Unit 7:  External dynamics of the Earth                                      1   2   3

2. External geological processes

The external geological agents act trough four external geological processes: weathering, erosion, transportation and sedimentation.

2.1. Weathering

      Weathering is the process which causes the alteration of the rocks located

      on the surface.

      It is mainly due to the action of atmospheric agents. These agents are changes

      in temperature and atmospheric gases such a water vapour, oxygen

      or carbon dioxide.

      It is a static process. The fragments produced by weathering do not displace.

      They remain in the same place where they are formed.

      There are three types of weathering depending on the agent which acts:

       - Mechanical weathering.

         It is physical process which provokes the breakdown of rocks into smaller pieces.

         It does not involve changes in the chemical composition of the rock.

         It is caused by changes in temperature.

         An example of mechanical weathering is gelifraction (or frost wedging).

         It is found in mountainous regions where temperature fluctuates widely

         between day and night.

Animation: Physical weathering (Harcourt)

        - Chemical weathering

       - Biological weathering

2.2. Erosion

      It is the removal of the weathering products from where they are produced.

      The erosion agents are wind, liquid or solid water and gravity.

      The effects of erosion depend on:

        - the intensity of the erosive agent. Although all agents always act,

          usually one is dominant in each climatic zone. For example in arid climate,

          wind acts with much force, while water does not have so much importance.

- the resistance of the eroded material. Some rocks are soft and very easy to erode, while others are extremely hard and erosion does not have almost effect on them. It is due to their mineral composition and to the disposition of these rocks in the terrain.

The particles swept along by water, ice or wind also exert an erosional action on the surface of rocks when they crash on them.

Gravity provokes the displacement of the weathering materials to lower areas.

Animation: Erosion (Edumedia)

2.3. Transportation

      It is the displacement of the eroded particles to other places. Transportation

      agents are usually the same ones that drive the erosion that is to say wind,

      water or gravity.

      The eroded particles can be transported further or nearer depending on:

- the agent force. The higher the stregth of the agent, the higher the distance. 

- the weight and size of the particles. The smaller the particle,

  the higher the distance.

Animation: Types of transportation (McGraw Hill)

2.4. Sedimentation

      It is the deposit of the transported particles. These particles are called sediments.

      Sedimentation takes place when the energy of the transportation agents

      decreases and the particles fall down by gravity, and accumulate.

Animation: How sediments are deposited (Exploring Earth)