Title: Igneous Rock Types
1Igneous 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
-
3First, 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
5Types 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
6Aphanitic 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.
10Igneous 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.
14Yet 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
15Pyroclastic Rock Volcanic Breccia
If pieces are smaller then referred to as
Volcanic Tuff
16Naming 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
17Summary - Igneous Rock Texture and Composition
18Begin Igneous Rock ID Lab