Igneous Rock

• Igneous means “born of fire”.

• Formed when magma or lava cools and hardens

through the process of crystallization

• Crystallization = the formation of mineral grains

• Classified by texture (cooling rate) and

composition (color)

• Usually made of 2 or more minerals

• Contains no fossils

• Has no layering

• Rarely reacts with acids

• Can be light-colored to dark-colored

• Can be fine-grained, coarse-grained, or glassy

• Resistant to weathering

Two Types of Igneous Rock:

1) Plutonic Igneous Rock (intrusive)

2) Volcanic Igneous Rock (extrusive)

• Formed from magma within Earth

• Magma is melted rock material composed of

common elements and fluids

• As magma cools, atoms and compounds in the

liquid rearrange themselves into new crystals

called mineral grains

• Rocks form as these mineral grains grow

together

• Rocks that form from magma below the surface

are called intrusive igneous rocks (PLUTONS)

• It takes a long time for these rocks to cool

because they form at great depths and are

surrounded by other rocks

• Slowly cooled magma forms individual mineral

grains that are large enough to be seen with the

unaided human eye

• Plutons – name given to intrusive rock bodies

• Plutons are classified by their size, shape, and

relationship to surrounding rocks.

Types of Plutons:

a) Batholith

• Largest plutons

• Primarily granite

• Common in the interior of major mountain chains

b) Stocks

• Massive pluton that formed from a magma

chamber

• About the size of a mountain

c) Laccolith

• A mushroom-shaped pluton with a round top and

flat bottom

• Occurs as a result of intense heat and pressure

d) Sill

• Forms when magma intrudes into parallel layers

of rock

• Few centimeters – hundreds of meters in

thickness

e) Dike

• Pluton that

cuts across

pre-existing

rock

Draw in and Label the Plutons (Igneous

Intrusions) in Notes!

• Formed from lava at the surface

• When lava cools at the surface, it is exposed to

air and water and cools quickly

• Quick cooling prevents mineral grains from

growing large

• Used as building materials

• Interlocking mineral grains make igneous rocks

strong

• The minerals that they are made up of are more

resistant to weathering

• Ores – a metal-bearing mineral or rock that can

be mined for a profit

• Ores are often associated with igneous intrusions

(magma that cools into rock inside Earth).

• Fluid left during magma crystallization contain

high levels of silica, water, and any leftover

elements that were not included into the

common igneous minerals (gold, lead, silver,

copper).

• These elements along with the dissolved silica are

released at the end of magma crystallization in a

hot, mineral-rich fluid that fills cracks and voids in

the surrounding rock.