Igneous rock classification
Igneous rock classification
Classifying igneous rocks
• Igneous rocks come in a range of different textures and chemical compositions.
• We can easily classify hand-specimens by colour and by grainsize.
Dark-coloured
Light-coloured
Fine-grained
Medium-grained
Coarse-grained
Simple classification
• As a rule of thumb, rocks rich in ferromagnesian minerals tend to have a darker colour and higher density than rocks rich in quartz and alkali feldspar. Obsidian is the obvious exception.
• Volcanic rocks tend to have very fine grains. Grainsize generally increases with the time taken for crystallisation.
Classifying igneous rocks
• Igneous rocks come in a range of different textures and chemical compositions
Ultramafic Mafic Intermediate Acid
Volcanic < 1mm
Basalt Andesite Rhyolite
Hypabyssal1-5mm
Dolerite Micro-diorite Micro-granite
Plutonic>5mm
Peridotite Gabbro Diorite Granite
Classification by chemistry
Classification by mineralogy
Classification summary