Physical Geology 101
Igneous Rocks


Igneous Rocks

Rocks that formed from the cooling and crystallization of magma
Igni- =
-ous =

Origin of Magma

Magma - liquid or molten material
Needed to make magma:

Heat

Increased temperature will cause melting
How?
Heat is kinetic energy
Heat in an object is related to how fast the atoms are moving in that object
faster motion = higher temperature
slower motion = lower temperature

Melting

Atoms in a solid rock are vibrating but not enough to break away from the other atoms
As the temperature goes up, what happens to the atoms?

Sources of heat in the Earth's interior: ?

Pressure

Increases in pressure will increase temperature.
Increased pressure means higher melting temperature.
How can changes in pressure lead to melting?

Volatiles

Products given off by material as a vapor or gas
Water and carbon dioxide
Volatiles will lower the melting temperature.

Partial Melting

Different materials melt a different temperatures.
Partial melting is the melting of some of the material in a rock but not the entire rock.
The composition of a magma will be different from the composition of the rock if the rock is not completely melted.

Crystallization

Reversal of melting resulting from:
A.
B.
C.

Sequence of Crystallization

Partial melting says that different material melt at different temperatures
Different materials solidify or crystallize at different temperatures
Bowens Reaction Series describes the crystallization sequence of common minerals found in igneous rocks

Classification of Igneous Rocks

Two main divisions:
1) Plutonic (Intrusive) Igneous rocks
2) Volcanic (Extrusive) Igneous rocks

Plutonic Rocks

Magma cools beneath the surface
Cools slowly - results in large, visible crystals
Phaneritic texture - individual crystals are visible to the naked eye; the entire rock is composed of crystals
Pegmatitic - extremely large crystals (>2-3 cm)

Volcanic Rocks

Magma reaches and cools on the surface
Cools rapidly - results in very small crystals or no crystals
Aphanitic Texture - no crystals are visible
Porphyritic Texture - some crystals are visible
Glassy - looks like a chunk of glass
Frothy - full of small bubbles
Scoriaceous - lots of bubbles but fewer than frothy
Pyroclastic - fragments of lava

Composition

Primarily based on SiO4 content
Ultramafic
Mafic - Fe, Mg, Ca, rich - SiO4 poor - tend to be dark to black in color
Intermediate - compositions between mafic and felsic
Felsic - K, Na rich - SiO4 rich - tend to be light colors

 

Igneous Rocks Identification

Additional information and identification exercises can be found at:
Atlas of Igneous Rocks
Igneous Rocks Identification Exercise

IUGS Igneous Rock Classification Diagrams
Plutonic Rocks
Volcanic Rocks
Ultramafic Rocks
Ultramafic Rocks with Hornblende
Le Maitre Plot