Geology 101 Week 4
Geology 101 Week 4 GEO 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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