Igneous rocks form when magma or lava cools and hardens. They are classified based on where they form, their texture, and composition. Igneous rocks that form inside Earth's interior through slow cooling are intrusive with coarse textures, while those forming outside through rapid cooling are extrusive with fine or glassy textures. The rate of cooling influences crystal size, with slow cooling leading to large crystals and fast cooling resulting in small or no crystals.

1. IGNEOUS ROCKS

2. Igneous Rocks
 Igneous rocks form when magma or lava cools
and hardens.
 Magma: Inside Earth’s Interior
 Lava: Outside Earth’s Crust
Igneous Rocks are classified according
to:
- Where they are made.
- The texture of the rock.
- What they are made of.

3. Igneous Rocks: Where they are
made?
 “in”side earth.
 Think about the
“interior” of a car.
 Think about an
“interior” designer.
 What cools? Magma
or Lava? Magma
 “Ex” sometimes
means to go “out”.
 Think about the
“exterior” of your
house.
 What cools? Magma
or Lava? LAVA
Inside Earth: INTRUSIVE IGNEOUS
ROCKS
Outside Earth: EXTRUSIVE IGNEOUS
ROCK
Igneous rocks can form inside Earth’s interior or outside Earth’s
surface.

4. Igneous Rocks: Where they are
made?

6. Igneous Rocks: Texture of the
Rocks?
 Igneous rocks have different textures.
 Texture is how a rock looks and feels.
 Texture is based on the size of crystal grains in
the rock.
 Sometimes they are large, sometimes small.

7. Igneous Rocks: Texture of the
Rocks?
 Pegmatite:
 Very large crystal grains.
 They are so big that some even join together to become even
bigger.
 You can see them with your eyes.
 Example: Pegmatite
 Coarse-grained:
 Large crystal grains.
 You can see them with your eyes.
 Example: Granite
 Fine-grained:
 Small crystal grains.
 Too small to be seen with your eyes – you need a microscope.
 Example: Basalt
 Glassy/Volcanic Glass:
 No Crystals at all.

8. Igneous Rocks: Rate of
Cooling
 Porphyry:
 Porphyry is when you have a mix of large and
small crystal grains.
 Think of the word potpourri…that’s a collection of
things (big and small crystals).
What could have caused this?
How do we get big crystals? – slow cooling inside
Earth.
How do we get small crystals? – quick cooling outside
Earth.
This rock experienced two cooling situations.

9. Porphyry

10. Igneous Rocks: Rate of Cooling
 Pegmatite and coarse-grained textures have very
large crystals.
 The slower the magma cools, the larger the mineral
crystals.
 Rocks that cool slowly have larger crystals.
 SLOW COOLING = LARGE CRYSTALS.
Example: Let’s say you are trying to cool a cake you made. Instead of
putting it outside to cool, you turn the oven off and keep it in there to cool.
You won’t be able to eat the cake for awhile because it will take longer to
cool since it is in the hot oven.
Example: Say you are a farmer. You plant your crops in spring and let them
grow for many months until fall. The crops will be big because they had lots
of time to grow before the ground froze.
Granite Diorite Gabbro
COARSE GRAINED IGNEOUS ROCKS

11. Igneous Rocks: Rate of Cooling
 Fine-grained texture means that there are very
tiny crystal grains.
 The faster the magma cools, the smaller the mineral
crystals.
 Rocks that cool fast have small crystals.
 FAST COOLING = SMALL CRYSTALS.
Example: Back to the cake, let’s say you put it in the fridge to cool. It will
certainly cool faster because of the cooler temperatures.
Example: Back to the crops. Instead of letting the crops grow all summer,
you harvest them early. They won’t get to grow as big because they didn’t
have enough time.
FINE GRAINED IGNEOUS ROCKS
Rhyolite Andesite BasaltPumice Scoria Obsidian
NO GRAINED IGNEOUS ROCKS

12. SAME MAGMA – DIFFERENT
ROCKS
RHYOLITE
GRANITEBOTH SAMPLES FORMED
FROM GRANITIC MAGMA
Intrusive – Large Crystals
Extrusive – Small Crystals

13. Granitic Magma
Pegmatite

14. Andesitic Magma

15. Basaltic Magma

16. Magma Types
 Magmas are hot liquid solutions of silicate rock
forming elements and volatiles.
 The major silicate rock forming elements are O, Si, Al, Fe,
Mg, K, Na and Ca.
 Volatiles are substances, dissolved in the magma,
which don't go into minerals as the magma solidifies.
 They are released in the form of liquids or gasses.
 H20 is by far the most abundant volatile in magmas.
 Others are CO2, SO2 and H2S.
 The more volatiles the magma has, the thicker and
more viscous it becomes (less fluid).

17. Igneous Rock: Magma Types
 High in
silica
 Low Iron.
 Light in
Color
 High
Volatile
 High
Viscosity
 Medium silica
 Medium Iron
 Not light, not
dark
 Medium Volatile
 Medium
Viscosity
Granitic Magma
(FELSIC)
Andesitic Magma
(INTERMEDIATE)
Basaltic Magma
(MAFIC)
 Low in silica
 High in Iron
 Dark Color
 Low Volatile
 Low
Viscosity
GraniteRhyolitePumice DioriteAndesiteScoria GabbroBasaltObsidian

18. Igneous Rock: Texture – Rate of
Cooling
 SUMMARY:
 Intrusive Rocks: Form inside
Earth where it is hot, so the
magma cools slowly producing
large crystals.
 Extrusive Rocks: Form outside of
the Earth where it is cool, so the
lava cools quickly producing
small or no crystals.