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.