Rocks:

Theearth’scrustiscomposedofrocks.A rockisanaggregate ofoneormoreminerals. Rock may be hard or soft and in varied colors.

FeaturesofRocks

1. Rockmaybehardorsoftand invaried colors. For example: Granite is hard, soapstone is soft. Gabbroisblackandquartzitecanbemilkywhite.

2. Rocksdonothaveadefinitecompositionofmineral constituents.
3. Rocksare mainlyclassified under 3groups suchasIgneousRocks, Metamorphic rocks and sedimentary rocks.

FormationofRocks:

Asthereare3maintypesofrocks,theformationoccursinvariouswaysthroughunique process for each of them. Such as,

1. Igneous Rocks:

• They are formed due to cooling down of molten magma or lava on the surface of earth. Also called as primary rocks.
• Forexample:Granite,gabbro, basalt
• This process of cooling down can occur above or below earth surface.
• Igneous rocks are classified based on Texture, which is dependent upon time taken for cooling down of magma. These can be given as follows:

1.Intrusiveigneousrocks(Plutonicrocks):

• When magma cools slowly at great depths, mineral grains formed in therocks may be very large. Such rocks are called intrusive rocks or plutonic rocks (e.g. Granite).
• Theserocksappearonthesurfaceonly afterbeing upliftedand denuded.

2.Extrusiveigneousrocks(LavaorVolcanicrocks)

• Sudden cooling of magma just below the surface or lava above the surface results in small and smooth grains in rocks as rapid cooling prevents crystallization,asaresult,suchrocksarefine-grained.Theyare called extrusive rocks or volcanic rocks (e.g. Basalt).
• The DeccantrapsintheIndianpeninsularregionisofbasaltic origin.
• Basic rocks contain a greater proportion of basic oxides, e.g. of iron,aluminum or magnesium, and are thus denser and darker in color

Basedon thepresenceof acid-forming radical,silicon,Igneous rocksarealsoclassifiedas Acidic and Basic rocks as follow,

AcidicRocks:

• Acidic rocks are characterized by high content of silica (quartz and feldspar) — up to 80 per cent. While rest of composition contains aluminium, alkalis, magnesium, iron oxide, lime etc.
• These rocks have a lesser content of heavier minerals like iron and magnesium. Hence, are less dense and are lighter in colour than basic rocks.
• Theserocksconstitute thesialportionofthecrust.
• Due tothe excessofsilicon,acidicmagma coolsfast, and itdoes notflowand spread far away.
• Highmountains areformedof thistypeofrock.
• Addrocksare hard,compact, massiveandresistantto weathering.
• Granite,quartzandfeldspararetypicalexamples.

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BasicRocks

• These rocks are poor in silica (about 40 per cent); magnesia content is up to 40 per cent, and the remaining is spread over iron oxide, lime, aluminium, alkalis, potassium etc.
• Due to low silica content, the parent material of such rocks cools slowly and thus, flows and spreads far away. This flow and cooling give rise to plateaus.
• Presence of heavy elements imparts to these rocks a dark colour. Not being very hard, these rocks are weathered relatively easily.
• Basalt,gabbroand dolerite aretypicalexamples.

2.Sedimentary Rocks

• Mechanically formed:(by mechanical agents like running water, wind, ocean currents, ice, etc.)

Ex.Sandstone,conglomerate,limestone,shale,loessetc.

• Organically formed: (remains of plants and animals are buried under sediments, and due to heat and pressure from overlying layers, their composition changes)

Ex.Geyserite,chalk,limestone,coaletc.

• Chemically formed: (Water containing minerals evaporate at the mouth of springs or salt lakes)

Ex.Chert,limestone,halite,potashetc

3.MetamorphicRocks:

èMetamorphism refers to change of form. These rocks form under the action of pressure, volume and temperature (PVT) changes.

èDue to metamorphism,already consolidated rocks undergo recrystallisation and reorganization of materials within original rocks.

èIt occurs when rocks are forced down to lower levels by tectonic processes or when molten magma rising through the crust comes in contact with the crustal rocks or the underlying rocks are subjected to great amounts of pressure by overlying rocks.

Causes of Metamorphism

èOrogenic (Mountain Building) Movements: movements with an interplay of folding, warping and high temperatures. These processes give existing rocks a new appearance.

èLava Inflow: The molten magmatic material inside the earth’s crust brings the surrounding rocks under the influence of intense temperature pressure and causes changes in them.

èGeodynamic Forces: The omnipresent geodynamic forces such as plate tectonics also play an important role in metamorphism.

TypesofMetamorphism:

ThermalMetamorphism

• Thechangeofformorre-crystallisationofmineralsofsedimentaryandigneousrocks under the influence of high temperatures is known as thermal metamorphism.
• A magmatic intrusion causing thermal metamorphism is responsible for thepeak of Mount Everest consisting of metamorphosed limestone.
• Asaresultofthermalmetamorphism, sandstonechangesintoquartziteand limestone into marble.

Dynamic Metamorphism

• This refers to the formation of metamorphic rocks under high pressure,sometimes accompanied by high temperatures and the action of chemically charged water.
• The combination of directed pressure and heat is very powerful in producing metamorphism because it leads to more or less complete recrystallisation of rocks and the production of new structures. This is known as dynamo thermal metamorphism.
• Under high pressure, granite is converted into gneiss; clay and shale aretransformed into schist.