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 ROCKSOutside Earth: EXTRUSIVE IGNEOUS ROCK
Igneous rocks can form inside Earth’s interior or outside Earth’s surface.
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.
Example: Obsidian or Pumice.
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.
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 Basalt
12. Igneous Rocks: Rate of Cooling
Glassy/Volcanic texture means that there are NO
crystal grains.
The faster the magma cools, the smaller the mineral crystals.
Rocks cooled in the air during eruption or under water.
Very FAST COOLING = No 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
13. SAME MAGMA – DIFFERENT ROCKS
RHYOLITE
GRANITEBOTH SAMPLES FORMED FROM
GRANITIC MAGMA
Intrusive – Large Crystals
Extrusive – Small Crystals
17. 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).
18. 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
19. 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
GraniteRhyolite DioriteAndesite GabbroBasalt
20. 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
Granite Diorite Gabbro
21. 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.
