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Igneous Rock Types

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Pyroclastic Rock Volcanic Breccia. If pieces are smaller then referred to as Volcanic Tuff ... Pyroclastic Volcanic Breccia or Tuff. Summary - Igneous Rock ... – PowerPoint PPT presentation

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Title: Igneous Rock Types


1
Igneous Rock Types
2
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

3
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?
4
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

5
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

6
Aphanitic texture
7
Phaneritic texture
8
Glassy textureVery rapid cooling of molten
rockwith no gas escaping. Resulting rock is
called obsidian
Obsidian
9
  • Glassy Texture with gas escaping Pumice.

10
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

11
Porphyritic texture
Granite
12
  • Pegmatitic texture
  • Exceptionally coarse grained
  • Form in late stages of crystallization of
    granitic magmas

13
  • Vesicular Texture holes in rock from escaping
    gases.

14
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

15
Pyroclastic Rock Volcanic Breccia
If pieces are smaller then referred to as
Volcanic Tuff
16
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

17
Summary - Igneous Rock Texture and Composition
18
Begin Igneous Rock ID Lab
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