Download Igneous Rocks

Igneous Rocks
After completing this section, the student will
differentiate between intrusive and extrusive igneous
rocks (Standard PI – 023)
Igneous rocks are formed from
the cooling of magma or lava
Granite is formed from magma
Basalt is formed from lava
Igneous rocks are classified by
composition – the minerals found inside
Igneous rocks are classified as mafic, felsic or
intermediate
Felsic –formed from light colored
silicate (quartz) rocks
Forms from thick, slow moving magma
Mafic – contains iron and magnesium,
and forms darker silicate rocks
Forms from thin fast moving magma
Intermediate - Has characteristics
of mafic and felsic magma
granodiorite
Igneous rocks are also classified
by texture
Texture – the size of the mineral crystals
Coarse grained – formed from slow
cooling
Intrusive – large crystals formed from slow
cooling of magma
Intrusive rocks are generally
formed in deposits called plutons
Pluton – a rock mass that forms when magma
cools inside the earth’s interior
Fine grained – formed from rapid
cooling
Extrusive – small crystals formed from quick cooling of
magma
Porphyry – has a combination of small
and large crystals
Porphyritic rocks started to cool slowly, but suddenly
forced to the surface
Igneous rocks can be grouped into
families according to their mineral
composition.
Granite Family
• Formed from felsic magma
• Contain light colored minerals
• Range in color from white to gray to
pink
Granite – intrusive, coarse grained
Obsidian – glassy volcanic rock
Dark colored, but contains the same minerals as
granite
Pumice – extrusive volcanic rock
Contains holes and air pockets
Gabbro Family
• mafic rocks – dark colored
• Denser than granitic rocks
• Contain pyroxene, olivine and plagioclase
Gabbro
coarse grained and dark colored
Basalt
fine grained and dark
Scoria – dark colored volcanic rock
Contains air holes, but doesn’t float like pumice
Diorite Family
medium grays and greens
Diorite
coarse grained medium colored
Andesite
fine grained, medium colored