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Igneous Rocks
Igneous = Fire.
Magma- Molten material below Earth’s surface (deep
or shallow). It is composed mostly of Si and O (we call
it silica, SiO2) and dissolved gases.
Why do we call this silica, and not quartz?
Where is the image?
Lava- Molten rock material that has reached
Earth’s surface. Some magmas become lavas,
but not all.
Igneous rock- A silicate-rich rock that forms
when molten rock (magma or lava) solidifies.
Two main types of
igneous rocks:
1) Intrusive/PlutonicFormed when magma
solidifies deep
underground. Includes
granites (main rock of the
continents).
Never directly witnessed!
2) Extrusive/VolcanicFormed when lava
solidifies at the Earth’s
surface. Includes basalts
(main rock of ocean
floors).
1) What are the two main categories of
igneous rocks?
2) Where does each category form?
3) What is the name of the liquid that each
solidifies from?
Classification of Igneous Rocks
The most useful system for classifying igneous rocks
utilizes texture and composition (color).
Felsic
(light)
Intermediate
(medium)
Mafic
(dark)
FineGrained
(Extrusive)
Coarsegrained
(Intrusive)
It is a binary classification system!
Ultramafic
Classification of Igneous Rocks
The most useful system for classifying igneous rocks
utilizes texture and composition (color).
FineGrained
(Extrusive)
Coarsegrained
(Intrusive)
Felsic
(light)
Intermediate
(medium)
Mafic
(dark)
Ultramafic
Rhyolite
Andesite
Basalt
Komatiite
Granite
Diorite
Gabbro
Peridotite
What is Texture?
• The size, shape and arrangement of crystal
grains within a rock.
• Igneous rocks have an interlocking texture
formed by growth of minerals from a melt.
• Crystal size is controlled by cooling rate.
Extrusive Textures
Extrusive/volcanic rocks cooled quickly at or near
Earth’s surface (little time for crystal growth). These are
fine-grained or aphanitic (most crystals <1 mm).
Extrusive Textures Cont.
An extreme case occurs when rocks are glassy (no
crystals at all). An example is obsidian.
Intrusive Textures
Intrusive/plutonic rocks cooled slowly deep within the
Earth. These are coarse-grained or phaneritic (most
crystals >1 mm).
Draw lines to match the appropriate geologic
concepts and terms.
Depth
Shallow
Cooling
Rate
Slow
Grain
Size
Small
Textural
Description
Phaneritic
Crystal
Size
<1mm
Deep
Fast
Coarse
Aphanitic
>1mm
• You find a rock that has interlocking
texture, is fine-grained and black.
• Name the rock.
Special Igneous Textures
1)Obsidian:
An extrusive rock with glassy (ultra-fast cooled) texture and rhyolite
composition (an unusually dark rhyolite).
Special Igneous Textures
2) Xenolith: A fragment of rock within an igneous rock
that differs compositionally from the host rock. The
host rock and zenolith inclusions formed from different
magmas.
3) Vesicule: A bubble or hole formed by escaping gas
(common in basalts).
Special Igneous Textures
4) Pegmatite: A very coarse grained
igneous rock (crystal sizes > 5 cm) in
which crystal growth was enhanced by the
presence of fluids.
Special Igneous Textures
5) Porphyritic: Igneous rock with large crystals (called
phenocrysts) in a fine-grained matrix.
Porphyritic rocks may represent a two-state cooling history:
1) slow cooling at depth followed by...
2) rapid uplift and fast cooling near Earth's surface.
Igneous Rocks Classified by
Composition
Four compositional categories:
• Felsic
• Mafic
• Intermediate
• Ultramafic
A rock's color tells us about the minerals
present and overall composition.
Mineral Composition vs Color
• Minerals With Light
Elements (Si, O, Al,
Na, K)…
• Minerals With Heavier
Elements (Ca, Mg,
Fe)…
• Tend to be
__________ colored.
• Tend to be
__________ colored.
Felsic Rocks Are:
• Rich in light-colored
minerals (quartz, alkali
feldspar, and some
plagioclase feldspar).
• Compositionally rich in Si,
Na, Al, and K (and poor in
Fe and Mg).
• The dark-colored minerals
biotite and amphibole are
present, but only in small
amounts).
Mafic Rocks Are:
• Rich in dark-colored
minerals (plagioclase
feldspar, pyroxene,
olivine, biotite, and
amphibole).
