Chapter 10
Igneous Rocks
Igneous Rock Classification
Classified according to WHERE the magma
cools and hardens
Intrusive: slow cooling below the crust
Extrusive: rapid cooling on earth’s surface
Differ mostly in the size of their crystal
grains– “texture”
Texture is determined by the cooling rate of
magma that formed the rock
Texture: Intrusive
Forms when magma cools and hardens
slowly, deep underground
Slow loss of heat allows for magma to form
large, well-developed crystals/mineral
grains
“Coarse-grained” rock
Core of continental crust
Intrusive Rocks
Ex: granite
Texture: Extrusive
Forms when lava cools rapidly on earth’s
surface.
Rapid loss of heat to air or sea water does
not allow time for large crystalline grains to
form.
“Fine-grained” rocks. Grains cannot be
seen by unaided eye
Oceanic crust composed mainly of basalt.
Extrusive Rocks
Ex: basalt
Texture: Porphyritic
Some igneous rock form when magma cools slowly at
first, then rapidly as it nears the surface
Large crystals embedded within mass of smaller ones
Porphyritic: rock with mixture of large and small crystals
Texture: Others
When highly viscous, silica-rich magma
cools rapidly, result is no crystals at all
Obsidian, or volcanic glass
When magma containing large amount of
dissolved gases cools rapidly- gases are
trapped in rock
Pumice
Composition
Mineral composition of igneous rocks is
determined by the chemical composition of
the magma from which it was developed
Different types of igneous rocks have
similar compositions
3 families: felsic, intermediate, mafic
Composition: Felsic
Magma high in silica
Light coloring
Main minerals:
orthoclase feldspar
and quartz
Ex:
Coarse: granite
Fine: rhyolite
obsidian
Composition: Mafic
Magmas low in silica, but
rich in iron and
magnesium
Dark in color
Main minerals:
plagioclase feldspar,
pyroxenes, olivine,
hornblende
Ex:
Coarse: gabbro
Fine: basalt
Composition: Intermediate
Medium-colored
Contain less quartz than
felsic rocks
Main minerals:
plagioclase feldspar,
hornblende, pyroxene,
biotite mica
Ex:
Coarse: diorite
Fine: andesite
Hmm….
If you know only that an igneous rock has
coarse grains… can you identify the family
it is in? Why?
Igneous Rock Structures:
Intrusions
Form underground
Batholith
Largest of all intrusions. At least 100 km2
“Deep rock”
Form cores of mountain ranges (Sierra Nevada, Coast
Range, British Columbia)
Wolf River Batholith
Stock
Similar to batholith, but covers less than 100 km2
Wolf River Batholith,
WI
Igneous Structures: Intrusions
Laccolith
“Lake of Rock”
Magma flows between rock layers, pushing them into
an arc. Stays parallel with underlying layers
Push small dome-shaped mountains onto surface
Sill
A sheet of magma that flows parallel to rock layers
surrounding it, no matter what angle they are at.
Big Bend National Park, TX
Dike
Magma forces its way through rock layers, following
existing fractures or forming new ones.
Cut across rock layers
Igneous Rock Structures:
Extrusions
Form on the surface
Volcanic neck
When a volcano stops erupting, it eventually wears
away. Softer parts of cone carried away by wind and
water. Solidified central vent, volcanic neck, remains
Shiprock, NM
Lava plateau
Lava flow: flat masses of rock
Lava plateau: develops from lava flowing out of cracks
on earths’ surface, spreading over vast area. Fills in
valleys and covers hills.
Shiprock, NM