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Geology 101 Week 4

by: Kayla Corbett

Geology 101 Week 4 GEO 101

Kayla Corbett
GPA 3.5

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Lecture #7 Metamorphism: A process of change
The Dynamic Earth
Dr. Natasha T. Dimova
Class Notes
Geology 101
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This 5 page Class Notes was uploaded by Kayla Corbett on Tuesday February 16, 2016. The Class Notes belongs to GEO 101 at University of Alabama - Tuscaloosa taught by Dr. Natasha T. Dimova in Fall 2016. Since its upload, it has received 18 views. For similar materials see The Dynamic Earth in Geology at University of Alabama - Tuscaloosa.


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Date Created: 02/16/16
Lecture #7: Metamorphism: A process of change Chapter 7: Metamorphism  Transition of one rock (igneous, sedimentary and metamorphic) into another metamorphic rock  by applying: 1.) high temperature and/or 2.) pressure;  3.) high temperature fluid. Rock remains essentially solid! 1.Temperature (heat)­dominant role! ­ nearby magma body ­ geothermal gradient Thermal metamorphism:analog from life Bake (“fire”) fundamentally and permanently changes the clay (not just drying it !) 2. Pressure Confining (Lithostatic) pressure ­ weight of the overlying rocks.  Applied equally in all directions  Increases with depth  Minerals grains become more closely packed and may re­crystallize becoming smaller  and denser minerals Pressure cont.­differential  Pressure that is not equal on all sides; it is directional  Rock is distorted; minerals align  Occur during deformation associated with mountain building 3. Fluids  Water and other volatiles   Enhance migration of ions and promotes recrystallization of existing minerals  Where does the water come from?  Pore spaces of sedimentary rocks  Fractures in igneous rocks  Hydrated minerals  Example: CaSO 4 ∙2H 2 O 4. Parent rock  Mineral make­up determines the degree to which change will occur Result of methamorphism 1.) Recrystallization: atoms re­arrange into new crystalline configuration Example: if a rock is composed by quartz 2SiO ) the tetrahedrons will reorganize in different  configuration. This will change visually the rock, i.e. its texture.  The chemical composition though stays the same. 2.) New mineral growth: bonds between atoms in the minerals break and new form, resulting in  totally different new mineral. This will change visually the rock, i.e. its texture.  The chemical composition will change to, totally different mineral. 3.) Metasomatism: rock composition alters by removing/adding new elements into the rock.     3.1 This can happen if we circulate hot water through the rock which will dissolve partially  certain chemicals from the rock.    3.2 As this water cools down, a new mineral may precipitate/add to the rock. Both texture and chemical composition will change! Types of metamorphism 1. Contact (thermal) metamorphism  Temperature of the hosting rock rises as magma flows through it due to friction between  them  The zone of alteration that is in contact with the magma is called metamorphic aureole Aureoles ­vary in width depending on the size, temperature, and composition of the intrusion, as  well as the composition of the surrounding country rock. 2. Burial Metamorphism • The changes are ONLY due to 1.) weight of overburden; 2.) temperature increases only  due geothermal gradient  – <10­15 km diagenesis, i.e. geochemical changes – >15 km low­grade metamorphism 3. Hydrothermal metamorphism  Chemical alteration caused when hot, ion­rich fluids circulate through cracks Types of metamorphic rocks  Divided into groups based on texture (i.e. the size, shape and orientation of the mineral  grains in the rocks) Metamorp Weakly- Foliat Non- foliated Example of metamorphism: shale Parent rock: shale* a fine­grained sedimentary rock; forms from the compaction of silt and clay­size mineral particles ("mudstones“) *contains a significant amount of organic material in the form of solid kerogen, part of the  composition of oil and gas.  How does shale change when we apply high temperature and pressure? 1.) Clay minerals start being replaced by mica crystals  As mica grows, it rotates and orients (aligns) perpendicular to the direction of the applied  stress/pressure: the rock forms a foliation called slaty cleavage.  Shale transforms into Slate! 2.) The parent rock now is slate. What happens when we continue applying pressure/ temperature (or increase the metamorphism)?  The mica crystals will continue to grow larger and align producing more defined  foliation.  Slate transforms into Phyllite! 3.) The parent rock now is Phyllite. What if we continue to increase the metamorphism?  The mica crystals grow so large that can be seen with naked eye (without magnification).  The rock has already this shiny luster  Phyllite transforms into Schist! 4.) The parent rock now is Schist. Let’s apply high­grade metamorphism?  Due to very high pressure and temperatures the atoms within the minerals will migrate  and form bands of dark minerals (amphibole and biotite) and light (quartz and feldspar)  Schist transforms into Gneiss! Summary shale metamorphism By changing pressure and temperature we were able to observe transformation of one  sedimentary rock (shale) into different rocks Main types of foliated textures* Slaty cleavage ­planar surfaces along which rocks split Schistosity ­ platy minerals exhibit layered structure Gneissic ­ high grade metamorphic, minerals segregate, distinctive banded appearance *Recognizing the texture in a rock help us understand the level of metamorphism this rock has  gone through. Foliated Metamorphic Rocks: summary properties Slate  Very fine­grained  Slaty cleavage  Generated from low­grade metamorphism of shale Phyllite  Minerals not large enough to be identified  Glossy sheen & wavy surfaces  Fine crystals of muscovite and/or chlorite Schist  Medium­ to coarse­grained   Micas predominate  Schistosity texture Gneiss  Medium­ to coarse­grained  Banded appearance  High­grade metamorphism Nonfoliated  Deformation is minimal   Equidimensional crystals Porphyroblastic   Large grains surrounded by fine­grained matrix of minerals Examples of non­foliated metamorphic rocks and their properties Marble  Coarse, crystalline  Composed of calcite or dolomite crystals Quartzite  Parent rock: quartz­rich sandstone  Quartz grains fused together Level of metamorphism? Metamorphic grade: indicates the intensity of metamorphism or the degree of metamorphic  change. Increasing  Temperature and Pressure in a  clay­rich sediment (shale) Slate   Low­grade metamorphic Phyllite Schist Gneiss      High grade metamorphic Metamorphism and plate tectonics Continent­Continent collisions  Edges of plate deforms  Major mountain ­ Alps, Himalayas, and Appalachians – are metamorphic Subduction Zones Distinct linear belts of metamorphic rocks  High­pressure, low­temperature zones nearest the trench  High­temperature, low­pressure zones further inland 


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