• Compositionally rich in
Ca, Fe, and Mg (lower in
Si, Na, Al, and K).
• There is no quartz or alkali
feldspar!
Intermediate Rocks Are:
• Composed of roughly equal amounts of dark- and lightcolored minerals.
Ultramafic Rocks Are:
• Composed almost
exclusively of Fe
and Mg-rich
minerals from the
mantle (olivine and
pyroxene).
• Compositionally
rich in Fe, Mg, and
Ca, but poor in Si.
Warmup
Without looking in your notes, fill in the following
rock names.
Felsic
(light)
FineGrained
(Extrusive)
Coarsegrained
(Intrusive)
Intermediate
(medium)
Mafic
(dark)
Ultramafic
SKIP
THIS
ONE
Classification of Igneous Rocks
The most useful system for classifying igneous rocks
utilizes texture and composition (color).
Felsic
(light)
FineGrained
(Extrusive)
Coarsegrained
(Intrusive)
Rhyolite
Intermediate
(medium)
Mafic
(dark)
Ultramafic
Classification of Igneous Rocks
The most useful system for classifying igneous rocks
utilizes texture and composition (color).
Felsic
(light)
FineGrained
(Extrusive)
Coarsegrained
(Intrusive)
Rhyolite
Granite
Intermediate
(medium)
Mafic
(dark)
Ultramafic
Classification of Igneous Rocks
The most useful system for classifying igneous rocks
utilizes texture and composition (color).
FineGrained
(Extrusive)
Coarsegrained
(Intrusive)
Felsic
(light)
Intermediate
(medium)
Rhyolite
Andesite
Granite
Mafic
(dark)
Ultramafic
Classification of Igneous Rocks
The most useful system for classifying igneous rocks
utilizes texture and composition (color).
FineGrained
(Extrusive)
Coarsegrained
(Intrusive)
Felsic
(light)
Intermediate
(medium)
Rhyolite
Andesite
Granite
Diorite
Mafic
(dark)
Ultramafic
Classification of Igneous Rocks
The most useful system for classifying igneous rocks
utilizes texture and composition (color).
FineGrained
(Extrusive)
Coarsegrained
(Intrusive)
Felsic
(light)
Intermediate
(medium)
Mafic
(dark)
Rhyolite
Andesite
Gabbro
Granite
Diorite
Ultramafic
Classification of Igneous Rocks
The most useful system for classifying igneous rocks
utilizes texture and composition (color).
FineGrained
(Extrusive)
Coarsegrained
(Intrusive)
Felsic
(light)
Intermediate
(medium)
Mafic
(dark)
Rhyolite
Andesite
Gabbro
Granite
Diorite
Basalt
Ultramafic
Classification of Igneous Rocks
The most useful system for classifying igneous rocks
utilizes texture and composition (color).
FineGrained
(Extrusive)
Coarsegrained
(Intrusive)
Felsic
(light)
Intermediate
(medium)
Mafic
(dark)
Ultramafic
Rhyolite
Andesite
Gabbro
(Komatiite)
Granite
Diorite
Basalt
Classification of Igneous Rocks
The most useful system for classifying igneous rocks
utilizes texture and composition (color).
FineGrained
(Extrusive)
Coarsegrained
(Intrusive)
Felsic
(light)
Intermediate
(medium)
Mafic
(dark)
Ultramafic
Rhyolite
Andesite
Basalt
Komatiite
Granite
Diorite
Gabbro
Peridotite
Classification of Igneous Rocks
Introduction to Bowen’s Reaction Series
Warmup
(use your cell phones
for 5 minutes).
What temperature does water freeze at?
F
C
What temperature does ice melt at?
F
C
Bowen’s Two Paths
Two minerals we NEVER expect to find in the same rock are:
Bowen’s Reaction Series: Minerals crystallize in
a predictable order over a large temperature range
(and melt in the reverse order).
Lessons from Bowen’s Series
• 1) The chemistry of a magma controls the
type of rock that can form from it.
Lessons from Bowen’s Series
• 2) First minerals to solidify:
– Olivine
– Pyroxene
– Ca-rich plagioclase.
This produces a mafic rock (rich in Fe, Mg, Ca)
like basalt or gabbro.
Lessons from Bowen’s Series
• 3) As magma rises it will “evolve” into
more a more felsic composition (rich in K,
Na, and Si) producing granite or rhyolite.
