CLASSIFICATION OF IGNEOUS ROCKS

Annotations:

• Rocks formed by the melting cooling and crystallization of other rocks.

1. TEXTURE
   1. Glassy

      Annotations:

      • -Formed by the very rapid cooling of magma. Usually around water bodies.  - No crystals
   2. Aphanitic

      Annotations:

      • -Formed by the rapid cooling of magma above the ground - Small homogenous crystals
   3. Phaneritic

      Annotations:

      • -Formed by the slow cooling of magma -Large crystals 
   4. Porphyritic

      Annotations:

      • -Large crystals embedded in small crystals -Phenocrysts embedded in groundmass 
2. COMPOSITIONAL CLASSIFICATION
   1. COLOUR
      1. Light

         Annotations:

         • -Termed 'felsic' because of the high silica content. -Very viscous and formed @ 850C -Non_Ferromagnesian
      2. Dark

         Annotations:

         • -Termed 'Mafic' cos of High Magnesium and Fe content -Formed at 1000C -Ferromagnesian
   2. SILICA CONTENT
      1. Felsic Granitic

         Annotations:

         • Composed of a lot of silica minerals
         1. Granite

            Annotations:

            • Phaneritic
         2. Rhyolite

            Annotations:

            • Aphanitic
         3. Obsidian

            Annotations:

            • Dark glassy texture
         4. Pumice

            Annotations:

            • Frothy, Glassy texture, contains a lot of air voids.
      2. Intermediate/ Andesitic

         Annotations:

         • -Contains 25% or More Dark silica  -due to explosive volcanic activity
         1. Andesite

            Annotations:

            • Aphanitic
         2. Diorite

            Annotations:

            • Phaneritic
      3. Mafic Basalt

         Annotations:

         • Dark silicate materials denser than Granitic rocks and is found on ocean floors. 
         1. Basalt

            Annotations:

            • -Aphanitic texture  -Composed of Calcium rich Plagioclase Feldspar and pyroxene 
         2. Gabbro

            Annotations:

            • Intrusive equivalent of Basalt