The Earth's External Processes and Landforms

TL;DR

External processes like weathering, erosion, and deposition constantly reshape Earth's surface. These forces, driven by solar energy and gravity, create a variety of distinctive landforms. Understanding them helps us see how landscapes evolve over time.

1. The Mental Model

Imagine Earth's surface as a giant sculpture always being worked on. External processes are the natural tools – wind, water, ice, and gravity – constantly chipping away, grinding down, and rebuilding the landscape.

2. The Core Material

External processes are those that occur on or near the Earth's surface, driven primarily by solar energy and gravity. They contrast with internal processes (like volcanism and plate tectonics) which originate from within the Earth.

There are three main categories of external processes:

Weathering

Weathering is the breaking down of rocks, soil, and minerals in situ (in place). It doesn't involve the removal of the material. There are two main types:

• Physical (Mechanical) Weathering: Breaks rocks into smaller pieces without changing their chemical composition.
  • Freeze-Thaw (Frost Shattering): Water enters cracks in rocks, freezes, expands (by about 9%), and widens the cracks. Repeated cycles eventually split the rock. Common in cold, wet climates.
  • Exfoliation (Onion-Skin Weathering): Outer layers of rock peel off due to repeated expansion and contraction from temperature changes, or the release of pressure from overlying material. Common in deserts or on exposed igneous rocks.
  • Salt Crystal Growth: Water evaporates, leaving salt crystals behind. These crystals grow, exerting pressure that breaks rock. Common in coastal and arid environments.
• Chemical Weathering: Changes the chemical composition of rocks, weakening them and making them more susceptible to erosion.
  • Carbonation: Carbon dioxide dissolves in rainwater to form weak carbonic acid, which reacts with calcium carbonate in rocks (like limestone) to dissolve them. Creates karst landscapes (caves, sinkholes).
  • Hydrolysis: Water reacts with minerals (especially silicates), altering their chemical structure. Common in feldspar-rich rocks, turning them into clay.
  • Oxidation: Oxygen reacts with minerals, especially iron-rich ones, forming oxides (rust). This weakens the rock and gives it a reddish-brown colour.

Erosion

Erosion is the process by which soil and rock materials are worn away and transported by natural agents like wind, water, ice, and gravity. It involves both the removal and movement of material.

• Fluvial Erosion (Rivers): Rivers erode their beds and banks through:
  • Hydraulic Action: The force of the water itself dislodging material.
  • Abrasion: Rock particles carried by the river rub against the bed and banks, wearing them away.
  • Attrition: Rock particles carried by the river collide with each other, breaking into smaller, rounder pieces.
  • Solution: Dissolved minerals are carried away in the water.
• Glacial Erosion (Ice): Glaciers are very powerful erosional agents.
  • Plucking (Quarrying): As a glacier moves, meltwater seeps into cracks, freezes onto the rock, and then the moving ice pulls pieces of rock away.
  • Abrasion: Rocks embedded in the base and sides of the glacier scrape against the bedrock, polishing and scratching it (leaving striations).
• Aeolian Erosion (Wind):
  • Deflation: Wind picks up and carries away loose, fine particles of sand and dust.
  • Abrasion: Wind-blown sand grains impact rock surfaces, grinding and polishing them.
• Marine Erosion (Waves): Waves erode coastlines through hydraulic action, abrasion, and solution.
• Mass Movement (Gravity): The downslope movement of rock and soil under the direct influence of gravity. This isn't strictly erosion by an agent like wind or water, but it's a critical process for transporting weathered material. Examples include landslides, mudflows, and rockfalls.

Deposition

Deposition is the process by which eroded material is dropped or laid down in a new location. This happens when the transporting agent (wind, water, ice) loses energy.

• Fluvial Deposition: Rivers deposit sediment when their velocity decreases, forming features like floodplains, deltas, and point bars.
• Glacial Deposition: Glaciers deposit unsorted material called till, forming moraines (ridges of till), drumlins (elongated hills of till), and outwash plains (sorted sediment deposited by meltwater).
• Aeolian Deposition: Wind deposits sand (forming dunes) and finer dust (forming loess deposits).
• Marine Deposition: Waves and currents deposit sediment along coastlines, creating beaches, spits, and bars.

Here's a simplified flow of these processes:

graph TD
    A["Initial Rock"] --> B["Weathering (breaking down)"]
    B --> C{"Material available for transport"}
    C --> D["Erosion (transportation by wind, water, ice, gravity)"]
    D --> E["Loss of energy by transport agent"]
    E --> F["Deposition (material laid down)"]
    F --> G["Formation of new landforms/sedimentary rocks"]

3. Worked Example

Let's trace the journey of a piece of granite from a mountain peak to a river delta.

1. Weathering: High on a mountain, water seeps into cracks in the granite. During winter, it freezes and expands (freeze-thaw weathering), eventually breaking off a fist-sized chunk.
2. Mass Movement: This chunk then tumbles down the steep slope (rockfall, a type of mass movement) until it reaches a stream bed.
3. Fluvial Erosion: The stream carries the granite piece. As it's carried, it bumps into other rocks and the stream bed (abrasion) and collides with other granite pieces (attrition), becoming smaller and more rounded over time.
4. Fluvial Deposition: Eventually, the river reaches a flatter area and slows down. The energy of the water decreases, and the now small, rounded granite grain (sand) is deposited along with other sediments on a floodplain or at the river's mouth, contributing to the formation of a delta.

4. Key Takeaways

• Weathering breaks down rocks in place, preparing material for transport.
• Erosion involves the removal and transportation of weathered material by natural agents.
• Deposition occurs when transporting agents lose energy, dropping the carried material.
• Each external process creates distinctive landforms, such as valleys, dunes, or deltas.
• These processes are continuous and interact with each another to constantly reshape landscapes.
• The type and rate of these processes are influenced by climate, geology, topography, and vegetation.

Common Mistakes to Avoid:
- Confusing weathering with erosion; weathering is breaking down, erosion is moving.
- Thinking erosion only happens in rivers; wind, ice, and waves are also powerful erosional agents.
- Forgetting the role of gravity (mass movement) in moving material, especially on slopes.
- Assuming chemical weathering doesn't involve water; water is often key.

5. Now Try It

Take 15 minutes to think about a specific landform you've seen or studied (e.g., a sandy beach, a 'V'-shaped river valley, a cave, a desert dune). For your chosen landform, write a short paragraph explaining which external processes (weathering, erosion, deposition, and their specific types) were most responsible for its formation and current appearance.

Success looks like: You can clearly link specific features of the landform to at least two distinct external processes, using appropriate geographical terminology.