DICHOTOMOUS KEY TO THE
IGNEOUS ROCKS
Igneous rocks break-up into two categories, Volcanic (extrusive) or Magmatic (underground from magmaintrusive). In general Extrusive or volcanic rocks either have holes (created from the gases that are released during and
eruption) and are light weight or can be glassy and sharp. Intrusive, cool slowly therefore can have small or large crystal
(mineral grains- usually chunky minerals such as feldspars and quartz). Texture is largely related to the rate of cooling
experienced by the rock. The more rapidly the magma cooled, the smaller the crystals formed and the finer the texture of
the resultant rock.
Igneous Textures are:
a) Aphanitic rocks have a fine texture because the rock cooled to a solid form before the crystals grew to
visible size. These rocks look dull and more or less uniform, therefore, crystals cannot be seen.
b) Phaneritic or coarse-textured rock, the cooling rate was sufficiently slow that visible crystals formed
before the rock solidified. This gives the rock a crystalline appearance. Even when the crystals are small,
they are still visible to the necked eye- such as Granite.
c) Porphyritic texture result from a two-stage cooling process. They contain larger crystals (phenocrysts) in a
fine-textured matrix or groundmass.
d) Vesicular texture is possessed of numerous holes. This is much like a form of glass or some fine-textured
rock.
e) Glassy texture, the melt solidified before any crystals formed, consequently the very fine, shiny look of the
rock.
Examples of Textures:
phenocrysts
Aphanitic
Vesicular
Porphyritic
Phaneritic
Glassy
1a. Vesicular texture - 2
1b. Texture other than vesicular - 3
2a. Dark reddish brown to black in color - Scoria
2b. Light white to gray in color - Pumice
3a. Glassy Texture - Obsidian
3b. Not glassy - 4
4a. Texture phaneritic (coarse) - 5
4b. Texture otherwise - 8
5a. Light colored, contains orthoclase - Granite
5b. Does not contain orthoclase - 6
6a. Mixture of light and dark crystals, orthoclase not evident - Diorite
6b. Dark minerals dominate - 7
7a. Hornblende and/or augite dominate - Gabbro
7b. Olivine the dominant mineral - Peridotite
8a. Texture aphanitic (Fine-textured) - 9
8b. Texture porphyritic - 11
9a. Color dark gray to black - Basalt
9b. Color not as above - 10
10a. Light colored (Pink to cream to white) - Rhyolite
10b. Not as above (Brownish to greenish) - Andesite
11a. Lighter-colored matrix with phenocrysts – porphyritic Rhyolite
11b. Matrix not light colored - 12
12a. Dark gray to black matrix with phenocrysts – porphyritic Basalt
12b. Gray to greenish matrix with phenocrysts – porphyritic Andesite