IGNEOUS ROCKS:

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IGNEOUS ROCKS

• The Rock Cycle
• Igneous Rocks Types and Formation
• Classifying Igneous Rocks

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Thought Question

• What are rocks? How are rocks different than
  minerals?
• Explain and diagram (draw it) the rock cycle.
• How do rocks form?

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Objectives

• Identify the three major types of rocks explain
  how each is formed
• Summarize and diagram the steps of the rock cycle
• Describe how the cooling rates of magma and lava
  affect the texture of igneous rocks
• Classify Igneous rocks according to their
  composition and specific rock structures
• Explain how rocks melt and crystallize discuss
  the factors that affect these rates.
• Classify the different types and textures of
  igneous rocks
• Discuss some common uses for igneous rocks

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THREE Rock Types

• Igneous- form when magma or lava cools can occur
  in or on the earths crust
• Magma-melted rock in the Earths surface
• Lava-melted rock that flows on the Earths
  surface
• Sedimentary-the deposition of broken down
  minerals, rocks, or organic matter that is
  hardened, cemented or compressed into rock
• Sediment-broken down minerals, rocks or organic
  material
• Metamorphic- rocks (igneous or sedimentary) that
  are changed by heat and/or pressure and/or
  chemical processes rocks that are chemically
  different from their parent material

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The Rock Cycle

• Rock Cycle- the geologic process that cause rocks
  to change from one form to another and back
  again

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Igneous Rock Structures

• Intrusive Igneous Rocks- Any rock that forms
  beneath the earths surface/ underground usually
  has visible/large crystals can be meters thick or
  can be several of kilometers thick
• Batholiths Stock Laccoliths Sills and Dikes

• Extrusive Igneous Rocks- Any igneous rock that
  forms on the surface of the earth usually has
  small, crystals not visible to the naked eye
  (lava pyroclastic material)
• Volcanic Neck Lava Plateau/field Bombs Ash

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Igneous Rock Samples
Intrusive Granite www.windows.ucar.edu
Extrusive Lava Flow-McGraw Hill Pub.
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Grain Size

• Cooling Rates
• When lava flows on Earths surface, it cools
  quickly and there is not enough time for large
  crystals to form.
• Extrusive igneous rocks have no visible mineral
  grains.
• When magma cools slowly beneath Earths surface,
  there is sufficient time for large crystals to
  form.
• Intrusive igneous rocks may have crystals larger
  than 1 cm.

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Large or Small Crystals?

• Texture- the appearance of a rock based on how
  visible the crystals/minerals in a rock are
• Coarse Grain- large visible crystals easily seen
  by the naked eye
• Fine Grain- small crystals that would only be
  visible with a powerful hand-lens or microscope
• Porphyritic- the mixture of large and small
  crystals

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Examples of Igneous Rx
Two Varieties of Granite
Fine Grain Basalt
Granite Porphory
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Composition and Origin of Magma

• Magma is a mix if molten rocks, dissolved gases,
  and minerals.
• Classified based on the amount of Silicates
  (SiO2) found in the magma three major types
• Rhyolitic- 70 SiO2 (usually light in color)
• Andesitic- 60 SiO2 (intermediate in color)
• Basaltic- 50 SiO2 (dark in color)

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Magma Formation

• Four major factors that affect the formation of
  magma are
• Temperature-different types of rocks melt at
  different temperatures (increases with depth)
• Pressure-the greater the pressure, the higher the
  melting point (increases with depth)
• Water Content-the addition of water tends to
  lower the melting point of most rocks
• Mineral Content-different minerals melt at
  different temperatures
• Silica content affects melting temperature and
  also impacts how quickly magma flows.

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For rocks to melt, the right combination of
temperature, pressure, and composition must be
present.
In general, oceanic crust is rich in iron and
magnesium and therefore melts at higher
temperatures than continental crust, which
contains higher levels of silicon and aluminum.
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Origins of Magma

• In the laboratory, most rocks must be heated to
  temperatures of 800C to 1200C before they melt.
  
• These temperatures are found in the upper mantle
  and lower crust.
• Scientists theorize that the remaining energy
  from Earths molten formation and the heat
  generated from the decay of radioactive elements
  are the sources of Earths thermal energy.

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• Granites higher water content and mineral
  composition cause it to melt at a lower
  temperature than basalt.

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How Rocks Melt-

• Partial melting is the process whereby some
  minerals melt at low temperatures while other
  minerals remain solid.
• Occurs because different minerals have different
  melting points not all parts of a rock melt at
  the same time.
• If temperatures are not great enough to melt the
  entire rock, the resulting magma will have a
  different chemistry from that of the original
  rock.

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Fractional Crystallization

• Fractional crystallization is the process wherein
  different minerals form at different
  temperatures.
• When magma cools, it crystallizes in the reverse
  order of partial meltingthe first minerals to
  crystallize from magma are the last minerals to
  melt during partial melting.

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Bowens Reaction Series

• Bowens reaction series illustrates the
  relationship between cooling magma and mineral
  formation.
• Bowen discovered two main patterns, or branches,
  of crystallization
• The first pattern is characterized by a
  continuous, gradual change of mineral
  compositions in the feldspar group.
• The second pattern is characterized by an abrupt
  change of mineral type in the iron-magnesium
  groups.

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Families of Igneous Rx

• Felsic- rx that are silica rich, are light in
  color (mainly quartz and orthoclase feldspar) can
  be coarse or fine grained
• Granite and obsidian (volcanic glass)
• Intermediate- rx that range in color from dark to
  light containg both felsic and mafic forming
  minerals (a mix of these types of rx) contain
  less quartz than felsic rx
• Diorites and Andesites are common forms
• Mafic- iron and magnesium rich rx that are low in
  silica tend to be dark in color and are mainly
  composed of feldspars and pyroxene minerals and
  lack quartz can be ferromagnetic
• Corse and fine grain basalts are common forms

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