Igneous

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Igneous Rocks

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Igneous Rocks Fire Formed

• Form when magma cools hardens

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Classification

• According to where molten rock cools hardens

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Intrusive

• Cooling of magma below surface
• Slow rate of cooling
• Large crystals formed

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Extrusive

• Cooling occurs on the surface
• Fast cooling rate
• Small crystals formed

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Texture

• Size of crystals
• Determined by cooling rate of magma or lava

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Cooling Rates Texture

• At the surface, cooling rates are rapid resulting
  in very fine-grained texture.

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Cooling Rates Texture

• Far below the surface, cooling is slow resulting
  in coarse-grained texture.

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Changing Rates

• Cooling rates can change, thus it is possible to
  have crystals of different sizes in the same rock.

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Porphyritic Texture

• An igneous rock which has large crystals embedded
  in a matrix of smaller crystals

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Glassy Texture

• Extremely rapid cooling may generate a rock with
  no crystals

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Mineral Composition

• Depends on the chemical composition of the magma
  from which it crystallizes
• 3 families

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Felsic Lava (Granite Family)

• Felsic lava high silica
• Light color
• Feldspar Quartz

granite
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Mafic LavaGabbro Family

• Mafic lava low silica, rich in iron
• Dark colored

basalt
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Intermediate Composition Lava Diorite Family

• Medium colored (half half)
• Little or no quartz

diorite
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Igneous Structures

• Intrusions or plutons batholiths, stocks,
  laccoliths, sills, dikes
• Extrusions volcanic neck lava plateau

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Volcanic necks

• When the cone erodes away, the solidified rock in
  the vent (neck) remains.