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Rock Classification

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					 Igneous Rocks (IR)
a.k.a Volcanic Rocks


     Ms. Tasneem
Rock cycle
Igneous Rock Formation
How do Intrusive IRs Form?
          Igneous Intrusions
Intrusion = liquid rock formed under the
surface of the earth
    Igneous Intrusions-Batholiths
 Batholith (from Greek bathos, depth + lithos, rock)
  a large inclusion of intrusive (also called plutonic)
  igneous rock that forms from cooled magma deep in
  the earth's crust
Yosemite National Park
El Capitan
  Igneous Intrusion – Stock & Sill
 Stock = Smaller Batholith with surface
  exposures around 40 km2
 Sill = parallel to “country rock” beds and usually
  horizontal in orientation
  Intrudes between
  layers of
  sedimentary rocks
   Igneous Intrusion – Laccolith & Dike

  Laccolith = dome shaped intrusion
  between two layers of sedimentary rock

Dike = any
geologic
body that
cuts across
other rock
formations
        Intrusive Rock Formation
 Intrusive (Plutonic) = crystallized from slowly
  cooling magma intruded within the Earth’s crust

       Granite                       Gabbro
How would you group these
      paperclips?
    Igneous Rock Classification
Color = (Proxy for silica content)

Size = (Proxy for grain size)
         Composition of IR
1. How are minerals related to rocks?
2. How would knowing the different
   types of crystals help you describe
   the characteristics of igneous rocks
   or distinguish among the three
   general types of rocks?
Composition of IR = MINERALS!
          Granite
                         Compositional
           Quartz
                         Classification
           Orthoclase

           Biotite
Granite
           Plagioclase
          Granite
                          Compositional
          Granodiorite    Classification

                    Quartz
Granite
                    Amphibole

                    Plagioclase
          Granite
                         Compositional
          Granodiorite   Classification
                    Diorite

Granite
                              Plagioclase

                              Amphibole
          Granite
                         Compositional
          Granodiorite   Classification
                    Diorite

Granite
                              Gabbro



                                       Plagioclase

                                       Pyroxene
           Granite
                         Compositional
            Granodiorite Classification


                       Diorite

Granite
                                 Gabbro
                                          Pyroxene
                                            Olivine




          Peridotite
    Size: Grain size        Color: Silica
 Coarse Grained = large    Content
  crystals                  Mafic = IR rich in dark
 Fine Grained = Small       colored elements (Mg &
  crystals                   Fe) = low silica
                            Felsic = IR rich in silica
Common Minerals
Rock Cycle
        How do Extrusive IRs form?
 Volcanic (extrusive) igneous rocks
    Cool   fast, small crystals
    form when molten rock (magma) in the earth’s interior rises to
    the surface through pipes or fractures in the crust.
       How do Extrusive IRs form?
 Extrusive= or erupted as pyroclastic material, i.e.,
 fragmented pieces of magma ejected and cooled in
 the air.
                              Ash
                                    Pumice



      Obsidian       Scoria
Classification of Igneous rocks
 Texture size of crystals most important
      Cooling rates
       – Surface, fast
             Small crystals
                                      Which formed at surface?
       – Below surface slow           Both are the same chemistry
             Larger crystals

a)                              b)




     Rhyolite                        Granite
    Chemical Composition
   Color
      Indicative of chemistry and temperatures of formation
          – Dark colors
               High temperatures (1000 to 1200 oC)
               Low silica content
                                                      Which is the high silica rock?
          – Light colors
               Lower temperatures                    Both formed below the surface
               High silica content

     a)                                        b)




          Gabbro                                      Granite
How did this form? Intrusive or Extrusive?
Formation of IR
The Legend of Finn McCool
What really happened…
More of Ms. Tasneem’s photos
  Six common Igneous Rocks
1000 C          Solidifying Temperature                     500 C
                 Increasing Grain Size

   Volcanic Silica         Minerals               Plutonic
                           Present (in order of
   Rocks    Content        abundance)             Rocks
   Basalt   low            pyroxene,              Gabbro




                                                                    Lighter Color
                           olivine, feldspar,
                           & amphibole

   Andesite intermediate   feldspar,              Diorite
                           amphibole,
                           pyroxene, biotite
                           mica
   Rhyolite   high         feldspar, quartz,      Granite
                           muscovite mica,
                           & amphibole
5) Imagine that all minerals found in
rocks were the same color. What
information would you no longer be able
to infer?

a. texture       b. cooling rate

c. composition
 Analysis Time
 Given:
    Bag rocks


           Identify the rocks

				
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posted:10/8/2012
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