Earth Science Standard 3c: Students know how to

explain the properties of rocks based on the physical

and chemical conditions in which they formed, including

plate tectonic processes.

Classifying Igneous Rocks

• As magma cools and crystallizes minerals

form in predictable patterns

• Bowen discovered 2 main patterns:

a) Right-branch – continuous, gradual change

of mineral compositions in the feldspar

group

b) Left-branch – minerals rich in iron and

magnesium change abruptly as the

temperature of the magma decreases

http://ees.as.uky.edu/sites/default/files/elearning/module03swf.swf

1. Texture (Rate of Cooling)

2. Color (Mineral Composition)

• Determined by rate of

cooling where and how

fast the lava or magma

cooled

2 Locations of cooling:

a) Intrusive (cooled inside

Earth)

b) Extrusive (cooled on

Earth’s surface)

• Not exposed to weather since inside Earth

• Very hot within Earth so cooling takes a long time!

a) Intrusive Igneous Rocks

Slow Cooling

• Large mineral grains (can see with human

eye)

• Hundreds to millions of years to cool

• The larger the mineral grains the slower it

cooled!

• Coarse-grained Texture (phaneritic)

Intrusive:

Course-grained,

interlocking

• Exposed to weather since form on top of Earth

• Cooler on Earth’s surface so cools more quickly!

b) Extrusive Igneous Rocks

• Very Fast Cooling = NO mineral grains &

Glassy Texture

• Hours to Days

• Fast Cooling = Fine-grained Texture (mineral

grains not seen with human eye (aphanitic)

• Days to Weeks

• Porphyritic = started cooling slowly and the

cooling sped up

• Some large crystals (phenocrysts)surrounded by a

fine-grained texture

Intrusive:

Course-grained,

interlocking

Extrusive:

Fine-grained, discrete

crystals, often glassy

• Determined by mineral composition

4 classifications:

a) Felsic

b) Mafic

c) Intermediate

d) Ultramafic

a) Felsic = Light colored; white to pink; high silica

b) Mafic = Dark colored; blackish-gray; low silica

c) Intermediate = Gray; between felsic and mafic

d) Ultramafic = black-green; very low silica

• Felsic = continental crust

• Mafic = oceanic crust

• Intermediate = coastlines

Ex

trusiv

e

Felsic Intermediate Mafic Ultramafic Texture

Obsidian

Basaltic

Glass

Glassy

(non-

crystalline)

Rhyolite

Andsesite

Basalt

Fine-

grained

Intru

sive

Granite

Diorite

Gabbro

Peridotite

Dunite

Coarse-

grained

Pegmatite

Very

Coarse-

grained

Classification of Igneous Rocks

Formation of Igneous Rocks

Pyroclasts

Extrusive

Intrusive

Porphyry:

partially crystalline

Compositional

Classification

Granite

Granite

Quartz

Orthoclase

Biotite

Plagioclase

Compositional

Classification

Granite

Granite

Granodiorite

Quartz

Amphibole

Plagioclase

Compositional

Classification

Granite

Granite

Granodiorite

Diorite

Plagioclase

Amphibole

Compositional

Classification

Granite

Granite

Granodiorite

Diorite

Gabbro

Plagioclase

Pyroxene

Compositional

Classification

Granite

Granite

Granodiorite

Diorite

Gabbro

Peridotite

Pyroxene

Olivine

Granite Gabbro

Intrusive Igneous Rocks

Rhyolite Basalt

Extrusive Igneous Rocks

Where did these rocks form?

How can you tell?

Color?

http://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousCompositions/5Andesite/AndesiteNCHblPorphyritic.JPG