Rocks - Igneous

IGNEOUS ROCKS

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.

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.

Igneous Rocks: Where they are made?

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.

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.

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.

Porphyry

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

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

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

SAME MAGMA – DIFFERENT ROCKS

RHYOLITE

GRANITEBOTH SAMPLES FORMED FROM GRANITIC MAGMA

Intrusive – Large Crystals

Extrusive – Small Crystals

Granitic Magma

Pegmatite

Andesitic Magma

Basaltic Magma

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). 

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

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

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

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.