Igneous RocksPages 99 - 113

Goal 1

Compare and contrast intrusive

and extrusive igneous rocks

All rocks are a mixture of minerals.

Mineral soup

Each type of rock has its own mix of minerals

Gabbro: minerals include hornblende, plagioclase, pyroxene, biotite, olivine, magnetite

Extrusive Igneous Rocks

1. Lava cooled quickly at surface

Kalapana, Hawaii 1990

Lava fountain height approx 10 m

Pu‘u Kahaualea , photo by J.D. Griggs, 02/25/83 USGS

2. Fine grain / glassy

3. Examples

Obsidian Pumice

Intrusive Igneous

Rocks

1. Cooled magma inside Earth

2. Cross cutting relationships

Photo credit: Eurico Zimbres

Photo credit: Wing-chi Poon at Devil's Lookout Point, Black Canyon

Gunnison National Park, Colorado, USA

3. Coarse Grain

+large, interlocking crystals

+naked eye

4. Examples

Granite porphyry: coarse grains surrounded by fine grains

Goal 3

Explain the 4 factors that affect how rocks melt and crystallize

Magma forms in the Earth’s Mantle

Slushy mix

Solids

Liquids

Gases

1. Temperaturea) Temp increases w/ depth

2. Pressure

a) pressure increases with depth

b) increased pressure = higher melting temp

3. Water content

a) water increase = melting point decrease

4. Mineral composition

a) minerals have individual unique melting points

Bowen’s Reaction seriesBowen’s Reaction series

Explains the order in which Explains the order in which minerals crystallize out of a minerals crystallize out of a

magma solutionmagma solution

Bowen’s Reaction Series

Image Credit:

James Madison University

Geology Dept.

Left side

Iron-rich minerals

Discontinous

crystallization

Right side

Feldspar minerals

Continuous

crystallization

Magma type SiO2 content Classification General color

Rhyolitic > 70% Felsic Light

Andesitic ~ 60% Intermediate ½ light ½ dark

Basaltic < 50% Mafic Dark

0% Ultramafic Dark / Green

SiO2 content is related to classification

Summary

1. Texture related to location of crystallization

a) Fine grain = extrusive

b) Coarse grain = intrusive

c) Porphyritic = intrusive

2. Mineral content related to crystallization series

a) Fe rich = discontinuous

b) Na, Ca Feldspars = continuous

3. Classification related to SiO2 content and color

a) Light = Felsic

b) ½ and ½ = Intermediate

c) Dark = Mafic

d) Very dark / green = Ultramafic

Determining if a rock is Igneous:

• Hard, scratches glass

• No banding or layers of colors

• Coarse-grain (large crystals) that interlock

• Fine-grain (small crystals), glassy, light / holey from gas pockets

• Common colors: black, white, gray, reds to pinks

Thanks to the following people who have generously shared photos on the creative

commons at Flickr:

The Shifted Librarian… Road ClosedJames_C… Red granite Cross

Kevinzim… gabbro sliceJeff gerhard… lava

AZAdam… lava tubeTravis S… obsidian pointAmbuj Saxena… pumice

Kevinzim… coarse grain intrusiveBasial… Intrusive examples