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Simplified Classification of the Igneous Rocks
Simplified Classification of the Igneous Rocks
In this introductory treatment, only the most common of the igneous rocks are classified. The simplified classification chart is at the bottom of this page, or here.
Crystalline Rocks
Crystalline Rocks
Phaneritic
Phaneritic
- Phaneritic means that rocks exhibit grains (crystals) large enough to be seen by the eye without magnification.
- Visibly sized crystals may be quite small, but still resolvable (millimeters), or quite large (centimeters +).
Click any image to enlarge.
granite
granite
Image credit: James St. John
diorite
diorite
Image credit: Amcyrus2012
gabbro
gabbro
Image credit: James St. John
dunite / peridotite
dunite / peridotite
Image credit: James St. John
Aphanitic
Aphanitic
- Aphanitic means that rocks exhibit grains (crystals) too small to be seen by the eye without magnification.
Click any image to enlarge.
rhyolite
rhyolite
Image credit: James St. John
andesite
andesite
Image credit: James St. John
basalt
basalt
Image credit: James St. John
Porphyritic
Porphyritic
- Porphyritic rocks exhibit grains (crystals) in two, or more, sizes (or populations).
- The larger sized crystals are called phenocrysts.
- Phenocrysts are always large enough to be seen by the eye without magnification. By definition, phenocrysts are phaneritic.
- The smaller sized crystals between the larger sized crystals constitute the groundmass.
- Crystals of the groundmass may be visible to the naked eye - phaneritic - or fine-grained - aphanitic.
Click any image to enlarge.
porphyritic granite
porphyritic granite
Image credit: James St. John
porphyritic granite (labeled)
porphyritic granite (labeled)
Image credit: James St. John
porphyritic rhyolite
porphyritic rhyolite
Image credit: James St. John
porphyritic rhyolite (labeled)
porphyritic rhyolite (labeled)
Image credit: James St. John
Vesicular
Vesicular
- Vesicles are cavities in the igneous rock.
- In extrusive igneous rock, cavities are caused by expanding gas.
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pumice
pumice
Image credit: James St. John
vesicular basalt
vesicular basalt
Image credit: James St. John
vesicular basalt - scoria
vesicular basalt - scoria
Image credit: James St. John
Fragmental
Fragmental
- Eruptive igneous events cause rock to break into fragments.
- Fragments may also form from the fast cooling of gassy liquid blasted into the atmosphere.
- Fragments may be microscopic to building-sized boulders. A classification and naming of fragments by size
- A technical term for this category of rock is pyroclastic (fire-broken).
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tuff
tuff
Image credit: James St. John
breccia
breccia
Image credit: James St. John
Glassy
Glassy
- Eruptive lava of a high silica composition that chills very quickly prevents the formation of mineral crystals.
- A solid without long-range order to the chemical structure is a glass.
- A distinctive feature of volcanic glass is the curvilinear fracture pattern, termed conchoidal fracture.
Click any image to enlarge.
obsidian
obsidian
Image credit: James St. John
Click to enlarge.
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