Igneous Rocks:
Forged By Fire
-Igneous rocks (from the Latin
ignis, or fire) form as molten
rock cools and solidifies.
- Igneous rocks and
metamorphic rocks, derived
from igneous “parents,” make
up about 95 percent of the
Earth’s crust.
- The mantle, which accounts for
more than 82 percent of the
Earth’s volume, is also
composed of igneous rock. Thus,
Earth can be described as a huge
mass of igneous rocks covered
with a thin veneer of
sedimentary rocks and having a
relatively small iron-rich core.
Where Do Igneous Rocks Come From?
Igneous rocks form from a
material called magma,
which is created when
intense heat and pressures
melt solid rock located in
the crust and upper mantle
of the Earth.
General characteristics of magma:
•Parent material of igneous rocks
•Forms from partial melting of rocks inside Earth
•Magma that reaches the surface is called lava
How Does Magma Make Igneous Rocks?
- Igneous rocks can be
described as intrusive
and plutonic (formed
from magma inside the
Earth), or extrusive and
volcanic (formed from
lava above the Earth’s
surface).
- Igneous rocks that form below the Earth’s surface are called
intrusive igneous rocks (or plutonic). The word “plutonic”
comes from Pluto, the name for the Greek god of the
underworld.
- They form when magma enters a pocket or chamber
underground that is relatively cool and solidifies into crystals as
it cools very slowly.
Characteristics of Magma
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
- A major portion of all magma is silica, which is a compound of
silicon (the second most abundant element on Earth), and oxygen
(the most abundant element on Earth). Magma also contains
gases, which expand as the magma rises.
- Magma that is high in silica resists flowing, so expanding gases
are trapped in it. Pressure builds up until the gases blast out in a
violent, dangerous explosion.
- Magma that is relatively
poor in silica flows easily, so
gas bubbles move up
through it and escape fairly
gently.
Characteristics of Magma
• Role of heat
- Temperature increases within
Earth’s upper crust (called the
geothermal gradient) average
between 20oC to 30oC per
kilometer
- 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
Characteristics 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
Characteristics 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
Characteristics of Magma
Sometimes as it is forming, magma can change its composition.
This may result in different types of magma coming out in one
volcanic event.
- Assimilation: Changing a
magma’s composition by
the incorporation of
foreign matter
(surrounding rock bodies)
into a magma
- Magmatic
differentiation:
Separation of a melt
from earlier formed
crystals to form a
different composition of
magma
Geology and Chemistry Connect!
- When scientists began observing different compositions of
lavas coming from various volcanic events, this led them to
wonder if there was a way to use this information to learn
more about the various igneous rocks that form.
• One scientist, N.L. Bowen, used this information to
demonstrate that as a magma cools, minerals
crystallize in a systematic fashion based on their
melting points.
• Bowen’s reaction series, as it is called today, shows us
temperature and mineral content are the main
determinants of how crystal structures will change and
evolve (and ultimately, which type of chemical
composition an igneous rock will have).
-During crystallization (as the magma cools), the composition of
the liquid portion of the magma continually changes. At the higher
temperatures associated with mafic and intermediate magmas,
the general progression can be separated into two branches: the
continuous branch and the discontinuous branch.
Bowen’s Reaction Series
Explained
The continuous branch describes the evolution of
the plagioclase feldspars as they evolve from being
calcium-rich to more sodium-rich.
Bowen’s Reaction Series
Explained
-The discontinuous branch describes the formation of the
mafic minerals olivine, pyroxene, amphibole, and biotite mica.
Bowen’s Reaction Series
Explained
-The weird thing that Bowen
found concerned the
discontinuous branch.
-At a certain temperature a magma might produce olivine, but if
that same magma was allowed to cool further, the olivine would
"react" with the residual magma, and change to the next mineral
on the series (in this case pyroxene). Continue cooling and the
pyroxene would convert to amphibole, and then to biotite.
-Mighty strange stuff, but if you consider that most silicate
minerals are made from slightly different proportions of the same
8 elements (O, Si, Al, Fe, Ca, Na, K, Mg), all we're really doing here
is adjusting the internal crystalline lattice to achieve stability at
different temperatures. Really no big deal.
So, What Does This Mean
For Igneous Rocks?
- Understanding the
way magma forms
and solidifies helps
us know WHY we
have different
textures and
compositions for
various igneous
rocks.
Properties of Igneous Rocks
Igneous rocks are composed primarily of silicate minerals
•Dark (or ferromagnesian) silicates
Olivine
Pyroxene
Biotite Mica
Amphibole
•Light (or nonferromagnesian) silicates
Quartz
Muscovite Mica
Feldspar
Properties of Igneous Rocks
Main Differences of Igneous Rocks
Granitic composition:
– Composed of lightcolored silicates
– Felsic (feldspar and
silica) in composition
– Contains high amounts
of silica (SiO2)
– Major constituents of
continental crust
– 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 and tend to produce large plutonic
structures
Main Differences of Igneous Rocks
Basaltic composition:
Composed of dark
silicates and calciumrich feldspar
Mafic (magnesium
and ferrum, for iron) in
composition
More dense than
granitic rocks
Comprise the ocean
floor as well as many
volcanic islands
- Basaltic magmas form at midocean ridges by decompression
melting or at subduction zones
- Large outpourings of basaltic
magma are common at Earth’s
surface
Main Differences of Igneous Rocks
• Other compositional groups:
• Intermediate (or andesitic)
composition
– Contain 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
Physical Properties of Igneous Rocks
Texture in igneous rocks is determined by the size and
arrangement of mineral grains, which is usually determined
by when/how a rock and crystals were formed.
Factors That Determine 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
• Amount of silica (SiO2)
present
• Amount of dissolved
gases
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 Basalt
Phaneritic Diorite
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 Andesite
Glassy Obsidian
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 Tuff
Pegmatite