Intro to Rocks

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					          Intro to Rocks
Major Rock Types:
     There are three major rock types
           1. Igneous—Rocks formed from
cooling of magma or lava.
           2. Sedimentary—Rocks formed
from sediments worn from other rocks.
           3. Metamorphic– rocks formed
by changing the chemistry, mineralogy, or
texture of other rocks.
               Rock Cycle
• Rock types are all connected in a cycle of
  formation, change, and destruction which
  we call the Rock Cycle.
• Let us start the rock cycle with molten rock
  (magma), which cools and forms igneous
  rocks. These rocks become uplifted as
  mountains are formed (orogeny). There it
  is attacked by the weather and starts to
                 Rock Cycle
• This weathered (eroded) material is carried
  away by streams, rivers, wind, glaciers and
  deposited elsewhere as sediments.
• The sediments are then buried and lithified
  (turned into solid rock) being subjected to heat,
  pressure, and fluids the sedimentary rocks
  becomes metamorphic rocks.
• Metamorphic rocks maybe uplifted and eroded
  or may become heated to the point that it again
  becomes Magma.
                              Rock Cycle
      What are Igneous Rocks?
• The term Igneous comes form the Latin
  word ignis meaning fire. These are rocks
  which form from cooling magma or lava.

• Magma: Molten or partially molten rock
  materials and dissolved gases beneath
  earth’s surface.
• Lava: Is also molten or partially molten
  rock material and dissolved gases which
  erupts at the Earth’s surface.
   Classifying Igneous Rocks
• Two methods are used: Texture and
  – Texture: a term which involves how a rock
    looks. This method involves:
    • 1. The size of the mineral grains (crystals)
    • 2. Does the rock have holes (vesicles) in it. If it has
      a lot of holes it is called a vesicular texture. This is
      an indication that the rock was lava and at the
      surface as it erupted or cooled.
    • 3. Is the rock a coherent mass of mineral grains or
      from smaller chunks of igneous rock which has
      been cemented or welded together (pyroclastic
• Using the texture characteristics Igneous
  rocks can be classified as either Intrusive
  or Extrusive.
  – Intrusive: Rocks composed of large crystals.
    This indicates slow cooling below the earth’s
  – Extrusive: Rocks composed of small,
    microscopic, or no crystals (obsidian)
    indicates rapid cooling at the earth’s surface.
      Chemical Classification
• Based on the Chemical Composition
  igneous rocks can be broken into 4
  general types.
  – Felsic: High in silica (65%+)
     • Usually light colored
     • Examples Rhyolite (extrusive) and Granite
  – Intermediate: Lower silica content (55-65%)
     • Darker than felsic, lighter than mafic
     • Example Andesite/dacite (extrusive) and
       Diorite/granodiorite (intrusive).
      Chemical Classification
– Mafic: low silica content (45-55%)
   • Ususally dark colored
   • Example basalt (extrusive) and gabbro (intrusive).

– Ultramafic: Extremly low silica content (less
  than 45%)
   • Usually dark colored, but high olivine content tend
     to produce green colors. There are also other rare
   • Example periodotite (intrusive)
Classification of Igneous Rocks
     How does Magma form?
• Magma originates from melting rocks. But
  rocks are made up of different minerals
  which melt at different temperatures (not
  uniformly). Some may not melt completely
  resulting in Partial melting.
• Melting temperatures may be affected by
  environmental conditions such as
  pressure, amount of water. Higher
  pressures increase melting temperatures,
  presence of water lowers the melting
       Magmatic Differentiation
• Partial melting also results in Partial
  freezing and called magmatic
  differentiation. Freeze does not mean cold.
  – Those minerals which have a higher melting
    temperature will also freeze first which will
    change the chemical composition of the
    remaining melt.
  – This process continues until all the magma
    has frozen or solidified.
       Chemical Classification
• There are three general paths that igneous rock
  may take as they cool depending on their
  chemical composition.
  – Continuous Reaction Series: deals with those melts
    that are calcium rich or sodium rich composition
    (plagioclase feldspars).
     • As the plagioclase feldspars crystallize, the first crystals are
       calcium rich which leaves the sodium rich melt behind. This
       continues till the melt reaches equilibrium again. Again
       calcium crystals come out followed by sodium. This
       continues till the entire melt is solid.
Bowen’s Reaction
       Chemical Classification
• Discontinous Series: As a mafic melt cools
  slowly the first crystals that form are olivine,
  followed by pyroxenes with the olivine being
  converted to pyroxenes. As the temperature
  farther lowers amphiboles crystallizes and all the
  pyroxenes convert to amphiboles. Farther lower
  of temperature results in Mica forming with the
  amphiboles all convert to Mica.
• Discontinous series can be seen as separate
  crystallizations and conversions, so that there is
  only one type of mineral present at a time.
      Chemical Classification
• Fractional crystallization says that after
  crystals form, they are somehow
  separated from the remaining molten
  material and don’t re-enter the reaction.
• One possible way that this might occur is
  the crystals as they form settle out due to
  gravity. Or as crystals form the melt moves
  to another location (squeezed out so to
Origin of Different types of Magmas
• Mafic Magmas (basaltic) has two sources:
  – 1. Mid-Ocean Ridges, where mafic magma
    rises up to form new ocean floor.
  – 2. Mid-Plate Volcanoes are sites where mafic
    magma rise up from great depths (where one
    plate is being subducted under another plate)
    this results in eruptions on the surface.
    • Mafic to intermediate volcanism can also occur in
      this location.
Origin of Different types of Magmas
• Felsic Magmas forms as oceanic plates move
  along picking up mud, silt, and wet sediments.
  As one oceanic plate is pulled under another
  plate the wet sediment are also pulled under. As
  the plate and sediments heat up due to pressure
  the water is driven out and into the mantle. This
  water lowers the melting point and rise (lower
  density), resulting in mixing with the material
  from the overlying continent (more melting
  occurs). Finally the melt reaches the surface as
  volcanoes or cools within the crust.
• You need to know the following:
   – 1. Different classes of rocks.
   – 2. How they are formed, and how they are tied together through
     the rock cycle. Able to draw out the rock cycle and explain
   – 3. What are igneous rocks?
   – 4. Classification and Types of igneous rocks (p.124 text)
       • Texture (intrusive vs. extrusive)
       • Chemical classification
   – 5. Magmatic differentation
       • General idea
       • Bowen’s series
   – 6. Fractional crystallization
   – 7. Where do different igneous rocks form (pp.122-123)
   – 8. Name and definitions of various intrusive bodies (p. 125 text)
                      SITES USED
Thanks to Greg Anderson for use
 of lecture notes.

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