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Igneous Rocks Chapter 4
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Half Dome, Yosemite National Park
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The Nature of Magma
Three Components of
Magma:
1. Liquid component –
melt composed of the
eight most common
elements.
2. Solid component –
already formed silicate
minerals.
3. Gaseous Phase volatiles that are
dissolved in magma but
separate from magma
into gases as pressure is
reduced.
Magma & Lava
  Rocks partially melt at different levels within
the Earth’s crust and mantle to form magma.
  Most magma never reaches the surface and
cools at depth forming (1) intrusive or
plutonic rocks, that are later uplifted and
exposed by erosion.
  Magma that reaches the surface is erupted as
lava, and cools to form (2) extrusive or
volcanic rocks.
Crystallization of Magma
  No two magmas are identical in composition, so
a variety of igneous rocks exist.
  The environment during crystallization can be
inferred from the size and arrangement of
mineral grains – called texture.
Igneous rocks are classified by their
1) Texture
2)  mineral composition.
Igneous Rock Composition
•  Ultramafic: contains almost all ferromagnesian
minerals, very dark in color, generally found
within the mantle. Low silica content.
•  Mafic (Basaltic): contains high amounts of
ferromagnesian minerals, dark in color, mostly
found erupting in the oceans, with some
erupting on land.
•  Intermediate (Andesitic) in color and
composition; found near continental margins.
•  Felsic (Granitic) contain more
nonferromagnesian minerals and are lighter in
color; almost always found on land. High silica
content.
Common Igneous Rocks
Composition
Texture
Felsic
Intermediate
Mafic
(feldspar &
(Mg and Fe
silica)
rich)
Intrusive
Granite
Diorite
Gabbro
Extrusive
Rhyolite
Andesite
Basalt
Mineralogy of Common Igneous Rocks
Low Temp
Silica Rich
Silica Poor
Iron Poor
Iron Rich
High Temp
Igneous Textures
Three factors contribute to the textures of
igneous rocks:
1.  Rate at which magma cools. The faster the
rate of cooling, the smaller the crystal size.
2.  The amount of silica present. The larger the
amount of silica, the more viscous the magma,
and the more likely to have a glassy texture
when cooled rapidly.
3.  The amount of dissolved gases in the
magma. Explosive gases eject rock fragments
during violent eruptions.
Texture
  The rate at which magma cools is the most
significant factor in determining the crystal size in
an igneous rock.
  Igneous rock textures from the smallest to the
largest textures are: glassy (no crystals) 
aphanitic (fine-grained)  phaneritic (coarsegrained)  porphyritic (two grain sizes).
Sample Textures
Aphanitic
Phaneritic
Sample Textures
Porphyritic
(Large crystals called phenocrysts
and matrix of smaller crystals
groundmass).
Glassy
Other Textures
Vesicular:
air bubbles
Pegmatitic:
very coarsegrained
Pyroclastic:
rock fragments
Igneous Rock Textures
Felsic (Granitic Rocks)
Both have the same composition, but different textures.
Granite
Rhyolite
Other Felsic Rocks
High in Silica content.
Obsidian
Pumice
Intermediate (Andesitic Rocks)
Both have the same composition, but different textures.
Diorite (Phanertic)
Andesite (porphyric)
Mafic (Basaltic Rocks)
Both have the same composition, but different textures.
Gabbro
Label the textures
Basalt
Common Igneous Rocks
Darker
Origin of Magma
Geothermal Gradient:
•  Change in temperature
with depth.
•  Predicts conditions where
rocks melt into magma!
Partial Melting!
What’s the problem with generating magma
based on the Geothermal Gradient above?
Fig.%6'1,%p.%123%
Origin%of%Magma%
Therefore, Earth’s crust & mantle are made of solid, not
molten rock!
Factors that can lower the melting point:
1.  Reduction of Pressure
2.  Addition of H20
Fig.%6'1,%p.%123%
Role of Pressure
Decompression
Melting - Example:
Mid-ocean Ridges
& Hotspots – As
tectonic plates move
apart, the confining
pressure is reduced
and triggers melting
of the mantle.
