L6 Metamorphic Rocks

8/11/2019 L6 Metamorphic Rocks

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What is Metamorphism?

Metamorphism is a change in form thathappens in Earths crustal rocks inresponse to changes in temperature

and pressure.


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Factors controlling Metamorphism

Simplest way to understand metamorphism is to remember cookingeffects

What you cook ?

Same is in the metamorphism process the end product will dependupon the initial composition and then the pressure/temperature

Factors that controls or influence metamorphism:

Chemical composition of the initial rock Intergranular fluids Temperature and pressure Time


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Type of Metamorphism Metamorphic changes thatoccur while temperature andpressure are rising (withabundant inter-granular fluid)

are termed progrademetamorphism

Metamorphic changes thatoccur as temperature andpressure are declining (withless inter-granular fluid) arecalled retrograde metamorphiceffects.

The intergranular fluid are not pure and plays important role inprocess of metamorphism (fluids mainly includes small amount ofgases, salts and mineral constituents dissolved from the host rock

If the fluids are absent or available in very less quantity the processof metamorphism is very slow and few changes in mineral

composition occurs


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The source of temperature is either from magma or dueto the depth factor

Metamorphism usually result into change in mineralcomposition and texture of rocks (Igneous andSedimentary Rocks) which are subjected to temp. >100 0 C and pr essu re >1000 s Mp a.

Low-grade metamorphism:Occ urs at abou t 100 0 C to 500 0 C

High-grade metamorphism:Occurs at > 500 0 C

Temperature


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Uniform Stress Differential Stress

min

inter

max

Pressure

Pressure (stress) result into change in texture of the originalrocks

Rock are solid, which can be squeezed more strongly in onedirection

Texture in metamorphic rocks are the result of differentialstress

Whereas, the texture resulted from uniform stress arecommonly seen in igneous rocks, because igneous rockscrystallize from Liquid magma.


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Development of differenttextures in rocks of the samecomposition different stresses

Granite Granite-Gneiss

Granite, consisting of quartz,feldspar, and mica (the darkmineral) will crystallized undera uniform stress and micagrains will be randomlyoriented.

Whereas, under adifferential stress. Majoras well as Mica grains willbe oriented parallel to themaximum stress direction,giving the rock a distinctfoliation


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Granite


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Gneiss


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Type of MetamorphicRocks classified based on

Textures

Foliated Non-foliated


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Foliated Rocks Foliation: when platy , lamellar or f laky minerals

(e.g., sheet silicate minerals the micas: biotite,muscovite, chlorite etc.), occurring in rock orientthemselves paral le l to one another (i.e.,

perpendicular to the direction of maximum pressure orstress).

Randomorientationof minerals

Preferredorientationof minerals


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Slate: fine grained foliated metamorphic rock formed under low-grade metamorphism (from mudstone, shales or claystones)


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Further deformation

Slate when metamorphosed further will give rise to morefoliated rock known as Phyllite

It is characterized by wavy texture/foliation Formed under low-grade metamorphism


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Foliation under microscope (mica and quartz

[darker grains])


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Phyllite to- Schist

Schist: is formed underintermediate to high grademetamorphism fromPhyllites


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Gneiss: parallel foliation; medium to coarse grained; quartz-feldspars

marked by lighter layers and darker are the ferromagnesian minerals


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Non foliated rocks These rocks lack foliation Common e.g., are Quartzite and Marble


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Quartzites

Medium to coarse grainedmade up of fused quartzgrains

Has sandy texture, alsotermed as Granulose texture.


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Marble

Metamorphosed from limestone Medium to coarse grained Crystalline texture Shows tight interlocking of calcite grains Formed under intermediate to high grade

metamorphism


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Other examples of use of MakranaMarble from India

Birla Mandir, Jaipur

Victoria Memorial, Calcutta

Sheikh Zayed Grand Mosque, Abu Dhabi


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Importance of Metamorphic rocks -SLATESFine grained impermeable, cleavable and softIncompetent; cannot withstand great loadsBut since they are impermeable and split easily; thin large sized slabs ofuniform thickness can be extracted for roofing purpose.Economic importance: Since they are bad conductor of electricity used in

electrical industries for switch board base

GNEISSGneissic rocks are rich in SILICA i.e. predominantly Quartz and Feldsparsalong with garnet, pyroxene, Hornblende etc.

Non-porous and impermeable nature increases the strength of the rockFoliated character to some extend improves workabilityLoad perpendicular to foliated planes gives more stronger foundation


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If mineral assemblage is more or less similar to Granite (with less % maficminerals) then:

It is used as building stone

As aggregate for making concreteAs road metals etc.

SCHISTMainly composed of prismatic or platy minerals, which contributes indevelopment of Schistose Structure. Eg. Hornblende, tourmaline, sillimaniteetc (prismatic); chlorite, muscovite, biotite, talc, kyanite etc. (platy)Cleavable nature of Schists is the main reason for their weakness; they areincompetent


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QUARTZITESANDSTONE (composed of quartz/feldspars/feldspathoid minerals) whenunder go metamorphism result into Quartzite.Granulose texture/structure (Granoblastic) makes them most competent rockamongst all other metamorphic rocks.Because metamorphism of Sst. Result disappearance of cementing material,

bedding planes, fossil content etc.

Quartzites are compact, hard and strong; very less porous and less permeablethan the parent Sst.Predominance of Quartz makes the rock very hard and suitable for roadmetal; can be used as concrete aggregate etc.Acts as strong foundation for any CE structure.

L6 Metamorphic Rocks

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