CLASSIFICATION OF IGNEOUS ROCKS

CLASSIFICATION OF IGNEOUS ROCKS

1. TEXTURE

   • Glassy
     • -Formed by the very rapid cooling of magma. Usually around water bodies.  - No crystals
   • Aphanitic
     • -Formed by the rapid cooling of magma above the ground - Small homogenous crystals
   • Phaneritic
     • -Formed by the slow cooling of magma -Large crystals 
   • Porphyritic
     • -Large crystals embedded in small crystals -Phenocrysts embedded in groundmass 

2. COMPOSITIONAL CLASSIFICATION

   • COLOUR
     • Light
       • -Termed 'felsic' because of the high silica content. -Very viscous and formed @ 850C -Non_Ferromagnesian
     • Dark
       • -Termed 'Mafic' cos of High Magnesium and Fe content -Formed at 1000C -Ferromagnesian
   • SILICA CONTENT
     • Felsic Granitic
       • Composed of a lot of silica minerals
       • Granite
         • Phaneritic
       • Rhyolite
         • Aphanitic
       • Obsidian
         • Dark glassy texture
       • Pumice
         • Frothy, Glassy texture, contains a lot of air voids.
     • Intermediate/ Andesitic
       • -Contains 25% or More Dark silica  -due to explosive volcanic activity
       • Andesite
         • Aphanitic
       • Diorite
         • Phaneritic
     • Mafic Basalt
       • Dark silicate materials denser than Granitic rocks and is found on ocean floors. 
       • Basalt
         • -Aphanitic texture  -Composed of Calcium rich Plagioclase Feldspar and pyroxene 
       • Gabbro
         • Intrusive equivalent of Basalt