Weathering, Erosion, Transport & Sedimentation: A Geological Overview
1. Weathering
1.1. Types of Weathering
1.1.1. Mechanical or Physical Weathering
The breakdown of rocks without altering their chemical composition. This process is caused by several factors:
- Ice Wedging (Cryoclasty): Water enters cracks in rocks, freezes, expands, and acts as a wedge, widening the cracks and breaking the rocks. This process thrives in cold climates with frequent freeze-thaw cycles.
- Temperature Variations (Thermoclasty): Sudden temperature changes cause successive contractions and expansions in the rock, leading to fragmentation. This is typical in desert climates with large temperature variations between day and night.
- Salt Crystallization (Haloclasty): Rocks are fragmented by the action of salt crystals. Water with dissolved salts enters cracks, evaporates, and leaves behind salt crystals that exert pressure, widening the cracks. This type of weathering is common in coastal areas.
- Decompression: Rocks formed under high pressure and temperature deep within the Earth can be uplifted to the surface. The decrease in pressure and temperature causes the rocks to expand and fracture.
- Action of Living Organisms (Bioclasty): Plant roots growing in cracks and animals burrowing can widen existing cracks and contribute to rock fragmentation.
1.1.2. Chemical Weathering
Chemical changes or transformations that rocks undergo due to the combined action of oxygen, carbon dioxide, and water. These reactions result in the rock losing its coherence and becoming crumbly. Water is the primary driver of these processes, making chemical weathering more prevalent in humid climates. Different types of chemical weathering include:
- Oxidation: Oxygen dissolved in water reacts with minerals containing iron and magnesium in the rocks, forming oxides. This is evident in the red-yellow surface layer that often covers rocks exposed to the atmosphere.
- Hydrolysis: A reaction between water and certain minerals, primarily silicates. This reaction alters the mineral structure and forms new minerals. Since silicates are abundant on Earth’s surface, hydrolysis is a common type of weathering. Ex: Granite, a very hard rock composed of quartz, mica, and orthoclase, undergoes hydrolysis. The orthoclase is converted into kaolin, a whitish powder, causing the rock to crumble as the quartz and mica are released.
- Hydration: Water molecules are incorporated into the crystal structure of some minerals, causing them to expand in volume. This is common in clayey soils, where minerals hydrate and swell during the wet season and dehydrate and crack during the dry season.
- Dissolution: Minerals formed by salts dissolve in contact with water. This is the case with minerals like halite (rock salt) and sylvite.
- Carbonation: A specific type of dissolution where water and carbon dioxide react to dissolve limestone.
1.2. Erosion
The wearing away of rocks by external agents like water or wind. Erosion is faster if the rock has been previously weakened by weathering. Erosion differs from weathering in that it involves the displacement or transportation of materials.
1.3. Transport
The movement of materials that have been weathered and eroded. This transport can occur through various mechanisms:
- Drag/Traction: Movement of large particles along the ground or riverbed.
- Saltation: Particles bouncing and hopping along the surface.
- Suspension: Fine particles (sand, dust, ash) carried within the air or water by turbulence and eddies.
- Dissolution: Transport of dissolved mineral salts (ions) in water.
1.4. Sedimentation
The deposition of transported materials when the transporting agent (air or water) loses energy. Sedimentation can occur:
- Gradually/in stages: Larger sediments are deposited first, followed by smaller ones, resulting in sorted deposits.
- Suddenly: Sediments of different sizes are mixed together, as in an alluvial fan.