WEEK-III Weathering & Erosion

True or False: The Earth’s surface has stayed the same for thousands of years

The Earth’s surface is always changing! True or False: The Earth’s surface has stayed the same for thousands of years False The Earth’s surface is always changing!

Surface Processes The wide range of conditions found in different parts of the globe at present reflect the variety of natural processes that operate to shape the surface of the Earth. Land areas are continually being reduced and their shape modified by weathering and erosion, and the general term for this is denudation.

Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters. Weathering occurs in situ, or "with no movement", and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, wind, and gravity. The materials left over after the rock breaks down combined with organic material creates soil.

Classification of Weathering The mineral content of the soil is determined by the parent material, thus a soil derived from a single rock type can often be deficient in one or more minerals for good fertility, while a soil weathered from a mix of rock types often makes more fertile soil. Classification of Weathering Two important classifications of weathering processes exist – (1) Physical 0r Mechanical weathering and (2) Chemical weathering.

Physical or Mechanical Weathering In Physical or Mechanical weathering, or disintegration, breaks down rocks into small particles by the action of temperature, by impact from raindrops and by abrasion from mineral particles carried in the wind. In very hot and very cold climates changes of temperature produce flaking of exposed rock surfaces. In areas of intense rainfall soil particles may be dislodged and the surface of the soil weakened by raindrops.

The processes most commonly involved in mechanical weathering are listed in the following table.

Pressure release (Mechanical unloading) could have caused the exfoliated granite sheets shown in the picture

Thermal loading Causes weathering Describe how ice causes weathering?

Wind causes weathering Why wasn’t this mass of land weathered away? What evidence of weathering do you see in this picture?

Chemical Weathering Chemical weathering  is the process by which rocks are decomposed, dissolved or loosened by chemical processes to form residual materials. Chemical weathering changes the composition of rocks, often transforming them when water interacts with minerals to create various chemical reactions. Comparison of unweathered (left) and weathered (right) limestone

Some of the Processes involved in chemical weathering are given below : CARBONATION  process by which dissolved carbon dioxide in rainwater or moisture in  surrounding air forms carbonic acid and reacts with the minerals in the rock. This process weakens the rock thus breaking it down in the process. e.g.: Calcium Carbonate + Water + Carbon Dioxide ---> Calcium bicarbonate (soluble) White carbonate veins where carbon dioxide has been transformed into rock show in a dark peridotite deposit in Oman

e.g. : Iron + Oxygen --> Iron Oxide (crumbles) HYDROLYSIS  Chemical reaction between the minerals in the rock and hydrogen in rain  water. For example, during hydrolysis, the feldspar in granite changes to clay mineral which crumbles easily, weakening the rock and causing it to break down. OXIDATION  The process by which oxygen combine with water and minerals in the rock  such as calcium and magnesium. When iron reacts with oxygen, reddish -brown   iron oxide is formed. The iron-oxide crust crumbles easily and weakens the rock. e.g. : Iron + Oxygen -->  Iron Oxide (crumbles) SOLUTION     process by which minerals in the rocks dissolve directly in water. HYDRATION process where minerals in the rock absorb water and expand, creating stress which causes the disintegration of rocks. EG: Unhydrated Calcium Sulphate + Water -->   Hydrated Calcium Sulphate (expands)

HYDROLYSIS 

HYDRATION OXIDATION 

Engineering Classification of Weathering The early stages of weathering are usually represented by discoloration of the rock material, which changes from slightly to highly discoloured as the degree of weathering increases. Because weathering brings about changes in engineering properties, in particular it commonly leads to an increase in porosity with a corresponding reduction in density and strength, these changes being reflected in the amount of discoloration. As weathering proceeds, the rock material becomes increasingly decomposed and/or disintegrated until a soil is ultimately formed. Numerous attempts have been made to devise engineering classifications of weathered rock and rock masses.

To recognize weathering grades Iliev (1967), has developed a coefficient of weathering, K, for granitic rock, based upon the ultrasonic velocities of the rock material according to the expression: