The Story of: Igneous
Rocks
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Section 4-2
Explain how the cooling rate of
magma affects the rock texture
Distinguish between Intrusive &
Extrusive Igneous rocks
Science Starter:
1. Using your notes
& section 4.2 in the
book, create a Tchart explaining the
differences between
Intrusive & Extrusive
Igneous rocks.
Intrusive
Extrusive
Igneous Rock
 Igneous
= Latin word for “fire”
 Rocks formed from the cooling of
magma (or lava).
Magma Formation
3
ways magma can form: (p. 98)
 Rock
is HEATed
 When PRESSURE is released
 When rock changes COMPOSITION
Classification of Igneous Rocks:
Composition: what minerals are in the
rock
 Texture: size of the rocks’ mineral
grains(crystals)
 Eg. Large grains or small grains

Composition of Igneous Rocks:
Composition
determines rock
color
 Dark Colored =
Mafic
 Made of elements:
Calcium, Iron &
Magnesium
 More Dense
Basalt
Composition of Igneous Rocks:
Light Colored =
Felsic
 Made of elements:
aluminum,
potassium, silicon
 Less Dense

Rhyolite
Texture of Igneous Rocks:
How quickly magma
cools determines
rock texture
 Cools slowly=
more time for
crystals to grow
 More time for
crystals to grow=
larger crystals
 Intrusive Igneous
Rock
Granite (coarse-grained)
Texture of Igneous Rocks:
Cools quickly=
little/no time for
crystals to grow
 no crystals or small
crystals
 Extrusive Igneous
Rock

Obsidian (volcanic glass)
4.2
Igneous rocks form from molten
rock.
As molten rock cools, minerals
crystallize and form Ig. rocks.
Extrusive igneous
rocks cool quickly
at Earth's surface.
Intrusive igneous
rocks cool slowly
within Earth.
intrusive igneous
rock
extrusive igneous
rock
4.2
Igneous rocks form from molten
rock.
As molten rock cools, minerals
crystallize and form Ig. rocks.
Extrusive igneous
rocks cool quickly
at Earth's surface.
Intrusive igneous
rocks cool slowly
within Earth.
intrusive igneous
rock
extrusive igneous
rock
Igneous Rock-Intrusive vs. Extrusive
Extrusive Igneous Rock:
Cools quickly at Earth’s
surface
What Texture will Extrusive rocks
have?
Intrusive Igneous Rock:
Cools slowly within
Earth
What Texture will Intrusive rocks
have?
Magma Formation, cont.

Temperature
 An
increase in temperature from the earth’s
interior can cause minerals in rock to melt.
 Different minerals have different melting
points.
Magma formation cont.

Pressure:
 Earth’s
interior=high pressure=rocks stay in
their solid form.
 As rocks move towards earth’s surface there
is less pressure and it sometimes causes them
to melt.
Magma formation cont.

Composition Sometimes
fluids enter a rock and change its
melting point, and ultimately the composition
of the rock itself.
Igneous Rock Vocabulary:
Composition:
Texture:
Igneous Rock Vocabulary
Composition:
Texture: