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Igneous Processes, Igneous Intrusions
I. Processes that
change the
composition of the
magma
A. Fractional
crystallization (crystal
settling)
B. Assimilation
C. Magma mixing
 Online rocks and mineral
table.
 Viscosity & melts
 Partial melt
 Bowen’s reaction series
Plate tectonics and Igneous Rocks
Basics
A. Basalt and Gabbro—
Low viscosity, so often reach
the surface (most
common rock)
1. Oceanic Basalts
a. Mid ocean ridge basalts
(MORBs)
65% of Earth’s surface
Divergent boundaries
b. Ocean Island Basalts
(OIBs)
Over hot spots
Plate tectonics and Igneous Rocks
A. Basalts and Gabbros
1. Oceanic Basalts
2. Continental Basalts
(rare)
a. continental rifting
b. continental hot
spots
c. over subduction
zones
rare because often undergo
fractional crystallization
 more felsic rocks
Plate tectonics and Igneous Rocks
B. Andesite and Diorite
Forms over subduction
zones
Basalt forms from partial
melting of ultramafic
mantle
Fractional crystallization and
assimilation of crust 
intermediate magmas
Also, melting of sediment on
top of slab  more
intermediate magmas
Plate tectonics and Igneous Rocks
C. Rhyolites and Granites
Continental hot spots
Over subduction zones
So the melts are very viscous,
so they rarely reach the
surface, usually
plutonic/intrusive.
Rhyolite and granite form by:
Melting of crust
Heat source from intermediate
and mafic magmas.
Fractional crystallization and
assimilation are important
Plate tectonics and Igneous Rocks
III. Intrusive Igneous Rocks
(plutonic rocks)
Plutons—formations created
by emplacement of
magma at depth
A. Tabular
1. Dike—discordant, cuts
cross pre-existing layers
Range in size from < centimeter to
hundreds of kilometers
Often more resistant to erosion than
surrounding rock, so remains like a wall
Spanish Peaks, southern Colorado,
Shiprock Peak, NM, all over SW US
Look lie sheets and generally
dip steeply or vertical
Dikes are magma injected into
fractures - pressure of
magma may open up
small fractures
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Plate tectonics and Igneous Rocks
III. Intrusive Igneous Rocks
(plutonic rocks).
Plutons—formations created by
emplacement of magma at
depth
A. Tabular
2. Sill: a concordant tabular
pluton
Lies parallel to layers of
preexisting rock.
Intrudes between layers and
pushed overlying rock up, so
shallow where overlying
pressure is low.
Generally basalt (low viscosity),
form uniform thickness
layers/large extent
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Plate tectonics and Igneous Rocks
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Plate tectonics and Igneous Rocks III. Intrusive Igneous Rocks
(plutonic rocks).
Plutons—formations created
by emplacement of magma
at depth
A. Tabular
3. Laccolith: concordant,
similar to sill, but mushroom
shaped/domed
Formed by more viscous
magma: intermediate - felsic
Shallow depth = low pressure
Henry Mountains of SE Utah
(draw erosion of overlying
rock to form mts)
Plate tectonics and Igneous Rocks III. Intrusive Igneous Rocks
(plutonic rocks).
Plutons—formations created
by emplacement of magma
at depth
A. Massive plutons—several
km thick and 10’s-100’s km
across
Examples: Sierra Nevada Mts., Yosemite,
White Mts in NH, Pikes Peak CO, Idaho
Batholith
Batholiths—massive
discordant plutons
When exposed, surface areas
of >hundred km2
Intermediate-felsic (too viscous
to reach surface)
Form ~25 km below the
surface
Plate tectonics and Igneous Rocks III. Intrusive Igneous Rocks
(plutonic rocks).
Plutons—formations created by
emplacement of magma at
depth
A. Massive plutons—several
km thick and 10’s-100’s km
across
Examples: Sierra Nevada Mts., Yosemite,
White Mts in NH, Pikes Peak CO, Idaho
Batholith
Batholiths: massive discordant
plutons
When exposed, surface areas
of >hundred km2
Intermediate-felsic (too viscous
to reach surface)
Form ~25 km below the surface
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Plate tectonic setting and igneous rock types
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Summary
Intrusive rocks
Cool slowly
Phaneritic texture
Extrusive rocks (volanic rx)
Cool rapidly
Aphanitic texture
Types of igneous rocks and tectonic settings
Divergent boundary
Convergent boundary
Concepts
Partial melting
Fractional crystallization & Crystal settling
Assimilation
Magma Mixing
Igneous rocks on and in the Earth
Dikes, sills, batholiths