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Igneous
Rocks
Pages 129-134
Origin of igneous rocks
• Igneous rocks are
formed by molten
material from a
volcano or molten
material within
Earth
• Temps are 1600 at
60-200 km beneath
the surface
Why is it so hot?
• The deep rocks are under pressure
from the rocks on top
• Also … radioactive elements
produce heat
• Leftover heat from formation of the
planet
Formation of magma
Three factors affect whether rock
melts
• Temperature
• Pressure
• Presence of fluids
Rocks have different
melting points based upon
the chemical
composition of their
minerals
This means they crystallize
(and melt) at different times
Partial Melting
• Different minerals have different
melting points, and minerals with
lower melting points are the first to
melt
• This creates lava with a specific
composition
• As the magma heats up, other
minerals melt and the magma’s
composition changes
Where do Igneous Rocks form?
Magma—Inside Earth
Lava—Outside Earth
Cooling time=crystal size
• Magma trapped underground is
insulated so it cools slowly
• As it cools, the atoms arrange
themselves in patterns to form crystals
• If the magma cools slowly, the atoms
have time to arrange large crystals.
• These crystals are called
mineral grains.
Rocks form when these
mineral grains grow together.
Intrusive Igneous Rocks
• Rocks that form beneath Earth's
surface are called intrusive
igneous rocks.
• They have crystals large enough to
see.
• We call this texture/grain size:
phaneritic
How am I ever going to
remember phaneritic?
Think of large “ph”annies
Granite
Granite is an
intrusive igneous
rock
It cooled slowly inside the earth
It has a phaneritic texture
Diorite—salt and pepper look
Typical Minerals
Na/Ca
PLAGIOCLASE &
MAFICS
(amphibole)
about 50/50
Quartz - absent to
trace
Gabbro
• A mafic,
phaneritic,
igneous rock
from the top of
Bowen's
Reaction
Series.
Extrusive Igneous Rocks
• When rocks form from molten lava
above Earth's surface we call them
extrusive igneous rocks.
• They cool quickly so the atoms don't
have time to arrange themselves into
patterns to form crystals. They have a
fine-grained texture.
• This texture is called aphanitic.
Rhyolite
Rhyolite is an
extrusive igneous
rock
It cooled quickly,It has small grains
This is called an
aphanitic texture
Andesite Porphyry
• This is an andesite
porphyry
• A porphyry has
big chunks in a
matrix
• Matrix—finegrained
background
Basalt
• Basalt makes
up the ocean
floors
• It is a mafic
igneous extrusive
rock
Common Igneous
rock terms
Type
of lava
Felsic
Intrusive Granite
Extrusive
Intermediate Mafic
Texture
term
Diorite
Gabbro
Phaneritic
Basalt
Aphanitic
Rhyolite Andesite
Vesicular Pumice
-Frothy
Scoria
Obsidian HyalineGlassy
Pairs
Granite
Cooled slowly
Rhyolite
cooled quickly
Others-- Scoria
formed when lava
cooled quickly above
ground. You can see
where little pockets of
air had been. Scoria
is actually a kind of
glass and not a
mixture of minerals.
Vesicular—bubbly
texture
Pumice
• Vesicular texture
• Uses
– lava soap
– Stone-washed jeans
• Glass. Pumice has
cooled too quickly for
minerals to form.
• Chemical composition,
however, will be from
the bottom of Bowen's
Reaction series (i.e.
felsic).
Obsidian
• Hyaline texture is
also called a glassy
texture
• Obsidian--Volcanic
glass
• Notice the
conchoidal fracture
Pegmatite
Rock cools
extremely
slowly, so very
big mineral
grains form.
Classification of Magmas
• Mafic—high in iron
and magnesium
• Felsic—high in feldspar and silica (quartz)
• Mafic—50% silica
– Colors-- black, green, red
• Intermediate—60% silica
• Felsic—70% silica
– Colors-- pink, white, light gray
Composition of Igneous Rocks
• Felsic—high in
silica (quartz and
feldspar)
– Light colored
• Mafic—high in
magnesium and
iron
– Dark colored
• Intermediate
– A mixture of the two
– In between colored
Review
Igneous Rocks
Melting point is also affected
by two factors
• The pressure
– As pressure
melting point
• The water content
– High water content=lower melting point
– Low water content=higher melting point
Remember a rock is made
of different minerals
• When rocks melt, those with lowest
melting points melt first
– Partial melting
• When rock crystallize, those with
highest melting points crystallize first
as the magma cools
– Fractional crystallization
Bowen’s Reaction Series
Bowen’s Reaction Series
Igneous Rock formation
beneath the Earth's surface
• Pluton-large
intrusive igneous
rock body
• Dike—cuts across
rock layers-vertical
• Sill—horizontal –
parallel to existing
rock layers
Caldera
Pillow Basalt
Igneous Rock Structures