The Geophile Pages: Lessons: Natural Disasters

Magma Compositions - Igneous rocks are often classified by mineral (chemical) composition.

Ultramafic
- Rare, high in magnesium and iron. Magmas with this composition are composed entirely of ferromagnesian silicates like olivine and pyroxenes.

Mafic (or basaltic)

Mafic: magnesium and ferrum (iron). Contains substantial dark silicate minerals like olivine and pyroxenes, and calcium-rich plagioclase feldspar.

Extrusive / Volcanic Rock: Basalt	Intrusive / Plutonic Rock: Gabbro
Aphanitic texture Composed mainly of pyroxene and calcium-rich plagioclase Most common extrusive igneous rock	Phaneritic texture Composed of pyroxene and calcium-rich plagioclase Comprises most of the oceanic crust

Intermediate (or andesitic)

Contains at least 25% dark silicate minerals. Associated with explosive volcanic activity

Extrusive / Volcanic Rock: Andesite	Intrusive / Plutonic Rock: Diorite
Aphanitic texture Dark grey color May have plagioclase phenocrysts	Phaneritic texture Composed of plagioclase & hornblende Black and white make DIORITE

Felsic (or granitic)

Felsic: feldspar and silica. Composed almost entirely of light-colored silicates

Extrusive / Volcanic Rock: Rhyolite	Intrusive / Plutonic Rock: Granite
Aphanitic texture Buff or pink color May have glass phenocrysts	Phaneritic texture >25 % quartz, about +65 % feldspar The term covers a wide range of rocks (Often incorrectly)

Varied Compositions
- Obsidian
  - Glassy texture
  - Dark in color
- Pumice
  - Glassy texture
  - Light in color
  - Frothy appearance with numerous voids due to escaping gases
- Pyroclastic rocks
  - Composed of fragments ejected during a volcanic eruption
  - Tuff – ash-sized fragments
  - Volcanic breccia – particles larger than ash

Common Minerals Found In Igneous Rocks

Igneous Rocks Lesson