Last class revision. (5:11 PM)

Exogenic Movements: (5:18 PM)

• These are the earth's movements caused due to various forces acting from outside the earth's surface.
• The source of energy for these movements are sunlight and gravity.
• Different agents of exogenic movements are wind, river, groundwater, waves, and glaciers.
• Exogenic processes involve aggradation and degradation.
• Aggradation is the action of various processes that cause deposition on the surface to bring uniformity of grade i.e. slope.
• Degradation or denudation involves the combined action of various processes that cause the wearing away of the earth's surface and results in general lowering and levelling out of the earth's surface.
• So, degradation involves weathering, erosion, and mass movements.

Weathering (5:41 PM):

• It is the total effect of various processes that cooperate in bringing about the decay and disintegration of rocks involving no large-scale transportation.
• Factors affecting weathering:
• 1. Rock type and structure:
• It includes characteristics like origin, massiveness, porosity, permeability, presence of fractures, etc.
• For example, sedimentary rocks are weathered faster than igneous rocks.
• 2. Slope and aspects:
• Steep slopes will greatly add weathering as disintegrated material gets removed faster.
• The slopes which are exposed to wind, rainfall, and sunlight are more prone to weathering.
• E.g. south-facing slopes of the Himalayas.
• 3. Temperature:
• The range of temperature, particularly the diurnal range subjects the surface layer to expansion and contraction continuously causing a physical breakdown.
• Temperature also increases the rate of certain chemical reactions.
• 4. Water:
• When water is added or removed, it imposes mechanical stress on the rocks and causes them to split.
• Water also decides the rate of chemical reactions.
• 5. Biological agents:
• These include the action of plant roots, micro-organisms, and burrowing animals.

Types of weathering (6:15 PM)

• Difference between Mechanical and Chemical weathering:

• Mechanical (Physical) Weathering	Chemical Weathering
  The disintegration of rocks takes place without any change in the chemical constituents of the rocks	The disintegration of rocks takes place with changes in the chemical constituents of the rocks
  Factors such as temperature, moisture, frost action, and wind cause the physical breakup of the rocks.	Temperature, moisture, etc. cause minerals in the rocks to either dissolve in water or change their composition
  It is more rapid in desert climates	It occurs in moist and cool climates
  Rocks are effect to great depth	It mostly takes place near the surface of the earth
  Its agents are temperature and moisture	Its agents are water, oxygen, carbon, and various organic acids

• Physical weathering:
• Crystal growth
• Growth of water crystal: When water freezes it expands and finds its way into cracks or pores, it wedges apart rock fissures and joints. In desert regions, it results in frost shattering, in glacial regions it causes freeze-thaw.
• Salt crystal: Growth of salt crystal by crystallization as moisture evaporates creating pressure in pore spaces and cracks. It is also called Haloclasty (salt weathering).
• Pressure or thermal expansion:
• 1. Exfoliation: Rapid expansion and contraction of surface rocks weaken the outer shell creating cracks and causing the rock shell to peel off 
• 2. Pressure release: Confining pressed from the weight of overlying rocks is released, when the upper layer of rocks is removed by degradation. This causes rocks to expand leading to the development of fractures.
• 3. Block disintegration: Here weathering causes the breakdown of rocks in blocks.
• 4. Granular disintegration: Here breakdown occurs in granules.
• Chemical Weathering:
• Hydrolysis:
• It is the chemical union of water and minerals.
• It produces solutions.
• Hydration:
• It is when minerals incorporate water into their molecular structure.
• It causes swelling.
• Oxidation and Reduction:
• It involves the addition or removal of oxygen.
• It causes discolouration. E.g. rusting of iron.
• They are the first visible sign of chemical weathering.
• Carbonation:
• Rainwater absorbs carbon dioxide and gets converted into carbonic acid.
• Limestone and dolomites are altered by the dissolving action of carbonic acid.
• Biological Weathering
• It is a type of weathering due to the action of biological agents.

Erosion: (7:20 PM)

• Erosion refers to the wearing away of the land surface by the mechanical action of debris derived from weathering as they are transported to a different location by agents of wind, water, glacier, etc.
• Types of Erosion:
• Abrasion:
• Wearing away surfaces by mechanical processes such as rubbing, scratching, polishing, etc.
• Abrasion in water is called corrasion.
• Attrition:
• It involves the reduction in the size of fragments by friction and impact during transportation.
• It breaks down the load into finer sediments.
• Cavitation:
• The collapse of water bubbles in rivers leads to an explosion sending a shock wave that tends to disintegrate the adjacent rocks which are cavitation.
• Corrosion:
• It involves the breaking of rocks by the solvent and the chemical action of water.
• Deflation:
• The lifting and removal of dust and sand by wind are called deflation.
• Hydraulic action:
• Breaking of rocks by fast-moving water.
• Plucking:
• As the glacier moves, they drag the rocks and disintegrate them into smaller pieces.

Mass Movement: (7:53 PM)

• The weathered material moving downhill slopes under the influence of gravity with or without the assistance of running water is called a mass movement.
• It is a large-scale breakdown or disintegration.
• It is also called mass wasting.

Topics for the next class: Types of mass movements, and different types of landforms.