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Igneous Rocks
Origin of Magma
Highly debated topic
Generating magma from solid rock
• Produced from partial melting of rocks in
the crust and upper mantle
• Role of heat
– Temperature increases within Earth’s upper
crust (called the geothermal gradient)
average between 20oC to 30oC per kilometer
Approximate temperatures in the
crust and mantle
Deep Potosi Mine Anecdote
Clicker Question 4.1
If geothermal gradient is 25 degrees C per km, and
temperature at surface is 10 degrees C,
What is temperature at a depth of 30 km?
A)
B)
C)
D)
E)
660 deg C
760 deg C
860 deg C
960 deg C
1060 deg C
Origin of Magma
• Role of heat
– Rocks in the lower crust and upper mantle
are near their melting points
– Any additional heat (from rocks descending
into the mantle or rising heat from the
mantle) may induce melting
Origin of Magma
• Role of pressure
– An increase in confining pressure causes an
increase in a rock’s melting temperature or
conversely, reducing the pressure lowers the
melting temperature
– When confining pressures drop,
decompression melting occurs
Decompression melting
Origin of Magma
• Role of volatiles
– Volatiles (primarily water) cause rocks to
melt at lower temperatures
– This is particularly important where oceanic
lithosphere descends into the mantle
Evolution of magmas
Processes responsible for changing a
magma’s composition
• Magmatic differentiation
– Separation of a melt from earlier formed
crystals to form a different composition of
magma
• Assimilation
– Changing a magma’s composition by the
incorporation of foreign matter (surrounding
rock bodies) into a magma
Assimilation and magmatic
differentiation
Which process is likely to change a
magmas chemical composition to a
chemistry more similar to the host rock?
A.
B.
C.
D.
Assimilation
Differentiation
Crystallization
None of above
Evolution of magmas
Processes responsible for changing a
magma’s composition
• Magma mixing
– Involves two bodies of magma intruding one
another
– Two chemically distinct magmas may
produce a composition quite different from
either original magma
Evolution of magmas
Partial melting and magma formation
• Incomplete melting of rocks is known as
partial melting
• Formation of basaltic magmas
– Most originate from partial melting of
ultramafic rock in the mantle
– Basaltic magmas form at mid-ocean ridges by
decompression melting or at subduction zones
Evolution of magmas
Partial melting and magma formation
• Formation of basaltic magmas
– As basaltic magmas migrate upward,
confining pressure decreases which reduces
the melting temperature
– Large outpourings of basaltic magma are
common at Earth’s surface (Deccan Traps)
– The presence of water promotes partial
melting of the mantle to create basalt
Evolution of magmas
Partial melting and magma formation
• Formation of andesitic magmas
– Interactions between mantle-derived basaltic
magmas and more silica-rich rocks in the
crust generate magma of andesitic
composition
– Andesitic magma may also evolve by
magmatic differentiation
Evolution of magmas
Partial melting and magma formation
• Formation of granitic magmas
– Most likely form as the end product of
crystallization of andesitic magma
– Granitic magmas are higher in silica and
therefore more viscous than other magmas
– Because of their viscosity, they lose their
mobility before reaching the surface
– Tend to produce large plutonic structures
(Pikes Peak, Rocky Mtn Natl Park)
Characteristics of magma
Igneous rocks form as molten rock cools
and solidifies
Characteristics of magma (molten rock)
• Parent material of igneous rocks
• Forms from partial melting of rocks
inside the Earth
• Magma that reaches the surface is lava
Geologic Environments Where Igneous Rocks Form
Igneous Rocks
Formed in Rift
Igneous Rocks
Formed Above
Sinking Plate
Clicker Question 4.2: How do you melt rock?
A) Lower pressure
B) Raise temperature
C) Add H20
D) B & C
E) A, B, & C
Characteristics of magma
General Characteristic of magma
• Rocks formed from lava at the surface are
classified as extrusive, or volcanic rocks
• Rocks formed from magma that
crystallizes at depth are termed intrusive,
or plutonic rocks
Is this an extrusive, or intrusive igneous rock?
A. Extrusive
B. Intrusive
Intrusive Igneous Rock (Granite) - Sorry no
pictures available from 25 kilometers down!
