Soil formation - Role of Weathering

Soil formation - Role of Weathering

Question:-Mechanical and chemical weather and its relationship with soil formation.

Weathering is a process consisting of the breakdown of rock and transportation of the material thus formed. The process works in the following four stages:

- Weathering
- Erosion
- Transport
- Deposition

Erosion, transport and deposition are effected by wind, ice and water. Weathering consists of two aspects, mechanical weathering and chemical weathering which assist each other in the process. Chemical weathering is the chemical alteration of minerals

Mechanical weathering: Mechanical weathering is the physical fragmentation of rock. It increases the area of the rock that is exposed on which chemical weathering can occur. The process is as follows-

- Frost wedging – This is the daily freeze-thaw cycle that takes place at higher altitudes. Water seeps into cracks and freezes, thus expanding the crack. This then melts and seeps deeper into the enlarged crack.

- Thermal expansion – Normal temperatures also make the rocks expand and contract causing fractures.

- Unloading – Number of rocks are formed deeper in the crust and on exposure and erosion the pressure on them is reduced causing them to expand and crack. Rocks like granite often crack in concentric layers which is a process known as exfoliation. This causes granite domes such as Half Dome in Yosemite, Enchanted Rock in Texas, and Stone Mountain in Georgia.

- Organic activity – Factors like plant roots, burrowing animals also contribute in weathering.

- Abrasion – As rocks get transported, they rub against one another, breaking corners and edges. Thus, rock fragments get rounded as transport time and distance increases.

Chemical weathering: Chemical weathering effects different minerals in different proportion. This weakens the material of the rock alleviating mechanical weathering. The process is as follows-

- Solution (or dissolution) – Soluble minerals which are generally ionically bonded get dissolved in water or weaker acids. Solutioning is effective in wet climates since it requires considerable amounts of water to remove some material.

- Oxidation – The reddish/tan coloration that rocks get are primarily due to iron oxides which is formed on oxidation of iron bearing minerals.

- Hydrolysis – Silicate minerals with ionically bonded metal ions are weathered by this reaction. Hydrolysis is a process by which, the H+ ion from water or weak acid, due to its small size, moves into the mineral structure and since H+ is highly reactive, it dislodges other metal ions causing the chemical breakdown of the crystal structure. Clay minerals are the most important byproduct of hydrolysis and the most abundant since silicates are most abundant.

Soil formation- Soil is said to consist of 45% mineral matter, 25% water, 25% air, and 5% organic matter. Wind blown or water transported sediments and organic matter act as a source for the formation of soil. This material is broken down is finer particles with chemical and mechanical weathering processes which is greatly affected by the climate. Dead plants and animals too add to the organic waste altering the material to an extent. The various processes thus make the topsoil and subsoil. This process may take hundred or thousands of years to produce even a slight thickness of productive soil.

Thus weathering process like chemical and mechanical play a greater role in formation of soil.