Weathering And Erosion
Weathering and erosion are closely related concepts. As weathering occurs, breaking up the rock, it can be transported through erosion.
There are two categories of weathering processes: Physical and Chemical. Although they are two very different processes, they work together to break down rocks and minerals.
Physical weathering and erosion can take place through any of the following processes:
- Development of Joints: Joints are fractures or cracks found in the rock formed by expansion due to cooling or relief of pressure. The space they create allows other agents of weathering to enter and begin a deeper process.
- Crystal Growth: Water containing ions percolates through fractures and form crystals that can expand and weaken the rocks.
- Heat: Sudden exposure to high temperatures can cause expansion and eventual breakage of the rock.
- Plant and Animal Activities: Plant roots can extend into fractures. This growth can cause expansion and further breakage. Animals can also break rock as the burrow or move through the cracks.
- Frost Wedging: As water freezes, it expands exerting force on the fractures.
When rocks and minerals are created, they are formed under conditions that are present deep within the Earth. When they arrive at the surface, the conditions are drastically different than their origin. At the surface there is lower temperature, lower pressure, higher free water, and higher free oxygen. The minerals react with these new conditions and produce new minerals that are stable under the new conditions.
The main agents in chemical weathering reactions are water and the weak acids formed in that water, the most common being carbonic acid, which is produced in rainwater.
Types of Chemical Weathering:
- Hydrolysis: H+ or OH- replaces an ion in the mineral.
- Oxidation: Since free Oxygen is more common on the surface of the earth, it may react with minerals to change the oxidation state of an ion.
- Leaching: Ions are removed by dissolution into water.
- Dehydration: The removal of H2O or OH- ion from a mineral.
- Complete Dissolution: All of the mineral is completely dissolved by the water.
Factors of Weathering and the Results:
- Rock Type and Structure
Weathering in rock usually starts at the surface and works inward. This can result in a variety of situations. Weathering Rinds are the outer weathered zone of rock. As weathering continues, the thickness of the weather rind increases. Exfoliation occurs when the concentrated shells of weathering become separated from the rock. Finally, Spheroidal Weathering takes place the rock fractures form a three-dimensional network. The rock is broken into cubes and begin to weather inward.
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