Characteristics of magma
The nature of magma
• Consists of three components:
– A liquid portion, called melt, that is composed
of mobile ions
– Solids, if any, are silicate minerals that have
already crystallized from the melt
– Volatiles, which are gases dissolved in the
melt, including water vapor (H2O), carbon
dioxide (CO2), and sulfur dioxide (SO2)
Characteristics of magma
Crystallization of magma
• Cooling of magma results in the
systematic arrangement of ions into
orderly patterns
• The silicate minerals resulting from
crystallization form in a predictable order
Olivene sand on the worlds
only “Green Sand” beach
on the southern Big Island Hawaii. Olivene is derived
from erosion of olivene
crystals in volcanic rock.
Crystallization of minerals in
magma bodies
Bowen’s reaction series and the
composition of igneous rocks
• N.L. Bowen demonstrated that as a
magma cools, minerals crystallize in a
systematic fashion based on their melting
points
Bowen’s Reaction Series
Granite
Hand Sample
Order of
Crystallization
Granite
Thin Section
Zoned feldspar (plagioclase) showing change in
composition with time in magma chamber
(calcic in core to sodic at rim)
Which
mineral
first?
Which
mineral
do crystallized
you think crystallized
first?
Orthoclase
A) A)
Feldspar
(Plagioclase)
Quartz
B) B)
Quartz
Olivene
C) C)
Hornblende
Characteristics of magma
Crystallization of magma
• Texture in igneous rocks is determined by the
size and arrangement of mineral grains
• Igneous rocks are typically classified by both:
– Texture
– Mineral composition
Igneous textures
Texture is used to describe the overall
appearance of a rock based on the size,
shape, and arrangement of interlocking
minerals
Factors affecting crystal size
• Rate of cooling
– Slow rate promotes the growth of fewer but
larger crystals
Igneous textures
Factors affecting crystal size
• Rate of cooling
– Fast rate forms many small crystals
– Very fast rate forms glass
• Amount of silica (SiO2) present
• Amount of dissolved gases
Hint: What are the small grey
minerals? Qtz, Plag or Biotite?
The big one is Plagioclase
Assuming little change in bulk chemistry or dissolved
volatiles, what does the texture of this rock tell you
about its cooling history?
A.
B.
C.
D.
E.
It cooled evenly deep in the Earth
It cooled slowly first (deep), and much faster later (shallow)
It first cooled quickly, then cooled slowly
It was heated and never cooled
None of the above
Types of Igneous textures
Types of igneous textures
• Aphanitic (fine-grained) texture
– Rapid rate of cooling of lava or magma
– Microscopic crystals
– May contain vesicles (holes from gas bubbles)
• Phaneritic (coarse-grained) texture
– Slow cooling
– Crystals can be identified without a
microscope
Aphanitic texture
Phaneritic texture
Igneous textures
Types of igneous textures
• Porphyritic texture
– Minerals form at different temperatures as
well as differing rates
– Large crystals, called phenocrysts, are
embedded in a matrix of smaller crystals,
called the groundmass
• Glassy texture
– Very rapid cooling of molten rock
– Resulting rock is called obsidian
Porphyritic texture
Granite
Glassy texture
Obsidian
Yet more types of Igneous textures
Types of igneous textures
• Pyroclastic texture
– Various fragments ejected during a violent
volcanic eruption
– Textures often appear to more similar to
sedimentary rocks
• Pegmatitic texture
– Exceptionally coarse grained
– Form in late stages of crystallization of
granitic magmas
Pyroclastic Rock - Superheated Superfast Flows
A beautiful pegmatite vein of orthoclase and quartz
Igneous Compositions
Igneous rocks are composed primarily of
silicate minerals that can include:
• Dark (or ferromagnesian) silicates
– Olivine
– Pyroxene
– Amphibole
– Biotite mica
Igneous Compositions
Igneous rocks are composed primarily of
light silicate minerals that include:
– Quartz
– Muscovite mica
– Feldspars
Igneous compositions
