Chapter 6 RocksChapter 6 Rocks

6.2 Igneous Rocks6.2 Igneous Rocks

Igneous Rock FormationIgneous Rock Formation• Form from the cooling

of molten rock – Extrusive (volcanic)-

form at or near the surface from lava

– Intrusive (plutonic) – form under the surface from magma

Igneous Rock FormationIgneous Rock Formation• Igneous rocks are

classified by mineral composition (mafic or felsic) and texture

• Texture – is the size, shape

and arrangement of grains (crystals)

TextureTexture

• Grain or crystal size depends on:– Rate of cooling

• Fast = small• Slow = large

– Dissolved gasses• More = larger

– Order of solidification

The Starting MaterialThe Starting Material• Magma is Classified as

Felsic, Mafic or and intermediate

• Felsic – Light colored– High silica magma with

abundance of quartz and feldspar.

– Thick slow flowing in molten state.

– Most intrusive rocks. – Acidic

The Starting MaterialThe Starting Material• Mafic

– Dark colored, – Low silica

ferromagnesian magma

– More fluid than felsic magma.

– Most extrusive igneous rocks.

Underground MagmaUnderground Magma• Magma can cool slow

or fast depending on where cooling occurs– Intrusive rocks cool

very slowly underground

– Result of slow cooling• Coarse (large) grains

INTRUSIVE – large grains

GRANITE GABBRO

At the SurfaceAt the Surface• Magma pouring onto

the surface is called lava– Extrusive igneous

rocks cool rapidly– Microscopic grains

or none at all

EXTRUSIVE – fine grains

BASALT OBSIDIAN

At the SurfaceAt the Surface• Porphyry

– Rocks that cool both underground and at the surface

– Results in large and small grains

PORPHYRY – large grains embedded in fine grains

RHYOLITE PORPHYRY

Igneous familiesIgneous families

• Grouped according to mineral composition

• Texture is determined where it cools – intrusive or extrusive– Each family can have coarse grained, fine

grained or glassy textures– Also can have vesicles

• Vesicles are gas cavities within the rock

Igneous FamiliesIgneous Families• Granite family

– Felsic magma– Light colored– Quartz and Feldspar

• Coarse– Granite

• Fine– Rhyolite

• Glassy– Pumice and

Obsidian

• Gabbro Family– Mafic magma– Dark colored– Pyroxene and Olivine

• Coarse– Gabbro

• Fine– Basalt

• Vesicular– Scoria

Igneous FamiliesIgneous Families• Diorite family

– Intermediate• Coarse

– Diorite• Fine

– Andesite

IGNEOUS INTRUSIONSIGNEOUS INTRUSIONS• PLUTONS – A ROCK MASS THAT FORMS

WHEN MAGMA COOLS UNDER GROUND– DIKES– SILLS– LACOLITHS– VOLCANIC NECKS– BATHOLITHS – LARGEST OF ALL

PLUTONS, FORM THE CORES OF MOST MOUNTAINS

Laccolith Volcanic neck

Sill Batholith

Stock

Dike

DIKE

SILL

LACOLITH

VOLCANIC NECK

BATHOLITH