Chapter 10
Rocks
Section 2
Igneous Rocks
Notes 10-2
Igneous Rocks
Two Types of Igneous Rocks
– Intrusive: magma cooled below the
surface to produce an igneous rock
Magma cools slowly
– Extrusive: lava cooled above the surface
to produce an igneous rock
Lava cools quickly
Texture of Igneous Rocks
Texture
– Determined by the cooling rate of the
magma or lava
– Intrusive rocks
Magma cools slowly
Allows time for the minerals to form large,
crystalline grains
Coarse grain texture
Ex: granite
– Core of continental crust is granite
Granite
Texture of Igneous Rocks
Extrusive Rocks
– Lava cools quickly
– Does not allow for large crystalline
structure to form
– Fine grained texture
– Ex: basalt
Most of oceanic crust and Hawaii
Basalt
Texture of Igneous Rocks
Porphyritic texture
– As magma moves to the surface, it cools
slowly.
– Then as it nears the surface, it cools
more rapidly.
– This mixes the large and small crystals
Porphyritic Texture
Texture of Igneous Rocks
Glassy texture
– Highly viscous, silica rich magma cools
very rapidly
– This results in no crystals
– Ex: obsidian (volcanic glass)
Vescular texture
– rock formed when gases are trapped in
the magma or lava.
– Produces holes in the rock
– Ex: Pumice
Obsidian
Pumice
Composition of Igneous
Rock
Felsic Rocks
– High in silica content
– Light in color
– Made mostly of orthoclase feldspar (k-
spar) and quartz
May have some plagioclase feldspar, biotite
mica, and muscovite mica
– Ex: granite, rhyolite, obsidian
Rhyolite
Composition of Igneous
Rock
Mafic Rocks
– Low in silica content, rich in Fe and Mg
– Dark in color
– Made mostly of plagioclase feldspar,
pyroxene, olivine and hornblende
– Ex: basalt, gabbro
Gabbro
Composition of Igneous
Rock
Intermediate Rocks
– Contain less quartz than the felsic family
– Mostly made of plagioclase feldspar,
hornblende, pyroxene and biotite mica
– Ex: diorite and andesite
Andesite
Diorite