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