Igneous Review
Marble demo, rock specimens, Petrographic Microscope, Olivine Porphyry or Gabbro
Trays of mafic and felsic minerals
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
Where?
 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
Geothermal
Gradient
Cool
Silica-rich rocks (with
Quartz, K-feldspar)
melt at cooler
temperatures.
Melts are viscous
Silica-poor rocks (with
Olivine, Pyroxene,
Ca-feldspar) melt at
higher temperatures
Melts are very fluid
Hot
Basaltic Magmas

Magmas
 Many initially form from partial melting of
mantle
- those are basaltic
Two Geologic Environments Where Igneous Rocks Form
Igneous Rocks
Formed in Rift
Igneous Rocks
Formed Above
Sinking Plate
Both melts are "Basaltic" i.e. Olivine, Pyroxene and Ca-Feldspars
Extrusive Igneous Rock - Lava (Hawaii)
Intrusive Igneous Rock (Granite) – This granite
cooled 30 kilometers under the surface
Plagioclase
Feldspar
Quartz
Amphibole
K Feldspar
Characteristics of magma
Three parts:
– Liquid portion, called melt, that is
mobile ions
– Solids, if any, are silicate minerals
already crystallized from the melt
– Volatiles, which are gases dissolved in the
melt, including water vapor (H2O), carbon
dioxide (CO2), and sulfur dioxide (SO2)
Bowen’s Reaction Series
Molten- VERY Hot
No solids
First mineral to crystallize out
Molten- Not so hot
100% Solid
Fine crystals
Need a microscope
Low silica, HOT, fluid
Course crystals
Easily seen
Intermediate
High silica, warm, viscous
Crystals can react with the melt if they touch it
If the first formed crystals of Calcium-rich (Ca) Plagioclase touch the melt they will react with it,
and 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)
If early (mafic) grains are removed, the melt
becomes richer in Silica
Remove
Fe, Mg, Ca
Some Si
Lesson:
You can start with a
Mafic (silica-poor)
magma
and end up with some
Felsic (silica-rich)
Granites.
Left with
K and Al
Most of Si
Marble Demo
A melt will crystallize its mafic components first, and the remaining melt may be granitic
Igneous textures

Most important is crystal size

Factors affecting crystal size
 Rate of cooling
– Slow rate promotes the growth of
fewer but larger crystals
– Fast rate forms many small crystals
– Very fast rate forms glass
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
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
 versus
…
“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

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
Igneous compositions

Naming igneous rocks – granitic (felsic)
rocks

fine grained because extruded,
so crystallized quickly
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
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
http://www.naturalsciences.org/microsites/education/Yellowstone/2006/pages/Yellow-bellied%20Marmot_side%20view.html
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
–Makes up a significant percentage of the
oceanic crust, beneath the basalt pillow
lavas.
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)
When Yellowstone
explodes, half of Wyoming
will perish
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
 Role of Pressure
– Reducing the pressure lowers the melting
temperature – the rock probably melts
– RIDGE: When confining pressures drop,
decompression melting occurs
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
Assimilation and magmatic
differentiation
Show Samples
Basalts forming in rifts and MORs
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
Basaltic here
Good diagram for the
Andes Mountains
Small blobs, not much heat in them
Assimilate some crust, fractionate
Plate Tectonics- Andesite Line
Andes
45
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 small amounts of granites from deep felsic rock passed by ascending magma