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.

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

èSedimentary rocks are types of rockthat are formed by the accumulation or depositionofmineralor organicparticlesat Earth'ssurface,followed by cementation. Sedimentationis the collective name for processes that cause these particles to settle in place

èTheir process of formation is called Lithification,which refers to transportation of rock fragments by different exogenous agencies and deposited. These deposits through compaction turn into rocks.

èDepending upon the mode of formation, sedimentary rocks are classified into three major groups:

• 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.

CausesofMetamorphism

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.

Rock Cycle:

èThe rock cycle describes the processes through which the three main rock types (igneous, metamorphic, and sedimentary) transform from one type into another.

èRocks do not remain in their original form for long but may undergo transformation. Rock cycle is a continuous process through which old rocks are transformed into new ones.

èIgneous rocks are primary rocks and other rocks (sedimentary andmetamorphic) form from them.

èIgneousrockscanbechangedintometamorphicrocks.

èThe fragments derived out of igneous and metamorphic rocks form into sedimentary rocks.

èSedimentary rocks themselves can turn into fragments andthe fragments canbe a source for formation of sedimentary rocks.

èThe crustal rocks once formed may be carried down into the mantle through subduction process and the same melt down due to increase in temperature in the interior and turn into molten magma, the original source for igneous rocks.

FactorsAffectingRockCycle:

The rock cycle is affected by various human activities and environmental phenomena, including:

• TheEarth’s internal heatand pressure, which cause rock to melt completely or transform it into a metamorphic rock.
• Theuplift of landcaused bytectonic processes, which exposes rock that was underground to weathering and erosion.
• Therateof weathering, whichisaffectedbyclimaticconditionssuch as precipitation and temperature.
• .Plant growth, especially roots can physically break up rocks and also change the environmental chemistry (for example, increase acidity), increasing the rate of chemical weathering. In turn, the kind of rock that is weathered determines soil quality, nutrient                                          levels (especially nitrogen and phosphorus levels),                                                                                  and local biodiversity.
• Ratesoferosion causedbywater, wind, ice,orgravity,whicharedrivenbythewater cycle, atmospheric and ocean circulation patterns, and regional topography (the structure of the landscape).
• Thesizeanddepthofthebodiesofwater,suchaslakes,rivers,ortheocean, where sediment is deposited.
• The extraction of           rocks           and fossil           fuels,           which           in          turn destabilize soils, increase erosion,                     and                    decrease water                 qualityby increasing sediment and pollutants in rivers and streams.
• Urbanization, can increase water runoff, increasing erosion and decreasing soil quality in the surrounding areas.

• Hydraulic fracking to remove oil and gas, which uses water, sand, and chemicals to create new or expand existing cracks in rocks that allow oil and gas to flow into drill holes for extraction.
• Humanlandandwateruse,includingdeforestationandagriculturalactivities.
• Example: Removing trees and other plants, plowing fields, and overgrazing by livestock destabilizes soils and can increase rates of erosion by 10 to 100 times.
• Damming rivers and extracting water from freshwater ecosystems for human use changes where and how much sedimentation occurs, which affects soil quality and causes changes in habitats.
• Plants and other organisms, such as those that build coral reefs, can trap sediment that otherwise might be deposited elsewhere.
• Extreme weather events, which can cause accelerated rates of erosion due to flooding or wave action

Socio-EconomicSignificanceOfRocks:

EconomicSignificanceofIgneous Rocks

• Sincemagmaisthechiefsourceofmetalores,manyofthemareassociatedwith igneous rocks.
• Themineralsofgreateconomicvaluefoundinigneousrocksaremagnetic iron, nickel, copper, lead, zinc, chromite, manganese, gold, diamond and platinum.
• Amygdales are almond-shaped bubbles formed in basalt due to escape of gases and are filled with minerals.
• The oldrocks of the great Indian peninsula are rich in these crystallised minerals ormetals.
• Many igneous rocks like granite are used as building materials as they come in beautiful shades.

EconomicSignificanceofSedimentaryRocks

• Sedimentaryrocksarenotas richinminerals ofeconomicvalueas the igneousrocks.
• Yetsomemineralssuchashematiteironore,phosphates,buildingstones,coals, petroleum and material used in the cement industry are found.
• Thedecayoftiny marineorganismsyieldspetroleum.Petroleumoccursinsuitable structures only.
• Minerals like bauxite, manganese, tin, are derived from other rocks but are found in gravels and sands carried by water.
• Sedimentaryrocksalsoyieldsomeof therichestsoils

EconomicSignificanceofMetamorphicRocks

• Slate,whichisusedasaroofingmaterialandforwritinginschools,isfoundover Rewari (Haryana), Kangra (Himachal Pradesh) and parts of Bihar.
• GraphiteisfoundinOrissaandAndhraPradesh.

GeologicalSignificanceofRocks:

Rocks havesignificantgeological importancedue totheirrolein understanding Earth’s history, processes, and evolution. Here are some key geological significances of rocks,

1. Rock Types and Formations: Different rock types and formations provide important clues about past geological processes, such as volcanic eruptions, sedimentation, erosion, and tectonic events.

Forexample,thepresenceofsedimentaryrockslikesandstoneandshalecan indicatepastenvironmentssuchasancientoceans,lakes,orriversystems,while igneous rocks like basalt and granite can reveal volcanic activities and magmacrystallization.

2. Fossil Record: Rocks can containfossils, which are the preserved remains or traces of ancient organisms. Fossils in rocksprovide crucial evidence of pastlife forms, theirevolution, distribution, and environmental conditions.

Example: Fossil-bearing rocks, such as limestone and shale, are invaluable in understanding Earth’s biological history, including the evolution of plants, animals, and ecosystems over millions of years.

3. Geologic Time Scale: Rocks and their formations are used to establish the geologic time scale, which is a framework for understanding Earth’s history and the relative ages of different rock layers.
4. Earth’s Processes: Rocks are integral to understanding the processes that shape and change the Earth’s surface, such as weathering, erosion, and sedimentation. The study of rocks helps us understand how landscapes evolve over time, including the formation of mountains, valleys, rivers, coastlines, and other landforms
5. Natural Resources: Rocks are the source of many important natural resources, includingminerals,ores,fuels,andwater.Understandingthegeological