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Types of Rock include Igneous, Sedimentary and Metamorphic
Igneous Rocks
Lecture 3
Marble demo, rock specimens, Petrographic Microscope, Olivine Porphyry or Gabbro
Characteristics of magma
• Igneous rocks form as molten rock cools
and solidifies
• Characteristics of magmas (molten rock)
depend on parent material and where they
crystallize
• Where determines speed of crystallization
• At surface, fast cooling makes small crystals
Geothermal
Gradient
Cool
Silica-rich rocks (with
Quartz) melt at cooler
temperatures.
Melts are viscous
Silica-poor rocks
(with Olivine,
Pyroxene) melt at
higher temperatures
Melts are very fluid
Hot
http://www.casdn.neu.edu/~geology/department/staff/colgan/iceland/usgs/
Characteristics of magma
• General Characteristics of molten rock
• Forms from partial melting of rocks inside
the Earth
• 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
Two Geologic Environments Where Igneous Rocks Form
Igneous Rocks
Formed in Rift
Igneous Rocks
Formed Above
Sinking Plate
Extrusive Igneous Rock - Lava (Hawaii)
Intrusive Igneous Rock (Granite) – This granite
cooled 30 kilometers under the surface
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
Rock-forming minerals crystallize with increasing complexity as the
magma cools. The most complex 3-D minerals crystallize last. The
hottest magmas can only crystallize Olivine (Independent
Tetrahedra), but as the magma cools, more complex minerals can
form.
Crystallization of minerals in
magma bodies
• Bowen’s reaction series and the
composition of igneous rock
• Norman L. Bowen demonstrated that as a
magma cools, minerals crystallize in a
systematic fashion based on their melting
points
http://vgp.agu.org/bowen_paper/bowen_paper.html
Bowen’s Reaction Series
Molten- VERY Hot
Molten- Not so hot
100% Solid
Fine crystals
Need a microscope
Low silica, HOT, fluid
Course crystals
Easily seen
Intermediate
High silica, warm, viscous
Bowens reactrion series
says: as a granitic melt
cools, Biotite Mica and
Plagioclase Feldspar
crystallize out before Quartz
Granite
Hand Sample
Granite
Thin Section
Order of
Crystallization
We can see the order
of crystallization
under the microscope
Crystals can react with the melt if they touch it
If the first formed crystals of Calcium-rich (Ca) Plagioclase are allowed to react with the melt,
they will become more sodium-rich on their outer rims
Zoned feldspar (plagioclase) showing change in
composition with time in magma chamber
(calcium-rich in core to sodium-rich at rim)
However, if early crystals are removed the
melt becomes richer in Silica
Remove
Fe, Mg, Ca
Some Si
Left with
K and Al
Most of Si
You can start with a
Mafic (silica-poor) magma
and end up with some
Felsic (silica-rich)
Granites.
Marble Demo
A rising melt may leave its mafic components behind, and the upper part may be granitic
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
• Most important is crystal size
• 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 (continued)
– Fast rate forms many small crystals
– Very fast rate forms glass
• Amount of silica (SiO2) present
• Amount of dissolved gases
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
Fine grained because it cooled
quickly at the surface
Phaneritic texture
Coarse crystals cooled slowly
at great depth
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
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
Pyroclastic Rock Superheated Flows
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
Still more types of Igneous
textures
• Types of igneous textures
• Pegmatitic texture
–Exceptionally coarse grained crystals
–Form in late stages of fractionation of
magmas
–This is often what prospectors are
looking for
A Pegmatite with Feldspar and Zircon
Zircon is very good for obtaining radiometric ages
Show tray of Mafic Minerals
Igneous Compositions
• Igneous rocks are composed primarily of
silicate minerals that include:
• dark (or ferromagnesian) colored silicates
– Olivine
– Pyroxene
– Amphibole
– Biotite mica
– And …
“MAFIC” Magnesium and Iron
Show tray of Felsic Minerals
Igneous Compositions
• Igneous rocks also contain light colored
silicate minerals that include:
– Quartz
– Muscovite mica
– Feldspars
“FELSIC” Feldspar and Silica
Igneous Rock Classification- Bowen’s Reaction Series on its side
Note Minerals in
Felsic rocks crystallize from warm melts
Note Minerals in
Mafic from hot melts
Igneous 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 20 percent quartz, about 25 percent or
more feldspar (usually much more feldspars).
