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Understanding Metamorphism & Metamorphic Rocks

Explore agents & types of metamorphism, rock classification, plate tectonics, & more. Learn how heat, pressure, & fluids transform rocks & create natural resources.

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Understanding Metamorphism & Metamorphic Rocks

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  1. GEOL: CHAPTER 7 Metamorphism and Metamorphic Rocks

  2. Learning Outcomes LO1: Identify the agents of metamorphism LO2: Identify the three types of metamorphism LO3: Explain how metamorphic rocks are classified LO4: Recognize the difference between metamorphic zones and facies LO5: Understand how plate tectonics affects metamorphism LO6: Understand the relationship between metamorphism and natural resources

  3. Metamorphism • Metamorphic rock: any rock that has been changed from its original condition by heat, pressure, and the chemical activity of fluids • Metamorphism: the phenomenon of changing rocks subjected to heat, pressure, and fluids so that they are in equilibrium with a new set of environmental conditions

  4. Agents of Metamorphism • Heat • Pressure • Fluid activity • Time for the above to chemically alter rocks is also an important factor

  5. Heat • Increases rates of chemical reactions • Can produce minerals different than those in the original rock • Lava • Magma • Burial: geothermal gradient; subducted rock

  6. Pressure • Pressure increases with depth • Lithostatic pressure: pressure exerted on rocks by the weight of overlying rocks • Differential pressure: • Pressure not applied equally to all sides of a body • Stresses stronger in some directions • Common with plate collisions

  7. Fluid Activity • Water (and carbon dioxide) are present in almost every region of metamorphism • Water increases reaction rates and thus metamorphism • Water also reacts with some minerals to create new minerals • Seawater moving through hot basaltic rock converts olivine to serpentine

  8. Fluid Activity, cont. • Sources of chemically active fluids: • Water trapped in pore spaces of sedimentary rocks • Volatile fluid in magma • Dehydration of water-bearing minerals

  9. Three Types of Metamorphism • Contact (thermal) metamorphism • Dynamic metamorphism • Regional metamorphism

  10. Contact Metamorphism • Metamorphism of country rock adjacent to a pluton • Magma from forming pluton: • Raises temperature of country rock • Releases hot fluids into country rock • Aureole: zone of metamorphism surrounding a pluton

  11. Contact Metamorphism, cont. • Important factors: • Initial temperature • Size of the intrusion • Fluid content of magma and country rock: hydrothermal alteration creates mineral deposits • Degree of metamorphic change decreases with distance from pluton • Can also occur with lava flows

  12. Dynamic Metamorphism • Occurs in fault zones where rocks are under high degrees of differential pressure • Mylonites frequently created • Hard, dense, fine-grained • Thin laminations

  13. Regional Metamorphism • Occurs over a large area • Caused by very high temperatures and pressures, occurring simultaneously with deformation • Convergent plate boundaries • Gradations of metamorphism based on severity of conditions

  14. Index Minerals and Metamorphic Grade • Index mineral: a mineral that forms within specific temperature and pressure ranges during metamorphism • Metamorphic grade: the degree to which a rock has undergone metamorphic change • Different rock compositions have different sets of index minerals

  15. Classifying Metamorphic Rocks • Foliated texture: Platy and elongate minerals aligned in a parallel fashion • Nonfoliated texture: no discernable preferred orientation of minerals

  16. Foliated Metamorphic Rocks • Foliated texture: Platy and elongate minerals aligned in a parallel fashion • From heat and differential pressure • Size and shape of mineral grains determine whether foliation is fine or coarse

  17. Slate • Low-grade metamorphism • Finely foliated • Minerals can only be seen with magnification • Slaty cleavage • Regional metamorphism of shale

  18. Phyllite • Coarser grained than slate, but similar composition • Need magnification to see minerals • Intermediate grain size between slate and schist

  19. Schist • Commonly produced in regional metamorphism • Type depends on intensity of metamorphism and characteristic of original rock • Many rock types yield schist • Minerals clearly visible • Schistosity/schistose foliation • Each type has associated minerals

  20. Gneiss • High-grade metamorphism • Segregated bands of light and dark minerals • Light bands: quartz and feldspar • Dark bands: biotite and hornblende • Forms from recrystallization of sedimentary rocks during regional metamorphism, or from igneous or metamorphic rocks

  21. Other Foliated Rocks • Amphibolite • Dark rock: hornblende and plagioclase • Slightly foliated • From basalt and ferromagnesian-rich mafic rocks • Migmatites • Igneous and high-grade metamorphic characteristics

  22. Nonfoliated Metamorphic Rocks • Nonfoliated texture: no discernable preferred orientation of minerals • Mosaic or roughly equidimensional minerals • Contact or regional metamorphism of rocks with no platy or elongate minerals

  23. Marble • Predominantly calcite or dolomite • Grains range from fine to coarsely granular • Contact or regional metamorphism of limestones or dolostones • Colors come from impurities in parent rock

  24. Quartzite • Formed from quartz sandstone • Intermediate to high-grade metamorphism • Typically complete recrystallization of quartz grains • Impurities add color

  25. Other Nonfoliated Rocks • Greenstone: dark-green, from mafic igneous rock; low- to high-grade metamorphism • Hornfels: many varieties, but mostly from sedimentary rocks; from contact metamorphism • Anthracite: hard coal with high % carbon; metamorphism of lower-grade coals

  26. Metamorphic Zones • Metamorphic zone: the region between lines of equal metamorphic intensity known as isograds • Metamorphic rocks divided into zones based on presence of distinctive silicate minerals

  27. Metamorphic Zones, cont. • Successive appearance of index minerals shows progression to higher-grade metamorphism • First appearance of a mineral indicates minimum temperature and pressure conditions • Isograds: lines of equal metamorphic intensity

  28. Metamorphic Facies • A group of metamorphic rocks characterized by particular minerals that formed under the same broad temperature and pressure conditions • Named after most characteristic rock or mineral • Not applicable when original rocks were pure quartz sandstones or pure limestones or dolostones

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