Igneous Rocks

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Igneous Rocks

• Basic Principles

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Igneous Rocks

• Igneous means fire formed
• Igneous rocks originate at high temperatures
• Temperatures are hot enough to melt rock
• Heat comes from radioactive decay of unstable
  elements such as U238 and K40

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Igneous Rocks

• Molten (melted) rock is Magma
• Magma on Earths surface is Lava
• Magma may form igneous rocks
• On the surface volcanic or extrusive rocks
• Within the Earth plutonic or intrusive rocks

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Hawaiian lava flow
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Igneous Rocks

• Igneous rocks form when magma
• Cools
• Crystallizes (forms crystals)
• Crystals interlock

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Cooled Lava becomesIgneous Rock
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Texture of Igneous Rocks

• Texture means the size of the crystals
• Crystal size is a result of the rate of cooling
• Slow cooling produces large crystals
• Rapid cooling produces small crystals

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Coarse and FineIgneous Textures

• Intrusive
  Extrusive

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Texture of Igneous Rocks

• Where is slow cooling likely to happen?
• Within the Earth INTRUSIVE
• Where is rapid cooling likely to happen?
• On Earths surface EXTRUSIVE

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Thin Sections showing Coarse and Fine Igneous
Texture

• Intrusive Extrusive

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Explosive eruptionfrom cooler magma
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Texture of Igneous Rocks

• Texture is a clue to rate of cooling and
  therefore place of cooling

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Composition of Igneous Rocks

• Composition is determined by
• What materials are present to be melted
• Temperature (different minerals have different
  melting points)
• Where the melting happens (related to temperature
  and type of material available)

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Composition of Igneous Rocks

• Mafic igneous rocks are enriched in
• Magnesium and Iron (Ferrum)
• Felsic igneous rocks are enriched in
• Feldspar (especially Na and K) and Silica
  (quartz)
• Intermediate igneous rocks are
• Intermediate in terms of Mg, Fe, Na, K, Si

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Composition of Igneous Rocks

• Mafic Felsic

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Classification of Igneous Rocks

• Igneous rocks are classified on the basis of
  their
• TEXTURE
• COMPOSITION