Earth’s Surface Shaping: Weathering and Erosion Processes
External Geological Processes on Earth’s Surface
The effect of external agents on the land surface begins with actions from the atmosphere, which disrupt and alter superficial rocks. This initial breakdown is known as weathering.
Erosion and Transport
Weathered materials rarely remain in their place of origin. Instead, they are moved by erosion and subsequent transport. The primary external geological agents responsible for erosion and transporting the resulting fragments include:
- Rivers and streams
- Ocean currents
- Glaciers
- Wind
Sediments can be transported as solid fragments of varying sizes (called debris) or in solution.
Sedimentation
Sedimentation is the process where debris, the remains of dead organisms, or dissolved chemical substances are deposited. This typically occurs in depositional environments on continents and in ocean basins, often referred to as sedimentary basins.
Understanding Weathering
Weathering is the process by which rocks are broken down and altered due to constant exposure to atmospheric action. There are three main types of weathering:
- Physical (or Mechanical) Weathering
- Chemical Weathering
- Biological Weathering
Physical Weathering
Physical weathering occurs when rocks break into smaller fragments without changing their chemical composition. Key processes include:
Frost Wedging (Freeze-Thaw Action)
Sometimes called gelifraction, the action of water freezing is a primary reason rocks break and disintegrate. Water seeps into small fractures or cracks in the rocks. When it freezes, it expands in volume, acting like a wedge that widens the cracks. If this process repeats many times over a long period, the rock eventually breaks apart.
Thermal Expansion and Contraction
Rocks can also break due to significant temperature variations between day and night, common in high mountain areas and deserts. Rocks expand when heated during the day and contract when they cool down at night. Repeated cycles of expansion and contraction can cause stress and fracturing.
Chemical Weathering
Chemical weathering involves alterations in the chemical composition of the minerals that form rocks. Water is the main agent driving this type of weathering. Important chemical weathering processes include:
Oxidation
Dissolved oxygen in water combines with various elements within minerals. Oxidation is particularly pronounced in iron-containing minerals because oxygen readily reacts with iron, often forming rust-like oxides.
Carbonation
Carbonation is the action of carbonic acid (formed when carbon dioxide dissolves in water) primarily on limestone rocks. Limestone is composed mainly of calcium carbonate, which is insoluble in pure water. However, when carbonic acid reacts with calcium carbonate, it forms calcium bicarbonate, which is soluble in water and can be carried away by it.
Biological Weathering
Living organisms also contribute to weathering, causing rocks to crack and alter. Examples include:
- Plant roots growing into cracks and widening them.
- Burrowing animals (like moles and various invertebrates) that live in the soil and disturb rock fragments.
- Microorganisms such as bacteria, fungi, and lichens, which can chemically alter rock surfaces.
Geological Action of Surface Water
Falling rainwater initially forms a sheet flow, which then gathers into small channels, creating wild water or runoff. Later, this water may collect into permanent courses (rivers) or temporary ones (wadis).
Wadis and Runoff Effects
When runoff flows down steep slopes over soft, poorly compacted materials with little or no vegetation, it can carve deep grooves into the surface. If water flows over soft, impervious ground like clay or marl, it can form large gullies known as badlands or cárcavas. In areas with heterogeneous materials, such as glacial sediments or volcanic deposits, erosion by water can form distinctive features like earth pillars or hoodoos (sometimes referred to as ‘fairy chimneys’).