Volcanic Rock Types
The appearance and form of Volcanic rock types is controlled by their mineral chemistry and mode of eruption. Volcanic rocks are often erupted and cool quickly this results in fine or no crystals present. All volcanic rocks have igneous equivalents ie basalt is erupted gabbro magma,rhyolite is erupted granitic magma. Rock types are categorised by percentage of Silica.Minerals called feldspar and quartz (Si O2) are silica rich minerals and dominant in felsic rocks, giving them a lighter colour.
Pyroxene and Olivine are Iron and magnesium rich minerals and give Mafic rocks their dark colour. This affects their viscosity, the more silica the more viscous the less the lava flows. Also viscous magmas produce explosive eruptions as gas bubbles cannot escape easily and pressure builds up, resulting in violent eruptions.
Mafic 48-52% silica, Basalt
Intermediate 52-68% Andesite to Dacite
Felsic 68-77% Rhyolite/Pumice/Obsidian
Crystals need time to grow in magma and when eruptions occur the magma way be violently erupted and quickly cooled allowing no crystal growth. It may be that crystals that formed in the magma chamber are trapped in the volcanic rock texture, these are called phenocrysts.
Volcanic rock fabrics can form from the nature of the eruption, ie pillow lavas are basalt lava erupted underwater, tuffs are the result of volcanic ash falling and compating, bombs are fragments of liquid lava which have cooled and become smooth as they travel through the air.