Metamorphism   and Metamorphic Rocks

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Introduction. Metamorphism - The transformation of rocks, usually beneath Earth's surface, as the result of heat, pressure, and/or fluid activity, produces metamorphic rocksDuring metamorphism, rocks are subjected to sufficient heat, pressure and fluid activity to change their mineral composition

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Metamorphism and Metamorphic Rocks

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1. Chapter 7 Metamorphism and Metamorphic Rocks

2. Introduction Metamorphism - The transformation of rocks, usually beneath Earth's surface, as the result of heat, pressure, and/or fluid activity, produces metamorphic rocks During metamorphism, rocks are subjected to sufficient heat, pressure and fluid activity to change their mineral composition or texture, or both. All this occurs below the melting point in the solid state.

3. Introduction Metamorphism is also responsible for producing a number of economically valuable materials, like marble, a favorite of sculptors throughout history. Metamorphism is an important process that is closely related to plate tectonics, the growth of continents, and even climate change.

4. Introduction Distribution of Metamorphic Rocks 1. Shields – oldest part of the continental crust 2. Cores of large mountain ranges

5. The Agents of Metamorphism The three principal agents of metamorphism are heat, pressure and fluid activity. Intrusive magmas or deep burial provide heat which causes metamorphism. Pressure is produced by overlying rocks (lithostatic) or is differential pressure produced by various stresses. Fluid activity increases the rate of metamorphism.

6. The Agents of Metamorphism Heat Heat is an important agent of metamorphism Heat Increases the rate of reactions Sources of heat include: Extrusive lava Intrusive magma Deep burial Temperature increase with depth. The geothermal gradient averages about 25 degrees C/ km.

7. The Agents of Metamorphism Pressure What are lithostatic and differential pressures, and why are they important? Lithostatic pressure is a uniform field of pressure experienced by most rocks beneath Earth’s surface. Like the hydrostatic pressure experienced by divers underwater, the pressure acting on a rock embedded in the crust “feels” the same from all directions.

8. The Agents of Metamorphism Pressure What are lithostatic and differential pressures, and why are they important? Differential pressure is a nonuniform field of pressure; the pressure acting on a rock in some directions is stronger than it is in others. Many metamorphic rocks form under conditions of differential pressure, which influences the development of metamorphic structures and textures in significant ways.

9. The Agents of Metamorphism Fluid Activity Fluids within sedimentary rocks or issuing from magmas can accelerate chemical changes which occur during metamorphism and can cause new minerals to form.

10. The Agents of Metamorphism The type of metamorphism that results largely depends on which of the three agents was dominant.

11. The Three Types of Metamorphism Contact Dynamic Regional metamorphism

12. The Three Types of Metamorphism Contact Metamorphism Contact metamorphic rocks form under conditions of high temperature and low pressure. They are arrayed in aureoles, or metamorphosed zones, around plutons and other intrusive igneous bodies.

13. The Three Types of Metamorphism Dynamic metamorphism Dynamic metamorphism is associated with faults and areas where high pressure builds up in the crust, but the temperature is low, such as in the accretionary wedges at convergent plate boundaries.

14. The Three Types of Metamorphism Regional metamorphism Regional metamorphism is the most common type of metamorphism. As the name implies, regional metamorphism has a broad range. Temperature and pressure both act as driving forces for metamorphic reactions in regional metamorphism.

15. The Three Types of Metamorphism Index Minerals and Metamorphic Grade Metamorphic grade – the degree of metamorphic change a rock has undergone. Index minerals – certain minerals are known to only form under specific temperatures and pressure.

16. How are Metamorphic Rocks Classified? Metamorphic rocks are classified principally according to texture. Foliated - Foliated texture is produced by the preferred orientation of platy minerals. Nonfoliated - Nonfoliated textures do not exhibit preferred orientation of minerals.

17. How are Metamorphic Rocks Classified? Classification of Common Metamorphic Rocks

18. How are Metamorphic Rocks Classified? Foliated Metamorphic Rocks Foliated texture is produced by the preferred orientation of platy minerals. Foliated metamorphic rocks form a graded series of grain size and/or development of foliation, from fine grained slate, to phyllite and coarser grained schist, to gneiss, with segregated bands of minerals. Amphibolite is another fairly common coarse grained foliated metamorphic rock.

19. How are Metamorphic Rocks Classified? Foliated Metamorphic Rocks Schist and Gneiss

20. How are Metamorphic Rocks Classified? Foliated Metamorphic Rocks Migmatite – contain streaks of granite

21. How are Metamorphic Rocks Classified? Nonfoliated Metamorphic Rocks Nonfoliated textures do not exhibit preferred orientation of minerals. Common nonfoliated metamorphic rocks are marble, quartzite, greenstone, and hornfels.

22. Metamorphic Zones and Facies Metamorphic zone – a belt of rocks showing roughly the same degree of metamorphism

23. Metamorphic Zones and Facies Metamorphic facies – refers to a group of rocks containing a distinctive mineral assemblage formed under similar conditions of temperature and pressure.

24. Metamorphic Zones and Facies How do metamorphic zones and metamorphic facies differ? Metamorphic zones show the gradational metamorphic change within a single rock composition. Metamorphic facies are groups of many different rock compositions whose mineral contents all indicate common temperature and pressure conditions during metamorphism.

25. Plate Tectonics and Metamorphism Metamorphism can occur along all types of plate boundaries, but is most common and extensive along convergent boundaries.

26. Plate Tectonics and Metamorphism Metamorphic rocks formed near the surface and within an oceanic-continental convergent plate boundary zone result from low temperature and high pressure conditions. Blueschist facies

27. Plate Tectonics and Metamorphism Higher temperatures and pressures existing at depth within such plate boundaries produce higher grades of metamorphism in a subducting oceanic plate.

28. Metamorphism and Natural Resources Mineral resources which are metamorphic rocks include marble and slate.

29. Metamorphism and Natural Resources Mineral resources which are metamorphic rocks include marble and slate.

30. Metamorphism and Natural Resources Mineral resources which are metamorphic minerals include graphite, talc, asbestos and garnet.

31. End

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