Along with endogenetic forces, Earth’s surface is being continuously subjected to by external forces originating within the earth’s atmosphere (exogenic forces) like weathering, mass movements, erosion and deposition.

Weathering

Weathering is defined as mechanical disintegration and chemical decomposition of rocks, through the action of various elements of weather and climate. Weathering occurs in situ, with little or no movement. Weathering can happen in three ways:

• Chemical Weathering - A group of weathering processes i.e. solution, carbonation, hydration, oxidation and reduction act on the rocks to decompose, dissolve or reduce them to a fine clastic state through chemical reactions by oxygen, surface /soil water and other acids.
  
  
• Physical Weathering - They are caused due to applied forces which can be gravitational forces, thermal expansion forces, water pressures etc. The repeated action of these processes cause damage to the rocks.
  
  
• Biological Weathering- It is contribution to or removal of minerals and ions from the weathering environment and physical changes due to growth or movement of organisms. Burrowing and wedging by organisms like earthworms, termites, rodents etc., help in exposing the new surfaces to chemical attack and assists in the penetration of moisture and Human beings by disturbing vegetation, ploughing, and cultivating soils also help in creating new contacts between air, water, and minerals in earth materials.
  
  

Mass Movements  

These movements transfer the mass of rock debris down the slopes under the direct influence of gravity. Mass movements are aided by gravity and no geomorphic agent like running water, glaciers, wind, waves and currents participate in the process of mass movements. Weak unconsolidated materials, thinly bedded rocks, faults, steeply dipping beds, vertical cliffs or steep slopes, abundant precipitation and torrential rains and scarcity of vegetation etc. favour mass movements. The movements of mass may range from slow to rapid, affecting shallow to deep columns of materials and include creep, flow (earthflow, mudflow), slide (rockslide, landslide) and fall.

Significance of Weathering and Mass Movement

1. Soil and Regolith: Weathering is crucial base to our ecology as it converts bed rock into regolith and soil. Regolith is also notable for being the basic source for the inorganic part of the soil.
   
   
2. Resources: Weathering produces natural resources like clay, sand, gravel etc. Practically all bauxite, most iron ore and some copper ore are formed and concentrated by weathering.
   
   
3. Landforms: Differential weathering helps in the evolution of different types of landforms like stone lattice, tors, buttes etc. Continuous removal and transfer of weathered materials through different processes of mass-translocation of rock wastes such as landslides and by the agent of erosion causes gradual lowering of the height of the affected area.
   
   
4. Disasters: In mountainous regions, mass wasting of weathered material on the slopes occurs in the form of landslides and debris avalanches leading to loss of life and property.
   
   

The effects of most of the exogenic geomorphic processes are small and slow and may be imperceptible in a short time span, but in the long run play an important role.