Igneous Rocks, Intrusive Activity,
and the Origin of Igneous Rocks
Physical Geology, Chapter 3
Quiz 1
Monday
Igneous Rocks, Intrusive Activity,
and the Origin of Igneous Rocks
Physical Geology, Chapter 3
Rocks and the Rock Cycle
• A rock is a naturally formed, consolidated
material usually composed of grains of one
or more minerals.
• There are three types of rocks:
– Igneous, metamorphic, and sedimentary
• Each type has different physical appearance (texture)
The Rock Cycle
• The rock cycle shows how
one type of rocky material
gets transformed into
another
– Representation of how rocks are
formed, broken down, and
processed in response to
changing conditions
– Processes may involve
interactions of geosphere with
hydrosphere, atmosphere and/or
biosphere
– Arrows indicate possible process
paths within the cycle
•Types of igneous rocks are closely related to the type
of magma and the tectonic environment.
•For the most part, volcanic activity is closely linked
to interaction between plates (tectonic activity).
The Rock Cycle and Plate Tectonics
Convergent plate boundary
• Magma is created by melting of rock above a subduction zone
• Less dense magma rises and crystallizes to form igneous rock
The Rock Cycle and Plate Tectonics
• Igneous rock exposed at surface gets weathered into sediment
• Sediments transported to low areas, buried and harden (undergo
lithification) into sedimentary rock
The Rock Cycle and Plate Tectonics
• Sedimentary rock heated and squeezed at depth to form
metamorphic rock
• Metamorphic rock may heat up and melt at depth to form magma
Igneous Rock Formation
• Igneous rocks form when molten rock
crystallizes (cools and solidifies)
– Molten rock underground
• magma
– Molten rock on the Earth’s surface
• lava
Diagram of a Magma Chamber
Cooling Basaltic Lava
1. Based on this information, what is
one way to classify igneous rocks?
Cooling Basaltic Lava on the surface of the Earth
Diagram of a Magma Chamber
Igneous Rocks Classification
• Texture or physical appearance is
one way to classify igneous rocks
– Texture depends on cooling rate
• Slower cooling rates mean that crystals
have more time to form and thus will
be larger
More
time
Less
time
Igneous Rocks Classification
• Texture or physical appearance is one way to
classify igneous rocks
– Texture depends on cooling rate
• Newton's law of cooling states that the rate of heat loss of a body is
proportional to the difference in temperatures between the body and
its surroundings
One more thing….
2. Match rock texture, cooling rate, and environment
1.
Extrusive
A. Fine-grained igneous rock
2.
Intrusive
B. Coarse-grained igneous rock
a.
b.
“Fast” cooling
“Slow cooling
Igneous Rocks Formation
• Intrusive igneous rocks form when
magma solidifies underground
• Granite is a common example
• Extrusive igneous rocks form when
lava solidifies at the Earth’s surface
Granite
• Basalt is a common example
3. What differences do you see in
these two rocks that could have
been used for classifying (naming)
them?
Basalt
Igneous Rock Classification
•
Igneous rock names are based on:
–
–
mineral (chemical)
composition
texture (grain size)
Igneous Rock
Chemistry
•
•
Rock chemistry, particularly
silica (SiO2) content, determines
mineral content and general
color of igneous rocks
Two general end members:
Mafic  Felsic
Mafic rocks
•
•
•
•
~50% silica by weight
contain dark-colored minerals that
are abundant in magnesium (Ma),
iron (Fe), and calcium
Gabbro = coarse-grain, intrusive
Basalt = fine-grain, extrusive
Igneous Rock
Chemistry
Felsic (silicic) rocks
•
•
•
•
>65% silica (SiO2) by weight
contain light-colored minerals,
such as feldspars and silica,
abundant in silica, aluminum,
sodium, and potassium
Granite = coarse-grain, intrusive
Rhyolite = fine-grain, extrusive
Igneous Rock
Chemistry
Intermediate rocks have silica
contents between those of mafic
and felsic rocks
•
•
Diorite = coarse-grain, intrusive
Andesite = fine-grain, extrusive
Igneous Rock Chemistry
Ultramafic rocks
•
•
•
< 45% silica by weight
composed almost entirely of dark-colored ferromagnesian minerals
Most common ultramafic rock is peridotite (intrusive)
–
Mostly olivine with minor amounts of Ca-rich feldspar and pyroxene
Igneous Rock Classification

Intrusive
SAME
chemical
composition
DIFFERENT
cooling rates and
environments
DIFFERENT
rock names!

