Basics of Metamorphic Rocks

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  • Metamorphic Rock
    • Change d by  Heat and/or pressure
      • Heat:Rocks close to an intrusion of magma will become heated, Rocks buried deep in the crust will become heated.
      • Pressure: Rocks in active zones (continental edges/mountain chains), Rocks buried deep will be affected by pressure
      • Mineralogy is changed, minerals react and recrystallize to suit new conditions
      • Texture is changed; as new minerals develop, their size, shape and arrangement    will change
    • Grades
      • High grade: greatly changed and looks very different from its original form
      • Medium grade or low grade metamorphism would mean less change has taken place
    • Thermal Metamorphism
      • Heat only
      • Occurs mostly around igneous intrusions
        • The amount of change depends on the amount of heat. Large batholiths cause more metamorphism than smaller dykes and sills.
        • Zone around a batholith is called a metamorphic aureole.
          • Variations can occur within an aureole because rocks closer to the intrusion will be changed more then those further away.
        • Around  smaller intrusions, or beneath lava flows, there is not enough heat to produce an aureole so it is called a baked margin
          • Against it you will find the rapidly cooled edge of the igneous rock called the chilled margin
        • The original  rocks into which a magma has intruded are called the country rocks.
    • Regional Metamorphism
      • Heat  and Presure
      • Whole region is altered-occurs when a large volume of     continental      crust is deformed into a mountain chain.
        • This formation        process is called an orogeny and regional. met. is a vital part of this process.
      • The most intense changes occur at great depth
        • Only in uplifted and eroded remains of ancient mountains can can the full effects be seen.
      • When a whole region of rock is altered. the grade differences throughout the area.
      • Because if the high amounts of pressure, minerals become aligned and show a foliated (banded) texture.
        • Example: Schist
        • Example: gneiss
    • Identifying Metamorphic Rocks
      • Because igneous     rocks where formed under high temperatures thermal met. has little effect on them.
        • Regional met. can cause fine-grained volcanics to recrystallise under pressure


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