IGNEOUS ROCKS
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WHAT IS AN IGNEOUS ROCK?
An igneous rock is a rock
that has formed from the
cooling and solidification
of magma or lava.
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MAGMA
LAVA
Melted rock
that is beneath
the surface of
the Earth.
Melted rock
that is at or
near the surface
of the Earth.
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Lava
Magma
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TYPES OF IGNEOUS ROCKS
Igneous rocks are classified
according to where they
cooled and solidified.
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INTRUSIVE
IGNEOUS ROCKS
EXTRUSIVE
IGNEOUS ROCKS
Rocks that form
from magma
cooling and
solidifying while
still inside the
Earth
Rocks that form
from lava
cooling and
solidifying while
at or near the
Earth
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P. 6 of ESRT
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6
Vesicular rhyolite
Vesicular
andesite
noncrystalline
Scoria
Vesicular basalt
Andesite
Basalt
Diabase
Granite
FELSIC
(rich in Si, Al)
75%
DARKER
DENSITY
HIGHER
Glassy
Nonvesicular
Vesicular
(gas
pockets)
Fine
Coarse
Nonvesicular
Very
coarse
100%
Potassium
feldspar
(pink to white)
Quartz
(clear to
white)
TEXTURE
MAFIC
(rich in Fe, Mg)
COMPOSITION
75%
Plagioclase feldspar
(white to gray)
Biotite
(black)
Amphibole
(black)
0%
Gabbro
COLOR
50%
25%
Peridotite
Pegmatite
LOWER
100%
Diorite
CRYSTAL
SIZE
10 mm 1 mm
less than
or
to
1 mm
larger 10 mm
Pumice
Dunite
EXTRUSIVE
(Volcanic)
Basaltic glass
LIGHTER
MINERAL COMPOSITION
(relative by volume)
CHARACTERISTICS
INTRUSIVE
Obsidian
(usually appears black)
Rhyolite
INTRUSIVE
(Plutonic)
IGNEOUS ROCKS
EXTRUSIVE
ENVIRONMENT OF FORMATION
Scheme for Igneous Rock Identification
50%
Pyroxene
(green)
Olivine
(green)
25%
0%
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Physical Setting/Earth Science Reference Tables — 2010
INTRUSIVE IGNEOUS ROCKS
Because these rocks form inside the
Earth, where it is very hot, it takes a
long time for the magma to cool
and solidify. This time allows mineral
crystals to grow large. So all intrusive
igneous rocks have inter-grown crystals
that are larger than 1mm.
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INTRUSIVE IGNEOUS ROCKS
PEGMATITE
GRANITE
*Notice the large mineral crystals
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Pumice
Scoria
noncrystalline
Basaltic glass
Vesicular
andesite
Vesicular rhyolite
Vesicular basalt
Andesite
Basalt
Diabase
Diorite
Granite
Pegmatite
Environment of Formation
LOWER
(where the rock formed)
FELSIC
ch in Si, Al)
DENSITY
P. 6 of ESRT
Coarse
Nonvesicular
Very
coarse
(description
of crystal
100%
sizes)
Non-vesicular means these rocks do
75%
not have any
trapped air bubbles
Plagioclase feldspar
(white to gray)
50%
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Fine
MAFIC
Texture
(rich
in Fe, Mg)
COMPOSITION
(pink to white)
Quartz
(clear to
white)
Vesicular
(gas
pockets)
Range of crystal sizes
HIGHER
Names of common
Potassium
intrusive igneous rocks
feldspar
75%
Glassy
Nonvesicular
DARKER
COLOR
LIGHTER
100%
Gabbro
Peridotite
10 mm 1 mm
less than
or
to
1 mm
larger 10 mm
INTRUSIVE IGNEOUS ROCKS
Dunite
EXTRUSIVE
(Volcanic)
Obsidian
(usually appears black)
Rhyolite
INTRUSIVE
(Plutonic)
ENVIRONMENT OF FORMATION
SIZE
Biotite
(black)
Pyroxene
(green)
50%
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EXTRUSIVE IGNEOUS ROCKS
Because these rocks form on or near the
surface of the Earth, where temperatures
are lower, the lava cools and
solidifies quickly. Without time,
crystals cannot grow very large, or may
not even form at all. So all extrusive
igneous rocks have either no crystals, or
crystals that are smaller than 1mm.
