Rocks – UPSC GS1

Describe the nature and mode of origin of the chief types of rock at the earth’s crust. How will you distinguish them?(150 Words)

Three types of Rocks: Igneous, Sedimentary, and Metamorphic.
Mode of origin:
Igneous Rocks: solidified from magma and lava,
Sedimentary Rocks: the result of deposition of fragments of rocks by exogenous process
Metamorphic Rocks – formed out of existing rocks undergoing recrystallization
Different Nature of these rocks:
1. Igneous Rocks:
They are rough hard rocks and water percolates with great difficulty along the joints. The igneous rocks are granular or crystalline rocks. They do not have strata like sedimentary rocks. The igneous rocks are less affected by chemical weathering. Igneous rocks do not contain fossils. They mostly associated with volcanic activities.
2. Sedimentary rocks:
Sedimentary rocks are formed of sediments derived from the older rocks, plant and animal remains and thus rocks contain fossils of plants and animals.
They are found over the largest surface area of the globe.
Sedimentary rocks contain several layers but these are seldom crystalline rocks.
Layers of sedimentary rocks are seldom found in original and horizontal manner.
They may well consolidate, poorly consolidated and even unconsolidated. They are characterized by different types of joints. These are generally perpendicular to the bedding planes.
3. Metamorphic rocks:
Foliation, streaking or parallel arrangement of the constituent crystals is the most
common characteristic feature of metamorphic rocks. They split along the bedding planes known as fissility. The structure of the presence of numerous closely spaced parallel planes of splitting is known as cleavage.


In what ways sedimentary rock is different from metamorphic and igneous rocks? Describe the various sources from which sedimentary rocks may be derived. (150 Words)
Difference between Sedimentary and igneous, Metamorphic rocks
Sedimentary rocks are formed in the earth surface from lithification; they are often contain fossils, usually have pores between pieces, often have layers, and are softer. Sedimentary rocks are porous to the water, and Tendency to react with acids is higher, have great colour variety.
Igneous rocks are formed from molten liquid minerals called magma. They are non-porous for water, have fossils very rarely, they are harder, and light or dark coloured.
Metamorphic rocks are formed deep in the earth. They rarely have pores or openings, may have bent or curved foliation and as hard as igneous rocks.
Sources of Sedimentary rocks are derived by their different formation like:
Chemically formed:
1. Gypsum
2. Salt rock
Organically formed:
1. Limestones
2. Dolomites
3. Coals
4. Peats
Formed On basis of transporting agents :
1. Marine rocks
2. Lacustrine rocks
3. Riverine rocks
4. Loess
5. Till
6. Moraines



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