Type of rocks – A presentation by ACWADAM

This presentation by ACWADAM deals with the three basic types of rocks that are found in nature – igneous, sedimentary and metamorphic. Rocks are classified on the basis of minerals & chemical composition, by texture and by the processes that formed them.

These three types are described below:

• Igneous rocks are formed from the consolidation of “magma” or molten rock material which exists within the earth at great depths and is often brought to the surface in the form of what is known as lava and other associated material through volcanoes and fissures in the earths crust, e.g. granite, basalt, rhyolite etc. They are formed either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.
• Sedimentary rocks are formed out of compaction of mineral or rock particles (sediments) that are deposited by wind and water, e.g. sandstones, limestones and shales.
• Metamorphic rocks are formed out of alteration (metamorphosis) of sedimentary, igneous or existing metamorphic rocks by the action of temperature and or pressure, e.g. schist, gneiss, quartzite, marble etc.,

A map showing an overlay of generalized hydrogeological settings with administrative boundaries (Districts and States) is shown for the country.

The nature of openings whether vertical/horizontal/inclined decide how groundwater flows through the rocks. The presentation deals with openings in rocks (pores/fractures), the action of water on ground (weathering and erosion).

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.