Weathering is the process of breaking down rocks, soils and their minerals through direct contact with the atmosphere. Weathering occurs in situ, or "without movement", and thus should not to be confused with erosion, which involves the movement and disintegration of rocks and minerals by processes such as water, wind, ice and gravity.
Weathering are of two types:
1) Mechanical or Physical Weathering.
By physical weathering, rocks are mechanically broken apart into smaller pieces. Agents which are responsible for breaking up rocks includes Water, Wind, Trees and Plants Roots.
Ø Frost Wedging:
The process of water freezing in rocks is probably one of the most important form of mechanical weathering. On freezing water expand 9%. If it is occupying a crack completely, the crack will grow. Continued cycles of freezing and thawing in rocks containing water will cause them to fragment into smaller pieces. This is called Forst Wedging.
Ø Exfoliation:
Removal of overlaying rocks which causes the underlaying rocks to expand and facture due to pressure release.
Ø Abrasion:
Particles moved by water, ice and air can be affective in wearing away rock.
Ø Salt Crystal Growth:
Similar to frost wedging, the force of growing salt crystals in fracture and pore spaces can cause loose grain and expand opening. Such process is important in arid and coastal areas.
Ø Action Of Plants:
Plants roots also wedge them into cracks of rocks and break them up.
Ø In addition Forest Fire cause the outside of rocks to expand, crack parallel to surface and eventually rock breaks. The rocks have shattered, but their minerals have remained the same.
Ø Examples of mechanical weathering are dominant in cold climate, where chemical weathering occurs at slow rates.
2) Chemical Weathering.
By Chemical Weathering, the minerals in rocks are dissolved into rainwater or changed from one type of mineral into another. Climate is an important factor determining the rate of chemical weathering. Warm, moist environment have more chemical weathering because water is needed from the chemical changes and the warmth facilitates the reaction. The rocks that are susceptible by chemical weathering are limestone and marble.
Ø Dissolution:
Water combines with carbondioxide in the soil to form weak acid called carbonic acid. Microbe respiration generates abundant soil carbondioxide, and rainwater percolating through the soil provides the water. Carbonic acid slowly dissolves away minerals in rocks, especially the carbonate minerals that make up limestone and marble. The weak acid decomposes the insoluble rocks into water soluble products that move into groundwater.
Ø Hydrolysis:
The water can break up minerals through Hydolysis.
The most common group of minerals, the silicates, is
decomposed by this process. Reactive hydrogen ions
that are liberated from the water, attacks the crystal
lattice of mineral, and the mineral decomposes.
Ø Oxidation:
Oxygen combines with the minerals to form oxides such as Hematite, Limonite etc.
Ø Air Pollution that contributes to weathering of rocks generally contains weak concentration of strong acids such as sulphuric acid and nitric acid. Strong acid escaping from steam vents around volcanoes and mine sites can also contribute to increased weathering of nearby rocks.
If the products from weathering of rocks are not
removed from the site by erosion, the accumulation
produces a Soil, if they are removed by erosion then
they will again recombine to form Rock.
Formation of Soil.
Soil a non renewable resource, is the final product of weathering, and is defined as that portion of rock which is capble of supporting plant life.
Accumulation of weathered products produces a Soil. Through times, vegetation grows on the land surface, and rainwater moves through the soil, carrying weathering products from the surface to lower depts and thereby forming soil horizons. Litter layer of plant residues in relatively undecomposed form forms the O Horizon. Organic matter accumulates at the surface when vegetation dies, forming the A Horizon, or the top soil. Then mineral subsoil begins to oxidize into C Horizon. As water translocate weathering products of clay, iron oxides, and caliche accumulates in the old C Horizon and becomes a B Horizon. With time, soil get thick, gain horizons, and become clay-rich.
Process of Reformation of Rocks.
Ø Erosion And Transport [Sediments on move]
Wind and Water can have enough force to move rocks. Larger particles can be carried in a strong current, like fast moving water. Very small particles like silt and clay settle very slowly at the bottom of waters.
Ø Deposition [Sediments Settling down]
When Water or Wind looses energy and slows down, sediments can no longer be carried in it. The particles of sediments fall through the water or air and form a blanket of sediments on the bottom of a river, a lake, an ocean, or on the surface of the land. This process is called Deposition.
Ø Lithification [From Pile of Sand to Solid Rock]
Sediments that are formed, been transported, and deposited, is not the sedimentary rock unless it is all bound together. The process of sediment becoming a rock is called Lithification. It can take tens to hundred of thousands of years. The material that holds sedimentary particle together into a rock is called Cement. It is made of minerals crystal that forms in-between the clasts and holds them together.
Thus Weathering and Formation of Rocks is a Complex process
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