– Plagioclase is Sodium-rich
– Abundant and often associated with mountain
building
– The term granite covers a wide range of mineral
compositions
Figure 3-7 continued
Igneous compositions
• Naming igneous rocks – granitic (felsic)
rocks
• Rhyolite
– Extrusive equivalent of granite
– May contain glass fragments and vesicles
– Aphanitic texture (means fine grained minerals)
– Less common and less voluminous than granite
– Phenocrysts can include quartz and feldspar
Rhyolite
fine grained because extruded,
so crystallized quickly
Igneous compositions
• Basaltic composition can be fine or coarse
– Composed of dark Olivine and Pyroxene and grey
calcium-rich plagioclase feldspar
– No Potassium-rich feldspar (no K-spar
‘Microcline’)
– Designated as being mafic (magnesium and
ferrum, for iron) in composition
– Much denser than granitic rocks - sinks
– Comprises the ocean floor as well as many volcanic
islands such as Hawaii. Also rift valley lavas
Igneous compositions
• Naming igneous rocks – basaltic (mafic) rocks:
Fine-grained
• Basalt
– Volcanic origin
– Aphanitic texture
– Composed mainly of pyroxene, some olivine and also
calcium-rich plagioclase feldspar
– Most common extrusive igneous rock
Scoria type Basalt note Gas Bubbles
Igneous compositions
• Naming igneous rocks – basaltic (mafic) rocks:
Coarse Grained
• Gabbro
–Intrusive equivalent of basalt
–Phaneritic texture consisting of pyroxene
and calcium-rich plagioclase (med. grain
“diabase”)
–Makes up a significant percentage of the
oceanic crust, beneath the basalt pillow
lavas.
Gabbro - a mafic
igneous rock. A largegrain version of Basalt
Igneous compositions
• Other compositional groups
• Intermediate (or andesitic) composition
– Contain at least 25 percent dark silicate minerals
– Associated with explosive volcanic activity
– Often gray
Igneous
compositions
• Intermediate rocks
• Andesite
– Volcanic origin
– Aphanitic texture
– Often resembles rhyolite
– Intermediate silica content
– Frequent composition in volcanoes above
subduction zones, e.g. in Andes Mountains
Igneous compositions
• Extrusive products can include:
• Pumice
– Volcanic
– Glassy texture, very light weight, mostly air
– Frothy appearance with numerous voids
(extrusive foam)
– Forms when lavas have a lot of water and other
volatiles
Common with
intermediate
compositions
Igneous
compositions
• Intermediate rocks
• Diorite
– Plutonic equivalent of andesite
– Coarse grained
– Intrusive
– Composed mainly of intermediate feldspar and
amphibole
Igneous compositions
• Silica content influences a magma’s
behavior
• Granitic magma
Plutonic
“Granite”
– High silica content
– Extremely viscous
– Liquid exists at temperatures as low as 700oC
– Huge explosion if it erupts (Yellowstone, Toba)
Volcanic
“Rhyolite”
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)
Origin of Magma
• 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 with increasing depth. The geothermal
gradient averages between 20oC to 30oC per
kilometer
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
– Reducing the pressure lowers the melting
temperature – the rock probably melts
– RIDGE: When confining pressures drop,
decompression melting occurs
Decompression melting
At a Mid-Ocean Ridge
Origin of Magma
• Role of volatiles - WATER
–Volatiles (primarily water) cause
rocks to melt at lower
temperatures
–This is particularly important
where oceanic lithosphere
descends into the mantle in a
subduction zone
The role of water
Pillow Lavas
Water promotes melting
despite high pressures
Evolution of magmas
• Processes responsible for changing a magma’s
composition
• Assimilation
– Changing a magma’s composition by the incorporation
of foreign matter (surrounding rock bodies) into a
magma
– The rising magma melts near surface rock, changing
the composition of the melt.
• Magmatic differentiation = Fractionation
– Separation of a melt from earlier formed crystals causes
a different composition in the remaining magma
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
Assimilation and magmatic
differentiation
Show Samples
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 of rising mantle
material
– also by partial melting of ocean crust +
sediments+ water in subduction zones
Basalts forming in rifts
Decompression Melting:
Magma under lithosphere heats and cracks
it. Mantle rock is exposed to low pressures
– it partially melts
Origin of Andesite & Diorite:
intermediate silica content
Good diagram for the
Andes Mountains
Small blobs, not much heat in them
Assimilate some crust, fractionate
Plate Tectonics- Andesite Line
Andes
62
Andesites form above the deep portions of a subduction zone
Origin of Granitic Rocks
Huge blobs w/ low
temps but lots of
magma, fractionation &
assimilation => Granite
Batholiths
Can also get granites from deep rock burial
End of Lecture 3