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 ROCKSOutside 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


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


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 moreviscous 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


High in silica

Low Iron.

Light in Color

High Volatile

High Viscosity

Medium silica

Medium Iron

Not light, not dark

Medium Volatile

Medium Viscosity




Low in silica

High in Iron

Dark Color

Low Volatile

Low Viscosity

GraniteRhyolite DioriteAndesite GabbroBasalt


High in silica

Low Iron.

Light in Color

High Volatile

High Viscosity

Medium silica

Medium Iron

Not light, not dark

Medium Volatile

Medium Viscosity




Low in silica

High in Iron

Dark Color

Low Volatile

Low Viscosity

Granite Diorite Gabbro

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.

rocks - igneous

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