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c. Classify rocks by their process of formation.
d. Describe processes that change rocks and the
surface of the earth.
 Identified
by
origin, texture,
and mineral
composition.
 From the Latin
word “ignis,”
which means fire.
 Tend to have
average densities.
Igneous Rocks
form from the cooling of
molten rock (lava and
magma).


LAVA is molten material found on the surface
of the earth.
MAGMA is molten material found beneath the
surface of the earth.
Intrusive rock
Extrusive Rock
At or near surface
Inside Earth
Magma
Lava
Cools slowly
Cools fast
Coarse-grained
Fine Grained
Large Crystals
Small Crystals
Classified by
Formation:
Extrusive Rock is formed
from lava.
Magma Exits the Earth to
form lava at the surface.
(Basalt)
Classified by Formation:
Intrusive Rock is formed
when magma hardens
beneath Earth’s surface.
Inside the Earth
(Granite)
TEXTURE
TEXTURE reflects cooling rate of magma or lava.
RAPID COOLING
=
SMALL(fine) Crystals
RAPID COOLING
=
VOLCANIC
SLOW COOLING
=
LARGE (coarse) Crystals
SLOW COOLING
=
PLUTONIC
Classified by Texture:
Fast Cooling Lava (Extrusive)
Fine-Grained, small crystals, smooth
Slow Cooling Magma (Intrusive)
Coarse-Grained, large crystals, rough
 Aphanitic
 Phaneritic
 Glassy
Grain Size
(Texture)
Aphanitic (fine grained) = grains are very fine
-such rocks are formed when the rock solidifies at
or near the surface (extrusive)
Small crystals < 2 mm
Fast cooling rates
Rhyolite
Felsite
Basalt
Scoria
TEXTURE
Types of Textures
Phaneritic (coarse grained) = large, identifiable
crystals
-are formed when rocks solidified far below the surface
(intrusive)
Large crystals > 2 mm
Slow cooling rate =
Plutonic (formed in a pluton – a cave of magma within
the volcano)
TEXTURE
Types of Textures
Glassy = crystal formation does not occur
-rocks are formed when molten rock is
suddenly ejected from a volcano
No crystals, lava cools too fast
Extremely fast cooling rate =
Obsidian
An intrusive igneous rock
would have what type of
texture? Why?
Coarse-Grained, large crystals.
Smooth & rough.
It is hotter in the earth than on the
outside of the earth so it takes
longer for magma to cool down.
An extrusive igneous
rock would have what
type of texture? Why?
Fine-Grained or glassy
texture
It is cooler on the surface of
the Earth so the rocks can
cool faster.
MINERAL COMPOSITION
You can determine basic igneous rock composition
just by the COLOR of the rock.
DARK COLORED igneous rocks
= MAFIC
LIGHT COLORED igneous rocks = FELSIC
GRAY COLORED igneous rocks
= INTERMEDIATE
 Lighter
colored
igneous rocks
will have lots of
silica.
 They tend to be
less dense than
the darker
versions.
 Less
silica means
these rocks will
be darker than
their felsic
counterpart.
 They will be
denser also.