Metamorphism
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Metamorphism

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    Metamorphism Metamorphism Presentation Transcript

    • Metamorphism – transformation of one rock type into another
      • Metamorphic rocks – produced from pre-existing sedimentary, igneous and metamorphic rocks
      • Parent rock – origin of a metamorphic rock
      See next slides for samples of metamorphic rocks
    • Norway
    • China
    • Lake Huron, Ontario, Canada
    • Michigan, North America
    • California, North America
    • California, North America
    • Metamorphism leads to changes in:
      • Mineral content
      • Texture
      • Chemical composition
      Metamorphic agents are:
      • Heat
      • Pressure
      • Chemically active fluids
    • Metamorphism occurs from a few km into the crust all the way down to the upper mantle.
    • Degrees of metamorphism
      • Low-grade metamorphism : little change in the parent rock
      • High-grade metamorphism : identity of parent rock becomes hard to distinguish
      • Remember : parent rock should not melt, if it does, then igneous processes (not metamorphic) become involved
    • Example of low-grade metamorphism (from shale to slate)
      Shale (sedimentary rock)
      Slate (metamorphic rock)
    • Example of high-grade metamorphism: from shale (top) to slate (top right) to gneiss (right)
    • Degrees of metamorphism
      Agents of metamorphism
      Time
      low-grade
      LOW
      SHORT
      Heat
      Pressure
      Chemical activity
      HIGH
      high-grade
      LONG
    • Heat as an agent of metamorphism
      Heat provides the energy of activation for the chemical transformation in metamorphic rocks.
      Chemical reactions result in:
      • recrystallization of existing minerals
      • formation of new minerals
      Chemical changes are all based on the increased kinetic energies of the ions.
    • Heat as an agent of metamorphism
      Temperatures increase with depth at a rate known as the geothermal gradient.
      Hence, degree of metamorphism increases as depth increases.
    • The deepest mine in the world
      The Western Deep Levels Mine in South Africa at 4 km is the deepest in the world.
      At these depths, the rock is actually hot enough to burn human skin. Miners work in pairs- one digging, the other operating a large fan to keep cool.
    • What heats up the Earth?
      • Primordial heat – trapped when Earth’s crust first cooled
      • Radioactivity – energy released when subatomic particles are spontaneously emitted by radioactive elements
      • Solar radiation – absorbed and converted to thermal energy by the Earth’s surface
    • Primordial heat
      Earth initially formed as a molten ball of rock. When the surface cooled to become the crust, the heat became trapped.
      This trapped heat now drives the convection cells in the mantle and the movement of the molten outer core round the inner core.
    • Solar radiation – the Earth’s crust absorbs about 50% of the Sun’s energy
    • The absorbed solar radiation is distributed unevenly. The greatest concentration occurs in the tropical belt (red zone).
    • Radioactivity
      Radioactive elements contribute to Earth’s internal heat. Background radiation (not enough to harm) exists all around us. Shown at right is a geologist measuring background radiation with a portable Geiger counter.
    • Pressure as an agent of metamorphism
      Two types of pressure:
      • Confining pressure – experienced by buried rocks; forces are applied equally in all directions
      • Differential stress – forces are unequal in different directions
    • Confining pressure is evenly distributed.
      • squeezes out the spaces between mineral grains
      • produces a more compact, denser rock
    • Confining pressure produces rock layers that are undeformed.
    • Directonial stress deforms rock layers as shown in C. (the series shows the deformation of sediments deposited in a river flood plain)
    • The layers of rock shown below have been deformed by directional stress. Such pressure is most active in convergent plate boundaries.
    • Rocks in shallow depths are pulverized when subjected to differential stress. At greater depths, on the other hand, rocks are more ductile and are compressed and elongated rather than crushed.
      A sample of conglomerate becomes metaconglomerate when subjected to differential stress at depth (note the elongated rock fragments).
    • Heat and pressure
      Heat increases with increasing pressure. This means that areas of mountain building (continental vs continental convergent plates) are hotter due to greater pressure.
    • METAMORPHIC ROCKS
      • definition
      • degrees of metamorphism
      • agents of metamorphism
    • Where does metamorphism occur?
      • contact metamorphism
      • regional metamorphism
      • dynamic metamorphism
    • Contact metamorphism
    • Contact metamorphism
    • Contact metamorphism
    • Contact metamorphism
    • Contact metamorphism
      Granite
      Gneiss
    • From limestone to marble
    • From igneous rocks to marble
      Syenite
      (igneous)
      marble
    • From sediments to metamorphic rocks
    • Regional metamorphism
      Mountain-building:
      • plate tectonics (convergent boundaries)
      • directional stresses are involved
      • greatest volume of metamorphic rocks are produced in this way
    • Review of plate tectonics
    • Regional metamorphism
    • Regional vs contact metamorphism
    • Textural changes
      Texture – size, shape, and distribution of particles that constitute a rock
      • foliated
      • non-foliated
    • Textural changes (foliation)
    • Types of foliation:
      • rock or slaty cleavage – minute crystals
      • schistosity – larger crystals
      • gneissic texture – segregation of minerals
    • Rock or Slaty cleavage
    • Rock or
      Slaty cleavage
    • Rock or Slaty cleavage
    • Schistocity
      mica schist – most abundant schist type
    • Schistocity
      mica schist
    • Schistocity
    • Schistocity
    • Schistocity
    • Schistocity
    • Schistocity
      talc schist
    • Schistocity
      talc schist
    • Schistocity
      talc schist
    • Gneissic texture
    • Gneissic texture
    • Gneissic texture
    • Gneissic texture
    • Nonfoliated texture
      marble
    • Nonfoliated texture
    • Nonfoliated texture
    • Nonfoliated texture
    • Nonfoliated texture
      quartzite
    • Nonfoliated texture
      quartzite