Powerpoint Presentation Physical Geology, 10/e

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Igneous Rocks, Intrusive Activity, and the Origin
of Igneous RocksPhysical Geology 13/e, Chapter 3
Tim Horner, CSUS Geology Department
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The Rock Cycle

• A rock is a naturally formed, consolidated
  material usually composed of grains of one or
  more minerals
• The rock cycle shows how one type of rocky
  material gets transformed into another
• Representation of how rocks are formed, broken
  down, and processed in response to changing
  conditions
• Processes may involve interactions of geosphere
  with hydrosphere, atmosphere and/or biosphere
• Arrows indicate possible process paths within the
  cycle

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The Rock Cycle and Plate Tectonics

• Magma is created by melting of rock
• above a subduction zone
• Less dense magma rises and cools
• to form igneous rock
• Igneous rock exposed at surface
• gets weathered into sediment
• Sediments transported to low areas,
• buried and hardened into sedimentary rock
• Sedimentary rock heated and squeezed at depth to
  form metamorphic rock
• Metamorphic rock may heat up and melt at depth to
  form magma

Convergent plate boundary
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Igneous Rocks

• Magma is molten rock
• Igneous rocks form when magma cools and
  solidifies
• Intrusive igneous rocks form when magma
  solidifies underground
• Granite is a common example
• Extrusive igneous rocks form when magma
  solidifies at the Earths surface (lava)
• Basalt is a common example

Granite
Basalt
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Igneous Rock Textures

• Texture refers to the size, shape and arrangement
  of grains or other constituents within a rock
• Texture of igneous rocks is primarily controlled
  by cooling rate
• Extrusive igneous rocks cool quickly at or near
  Earths surface and are typically fine-grained
  (most crystals lt1 mm)
• Intrusive igneous rocks cool slowly deep beneath
  Earths surface and are typically coarse-grained
  (most crystals gt1 mm)

Fine-grained igneous rock
Coarse-grained igneous rock
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Special Igneous Textures

• A pegmatite is an extremely coarse-grained
  igneous rock (most crystals gt5 cm) formed when
  magma cools very slowly at depth
• A glassy texture contains no crystals at all, and
  is formed by extremely rapid cooling
• A porphyritic texture includes two distinct
  crystal sizes, with the larger having formed
  first during slow cooling underground and the
  small forming during more rapid cooling at the
  Earths surface

Pegmatitic igneous rock
Porphyritic igneous rock
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Igneous Rock Identification

• Igneous rock names are based on texture (grain
  size) and mineralogic composition
• Textural classification
• Plutonic rocks (gabbro-diorite-granite) are
  coarse-grained and cooled slowly at depth
• Volcanic rocks (basalt-andesite-rhyolite) are
  typically fine-grained and cooled rapidly at the
  Earths surface
• Compositional classification
• Mafic rocks (gabbro-basalt) contain abundant
  dark-colored ferromagnesian minerals
• Intermediate rocks (diorite-andesite) contain
  roughly equal amounts of dark- and light-colored
  minerals
• Felsic rocks (granite-rhyolite) contain abundant
  light-colored minerals

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Igneous Rock Identification

• Igneous rock names are based on texture (grain
  size) and mineralogic composition

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Chemistry (mineral content)
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Igneous Rock Chemistry

• Rock chemistry, particularly silica (SiO2)
  content, determines mineral content and general
  color of igneous rocks
• Mafic rocks have 50 silica, by weight, and
  contain dark-colored minerals that are abundant
  in iron, magnesium and calcium
• Intrusive/extrusive mafic rocks - gabbro/basalt
• Felsic (silicic) rocks have gt65 silica, by
  weight, and contain light-colored minerals that
  are abundant in silica, aluminum, sodium and
  potassium
• Intrusive/extrusive felsic rocks -
  granite/rhyolite
• Intermediate rocks have silica contents between
  those of mafic and felsic rocks
• Intrusive/extrusive intermediate rocks -
  diorite/andesite
• Ultramafic rocks have lt45 silica, by weight, and
  are composed almost entirely of dark-colored
  ferromagnesian minerals
• Most common ultramafic rock is peridotite
  (intrusive)

