Metamorphic Rocks

Metamorphic RocksWhat are metamorphic rocksFactors controlling metamorphismKinds of metamorphismClassification of metamorphic rocks

What are metamorphic rocks?Increases in heat and pressure and changes in chemical environment can alter the mineral composition and crystalline textures of sedimentary and igneous rocks even though they remain solid all the while (i.e. they do not melt)

What are metamorphic rocks?Heat & PressureCauses of metamorphism

Factors Controlling MetamorphismTemperatureRe-crystalisation new mineral assemblagesSegregation of new minerals bandingPressureConfining pressure acts in all directions alters mineralogy by squeezing atoms into a denser stateDirected pressure acting in a particular direction convergent plate boundaries causes preferred orientation - foliationMetamorphic changes bring a pre-existing rock into equilibrium with new surroundings

Factors Controlling MetamorphismChemical metamorphic changesIntroduction & removal of chemical componentsHydrothermal fluids associated with magma cooling will react with existing rocks replacing one mineral with another without changing the texture of the rock metasomatismMany valuable deposits of Cu, Pb, Zn etc formed in this way

Factors Controlling MetamorphismFluids in MetamorphismMost chemical & mineralogical changes are associated with fluids which permeate the solid rockWater containing carbon dioxide is most commonAccelerates metamorphic chemical reactions by allowing movement of ions. Water can migrate through the rock.Pore water and chemically bound water are lost during metamorphism the higher the metamorphic grade the lower the water content.

Kinds of MetamorphismRegional RegionalContact ContactCataclasticHydrothermalBurial

Kinds of MetamorphismOccurs where both high temperature and high pressure are imposed over large belts of the crustDestroys all original sedimentary or igneous textures through growth of new mineralsOccurs in deeper levels of the crust along convergent plate boundaries particularly continental-continental boundaries where active mountain building is taking place. Regional Metamorphism

Kinds of MetamorphismHeat and pressure generated by igneous intrusionsAffects only a thin region of intruded rock.At shallow depths the mineral transformations generally associated with heat pressure becomes important for intrusion at great depthContact Metamorphism

Kinds of MetamorphismFound along faultsFault movement causes rocks on either side to fragmentThe product is a rock with a broken and pulverised textureGenerally found in strongly deformed mountain belts often found with regionally metamorphosed rocksCataclastic Metamorphism

Kinds of MetamorphismOften associated with mid-ocean ridgesSeawater heated by upwelling magma promotes chemical reactions with basalt causing alterationAlteration is also common in other igneous rocks such as granite.Alteration can result in the rock becoming weaker, more compressible and more permeable the end product is very similar to that produced by chemical weatheringHydrothermal Metamorphism

Kinds of MetamorphismDiagenisis grades into burial metamorphismLow grade metamorphisms resulting from heat and pressure exerted by overlying sediments and sedimentary rocksBedding and other sedimentary structures are preservedBurial metamorphism can grade into regional metamorphismBurial Metamorphism

Classification of Metamorphic RocksFoliated RocksA set of flat or wavey parallel planes produced by the preferred orientation of minerals, particularly platey minerals like mica

Classification of Metamorphic RocksFoliated RocksCriteria

Nature of foliationGrain sizeDegree of bandingMetamorphic grade

Classification of Metamorphic RocksSlateSchistGneissFoliated Rocks

Classification of Metamorphic RocksHornfelsProduct of contact metamorphismUniform grain size- little or no deformation plate/elongate crystals are randomly orientatedOften harder and more brittle than parent rockMarbleProduct of heat & pressure acting on limestone or dolomiteOften show irregular banding or mottling due to impuritiesGranular textureQuartziteDerived from quartz-rich sandstones (contact or regional metamorphism) often very hard and extremely strongNon-foliated Rocks