block fields.

Biological Weathering Processes

Biological weathering involves the breakdown of rocks through the activity of plants and animals.

Plants:
- Roots: Plant roots anchor into bedding planes or joints. As the roots grow and expand, they exert pressure that causes joints to widen and the rock to eventually split.
- Lichens and Mosses: These organisms produce chelating agents (organic acids) that chemically decompose the rocks they grow on.
Animals:
- Burrowing: Species that dig into the earth loosen joints and break apart rock fragments.
- Earthworms and Termites: These organisms loosen and expose subsurface materials to the surface, where they are more vulnerable to weathering.
- Nutrient Cycling: Animal wastes (excrement) and the decay of organic matter create organic acids that assist in the chemical weathering of rocks.