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EXTRUSIVE IGNEOUS ROCKS
BASALT
RHYOLITE
*Notice the mineral crystals are so small, you can barely see them
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EXTRUSIVE IGNEOUS ROCKS
BASALTIC GLASS
OBSIDIAN
*These extrusive igneous rocks formed so quickly, no crystals were
able to form. They are called “glassy” or “non-crystalline.”
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STREAM VELOCITY (cm/s)
This generalized graph shows the water velocity
needed to maintain, but not start, movement. Variations
occur due to differences in particle density and shape.
EXTRUSIVE IGNEOUS ROCKS
Scoria
Vesicular
andesite
Vesicular rhyolite
Andesite
noncrystalline
Pumice
Vesicular basalt
Basalt
Diabase
Diorite
Granite
Environment of Formation
Pegmatite
(where the rock formed)
LIGHTER
P. 6 of ESRT
100%
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COLOR
DENSITY
FELSIC
ich in Si, Al)
Potassium
feldspar
(pink to white)
Peridotite
Range of crystal sizes
Names of common
intrusive igneous rocks
LOWER
Gabbro
CRYSTAL
SIZE
10 mm 1 mm
less than
or
to
1 mm
larger 10 mm
Basaltic glass
Dunite
EXTRUSIVE
(Volcanic)
Obsidian
(usually appears black)
Rhyolite
INTRUSIVE
(Plutonic)
ENVIRONMENT OF FORMATION
Scheme for Igneous Rock Identification
COMPOSITION
TEXTURE
Glassy
Nonvesicular
Vesicular
(gas
pockets)
Fine
Coarse
Nonvesicular
Very
coarse
Texture
(description of crystal
DARKER
sizes)
HIGHER
Some of these
rocks have trapped gas
MAFIC
(rich in Fe,
Mg)do not
bubbles,
some
100%
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A quick review...
✓
Igneous rocks form from cooled and solidified magma
or lava.
✓
When magma cools slowly inside Earth, we get intrusive
(plutonic) igneous rocks with large crystals.
✓
When lava cools quickly at or near Earth’s surface, we
get extrusive (volcanic) igneous rocks with small or no
crystals. These may be vesicular, meaning there are
trapped gas bubbles.
✓
Page 6 of the ESRT shows common igneous rocks and
their characteristics.
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6
Vesicular rhyolite
Vesicular
andesite
noncrystalline
Scoria
Vesicular basalt
Andesite
Basalt
Diabase
Granite
FELSIC
(rich in Si, Al)
75%
DARKER
DENSITY
HIGHER
Glassy
Nonvesicular
Vesicular
(gas
pockets)
Fine
Coarse
Nonvesicular
Very
coarse
100%
Potassium
feldspar
(pink to white)
Quartz
(clear to
white)
TEXTURE
MAFIC
(rich in Fe, Mg)
COMPOSITION
75%
Plagioclase feldspar
(white to gray)
Biotite
(black)
Amphibole
(black)
0%
Gabbro
COLOR
50%
25%
Peridotite
Pegmatite
LOWER
100%
Diorite
CRYSTAL
SIZE
10 mm 1 mm
less than
or
to
1 mm
larger 10 mm
Pumice
Dunite
EXTRUSIVE
(Volcanic)
Basaltic glass
Rhyolite
INTRUSIVE
(Plutonic)
ENVIRONMENT OF FORMATION
IGNEOUS ROCKS
CHARACTERISTICS
Obsidian
(usually appears black)
LIGHTER
MINERAL COMPOSITION
(relative by volume)
IGNEOUS ROCK COLORS
Scheme for Igneous Rock Identification
50%
Pyroxene
(green)
Olivine
(green)
25%
0%
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Physical Setting/Earth Science Reference Tables — 2
6
Vesicular rhyolite
Vesicular
andesite
noncrystalline
Scoria
