Igneous Rocks
Rocks Brain-Pop
Igneous Rocks
Rocks that form when
natural, molten rockforming material cools and
turns into a solid
Igneous Rock Formation
Video
Liquid (molten) rocks
Lava – On the Earth’s
surface (Extrusive)
Magma – Beneath the
Earth’s surface (Intrusive)
Formation
Solidification – the change from a
liquid to a solid
Crystallization – molten magma or
lava cools and form solid rocks
composed of intergrown mineral
crystals
Cooling Rates Animation
Intrusive Igneous Rocks:
• forms underground
• magma never reaches the
surface
• cools slowly
• form large crystals
• medium to coarse
grained texture
• Get exposed at the
surface only when rock is
uplifted and exposed
Intrusive Igneous Rocks
Video
Extrusive Igneous Rocks:
• Forms on the surface
• Lava comes out of a
volcano
• cools rapidly
• forms small crystals
• glassy or fine-grained
• Covers the surface
as lava flows or
pyroclastic debris
Extrusive Igneous Rocks
Video
CRYSTAL SIZE
VS.
COOLING TIME
C
r
y
s
t
a
l
As cooling
time
increases,
crystal size
increases.
S
I
Z
E
Cooling TIME
Classification of Igneous Rocks:
They are classified according to their
chemical composition and texture
Felsic
Mafic
Rich in:
• Rich in:
• Silica
• Iron
• Aluminum
• Pyroxene
• Potassium Feldspar
• Olivine
• Quartz
The % of different minerals affect the color of the rock
More silica the lighter the rock, less silica the darker
More Iron the darker the rock, less Iron the lighter
Classify the following rocks as either:
Extrusive or Intrusive
Mafic or Felsic
Intrusive + Mafic = Gabbro
Extrusive + Mafic = Basalt
Intrusive + Felsic = Granite
Extrusive + Felsic = Rhyolite
Vesicles (Vesicular)
• As lava rises and cools to form extrusive
igneous rock,
• Pressure drops which causes the gases
that are in the lava to expand.
• These gases form vesicles (cavities)
which are the remains of gas bubbles.
• Intrusive rocks rarely contain vesicles
because they are formed at depth where
high pressure stops the gases from
expanding.
Vesicular Rocks - Pumice
Compare Granite and
Basalt
• Granite Intrusive (large crystals)
Felsic (Light In Color)
• Basalt Extrusive (Small Crystals)
Mafic (Dark In Color)
Crystal Size Animation
Crystallization:
• Magma is a mixture of different substances.
They all have different melting points.
• So as magma cools, some substances will form
crystals before other substances. They have a
head start in growth and tend to be bigger.
•If the magma already has crystals present in it
and then is cooled rapidly then you will see large
crystals embedded in finer-grained material.
This is called a porphyritic texture
Basaltic lava in Hawaii
Top: Basaltic lava flow
Bottom: Slow moving lava
Magma & Lava Video
Mt. St. Helens
Pre eruption
Post eruption
(May 1980)
Igneous Rock Review
1. What is the difference between magma
and lava?
2. What are the two main types of igneous
rocks?
3. How can you tell them apart?
4. Why is it not common for Intrusive rocks
to have vesicles?
Vocabulary: magma, lava, intrusive, extrusive, vesicles,
felsic, mafic