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Chapter 3
Igneous Rocks, Intrusive Activity, and the
Origin of Igneous Rocks 
Photo credit G. Mattioli
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The Rock Cycle

• A Plate Tectonic Example 
• Igneous Rocks
• Igneous Rock Textures
• Identification of Igneous Rocks
• Varieties of Granite
• Chemistry of Igneous Rocks 

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Fundamental Questions

• How are rocks sampled in the field and analyzed
  in the lab to determine their chemical, modal,
  and mineralogical composition?
• What do these analyses tell us about the
  composition of magmatic rocks?
• How can the data be presented to elucidate
  compositional patterns and contrasts?
• How do we classify magmatic rocks to convey
  meaningful petrogenetic information on the origin
  and evolution of the magma from which they
  solidified?

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The Rock Cycle
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Intrusive Rocks in Northern Victoria Land,
Antartica
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Plate Tectonics and the Rock Cycle
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Igneous Rocks Terminology

• Igneous rocks are formed as a result of cooling
  and crystallization from a magma.
• Magma is molten rock (fluid), rich in silica
  (SiO2), which contains dissolved volatiles (e.g.
  CO2 and H2O).
• Lava is magma extruded on or very near the
  Earths surface. Most lavas have been
  significantly degassed en route to the surface.

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Classification of Igneous and Volcanic Rocks

• Based on hand specimen fabric
• Based on field relationships and textures
• Based on mineralogy and chemistry
• Color Index

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Classification Schemes I

• Based on Fabric
• Phaneritic rocks with mineral grains that are
  large enough to be identified by eye. Texture is
  typical of slowly cooled intrusive rocks.
• Aphanitic rocks with grain too small to be
  identified by eye. Texture is most common in
  rapidly solidified extruded magma and marginal
  facies of shallow intrusions.

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Classification based on Field Relations and
Textures

• Extrusive or volcanic rocks typically aphanitic
  or glassy. This means that they are generally
  fine grained in texture. Grains are typically 0.5
  to 1 mm. Common example is basalt.
• Many varieties are porphyritic. This means that
  the grain size is bimodal, with a fine grained
  matrix surrounding larger grains that are called
  phenocrysts. Common example is andesite.
• Intrusive or plutonic rocks typically
  phaneritic. This means that they are generally
  coarse grained and this texture is often quite
  uniform. Grains can range in size but are often
  clearly visible to the naked eye (gt2-3 mm).
  Common example is granite.
• Amphiboles and biotites are commonly altered to
  chlorite. Muscovite found in some granites, but
  rarely in volcanic rocks. Perthitic feldspar,
  reflecting slow cooling and exsolution, is
  widespread.

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Rhyolite Hand Specimen
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Granite Hand Specimen
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More on Fabric Classification

• Porphyritic texture magmatic rocks with bimodal
  grain size distributions.
• Larger grains are called phenocrysts
• Smaller grains constitute the groundmass or
  matrix
• Porphyritic aphanitic rocks are more common than
  porphyritic phaneritic rocks
• Glassy or vitric texture rocks that contain
  variable proportions of glass.
• Holocrystalline rocks wholly composed of
  crystals
• Vitrophyric rocks porphyritic rock with
  phenocrysts in a glassy matrix

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Magmatic Intrusion
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Deeply Eroded Intrusions
GRANITE INTRUSION
SHALE COUNTRY ROCK
Torres del Paine, Chile
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Coarse Grained Intrusive Rock Texture
Potassium Feldspar (stained yellow)
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Photomicrograph - Phaneritic Texture
Interlocking grains
Photo credit C.C. Plummer
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Andesite Hand Specimen
Matrix or groundmass
Plagioclase Feldspar phenocrysts
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Diorite Hand Specimen
Interlocking grains with uniform size
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Classification based on Mineralogy Chemistry

• Felsic rocks mnemonic based on feldspar and
  silica. Also applies to rocks containing
  abundant feldspathoids, such as nepheline.
  GRANITE
• Mafic rocks mnemonic based on magnesium and
  ferrous/ferric. Synonymous with ferromagnesian,
  which refers to biotite, amphibole, pyroxene,
  olivine, and Fe-Ti oxides. BASALT
• Ultramafic rocks very rich in Mg and Fe.
  Generally have little feldspar. PERIDOTITE
• Silicic rocks dominated by quartz and alkali
  fsp. Sometimes referred to as sialic (Si Al).

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Color Index

• Defined as the modal proportion of dark-colored
  minerals in a rock. Should really be based on
  the proportion of ferromagnesian minerals as
  feldspars may range in color.
• Leucocratic 0-30 mafics
• Melanocratic 60-100 mafics

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Gabbro Hand Specimen
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Basalt Hand Specimen
Fined grained mafic volcanic rock
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Summary Classification for Igneous Rocks
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Igneous Rock Identification Summary
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Intrusive Bodies

• Shallow Intrusive Structures
• Sills
• Dikes
• Volcanic necks
• Intrusive Rocks That Crystallize at Depth
• Abundance and Distribution of Plutonic Rocks
• Sierra Nevada Batholith
• Pegmatites

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San Cristobal Volcano, Nicaragua
Photo Credit G. Mattioli
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Volcanic Neck and Dike
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Aerial Photo of Shiprock, New Mexico
From http//www.ngdc.noaa.gov/seg/hazard
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Dikes vs. Sills
Sills Concordant structures Parallel to
pre-existing layers
Dikes Discordant structures Cross-cut
pre-existing layers
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2 m Dike in Dominica, West Indies
Photo Credit G. Mattioli
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Dominica Dike along Strike View
Photo Credit G. Mattioli
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Dikes in northern Victoria Land, Antartica
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Edinburgh Sill
From http//www.ngdc.noaa.gov/seg/hazard
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Magmatic Diapirs
A diapir is a dome that is cored by
plastic material, in this case, partially molten
rock. Concept first applied to salt domes.
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Coalescing Diapirs andPlutons
But siliceous magmas have high viscosity or
resistance to flow, which makes
eruption difficult without high gas contents.
Rise because of buoyancy-magma is lower
density than rocks.
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Pegmatite Outcrop
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Sierra Nevada Batholith