Igneous Rock Types

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Igneous Rock Types
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Characteristics of magma

• Crystallization of magma
• Texture in igneous rocks is determined by the
  size and arrangement of mineral grains
• Igneous rocks are typically classified by both
• Texture
• Mineral composition

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First, Remember Composition

• Rocks from Felsic Magma
• Silica Rich
• Light in color
• Granitic in composition
• Intrusive Granite
• Extrusive Rhyolite
• -Silica rich magma is very viscous (thick). This
  traps gases and makes it difficult to flow.

• Rocks from Mafic Magma
• Silica poor, ferro-magnesian rich
• Dark in color
• Basaltic in composition
• Intrusive Gabbro
• Extrusive Basalt
• -Mafic magma has a low viscosity and therefore
  can flow easily and allow gases to escape.

What about the intermediate magma composition?
What would an intrusive rock of this composition
be called? What about its extrusive equivalent?
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Igneous textures

• Texture is used to describe the overall
  appearance of a rock based on the size, shape,
  and arrangement of interlocking minerals
• Factors affecting crystal size
• Rate of cooling
• Slow rate promotes the growth of fewer but larger
  crystals

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Types of Igneous textures

• Aphanitic (fine-grained) texture
• Rapid rate of cooling of lava or magma
• Microscopic crystals
• May contain vesicles (holes from gas bubbles)
• Phaneritic (coarse-grained) texture
• Slow cooling
• Crystals can be identified without a microscope

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Aphanitic texture
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Phaneritic texture
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Glassy textureVery rapid cooling of molten
rockwith no gas escaping. Resulting rock is
called obsidian
Obsidian
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• Glassy Texture with gas escaping Pumice.

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Igneous textures

• Other types of igneous textures. These names go
  before the rock type name.
• Porphyritic texture
• Minerals form at different temperatures as well
  as differing rates
• Large crystals, called phenocrysts, are embedded
  in a matrix of smaller crystals

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Porphyritic texture
Granite
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• Pegmatitic texture
• Exceptionally coarse grained
• Form in late stages of crystallization of
  granitic magmas

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• Vesicular Texture holes in rock from escaping
  gases.

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Yet more types of Igneous textures

• Pyroclastic texture
• Various fragments ejected during a violent
  volcanic eruption
• Textures often appear to be more similar to
  sedimentary rocks

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Pyroclastic Rock Volcanic Breccia
If pieces are smaller then referred to as
Volcanic Tuff
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Naming Igneous Rocks

• Is it Intrusive or Extrusive (Crystal size?)
• What is the composition? (Mafic / Intermediate /
  Felsic)
• Any special textures
• Porphorytic
• Pegmatitic
• Vesicular
• Is it a special type
• -Glassy texture Obsidian or Pumice
• - Pyroclastic Volcanic Breccia or Tuff

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Summary - Igneous Rock Texture and Composition
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Begin Igneous Rock ID Lab