Extrusive
Igneous Rock Textures
• Texture refers to the physical appearance of the rock
– size, shape and arrangement of grains or other constituents within a rock
– Fine-grained texture = aphanitic
– Coarse-grained texture = phaneritic
• Texture of igneous rocks is primarily controlled by cooling rate
Aphanitic igneous rock
Phaneritic igneous rock
Igneous Rock Textures
• Extrusive igneous rocks cooled
quickly at or near Earth’s surface are
typically aphanitic or fine-grained
(most crystals <1 mm)
• Intrusive igneous rocks cooled
slowly deep beneath Earth’s surface
are typically phaneritic or coarsegrained (most crystals >1 mm)
Fine-grained (aphanitic), extrusive igneous rock
…But, what if magma started cooling
underground, forming a “mush” of
molten rocks, and then erupted?
Coarse-grained (phaneritic), intrusive igneous rock
Porphyritic
Rhyolite
Some Other
Igneous Textures
• Igneous rocks with
porphyritic texture have
two distinct crystal sizes,
indicating that the rock
underwent a two-stage
cooling process.
– Larger crystals (phenocrysts)
formed first during slow
cooling underground
– Smaller crystals (matrix or
ground mass) formed during
more rapid cooling on or
near the Earth’s surface
phenocryt
Some Other Igneous Textures
• A pegmatite is an extremely
coarse-grained igneous rock
(most crystals >5 cm) formed
when magma cools very slowly at
depth
• A glassy texture contains no
crystals at all, and is formed by
extremely rapid cooling
(quenching)
Intrusive Rock Bodies
•
Intrusive rocks exist in bodies or structures that penetrate or
cut through pre-existing country rock
Intrusive bodies are given names based on their size, shape
and relationship to country rock
•
–
–
Deep intrusions: Plutons
Shallow intrusions: Dikes, sills, volcanic necks
Intrusive Rock Bodies
•
Intrusive rocks exist in bodies
or structures that penetrate or
cut through pre-existing
country rock
Intrusive bodies are given
names based on their size,
shape and relationship to
country rock
•
–
Deep intrusions: Plutons
•
•
•
Form at depth beneath Earth’s
surface when rising blobs of
magma (diapirs) get trapped
within the crust
Crystallize slowly in warm
country rock
Generally coarse-grained rocks
Deep Intrusive Rock Bodies
•
Pluton
–
–
Large, blob-shaped intrusive body formed of coarse-grained igneous
rock, commonly granitic in composition
Small plutons (exposed over <100 km2) are called stocks, large plutons
(exposed over >100 km2) are called batholiths
Sierra Nevada batholith
Intrusive Rock Bodies
•
Intrusive bodies are given names based on their size, shape and
relationship to country rock
–
Shallow intrusions: Dikes, sills, volcanic necks
•
•
•
Form <2 km beneath Earth’s surface
Chill and solidify fairly quickly in cool country rock
Generally composed of fine-grained rocks
Intrusive Rock Bodies
•
Volcanic neck
–
•
Shallow intrusion formed
when magma solidifies in
throat of volcano
Dike
–
•
Tabular intrusive structure
that cuts across any layering
in country rock
Sill
–
Tabular intrusive structure
that parallels layering in
country rock
Igneous Rock Identification
•
Igneous rock names are based on texture (grain size) and
mineralogic composition
Textural classification
•
–
–
•
Plutonic rocks (gabbro-diorite-granite) are coarse-grained and cooled
slowly at depth
Volcanic rocks (basalt-andesite-rhyolite) are typically fine-grained and
cooled rapidly at the Earth’s surface
Compositional classification
–
–
–
Mafic rocks (gabbro-basalt) contain abundant dark-colored
ferromagnesian minerals
Intermediate rocks (diorite-andesite) contain roughly equal amounts of
dark- and light-colored minerals
Felsic rocks (granite-rhyolite) contain abundant light-colored minerals
How Magma Forms
•The Earth is not homogeneous (the same throughout).
It has a layered structure, and the layers have different
chemical compositions (differentiated).
