Download Classification of Igneous Rock

Classification of Igneous
Rock
Section 5.2
Mineral Composition

Intrusive
– Igneous intrusion
– Large crystals (macroscopic)

Extrusive
– Lava flow or flood basalts
– Small crystals (requires magnification)
Mineral Composition

Basaltic rocks
– Dark colored
– Lower silica content
– Contain mostly plagioclase and pyroxene

Granite rocks
– Light colored
– High silica content
– Contain mostly quartz, K+ & plagioclase
feldspar
Mineral Compositions
Intermediate rocks have a composition
of minerals between baslatic and granite
rocks
 Ultrabasic rocks

– Contain only iron-rich minerals
– Always dark
Texture
Texture refers to the size, shape, and
distribution of the crystals or grains that
make up a rock
 Rhyolite – fine-grained
 Granite – coarse-grained
 How is crystal size affected by cooling
rate?

Texture
Porphyritic texture – large well formed
crystals surrounded by finer grained
crystals of the same or different mineral
 Forms when slowly cooling magma
suddenly cooled rapidly
 Vesicular texture – spongy appearance;
Pumice is an example

Thin Sections
A slice of rock usually 2cm X 4 cm and
only 0.03 mm thick
 Light is able to pass through it
 A petrographic microscope is used to
view thin sections

Veins
Valuable ores occur within igneous
intrusions; Ores also occur in the rocks
surrounding intrusions called veins
 Important metallic elements include gold,
silver, lead, and copper

Pegmatites
Pegmatites – veins of extremely largegrained minerals
 Lithium and Beryllium ores form in
pegmatites
 Produce beautiful crystals
 Mount Rushmore in South Dakota

Kimberlites
Kimberlites are ultrabasic rocks
 Form at depths of 150 to 300 km
 Named for Kimberly, South Africa
 Diamonds form in kimberlites
 Where in the US have kimberlites been
found?

Construction Uses
Interlocking grain textures make igneous
rocks strong
 Resistant to weathering
 Granite is used in countertops, floors, and
statues