Role of Volatiles
Role of Volatiles –
water lowers the
melting
temperatures. At a
Subduction Zone,
the oceanic plate
descends into the
mantle, water is
driven from the plate
and lowers the
melting temperature,
creating melt.
How Magmas Evolve
•  A single volcano may extrude lavas that vary in
composition
•  Bowen’s reaction series
•  Minerals crystallize in a systematic fashion
based on their melting points.
•  As minerals crystallize, the composition of the
liquid portion of the magma continually
changes
•  The minerals rich in iron (Fe) and magnesium
(Mg) (ferromagnesians) crystallize first.
•  The melt becomes enriched in Na, K, Al, and
SiO2
Diversity of Igneous Rocks
There are 3 ways in which diverse igneous rocks
are formed.
1.  Magmatic Differentiation:
•  When crystals form, they are heavier (crystal
settling) and tend to sink toward the bottom of
the magma chamber.
•  This changes the composition of the melt.
Thus, more than one igneous rock type is
produced from a single parent magma.
Diversity of Igneous
Rocks
 More Silica Rich Magma
Basalt
Andesite
Rhyolite
Diversity of Igneous Rocks
  2. Assimilation – incorporate surrounding host rock into
magma – tends to the make magma more silica rich.
  3. Magma Mixing – The mixing of two magma chambers
would produce one magma chamber with a different
composition.
Assimilation and Magma Mixing
Partial Melting & Magma Composition
  As a rock is heated, the minerals with the lowest
melting points begin to melt. If melting continues
and all of the rock is melted, then the composition
of the melt matches that of the rock.
  In most cases, melting is not complete and only
partial melting occurs.
  What minerals have the lowest melting points?
  Bowen’s Reaction Series predicts both
crystallization and melting of minerals.
Bowen’s Reaction Series
When you partially melt a rock, the last formed minerals are the
first to melt – the silica-rich minerals.
Partial Melting
Partial Melting & Magma Composition
  An important result of partial melting is the
production of melt with a higher silica
content than the original rock.
4.7 Partial Melting
Silica-rich
magma.
50% White Chocolate
100% White Chocolate:
Melt always has higher
chocolate content than
the original cookie.
Silica-poor
unmelted.
0% White Chocolate:
All that remains are
the nuts & cookie.
Partial Melting & Basalts at MOR
  Partial melting of peridotites (upper mantle rocks)
produce basaltic rocks (oceanic seafloor).
Making Granite at Subduction Zones
  (1) Partial melting of upper mantle peridotite produces
basaltic magma.
  (2) THEN with magmatic differentiation in large magma
chambers, andesitic magma forms.
  (3) THEN the andesitic magma must assimilate continental
rocks to get additional silica to produce granitic rocks.
(3)
(2)
(1)
Making Granite at Subduction Zones
  At Subduction Zones:
(1) Partial melting of
upper mantle peridotite
produces primary
(2) & (3)
basaltic magma.
  (2) THEN with
magmatic
(2) & (3)
differentiation in large
magma chambers,
secondary andesitic
magma forms.
  (3) THEN the andesitic
magma must assimilate
(1)
continental rocks to get
additional silica to
produce granitic rocks.
Granitic Batholiths
Q: Why are granitic
batholiths only formed
along oceaniccontinental convergent
plate margins?
A: Remember: you need
the extra silica from
continental rocks to
assimilate (Step 3) into the
andesitic magma to
produce felsic magma.
Intrusive Igneous Structures
Tabular Intrusive Bodies: Dikes and Sills
Dike: a tabular, discordant pluton
Serves as tabular conduits to transport magma
Sill—a tabular, concordant pluton
Tend to accumulate magma and increase in
thickness.
Closely resembles buried lava flows
Intrusive Igneous Activity
Exposed Dike
Exposed Sill
Intrusive Igneous Activity
•  Massive Intrusive Bodies: Batholiths,
Columnar joints form as igneous rocks cool and
develop shrinkage fractures that produce elongated
pillar-like columns.
Devil’s Post Pile, eastern Sierra Nevada.
Xenoliths
• Xenoliths are suspended
blocks of country rocks
found in plutons
Xenoliths
• Xenoliths are suspended
blocks of country rocks
found in plutons