How Magma Forms
•
•
The mantle is solid and is has a basaltic chemical
composition high in ferromagnesium silicates
Basically, magmas form through melting of the mantle
4. How can a solid become molten?
How Magma Forms
•
Addition of heat
–
–
Heat transferred upward (by
conduction and convection)
from the very hot (>5000°C)
core through the mantle and
crust
Rate at which temperature
increases with increasing depth
beneath the surface is the
geothermal gradient
•
Gradient is not the same
everywhere
How Magma Forms
•
Decrease pressure
–
–
Melting point of minerals
generally increases with
increasing pressure
Decompression melting can
occur when hot mantle rock
moves upward and pressure is
reduced enough to drop melting
point to the temperature of the
rising rock body
How Magma Forms
•
Add water or other contaminant
–
Hot water under pressure
•
•
–
Water becomes increasingly
reactive at higher temperatures
At sufficient pressures and
temperatures, highly reactive water
vapor can reduce the melting point
of rocks by over 200°C
Mineral mixtures
•
Mixtures of minerals, such as
quartz and potassium feldspar, can
result in the melting of both at
temperatures hundreds of degrees
lower than either mineral would
melt on its own
How Magma Forms
•
The mantle is solid and is has a basaltic
chemical composition
• Basically, magmas form from melting of the
mantle
• Mantle melts due to:
– Increased heat
– Decreased pressure
– Addition of water
Magma Crystallization and
Melting Sequence
•
Minerals crystallize in a predictable order (and melt
in the reverse order), over a large temperature range,
as described by Bowen’s Reaction Series
Magma Crystallization and
Melting Sequence8
•
Bowen’s Reaction Series has two
branches.
–
Discontinuous branch
•
•
–
Ferromagnesian minerals (olivine,
pyroxene, amphibole, biotite)
crystallize in sequence with
decreasing temperature
As one mineral becomes chemically
unstable in the remaining magma,
another begins to form
Continuous branch
•
Plagioclase feldspar forms with a
chemical composition that evolves
(from Ca-rich to Na-rich) with
decreasing temperature
–
Often forms zoned crystals
Bowen’s Reaction Series
Lessons from Bowen’s Reaction Series
•
Large variety of igneous rocks is produced by large
variety of magma compositions
Mafic magmas will crystallize into basalt or gabbro if
early-formed minerals are not removed from the magma
Intermediate magmas will similarly crystallize into
diorite or andesite if minerals are not removed
Separation of early-formed ferromagnesian minerals
from a magma body increases the silica content of the
remaining magma
•
•
•
–
–
•
Differentiation or fractional crystallization
Can produce granites from basaltic magma
Minerals melt in the reverse order of that in which they
crystallize from a magma
Magma Evolution
•
•
A change in the composition of a
magma body is known as magma
evolution
Magma evolution can occur by:
–
–
–
–
differentiation,
partial melting,
assimilation,
or magma mixing
Magma Evolution
•
•
Differentiation or fractional crystallization
involves the changing of magma composition by
the settling of denser early-formed
ferromagnesian minerals
The removal of some components means that the
relative % of the other components remaining in
the magma increases (enriched)
Magma Evolution
•
Incomplete or partial melting produces magmas less mafic
than their source rocks\
–
–
Minerals melt in the reverse order of that in which they crystallize
from a magma
Lower melting point minerals are more felsic in composition
Magma Evolution
•
Assimilation occurs when
a hot magma melts and
incorporates more felsic
surrounding country rock
Magma Evolution
•
Magma mixing involves
the mixing of more and
less mafic magmas to
produce one of
intermediate composition
Putting it all together:
Igneous Activity and
Plate Tectonics
•
Igneous activity occurs primarily at or
near tectonic plate boundaries
•
Mafic igneous rocks are commonly
formed at divergent boundaries
–
Increased heat flow and decreased
overburden pressure (decompression
melting) produce mafic (basaltic) magmas
from melting of the mantle
Igneous Activity and
Plate Tectonics
•
Intermediate igneous rocks are commonly formed at
convergent boundaries
–
Partial melting of basaltic oceanic crust due to the addition of water
produces intermediate magmas
Igneous Activity and
Plate Tectonics
•
Felsic igneous rocks are
commonly formed
adjacent to subduction
zones (convergent
boundary)
–
–
Water from subducting
crust lowers melting
point
Resulting hot rising
magma causes partial
melting and assimilation
of the granitic
continental crust
Igneous Activity and
Plate Tectonics
•
Intraplate volcanism
–
Rising mantle plumes can
produce localized hotspots
and volcanoes when they
produce magmas that rise
through oceanic or
continental crust
•
•
Hawaii is an example of
intraplate volcanism in
oceanic crust
Yellowstone is an example
of intraplate volcanism in
continental crust
Magma, Rock Types, and Plate Tectonic Setting
Rock Starting Ending Melting Processes
Magma Magma
Basalt
Gabbro
Mafic
Andesite
Diorite
Mafic
(usually)
Rhyolite
Granite
Felsic
(silicic)
Mafic
Intermediate
Tectonic
Setting
Mantle melting due to
decreased pressure
or
increased heat
Divergent
Hot spot
(intraplate)
Partial melting of mantle
through addition of water
Subduction
zones
(assimilation, differentiation,
or magma mixing)
Felsic
Partial melting of lower crust
Convergent
boundaries
Hot spot
End of Chapter 3…
…Wednesday, Chapter 4
Volcanism and Extrusive Igneous rocks