Vesicular basalt
Andesite
Basalt
Diabase
Granite
FELSIC
(rich in Si, Al)
75%
DARKER
DENSITY
HIGHER
Glassy
Nonvesicular
Vesicular
(gas
pockets)
Fine
Coarse
Nonvesicular
Very
coarse
100%
Potassium
feldspar
(pink to white)
Quartz
(clear to
white)
TEXTURE
MAFIC
(rich in Fe, Mg)
COMPOSITION
75%
Plagioclase feldspar
(white to gray)
Biotite
(black)
Amphibole
(black)
0%
Gabbro
COLOR
50%
25%
Peridotite
Pegmatite
LOWER
100%
Diorite
CRYSTAL
SIZE
10 mm 1 mm
less than
or
to
1 mm
larger 10 mm
Pumice
Dunite
EXTRUSIVE
(Volcanic)
Basaltic glass
Rhyolite
INTRUSIVE
(Plutonic)
ENVIRONMENT OF FORMATION
IGNEOUS ROCKS
CHARACTERISTICS
Obsidian
(usually appears black)
LIGHTER
MINERAL COMPOSITION
(relative by volume)
IGNEOUS ROCK DENSITY
LAST
Scheme for Igneous Rock Identification
50%
Pyroxene
(green)
Olivine
(green)
25%
0%
NEXT
Physical Setting/Earth Science Reference Tables — 2
6
Vesicular rhyolite
Vesicular
andesite
noncrystalline
Scoria
Vesicular basalt
Andesite
Basalt
Diabase
Granite
FELSIC
(rich in Si, Al)
75%
DARKER
DENSITY
HIGHER
Glassy
Nonvesicular
Vesicular
(gas
pockets)
Fine
Coarse
Nonvesicular
Very
coarse
100%
Potassium
feldspar
(pink to white)
Quartz
(clear to
white)
TEXTURE
MAFIC
(rich in Fe, Mg)
COMPOSITION
75%
Plagioclase feldspar
(white to gray)
Biotite
(black)
Amphibole
(black)
0%
Gabbro
COLOR
50%
25%
Peridotite
Pegmatite
LOWER
100%
Diorite
CRYSTAL
SIZE
10 mm 1 mm
less than
or
to
1 mm
larger 10 mm
Pumice
Dunite
EXTRUSIVE
(Volcanic)
Basaltic glass
Rhyolite
INTRUSIVE
(Plutonic)
ENVIRONMENT OF FORMATION
IGNEOUS ROCKS
CHARACTERISTICS
Obsidian
(usually appears black)
LIGHTER
MINERAL COMPOSITION
(relative by volume)
IGNEOUS ROCK COMPOSITION
LAST
Scheme for Igneous Rock Identification
50%
Pyroxene
(green)
Olivine
(green)
25%
0%
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Physical Setting/Earth Science Reference Tables — 2
IGNEOUS ROCK MINERALS
This generalized graph shows the water velocity
needed to maintain, but not start, movement. Variations
occur due to differences in particle density and shape.
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Find the rock name and look below it to see
what minerals
may be present in that rock.
Scheme for Igneous Rock Identification
Vesicular rhyolite
Vesicular
andesite
noncrystalline
Scoria
Vesicular basalt
Andesite
Basalt
INTRUSIVE
(Plutonic)
Diabase
Granite
FELSIC
(rich in Si, Al)
75%
Gabbro
COLOR
DARKER
DENSITY
HIGHER
Glassy
Nonvesicular
Vesicular
(gas
pockets)
Fine
Coarse
Nonvesicular
Very
coarse
100%
Potassium
feldspar
(pink to white)
Quartz
(clear to
white)
TEXTURE
MAFIC
(rich in Fe, Mg)
COMPOSITION
75%
Plagioclase feldspar
(white to gray)
50%
25%
Peridotite
Pegmatite
LOWER
100%
Diorite
10 mm 1 mm
less than
or
to
1 mm
larger 10 mm
Pumice
Dunite
EXTRUSIVE
(Volcanic)
Basaltic glass
Rhyolite
Biotite
(black)
Amphibole
(black)
0%
6
Obsidian
(usually appears black)
LIGHTER
MINERAL COMPOSITION
(relative by volume)
CHARACTERISTICS
IGNEOUS ROCKS
ENVIRONMENT OF FORMATION
CRYSTAL
SIZE
50%
Pyroxene
(green)
Olivine
(green)
25%
0%
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Physical Setting/Earth Science Reference Tables — 2010