Processes that shape the earth: II – A presentation by ACWADAM

The presentation deals with weathering, erosion and deposition of rocks -

• The process of breaking down and alteration of rocks and formation of new materials from them is known as weathering of rocks.
• The group of processes whereby the materials of the earth’s crust are moved from one place to another by running water (including rainfall), waves and current, glacier ice and wind is known as erosion.  
• The processes whereby the material being carried during the process of erosion is deposited either through physical processes (such as drop in the velocity of flowing water) or precipitation of dissolved material under a set of conditions for instance, the deposition of limestones from water rich in calcium carbonate is known as deposition.

Thus erosion and deposition are interchangeable processes.  Erosion cuts down the relief while deposition builds.  As material is successively deposited at a place the first deposited material is loaded with material deposited later. Due to this piling there is a pressure exerted on earlier deposits due to which they tend to become compact. Shale (fine grained sediments) undergo a large amount of compaction while sandstones and conglomerates do not. 

Once a deposit is covered by subsequent one, it is often sealed and the chemistry of the fluids contained in the pores changes. Through this process new minerals are formed and earlier formed minerals are destroyed. This process is called diagenesis. As the rock pile increases the field of diagenesis changes to metamorphism. After a limit of temperature and pressure, the rocks tend to melt and form molten rock material in the interior of the earth. This material then cools down to form igneous rocks.   

This presentation is part of the training modules on planning, development and management of groundwater with special reference to watershed management programmes by ACWADAM. Please write to ACWADAM at acwadam@vsnl.net for sourcing these presentations.