Lessons from Bowen’s Series
• 4) During heating, the order of mineral
melting will be reversed.
The Feldspar Triangle
Alkali Feldspar is also
known as orthoclase.
What Favors Melting?
1) Increase the Temperature! The temperature of
the Earth increases from crust to core at
approximately 30 C/km (the geothermal
gradient). The core temperature is > 5000 C, and
heat moves upward and melts the upper mantle and
crust.
What Favors Melting?
2) Decrease the pressure! Since pressure favors
solids, mineral melting temperatures decrease with
decreasing pressure. This decompression melting
occurs when hot mantle rock moves upward.
P
T
What Favors Melting?
3) Add water! Water reduces the melting point of
rock.
Dry granite
melting
temperature:
900°C.
What Favors Melting?
3) Add water! Water reduces the melting point of
rock.
Dry granite
melting
temperature:
900°C.
Wet granite
melting
temperature:
700°C.
What Favors Melting?
4) Mix minerals!
Mixtures of minerals
always have lower
melting points than the
pure minerals would.
Quartz melts at ~1650°C
and K-Feldspar melts
at ~1300°C. However,
a 50/50 mixture of
these two minerals
will melt at ~1150°C.
Hints for the Igneous Rocks Lab
We will have a quiz on igneous rocks.
Make SURE you know the difference between
a rhyolite and basalt!
Magma Evolution
Magmas (liquids) evolve from mafic  felsic.
Magmas that solidify close to their source rock will be the most like
the source rock.
Magmas that solidify far from the source rock will be changed by
magma differentiation. This occurs by 4 different processes:
1) Partial melting: Produces magmas more felsic than the source
rock.
Magma Evolution
3) Assimilation: A hot magma can incorporates surrounding
“country rock”. If mafic magma assimilates more felsic rocks
continental crust an intermediate rock will result.
4) Magma mixing involves the mixing of more and less mafic
magmas to produce a magma of intermediate composition.
Magma Evolution
2) Fractional crystallization: The magma composition changes by
the removal of denser early-formed ferromagnesian minerals
(crystal settling). The remaining magma becomes more felsic.
Intrusive Rock Bodies
Intrusive rocks exist in intrusions that penetrate or cut through preexisting country rock.
We name these bodies based on:
1) Size
2) Shape
3) Geometric relationship to the country rock
Intrusions are:
A. Shallow if formed < 2 km deep (relatively fast cooling produces
finer textures (but coarser than extrusive rocks).
B. Deep if formed > 2 km deep (slow cooling produces coarsegrained rocks).
SHALLOW BODIES
Dike: Tabular
structure that cuts
across the layering in
the country rock.
SHALLOW BODIES
Sill: Tabular structure
that parallel to
layering in country
rock.
SHALLOW BODIES
Volcanic Neck: Shallow intrusion
formed when magma solidifies in the
throat of a volcano (Ship Rock, New
Mexico).
DEEP BODIES
Plutons: Large, blob-shaped
bodies formed when rising blobs
of magma (diapirs) get trapped
within the crust.
Stock: A small pluton
(exposed over <100
km2).
Batholith: A large
pluton (exposed over
>100 km2).
The interfaces between
intrusions and country
rock are called contacts.
Where Does Most Igneous Activity
Occur?
Mainly at or near tectonic plate boundaries.
Mafic Rocks are common at
divergent boundaries.
• The magma comes from the upper mantle
(asthenosphere) and rises through thin crust.
• Little opportunity to differentiate  mafic.
• The ocean floors are basalt!
Felsic Rocks are Common on
Continents.
• The thick continental crust means longer
magma journey.
• More magma differentiation  more felsic.
• Continental crust is rich in granite!
Intermediate Rocks.
• Common at convergent boundaries.
• Partial melting of subducted ocean crust
(mafic) produces basaltic magma; this
evolves into more felsic magma by the
assimilation of felsic crust.
Intermediate igneous rocks commonly form at convergent
boundaries.
Here, partial melting of subducted asthenosphere produces basaltic
magma which evolves into more felsic magma by the assimilation of
felsic crust.
Some igneous rocks form within plates (not at a plate
boundary). Rising mantle plumes (of controversial
origin) can produce localized hotspots and volcanoes
as they rise through continental or oceanic crust.
End of Chapter 3