Chapter 4,
“Weathering, Mass Movement and Groundwater,” explains the “exogenic” (external) forces that wear away the Earth’s surface. While internal forces build landforms, these processes level them down.
1. Weathering: The Process of Disintegration
Weathering is the mechanical breakup or chemical decay of rocks in situ (without movement).
- Physical (Mechanical) Weathering:
- Insolation (Thermal Expansion): In deserts, rocks expand by day and contract by night, leading to Exfoliation (peeling of rock layers).
- Frost Action: Water enters cracks, freezes, expands, and shatters the rock. This creates Screes or Talus (rock fragments at mountain bases).
- Chemical Weathering:
- Oxidation: Reaction with oxygen (rusting of iron-rich rocks).
- Carbonation: Rainwater mixes with
𝐶𝑂2 to form a weak carbonic acid, which dissolves Limestone.
- Hydration: Rocks absorb water and expand, weakening their structure.
- Biological Weathering: Caused by plant roots, burrowing animals, or human activity.
2. Mass Movement
The movement of weathered material down a slope due to gravity.
- Soil Creep: A very slow, continuous movement of soil down a gentle slope.
- Soil Flow (Solifluction): Occurs in cold climates when the surface layer thaws and slides over a frozen subsoil.
- Landslides: Sudden, rapid movement of large masses of bedrock or soil. Usually triggered by heavy rain or earthquakes.
3. Groundwater: The Hydrological Cycle
Water that soaks into the ground through pore spaces and cracks.
- Permeability: The ability of a rock to allow water to pass through (e.g., Sandstone).
- Impermeability: Rocks that do not allow water to pass (e.g., Clay, Granite).
- Water Table: The upper level of the saturated zone of underground water. It fluctuates with the seasons.
- Aquifer: A layer of permeable rock that stores and transmits groundwater.
4. Springs and Wells
- Springs: Occur where the water table reaches the surface.
- Hot Springs: Groundwater heated by volcanic activity (e.g., Yellowstone).
- Geysers: Fountains of hot water and steam ejected at intervals (e.g., Old Faithful).
- Artesian Wells: A well bored into an aquifer confined between two impermeable layers. Water rises under its own pressure.
5. Quick Revision Facts
- Exfoliation is often called “onion-skin weathering.”
- Limestone is uniquely susceptible to chemical weathering via carbonation.
- The Zone of Saturation is the area below the water table where all pores are filled with water.
- Permeable rocks are not always porous (e.g., cracked granite is permeable but not porous).
