1Essentials of GeologyEssentials of GeologyDavid SalleeDavid SalleeChapter 7Chapter 7Metamorphic RocksMetamorphic RocksWhy Should You Study Why Should You Study Metamorphic Rocks?Metamorphic Rocks?Exposed Exposed metamorphic rocks metamorphic rocks make up large make up large parts of continentsparts of continentsCertain minerals in Certain minerals in metamorphic rocks metamorphic rocks give clues about give clues about the conditions the conditions existing when they existing when they formedformedUsed widely as Used widely as building materials building materials and manufacturingand manufacturingWhat are the Agents of What are the Agents of Metamorphism?Metamorphism?HeatHeat••increases the increases the rate of chemical rate of chemical reactions that reactions that produce produce different different mineralsmineralsHeat sourcesHeat sources••intrusive magmaintrusive magma••deep burial deep burial along subduction along subduction zoneszonesWhat are the Agents of What are the Agents of Metamorphism?Metamorphism?PressurePressure••lithostatic pressure lithostatic pressure results from the results from the weight of overlying rocksweight of overlying rocks••mineral grains are more closely packedmineral grains are more closely packed••recrystallization may occur, producing smaller and recrystallization may occur, producing smaller and denser mineralsdenser minerals••differential pressure results from unequal forces differential pressure results from unequal forces applied to the rockapplied to the rockWhat are the Agents of What are the Agents of Metamorphism?Metamorphism?Fluid ActivityFluid Activity••water and carbon dioxide are almost always water and carbon dioxide are almost always present in some amount in metamorphic regionspresent in some amount in metamorphic regions••these fluids enhance metamorphism by increasing these fluids enhance metamorphism by increasing the rate of chemical reactionsthe rate of chemical reactionsFluid SourceFluid Source••water trapped in the pore space of sedimentary water trapped in the pore space of sedimentary rocksrocks••volatile magmatic fluidsvolatile magmatic fluids••dehydration of waterdehydration of water--bearing minerals that are bearing minerals that are subjected to heat and pressuresubjected to heat and pressure2What are the Three Types of What are the Three Types of Metamorphism?Metamorphism?Contact Contact MetamorphismMetamorphism••produced when a produced when a body of magma body of magma alters the alters the surrounding surrounding country rockcountry rockFactors in contact metamorphismFactors in contact metamorphism••initial temperature and size of the intrusioninitial temperature and size of the intrusion••presence and chemistry of fluidspresence and chemistry of fluidsMetamorphic aureolesMetamorphic aureoles••zones of mineral assemblages surrounding intrusionzones of mineral assemblages surrounding intrusionWhat are the Three Types What are the Three Types of Metamorphism?of Metamorphism?Dynamic Dynamic MetamorphismMetamorphism••results from results from high high differential differential pressurespressures••most often most often found near found near faultsfaults••mylonites are mylonites are hard, dense, hard, dense, and fine and fine grainedgrainedWhat are the Three Types of What are the Three Types of Metamorphism?Metamorphism?Regional Regional MetamorphismMetamorphism••produces most produces most metamorphic rocks; metamorphic rocks; makes up the shieldsmakes up the shields••results from extreme results from extreme heat and pressure heat and pressure generated by the generated by the processes at processes at convergent plate convergent plate boundariesboundaries••covers large covers large geographic areas, and geographic areas, and usually shows a usually shows a gradation of gradation of deformation deformation corresponding to areas corresponding to areas of the most intense of the most intense heat/pressureheat/pressureWhat are the Three Types of What are the Three Types of Metamorphism?Metamorphism?Regional Regional MetamorphismMetamorphism••certain certain minerals are minerals are known to form known to form under specific under specific conditions of conditions of temperature temperature and pressure and pressure --index mineralsindex minerals••low, intermediate, and high grades of low, intermediate, and high grades of metamorphism are recognized, based on conditions metamorphism are recognized, based on conditions and the resulting mineral assemblagesand the resulting mineral assemblagesHow are Metamorphic Rocks How are Metamorphic Rocks Classified?Classified?Foliated Foliated Metamorphic RocksMetamorphic Rocks••Heat and differential Heat and differential pressure cause minerals pressure cause minerals to arrange themselves to arrange themselves in a parallel fashion in a parallel fashion called foliationcalled foliation••size and shape of size and shape of crystals determines crystals determines texture; fine to coarsetexture; fine to coarse••with increasing grain with increasing grain size, common foliated size, common foliated metamorphic rocks metamorphic rocks include:include:QQSlateSlateQQPhyllitePhylliteQQSchistSchistQQGneissGneissFoliated TexturesFoliated TexturesSlateSlateGneissGneiss3How are Metamorphic Rocks How are Metamorphic Rocks Classified?Classified?Nonfoliated Nonfoliated Metamorphic Metamorphic RocksRocks••minerals do not minerals do not show a discernible show a discernible preferred preferred orientationorientation••most common are most common are those composed those composed mainly of one mainly of one mineral mineral --marble or marble or quartzite quartzite --or very or very fine grained fine grained textures as in textures as in greenstone or greenstone or hornfelshornfelsWhat are Metamorphic Zones What are Metamorphic Zones and Facies?and Facies?Metamorphic Metamorphic zones in the zones in the Upper Upper peninsula of peninsula of MichiganMichiganWhat are Metamorphic What are Metamorphic Zones and Facies?Zones and Facies?A pressure A pressure temperature temperature diagram shows diagram shows where various where various metamorphic metamorphic facies occurfacies occur••facies are groups facies are groups of rocks of rocks characterized by characterized