Igneous Rocks and Classifying Igneous Rocks

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Igneous Rocks and Classifying Igneous Rocks

• Chapter 5

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Igneous Rocks

• Igneous rocks are formed from the crystallization
  of magma.

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Igneous Rocks

• Extrusive igneous rocks are fine-grained rocks
  that cool quickly on Earths surface.

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Igneous rocks

• Basalt is an extrusive igneous rock that is very
  dark in color. It is the most common type of rock
  in the Earth's crust and it makes up most of the
  ocean floor.

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Igneous Rocks

• Intrusive igneous rocks are coarse-grained and
  cool slowly beneath Earths surface.

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Igneous Rocks

• Granite
• The most common intrusive igneous rock
• Many granite deposits cross-cut into other rock
  formations
• This cross-cutting is evidence that granite was
  intruded into existing rocks

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Igneous Rocks

• Review of magma
• A slushy mix of molten rock, gases, and mineral
  crystals.
• Silica (SiO2) is the most abundant compound in
  magma and has the greatest effect on its
  characteristics.
• Basaltic 50 silica, Andesitic 60 silica,
  Rhyolitic 70 silica
• Silica affects melting temp. and viscosity

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Igneous Rocks

• Factors that affect magma formation
• Temperature
• Temperature generally increases with depth in
  Earths crust.
• Pressure
• Pressure also increases with depth
• As the pressure on a rock increases, its melting
  point also increases

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Igneous Rocks

• Water content
• As water content increases, melting pt. decreases
• Mineral content
• Different minerals have different melting points
• In general, oceanic crust melts at higher
  temperatures than continental crust
• Rocks melt only under certain conditions the
  right combination of temperature, pressure, and
  composition

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Igneous Rocks

• How rocks melt
• Different parts of a rock may melt at different
  temperatures due to the different minerals
  present in the rock
• Partial melting the process whereby some
  minerals melt at low temperatures while other
  minerals remain solid

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Igneous Rocks

• How rocks melt
• As each group of minerals melts, different
  elements are added to the magma stew, thereby
  changing its composition
• 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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Igneous Rocks

• How rocks melt
• Fractional crystallization
• The process wherein different minerals form at
  different temperatures
• When magma cools, it crystallizes in the reverse
  order of partial melting (the first minerals to
  crystallize from magma are the last minerals to
  melt during partial melting)

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Igneous Rocks

• Feldspars
• Feldspar minerals undergo a continuous change of
  composition
• As magma cools, the first feldspars to form are
  rich in calcium
• As cooling continues, these feldspars react with
  magma, and their calcium-rich compositions change
  to sodium-rich compositions

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Igneous Rocks

• Feldspars
• In come instances, as when magma cools rapidly,
  the calcium-rich cores are unable to react
  completely with the magma.
• The result is a zoned crystal that has
  sodium-rich outer layers and calcium-rich cores

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Igneous Rocks

• Iron-rich minerals
• These minerals undergo abrupt changes during
  fractional crystallization.
• As minerals form, elements are removed from the
  remaining magma
• Silica and oxygen are left over
• When the remaining magma finally crystallizes,
  quartz is formed.

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Igneous Rocks

• Crystal separation
• Crystal separation can occur when
• Crystals settle to the bottom of the magma body
• Liquid magma is squeezed from the crystal mush to
  form two distinct bodies with different
  compositions.
• Layered intrusions
• Formed when minerals form into distinct bands

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Intermission Part II next class
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Classifying Igneous Rocks

• Mineral composition
• Felsic
• Light-colored, have high silica contents
• Contain quartz and feldspars orthoclase, and
  plagioclase
• Example Granite

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Classifying Igneous Rocks

• Mineral composition
• Mafic
• Dark-colored, have lower silica contents, rich in
  iron and magnesium
• Contain plagioclase, biotite, amphibole,
  pyroxene, and olivine.
• Example Diorite

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Classifying Igneous Rocks

• Ultramafic
• Low silica content and very rich in iron and
  magnesium
• Theory formed by the fractional crystallization
  of olivine and pyroxene
• The minerals may have separated from magma and
  did not convert to another mineral upon reaching
  a particular temperature

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Classifying Igneous Rocks

• Grain size
• Fine-grained vs. coarse-grained
• Cooling rates
• When lava cools on Earths surface, there is not
  enough time for large crystals to form.
• Thus, extrusive igneous rocks have no visible
  mineral grains
• When magma cools beneath the surface, large
  crystals form.
• Thus, intrusive igneous rocks may have crystals
  larger than 1cm.

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Classifying Igneous Rocks

• Texture
• Porphyritic texture when a rock has grains of
  two different sizes.
• Large, well-formed crystals surrounded by
  finer-grained crystals of the same mineral or
  different minerals.
• Porphyritic textures indicate a complex cooling
  history wherein a slowly cooling magma suddenly
  began cooling rapidly.

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Classifying Igneous Rocks

• Ore deposits
• Veins
• The fluid left over during fractional
  crystallization contains any leftover elements
  that were not incorporated into the common
  igneous minerals
• They include gold, silver, lead, and copper.
• These elements are released at the end of magma
  crystallization in a hot, mineral-rich fluid that
  fills cracks and voids in the surrounding rock
• This fluid solidifies to form metal-rich quartz
  veins.

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Classifying Igneous Rocks

• Pegmatites
• Veins of extremely large-grained minerals are
  called pegmatites.
• Ores of rare elements such as lithium and
  beryllium are found in pegmatites
• Pegmatites can also produce beautiful crystals
• Because these veins fill cavities and fractures
  in rock, minerals grow into voids and retain
  their shapes.

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Classifying Igneous Rocks

• Kimberlites
• Rare, ultramafic rocks where minerals such as
  diamonds are found
• Kimberlites are a variety of periodite
• They likely form deep in the crust at depths of
  150-300km or in the mantle because diamond and
  other minerals found in kimberlites can form only
  under very high pressures.