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Intrusive Rock Bodies

• Intrusive rocks exist in bodies or structures
  that penetrate or cut through pre-existing
  country rock
• Intrusive bodies are given names based on their
  size, shape and relationship to country rock
• Shallow intrusions Dikes and sills
• Form lt2 km beneath Earths surface
• Chill and solidify fairly quickly in
  cool country
  rock
• Generally composed of
  
  fine-grained rocks

Insert new Fig. 3.11 here
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Intrusive Rock Bodies

• Intrusive rocks exist in bodies or structures
  that penetrate or cut through pre-existing
  country rock
• Intrusive bodies are given names based on their
  size, shape and relationship to country rock
• Deep intrusions Plutons
• Form at considerable depth beneath
  Earths surface when
  rising blobs of
  magma (diapirs) get trapped within
  the
  crust
• Crystallize slowly in warm
  country rock
• Generally composed of
  coarse-grained
  rocks

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Intrusive Rock Bodies

• Volcanic neck
• Shallow intrusion formed when magma solidifies in
  throat of volcano
• Dike
• Tabular intrusive structure that cuts across any
  layering in country rock
• Sill
• Tabular intrusive structure that parallels
  layering in country rock
• Pluton
• Large, blob-shaped intrusive body formed of
  coarse-grained igneous rock, commonly granitic
• Small plutons (exposed over lt100 km2) are called
  stocks, large plutons (exposed over gt100 km2) are
  called batholiths

Light-colored dikes
Basaltic sill
Sierra Nevada batholith
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Deep intrusive structuresBatholiths

• Surface exposure gt 100 km2

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The Big Picture

• What causes rocks to melt?
• The internal Earth is hot.
• Temperature increases downward
• Yet the interior of the Earth is mostly solid
• Melts occur by three processes
• Decompression melting
• Heating
• Water-flux melting

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1) Decrease Pressure
Decompression melting
LIQUID
Normal conditions Mantle is not hot enough to
melt
SOLID
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1) Decrease Pressure
Decompression melting
Move mantle rocks up toward the surface
decrease the pressure at a given temperature
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2) Increase Temperature
Normal conditions Mantle is not hot enough to
melt
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2. Increase Temperature
Increase temperature of rocks at a given depth
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3. Change composition (add H2O)
Water decreases the melting temperature of hot
rock FLUX MELTING Fluxing effect- Used in
foundries. Add flux and metal melts at a lower
temperature
Normal conditions Mantle is not hot enough to
melt
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3. Change composition (add H2O)
Water decreases the melting temperature of hot
rock FLUX MELTING Fluxing effect- Used in
foundries. Add flux and metal melts at a lower
temperature
Add water to the mantle. Change its composition
and thus its melting temperature
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Plate tectonics and melting

• Decompression melting
• Divergent margins
• Hot spots
• Heating
• Hot spots
• Flux melting
• Convergent margins

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Divergent Boundary
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Hot Spots (e.g. Hawaii)
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Convergent Margins- flux melting
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Most magma is generated by melting the mantle
(makes a mafic melt) but we see a whole range of
compositions from mafic to felsic. How do we get
different compositions? or Why is
continental crust felsic, not mafic?

• Crystallization (differentiation)
• Assimilation
• Magma mixing

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Bowens Reaction Series
1) Crystallization
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Magma Crystallization and Melting Sequence

• Minerals crystallize in a predictable order (and
  melt in the reverse order), over a large
  temperature range, as described by Bowens
  Reaction Series
• Discontinuous branch
• Ferromagnesian minerals (olivine, pyroxene,
  amphibole, biotite) crystallize in sequence with
  decreasing temperature
• As one mineral becomes chemically
  unstable in the remaining magma,
  another begins to form
• Continuous branch
• Plagioclase feldspar forms with a
  chemical composition that evolves
• (from Ca-rich to Na-rich) with
  decreasing temperature

Bowens Reaction Series
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Lessons from Bowens Reaction Series

• Large variety of igneous rocks is produced by
  large variety of magma compositions
• Mafic magmas will crystallize into basalt or
  gabbro if early-formed minerals are not removed
  from the magma
• Intermediate magmas will similarly crystallize
  into diorite or andesite if minerals are not
  removed
• Separation of early-formed ferromagnesian
  minerals from a magma body increases the silica
  content of the remaining magma
• Minerals melt in the reverse order of that in
  which they crystallize from a magma

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2) Crustal Assimilation
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Evidence for crustal assimilation

• Xenoliths (xeno foreign lith rock)

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3) Magma mixing