Igneous Rocks

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Chapter 5-2

• Igneous Rocks

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Igneous Rocks

• Classified according to
• Origin
• Texture
• Mineral composition

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Origin

• Where they are formed
• Extrusive Rock
• Formed from lave that erupted onto Earths
  surface (Basalt forms much of the crust,
  oceanic crust)
• Intrusive Rock
• Formed when magma hardens below the surface
  (Granite continental crust)

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Texture

• Depends on size and shape of its mineral crystals
• Fine-grained (rapid cooling), coarse-grained
  (slow cooling), glassy, or porphyritic
• Intrusive has larger crystals than extrusive
• Porphyritic Texture large crystals scattered on
  a background of much smaller crystals forms
  when rock cools in two different stages
• Extrusive rocks fine-grained, glassy texture

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Mineral Composition

• Basalt lava that is low in silica content,
  dark-colored rock (contains feldspar, but no
  quartz)
• Granite lava that is high in silica content,
  light-colored rock
• Comes in many shades and colors
• Vary immensely depending upon amounts of
  feldspar, hornblende, mica, and quartz

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Uses of Igneous Rocks

• Hard, dense, and durable
• Tools and building materials
• Granite statues, bridges, pavement
• Basalt gravel (construction)
• Pumice cleaning and polishing
• Perlite formed from heating obsidian (mixed in
  soil for starting vegetable seeds)