Granitic versus basaltic compositions
• Granitic composition
– Composed of light-colored silicates
– Designated as being felsic (feldspar and silica)
in composition
– Contains high amounts of silica (SiO2)
– Major constituents of continental crust
Igneous compositions
Naming igneous rocks – granitic (felsic)
rocks
• Granite
– Phaneritic
– Over 65 percent quartz, about 25 percent or
more feldspar
– Abundant and often associated with
mountain building
– The term granite covers a wide range of
mineral compositions
Granite
Igneous compositions
Naming igneous rocks – granitic (felsic)
rocks
• Rhyolite
– Extrusive chemical equivalent of granite
– May contain glass fragments and vesicles
– Aphanitic texture
– Less common and less voluminous than
granite
– Phenocrysts can include quartz and feldspar
Rhyolite
Igneous compositions
Granitic versus basaltic compositions
• Basaltic composition
– Composed of dark silicates and calcium-rich
feldspar
– Designated as being mafic (magnesium and
ferrum, for iron) in composition
– More dense than granitic rocks
– Comprise the ocean floor as well as many
volcanic islands
Igneous compositions
Naming igneous rocks – basaltic (mafic)
rocks
• Basalt
– Volcanic origin
– Aphanitic texture
– Composed mainly of pyroxene and calciumrich plagioclase feldspar
– Most common extrusive igneous rock
Basalt
Basalt (scoria with gas bubbles)
Igneous compositions
Naming igneous rocks – basaltic (mafic)
rocks
• Gabbro
– Intrusive equivalent of basalt
– Phaneritic texture consisting of pyroxene and
calcium-rich plagioclase
– Makes up a significant percentage of the
oceanic crust
Gabbro - a mafic
igneous rock
Igneous compositions
Other compositional groups
• Intermediate (or andesitic) composition
– Contains at least 25 percent dark silicate
minerals
– Associated with explosive volcanic activity
• Ultramafic composition
– Rare composition that is high in magnesium
and iron
– Composed entirely of ferromagnesian
silicates
Igneous compositions
Naming igneous rocks – intermediate
rocks
• Andesite
– Volcanic origin
– Aphanitic texture
– Often resembles rhyolite
Andesite
Igneous compositions
Naming igneous rocks – intermediate
rocks
• Diorite
– Plutonic (intrusive) equivalent of andesite
– Coarse grained
– Composed mainly of intermediate feldspar
and amphibole
Diorite
Clicker Questions 4.3
Name the aphanitic volcanic
rock that is probably basalt.
C)
A)
D)
B)
Name the phaneritic intrusive
igneous rock that is granite.
A)
C)
D)
B)
Name the aphanitic volcanic
rock that is probably rhyolite.
C)
A)
D)
B)
Igneous compositions
Silica content influences a magma’s
behavior
• Granitic magma
– High silica content
– Extremely viscous
– Liquid exists at temperatures as low as 700oC
– Goes Boom when it erupts (Yellowstone)
Igneous compositions
Silica content influences a magma’s
behavior
• Basaltic magma
– Much lower silica content
– Fluid-like behavior
– Crystallizes at higher temperatures
– Gurgles when it erupts (Hawaii)
Comparison of Igneous Rocks
Summary - Igneous Rock Classification
Igneous Textures
Naming igneous rocks – granitic (felsic)
rocks. Extrusive products can include:
• Obsidian
– Dark colored
– Glassy texture
• Pumice
– Volcanic
– Glassy texture
– Frothy appearance with numerous voids
(extrusive foam)
Obsidian
Pumice
Frothy appearance with numerous voids (extrusive foam)
Guinness
at Night
Head of gas bubbles on
a pint of Guinness is a
perfect analog for
pumice (and it tastes
a lot better!)
Clicker Question
B)
Name
rock
Name the
the intrusive
following igneous
rock types.
List two key characteristics that
A)
identify
each specimen
Gabbro
Obsidian
Pumice
C)
D) A) and B)
Igneous compositions
Naming igneous rocks – pyroclastic rocks
• Composed of fragments ejected during a
volcanic eruption
• Varieties
– Tuff – ash-sized fragments
– Volcanic breccia – particles larger than ash
Ash and pumice layers
Summary - Igneous Rock Texture and Composition