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GY111 Physical Geology
Lecture 4: Igneous Rocks
Types of Rocks
• Rock: an aggregate of one or more minerals
• Igneous Rocks: crystallize from a magma
• Sedimentary Rocks
– Clastic: formed by the erosion of pre-existing rocks
– Chemical/Biochemical: precipitated from chemical
reactions
• Metamorphic Rocks: formed by exposure to
extreme heat & pressure below the melting point
Magma
• Magma is generated in the interior earth by heat
from radioactive minerals
• Volcanic eruptions prove that magma exists near
the surface of the earth
• Laboratory studies verify that common rocks will
melt at the T & P inside the earth
• Coarse grained igneous rocks prove that
magma must cool slowly, and the only way that
that can happen is that the surrounding rocks
must be almost as hot as the magma itself
Intrusive Igneous Rocks
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Cool slowly at depths > 1 km
Form coarse-grained textures
Surrounding rock is termed “country” rock
May contain portions of the country rock
that “fall” into the original magma chamber
forming a xenolith
Extrusive Igneous Rocks
• Form on the Earth’s surface
• Lava: flow of magma onto the Earth’s surface
– Pahoehoe: ropy surface (low viscosity)
– Aa: fragmental surface (high viscosity)
• Pyroclastic rocks: form from the explosive
eruption of volcanoes
– Ash: particles of glass
– Tuff: a rock composed of fragments of pre-existing
rock in an ash matrix
– Pumice: a rock so full of voids (vesicles) that it can
float in water (S.G. < 1.0)
– Obsidian: massive volcanic glass
Lava Flow Types
• Pahoehoe: ropy
• Aa: fragmented
Igneous Textural Terms
• Aphanitic: mineral grains in rock are too small to
be identified with a hand lens (rock cooled from
magma rapidly)
• Phaneritic: minerals grains in rock are large
enough to be identified with a hand lens (rock
cooled relatively slowly)
• Phenocrysts: crystals that are distinctly larger
than surrounding mineral grains
• Porphyritic: a texture where relatively large
phenocryst mineral grains are surrounded by
smaller grains
View of Textural Types
• Aphanitic
• Phaneritic
Composition
• Felsic: light colored igneous rock relatively rich
in Si, Na and K.
• Intermediate: rock made up of equal proportions
light and dark minerals
• Mafic: dark colored rock relatively rich in Ca, Fe
and Mg
• Ultramafic: dark colored rock relatively rich in Fe
and Mg
• Note: red is considered a felsic (light) color;
green is considered a mafic (dark) color
Where Different Igneous Textures
Form
Common Igneous Minerals
Classification of Igneous Rocks
• Based on Mineral Content & Texture
Magma Formation
• Magma formation is favored by increasing
temperature and decreasing pressure
• Magma formation is favored by increasing H2O
content because it effectively lowers the melting
point of minerals in rocks
• Several tectonic environments favor magma
formation:
– Divergent boundaries, Hot Spots: pressure reduction
in upwelling mantle (Decompression melting)
– Convergent boundaries: increasing temperature and
water content in subducting slab; frictional heating
Granite Melting Curves
• Experimental results with actual granite rock
displays effect of pressure and water
Divergent decompression
10
solid
8
P
Kbar
melt
35 km
Dry melting
curve
Convergent Subduction
20 km
6
Wet (H2O) melting
curve
solid
melt
4
500
600 T Deg. C
800
Fractional Crystallization
• Controlled by Bowen’s Reaction Series
Discontinuous
Series
Continuous
Series
Palisades Sill: Example of
Fractional Crystallization
• Early high-temp crystals settle to the base of the
magma chamber
Palisades Sill cont.
• The end result is a layered intrusion- different
layers have different compositions
Forms of Magma Intrusions
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Batholith: discordant; >= 100 km2
Stock: discordant; >= 1 and < 100 km2
Pluton: discordant; < 1 km2
Dike: discordant; tabular
Sill: concordant; tabular
Laccolith: concordant; shield shaped
Lopolith: concordant; saucer shaped
Intrusive Forms
• Note: laccoliths and lopoliths are not shown in this
schematic
Plate Boundary Associations:
Divergent
• Divergent Boundaries:
production of ophiolite
sequences
• Ultramafic mantle partially
melts to form basalt and
gabbro (mafic rocks)
• While in contact with ocean
water the ocean crust is
hydrated and altered
chemically (seawater
alteration)
Plate Boundary Associations:
Convergent
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Subducted ocean
lithosphere
partially melts to
produce
intermediate and
felsic magma
The hydration of
the ocean
lithosphere
dramatically
lowers its melting
point leading to
abundant felsic to
intermediate
magma generation
Volcanic Landforms
• Central Vent Eruptions
– Shield Volcanoes: low viscosity lava flows
– Volcanic domes: viscous lava extruded as a dome after major
pyroclastic eruption
– Cinder cones: small low viscosity eruptions that spatter small
fragments of lava that solidify as cinders
– Stratovolcanoes: high viscosity pyroclastic eruptions build a
steep-sided cone
– Craters/Calderas: explosive eruptions will blast a small crater at
the summit of a volcano, or a large caldera for more violent
eruptions
– Diatremes: rapid intrusion of a very low viscosity carbonate-rich
magma. Diamond bearing diatremes are termed “Kimberlites”
Volcanic Landforms cont.
• Central vent
eruptions
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Shield
Lava dome
Cinder cone
Stratovolcano
(Composite)
– Caldera
Caldera Formation
• Result from very large
pyroclastic eruptions
(Super Eruptions)
• The Yellowstone Caldera
is one example
Fissure Eruptions
• Flood Basalts: large outpourings of low
viscosity basaltic lava fills in low areas
• Ash Flow deposits: result from the fissure
eruption of felsic magma to produce
extremely large pyroclastic flows
(Yellowstone)
Columbia River Flood Basalts
• An example of a fissure eruption of mafic
lava
Hydrothermal Vents
• Water-rich liquid at high temperature
• Under high pressure water may have a
temperature of over 400 deg. C and still be
a liquid phase
• Geysers: interaction between groundwater
and a volcanic magma chamber
• Hydrothermal veins: important economic
mineral sources; boil off from magma
during fractional crystallization
Global Patterns of Volcanism
• Divergent: low viscosity mafic magma with little
or no H2O; generate shield volcanoes (Iceland)
• Convergent: high viscosity intermediate and
felsic magma with abundant H2O; generate
stratovolcanoes (Cascade Range)
• Hot Spot: low viscosity dry mafic magma
produces shield volcanoes under ocean
lithosphere (Hawaii); high viscosity wet felsic
magma under continental lithosphere
(Yellowstone)
Exam Summary
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Know intrusive geometry classes
Know textural terms (aphanitic, phaneritic, etc.)
Know common rock-forming silicates in felsic, intermediate, etc.,
compositions
Know the characteristics of Shield versus Composite Cone volcanoes.
Be able to diagram Bowen’s Reaction Series and describe the Palisades Sill
as an example or fractional crystallization.
Be able to describe the conditions that lead to the formation of aa,
pahoehoe, pumice, obsidian, welded tuff, scoria.
Be able to explain why some volcanoes extrude low-viscosity lava whereas
others tend to erupt explosively. Relate low- versus high-viscosity magma to
types of plate tectonic boundaries.