Geological Classification of Rocks

It is estimated that three-fourth of the land area of the globe is underlain by sedimentary rocks and the other fourth by igneous and metamorphic rocks. Main three geological classification of rocks are, :

Igneous rocks:

Are of volcanic origin, formed as a result of consolidation of molten materials either in the interior of the earth’s crust or upon its surface. They represent a crystalline glassy or fused texture. Generally, igneous rocks are hard, tough, dense, impervious, strong and durable.

Granite, Dolerite, Basalt, Trap, are examples. Form excellent concrete aggregate.

Acid rocks:

Igneous rocks containing over 65 per cent of silica. Compared with basic rocks, acid rocks are of lighter colour and in coarsely crystalline varieties ; free silica or quartz can be seen without the use of a lens. Granite is an acid rock. Basic rocks: Igneous rocks containing less than 52 per cent of silica, Compared with acid rocks they are of darker colour, and only rarely show free silica or quartz. Basalt and dolerite are basic rocks,

Sedimentary or Aqueous rocks .:

Are formed by the sediments deposited chiefly by water and to some extent by wind and ice, (sand, gravel, clay, cemented together by silica, lime, etc.) They represent a bedded or stratified structure in general, the individual beds lying one above another, often being distinguishable by differences in colour, texture or composition, May be close grained, compact or open textured. Sandstones, Limestones and Shale are examples, Gravel, sand, silt, clay and peat are considered as uncemented and unconsolidated sedimentary rocks.

Metamorphic rocks:

Are either igneous or sedimentary in their origin but subsequently changed due to movements of the crust as a result of metamorphic action of heat and pressure. Metamorphic rocks have a foliated structure in general and also show layers of stratification which are not always uniform. These rocks are hard and durable, Slates, Schists Gneisses, quartzite, some hard Shales and Marbles, etc., are formed in this way.

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Geological Classification of Rocks


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