Igneous Rocks
BIG Idea
 Igneous
rocks were the first
rocks to form as Earth cooled
from a molten mass to the
crystalline rocks of the early
crust.
The word ‘Igneous’ means:

of or like fire [Latin ignis fire]
What are igneous rocks?
IGNEOUS rocks are…
 rocks
that form when
molten material cools and
hardens either below or on
Earth’s surface.
How do we define MOLTEN?
melted
 changed into liquid form by heat
 glowing ‘red-hot’


MAGMA: THE
PARENT TO ALL
ROCKS

Molten rock
beneath the
surface of the
Earth (forms
underground)

LAVA: molten rock that
flows out ON to Earth’s
surface
Igneous rocks are classified based
on:
1. mineral composition
2. crystal size
3. texture
Two (2) Types of Igneous
Rocks:
1.
EXTRUSIVE: RAPID (fast) COOLING
LAVA ON THE EARTH’S SURFACE


2.
small or no crystals
fine-grained or glassy textures
INTRUSIVE: SLOW COOLING MAGMA
DEEP WITHIN THE EARTH’S CRUST


larger crystals
coarser texture
TEXTURE
 What
is the ‘texture’
of an igneous rock?
 the
size, shape, and arrangement of
the crystals
1. EXTRUSIVE: SMALL (or no)
CRYSTALS - FINE GRAIN (fast
cooling, no time to grow)

EXTRUSIVE rocks can have air
pockets due to fast cooling…
Can you name this rock?

Hints:

it’s the only rock that will float on
water, and
 it’s used for pedicures…

Pumice!
 This texture is described as
‘vesicular’
 Gas pockets (holes) make the
rock look ‘spongy’
pumice
vesicular basalt
scoria
Obsidian: cooled too quickly
for crystals to form

Also known as
‘volcanic
glass’
2. INTRUSIVE:
LARGE WELLDEVELOPED
CRYSTALS =
COARSE GRAIN
TEXTURE (slow
cooling, a lot of
time to grow)
PORPHYRITIC: LARGE AND
SMALL CRYSTALS
Magma cooled in two stages
COMPOSITION OF IGNEOUS
ROCKS
1. FELSIC:
 high silica (Si
and O) and
aluminum (Al)
content
 light color
 low density
Rhyolite
2. MAFIC:
 high iron (Fe)
and
magnesium
(Mg) content
 dark color
 high density
Basalt
3. INTERMEDIATE:
 medium color
 medium density
Diorite
Lava Plateaus