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

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Chapter 3

Igneous Rocks, Intrusive Activity,

and the Origin of Igneous Rocks 

Photo credit: G. Mattioli

the rock cycle
The Rock Cycle
  • A Plate Tectonic Example 

Igneous Rocks

  • Igneous Rock Textures
  • Identification of Igneous Rocks
  • Varieties of Granite
  • Chemistry of Igneous Rocks 
fundamental questions
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?
igneous rocks terminology
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 Earth’s surface. Most lavas have been significantly degassed en route to the surface.
classification of igneous and volcanic rocks
Classification of Igneous and Volcanic Rocks
  • Based on hand specimen fabric
  • Based on field relationships and textures
  • Based on mineralogy and chemistry
    • Color Index
classification schemes i
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.
classification based on field relations and textures
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 (>2-3 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.
more on fabric classification
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
deeply eroded intrusions
Deeply Eroded Intrusions



Torres del Paine, Chile

coarse grained intrusive rock texture
Coarse Grained Intrusive Rock Texture

Potassium Feldspar (stained yellow)

photomicrograph phaneritic texture
Photomicrograph - Phaneritic Texture

Interlocking grains

Photo credit: C.C. Plummer

andesite hand specimen
Andesite Hand Specimen

Matrix or groundmass

Plagioclase Feldspar


diorite hand specimen
Diorite Hand Specimen

Interlocking grains

with uniform size

classification based on mineralogy chemistry
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).
color index
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
basalt hand specimen
Basalt Hand Specimen

Fined grained mafic volcanic rock

intrusive bodies
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
san cristobal volcano nicaragua
San Cristobal Volcano, Nicaragua

Photo Credit: G. Mattioli

aerial photo of shiprock new mexico
Aerial Photo of Shiprock, New Mexico


dikes vs sills
Dikes vs. Sills

Sills:Concordant structures

Parallel to pre-existing layers

Dikes:Discordant structures

Cross-cut pre-existing layers

2 m dike in dominica west indies
2 m Dike in Dominica, West Indies

Photo Credit: G. Mattioli

dominica dike along strike view
Dominica Dike along Strike View

Photo Credit: G. Mattioli

edinburgh sill
Edinburgh Sill


magmatic diapirs
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.

coalescing diapirs and plutons
Coalescing Diapirs andPlutons

But siliceous

magmas have

high viscosity

or resistance

to flow, which

makes eruption

difficult without

high gas contents.

Rise because of


is lower density

than rocks.