Difference Between Physical and Chemical Weathering

Physical vs Chemical Weathering

We see mountains or large rocks stay as it is for years without changing. May be, for hundreds of years, we may not see them changing. However, changes are taking place there which we cannot see because those changes are very small and taking place very slowly. Weathering is such a process that rocks, soils, and any material go through. This is the process by which rocks are breaking down into smaller particles. Due to wind, water, or biota a slow breakdown happens which is known as weathering. There’s no visible movement in this form. After weathering the materials are combined with other organic material and forms soil. The content of the soil is determined by the parent rock which undergoes weathering. Weathering can be divided into two as physical weathering and chemical weathering. Usually both processes are taking place at the same time, and both are responsible for the whole weathering process.

What is Physical Weathering?

Physical weathering is also called as mechanical weathering. This is the process where rocks breakdown without altering their chemical composition. Physical weathering can occur due to temperature, pressure or snow. There are two main types of physical weathering. They are freeze thaw and exfoliation.

Freeze-thaw is the process where water goes into the cracks of the rock, then freezes and expands. This expansion causes rock to break apart. Changing temperature also causes rocks to expand and contract. When this happens over a period of time, rock parts starts to break down. Due to the pressure, cracks can be developed parallel to the land surface which leads to exfoliation.

Physical weathering is prominent in the places where there is little soil and few plants. For example, in desserts surface rocks are subjected to regular expansion and contraction due to temperature changes. Also, in mountain tops, snow keeps melting and freezing which causes physical weathering there.

What is Chemical Weathering?

Chemical weathering is the decomposition of rocks due to chemical reactions. This changes the composition of the rock. This often takes place when rain water reacts with minerals and rocks. Rain water is slightly acidic (due to dissolution of atmospheric carbon dioxide, carbonic acid is produced), and when the acidity increases chemical weathering also increases. With the global pollution, acid rains occur now, and this increases chemical weathering more than the natural rate.

Other than water, temperature is also important for chemical weathering. When the temperature is high, the weathering process is also high. This releases minerals and ions in rocks into surface waters. There are three main types as to how the chemical weathering occurs. They are solution, hydrolysis and oxidation. Solution is the removal of rock in solution due to acidic rain water, as above described. This is sometimes called carbonation process, since the rain water acidity is due to carbon dioxide. Hydrolysis is the breakdown of rock to produce clay and soluble salts by acidic water. Oxidation is the breakdown of rock due to oxygen and water.

Physical Weathering vs Chemical Weathering

  • Physical weathering does not change the chemical composition of the rock whereas chemical weathering changes the composition.
  • Physical weathering may result due to temperature, pressure, snow, etc. whereas chemical weathering mainly takes place due to rain.