Classification of Igneous Rock

1
Classification of Igneous Rock

• Section 5.2

2
Mineral Composition

• Intrusive
• Igneous intrusion
• Large crystals (macroscopic)
• Extrusive
• Lava flow or flood basalts
• Small crystals (requires magnification)

3
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

4
Mineral Compositions

• Intermediate rocks have a composition of minerals
  between baslatic and granite rocks
• Ultrabasic rocks
• Contain only iron-rich minerals
• Always dark

5
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?

6
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

7
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

8
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

9
Pegmatites

• Pegmatites veins of extremely large-grained
  minerals
• Lithium and Beryllium ores form in pegmatites
• Produce beautiful crystals
• Mount Rushmore in South Dakota

10
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?

11
Construction Uses

• Interlocking grain textures make igneous rocks
  strong
• Resistant to weathering
• Granite is used in countertops, floors, and
  statues