Igneous Rocks

Definition, Classification, Formations

Igneous Rocks:Outline

• Rocks in General– Intro. to the Rock Cycle

• Definition of an Igneous Rock– How igneous rocks “fit in” the Cycle

• Classification of an Igneous Rock– Crystal Size and Implications– Composition and Implications

• Igneous Rock Formations– Intrusions– Other

What is rock?

• Rock = collection of minerals– i.e., several natural, pure

compounds in a solid mixture

• Type of rock depends on process of formation

http://en.wikipedia.org/wiki/File:Rhcp-live-pinkpop05.jpg

Intro. to the Rock Cycle

http://www.washington.edu/uwired/outreach/teched/projects/web/rockteam/WebSite/rcycle.gif

Igneous Rocks:Definition

• Igneous rocks are solidified from hot liquid materials– i.e., cooled from magma OR lava

http://en.wikipedia.org/wiki/File:Pahoeoe_fountain_edit2.jpg

Igneous Rocks:Classification

• Classified by two criteria

– “Texture”: size of the crystals• three distinctions (see upcoming slides)

– Composition: the minerals present in the rock• especially how much SILICA (i.e., SiO2)

• multiple distinctions (see upcoming slides)

Texture Vocabulary

• Crystal Size involves three main distinctions

Coarse Grained Fine Grained No Crystals

http://upload.wikimedia.org/wikipedia/commons/thumb/1/17/Lipari-Obsidienne_%285%29.jpg/640px-Lipari-Obsidienne_%285%29.jpg

http://en.wikipedia.org/wiki/File:BasaltUSGOV.jpghttp://upload.wikimedia.org/wikipedia/commons/thumb/b/bc/Fj%C3%A6regranitt3.JPG/640px-Fj%C3%A6regranitt3.JPG

Rock Cycle: What Can Texture Tell Us?

http://www.washington.edu/uwired/outreach/teched/projects/web/rockteam/WebSite/rcycle.gif

Texture Indicates Cooling Time

Intrusive Igneous Rocks• Cooled from magma under

the surface• Can take millions of years• Coarse grains

Extrusive Igneous Rocks• Cooled from lava above the

surface– from volcanoes, e.g.

• Can cool in a few days or even minutes

• Fine grains or no crystals

http://en.wikipedia.org/wiki/File:Diorite2.tif

http://en.wikipedia.org/wiki/File:Teidepumice.jpg

Porphyry – Mix and Match!What could cause coarse AND fine grains?

http://en.wikipedia.org/wiki/File:Rhyolite_porphyry.jpg

Composition Vocabulary

• More Silica = Felsic• Less Silica = Mafic

– Usu. more iron and magnesium

• Other distinctions– Intermediate– Ultramafic!

• FELSIC IS LIGHT almost always

• MAFIC IS DARK

http://upload.wikimedia.org/wikipedia/commons/thumb/0/0b/PinkRhyolite.tif/lossy-page1-640px-PinkRhyolite.tif.jpg

http://en.wikipedia.org/wiki/File:BasaltUSGOV.jpg

What Composition Tells Us

• More silicates (felsic) are more viscous when liquid– i.e., they resist flow and

cause different formations

• Less silicates (mafic) are denser– may indicate the source

of the magma/lava

http://en.wikipedia.org/wiki/File:Pahoeoe_fountain_edit2.jpg

http://en.wikipedia.org/wiki/File:Mayon_0052.jpg

Six Common Igneous Rocks(In Words)

Texture →

Composition ↓

Fine-Grained Coarse-Grained

Felsic (High Silica)

Rhyolite GraniteIntermediate (Mid Silica)

Andesite DioriteMafic (Low Silica)

Basalt Gabbro

Six Common Igneous Rocks(In Pictures)

Texture →

Composition ↓

Fine-Grained Coarse-Grained

Felsic (High Silica)

Intermediate (Mid Silica)

Mafic (Low Silica)http://en.wikipedia.org/wiki/File:Amygdaloidal_andesite.jpg

http://en.wikipedia.org/wiki/File:GabbroRockCreek1.jpg

Igneous Rock Formations:Intrusions

• Intrusions form from intrusive rock• Can take millions of years to form, followed by

a very long time to surface• Types that we study:

1. Batholiths2. Stocks3. Laccoliths4. Dikes5. Sills

Batholiths and Stocks• Batholith from Greek,

“deep rock”– HUGE intrusions– form deep in the Earth’s

crust– most are granite

• Stocks are just smaller versions (may be tip of a batholith)

http://en.wikipedia.org/wiki/File:Enchanted_rock_2006.jpg

http://en.wikipedia.org/wiki/File:Half_Dome_from_Glacier_Point,_Yosemite_NP_-_Diliff.jpg

Laccoliths and Dikes

• Laccolith from Greek, “pond rock”– magma intrudes into a

space, causing layers of rock to bulge into an arch

• Dikes are thin intrusions that cut across layers

http://en.wikipedia.org/wiki/File:Laccolith.svg

Examples

http://en.wikipedia.org/wiki/File:Laccolith_Montana.jpg

http://en.wikipedia.org/wiki/File:Dike_diabase_AZ.jpg

Sills

• Intrusions that get in between layers

• Resemble laccoliths, but remain flat (no arch)

http://en.wikipedia.org/wiki/File:Sill.JPG

Example

http://en.wikipedia.org/wiki/File:Edinburgh_Salisbury_Crags_2004-05-18.jpg

Igneous Rock Formations:Extrusions

• Lava Plateaus– Layers of lava that have

built up– Formed as lava flows

over flat areas

• Any volcanoes are also extrusions (details in a later unit) http://en.wikipedia.org/wiki/File:Pahoeoe_fountain_edit2.jpg

Example