Igneous Rock

• Igneous means “born of fire”.

Igneous Rocks

Formation:• 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

Main Characteristics:

Two Types of Igneous Rock:

1) Plutonic Igneous Rock (intrusive)

2) Volcanic Igneous Rock (extrusive)

Formation of Igneous Rocks

• 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)

1. Plutonic Igneous Rock

• 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

2. Volcanic Igneous Rock

Igneous Rocks as Resources:• Used as building materials

WHY?• 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).

Igneous Rocks as Resources:

• 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.

Gold-bearing vein in Sierra

Nevadas